advances in the production and chemistry of the …...advances in the production and chemistry of...

Advances in the Production and Chemistry of the Heaviest ElementsAndreas Turler*,†,‡ and Valeria Pershina§

†Laboratory of Radiochemistry and Environmental Chemistry, Department of Chemistry and Biochemistry, University of Bern,Freiestrasse 3, CH-3012 Bern, Switzerland‡Laboratory of Radiochemistry and Environmental Chemistry, Department Biology and Chemistry, Paul Scherrer Institute, CH-5232Villigen PSI, Switzerland§GSI Helmholtzzentrum fur Schwerionenforschung GmbH, Planckstrasse 1, D-64291 Darmstadt, Germany

CONTENTS

1. Introduction 12382. History of the Discovery of the Transuranium

Elements 12392.1. Actinides 12392.2. Transactinides 1240

3. Nuclear Properties 12424. Nuclear Reactions of Colliding Nuclei 12435. Nuclear Experimental Techniques 12446. Relativistic Effects on Chemical Properties 1246

6.1. Relativistic Effects on Atomic ElectronicShells 1246

6.2. Current Relativistic Quantum-ChemicalMethods 1247

6.3. Relativistic Effects and the Future PeriodicTable of the Elements 1248

7. Chemical Techniques To Investigate Transacti-nides 12497.1. Prerequisites for a Chemical Isolation of

Heaviest Elements 12497.1.1. Synthesis of Heaviest Elements 12497.1.2. Rapid Transport 12517.1.3. Chemical Isolation, Sample Preparation,

and Detection 12527.2. Gas-Phase Chemistry: Isothermal Chroma-

tography and Thermochromatography 12527.2.1. Isothermal Chromatography 12537.2.2. Thermochromatography 1255

7.3. Liquid-Phase Chemistry 12567.3.1. Manual Liquid−Liquid Extractions and

Column Chromatography 12577.3.2. Automated Column Separations 12577.3.3. Automated Liquid−Liquid Extractions 1258

8. Methods To Predict Experimentally MeasurableProperties of Transactinides 12598.1. Volatility 1259

8.1.1. Physisorption and Chemisorption Mod-els 1259

8.1.2. Empirical Correlations and Extrapola-tions 1260

8.2. Complex Formation in Aqueous Solutions 12619. Rutherfordium (Z = 104) 1262

9.1. Theoretical Predictions 12629.2. Experimental Results 1264

9.2.1. Gas-Phase Chemistry of Rutherfordium 12659.2.2. Liquid-Phase Chemistry of Rutherfordi-

um 126810. Dubnium (Z = 105) 1273


10.2.1. Gas-Phase Chemistry 127510.2.2. Liquid-Phase Chemistry of Dubnium 1277

11. Seaborgium (Z = 106) 127811.1. Theoretical Predictions 127811.2. Experimental Results 1279

11.2.1. Gas-Phase Chemistry 127911.2.2. Liquid-Phase Chemistry 1282

12. Bohrium (Z = 107) 128212.1. Theoretical Predictions 128212.2. Experimental Results 1283

12.2.1. Gas-Phase Chemistry of Bohrium 128313. Hassium (Z = 108) 1284


13.2.1. Gas-Phase Chemistry 128513.2.2. Liquid-Phase Chemistry 1286

14. Meitnerium (Z = 109), Darmstadtium (Z = 110),Roentgenium (Z = 111) 128714.1. Theoretical Predictions 1287

15. Copernicium (Z = 112) 128815.1. Theoretical Predictions 128815.2. Experimental Results 1290

15.2.1. Gas-Phase Chemistry 129016. Element 113 1292


16.2.1. Gas-Phase Chemistry 129317. Flerovium (Z = 114) 1293


17.2.1. Gas-Phase Chemistry 1294

Special Issue: 2013 Nuclear Chemistry

Received: June 15, 2012Published: February 13, 2013

Review

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© 2013 American Chemical Society 1237 dx.doi.org/10.1021/cr3002438 | Chem. Rev. 2013, 113, 1237−1312

pubs.acs.org/CR

18. Element 115, Livermorium (Z = 116), Element117, and Element 118 129618.1. Theoretical Predictions 1296

19. Element 119 and Element 120 129719.1. Theoretical Predictions 1297

20. Elements beyond Z = 120 129820.1. Theoretical Predictions 1298

21. Conclusions and Outlook 129821.1. Summary of Volatility Studies of the

Heaviest Elements and Their Compounds 130021.2. Summary of Aqueous Chemistry Studies of

the Heaviest Elements 130021.3. Future Developments 1300

Author Information 1300Corresponding Author 1300Notes 1300Biographies 1300

Acknowledgments 1301Abbreviations and Quantities 1301References 1302

1. INTRODUCTION

After Dmitri Ivanovich Mendeleev and, independently, JuliusLothar Meyer, and others discovered the ordering principles ofthe elements, Mendeleev proposed the first valid Periodic Tableof the Elements nearly 150 years ago.1 We are now at a pointwhere the discovery of the last missing element in the seventhperiod, namely synthesis of the element with atomic number117, has recently been announced.2,3 Therefore, the PeriodicTable currently contains 118 elements, the lightest beinghydrogen and the heaviest yet unnamed eka-radon (or Uuo,ununoctium) as provisional name in IUPAC (InternationalUnion of Pure and Applied Chemistry) terminology. However,among heavy element chemists, the IUPAC provisional namesare only rarely used. Instead, for elements that have beenreported, but not fully authenticated, quite often only theatomic number is being used (i.e., element 113 or E113). Amodern Periodic Table of the Elements is shown in Figure 1.But has the far end of the Periodic Table of the Elements

now been reached? What is the heaviest element in the PeriodicSystem? Are there still undiscovered ones which might even befound in nature? Is there an eighth period and how many

elements will it contain? Will we need to introduce the g-orbitals, and will the current principles governing the groupsand periods of the Periodic Table still be valid for the heaviestelements? These intricate questions are the topic of currentresearch in the chemistry of the heaviest elements.For increasingly heavy nuclei, the electrostatic repulsions of

protons cannot be sufficiently compensated by the attractivenuclear force through an increasing number of mediatingneutrons. Therefore, the heaviest stable known nucleus isalready reached with 208Pb. All isotopes of heavier elements,including some elements such as Bi, Th, and U that still can befound in nature as remnants of the last nucleosynthesis process,are radioactive and decay preferentially by successive α-particleand β-particle emissions back to the last stable element Pb. Theheaviest nuclide, which was reported to be detected in traces innature, is 244Pu, with a radioactive half-life (t1/2) of 81.2 millionyears. It is conceivable that 244Pu could still be present on earthas primordial element or be of cosmic origin due to explosivestellar nucleosynthesis, for example from a nearby supernovathat occurred after the formation of our solar system. In anattention attracting article, Hoffman et al.5 reported aconcentration of about 2350 atoms of 244Pu per gram ofBastnaesite, a mineral highly enriched in rare earth elements.However, a recent search in Bastnaesite from the same mineusing accelerator mass spectrometry remained negative with adetection limit of less than 370 atoms per gram (99% upperconfidence limit).6 Well established is the presence of smallquantities of 237Np and 239Pu in nature due to neutron captureprocesses on 235U and 238U, respectively.7 Furthermore, recentevidence presented by Marinov et al.8 about the existence of along-lived superheavy element with atomic number Z = 122 (orneighboring element) and mass number A = 292 with anabundance of about 1 × 10−12 relative to Th in natural Thcould not be authenticated.9,10 Also the claimed observation ofextremely long-lived, high spin, super- or hyperdeformedisomeric states in neutron deficient heavy nuclei,11 which wasused as an argument to explain the observation of long-lived292122, could not be observed in independent experiments.12,13

Therefore, all elements heavier than Pu (Z = 94) are man-made. Elements up to Fm (Z = 100) were synthesized insuccessive irradiations of ever heavier elements with α-particles(4He2+) and neutrons. Weighable amounts of various actinideswere then produced by successive neutron capture and β−-

Figure 1. Current Periodic Table of the Elements with IUPAC approved numbering of groups and element symbols. The names for elements 114(Flerovium, Fl) and 116 (Livermorium, Lv) suggested by the team of discoverers have recently been officially accepted by IUPAC4.

Chemical Reviews Review

dx.doi.org/10.1021/cr3002438 | Chem. Rev. 2013, 113, 1237−13121238

decay in nuclear reactors. Transactinide elements aresynthesized in heavy ion fusion reactions at high poweraccelerators on a “one atom at a time” level with beams rangingfrom O to Zn, with 294118 currently being the heaviestobserved nucleus.14 Some theoretical studies place the limits ofnuclei that can no longer exist as bound entities beyond Z ≅300 and A ≅ 960.15 Even though such nuclei could theoreticallyexist as so-called hyperheavy bubble nuclei, the heaviestelements should exist in the form of atoms. To qualify as achemical element, the nucleus of the longest lived isotopeshould exist >10−14 s,16 which is the time needed for theformation of an electron shell. The number of electrons thatcan be arranged around a nucleus is limited. Modern relativisticelectronic structure theory that takes into account quantumelectrodynamic effects (QED) predicts this to happen atelement Z = 173, where the energy of the 1s electron falls intothe negative energy continuum;17 that is, it becomes less than−2mec

2.The place an element occupies in the Periodic Table is not

only defined by its atomic number, i.e. the number of protonsin the nucleus, but also by its electronic configuration, whichdefines its chemical properties. Strictly speaking, a new elementis assigned its proper place only after its chemical propertieshave been sufficiently investigated. In some cases it has beenpossible to experimentally investigate chemical properties oftransactinide elements and even synthesize simple compounds.Due to the predicted strong influence of relativistic effects, theexperimental investigation of superheavy elements is especiallyfascinating.The study of the chemical properties of the heaviest known

elements in the Periodic Table is an extremely challenging taskand requires the development of unique methods but also thepersistence to continuously improve all the processes andcomponents involved in order to achieve the ultimate goal ofchemically isolating one single atom that lives for only a fewseconds. At first sight, the study of the chemical properties ofthe heaviest elements appears to be of purely academic interest.Indeed, as of today, it is not conceivable that weighablequantities of any transactinide element will be produced in thenear future, and their immediate practical use appearsquestionable. Nevertheless, chemical studies of the heaviestelements open up possibilities for a deeper insight into theregularities of the Mendeleev Periodic System. Recent experi-ments have demonstrated that the chemical properties of theheaviest elements can no longer be predicted from simpleextrapolations of the regularities in the groups and periods ofthe Periodic Table.Due to the low production rates of the heaviest elements,

chemical information obtained from experiments is limited tothe knowledge of very few properties. It mainly answers thequestion whether a new element behaves similarly to its lightercongeners in a chemical group, or whether some deviationsfrom the trends occur due to very strong relativistic effects onits valence electron shells. Thus, in this area of the PeriodicTable, theory starts to play an extremely important role and isoften the only source of useful chemical information. Forexample, electronic configurations can only be calculated at themoment. Properties such as a chemical composition, stabilityand geometrical configuration, ionization potential (IP),electron affinity (EA), etc. can also be obtained onlytheoretically. Theoretical studies are also invaluable inpredicting and/or interpreting the outcome of sophisticatedand expensive experiments with single atoms. Moreover, it is

only theory that can reveal how relativistic effects influencechemical properties: only a comparison of the observedbehavior with that predicted on the basis of relativistic vsnonrelativistic calculations does allow assessing the importanceand magnitude of relativistic effects.Theoretical chemical research on the heaviest elements is not

less challenging than the experimental one and should be basedon the most accurate relativistic electronic structure calculationsin order to reliably predict properties and experimentalbehavior of the new elements and their compounds. It alsorequires the development of special approaches that bridgecalculations with quantities that cannot be so easily predictedfrom calculations. Due to the recent spectacular developmentsin relativistic quantum theory, computational algorithms, andtechniques, very accurate calculations of properties of thetransactinide elements and their compounds are now possible.The experimental verification of these predictions is highly

desirable, but also very demanding. The chemical identificationof transactinide elements is, up until now, always accomplishedby the detection of the characteristic radioactive decayproperties of their isotopes. The knowledge about the nucleardecay properties of transactinide nuclei is by no meanscomplete, and a great number of them have not even beendiscovered. The field of heavy element chemistry is thereforeclosely related to low energy nuclear physics. Actually, achemical separation can be regarded as a somewhat slow (interms of nuclear physics) but powerful Z separator. As will bediscussed, the discovery of new elements and the identificationof new nuclides were not always straightforward. Especially thecorrect assignment of mass numbers proved difficult, sincenuclear isomeric states are frequent in the heaviest nuclides andthe experimental determination of masses has not progressedyet into the region of transactinide nuclides.In the current article, recent advances in the synthesis,

chemical characterization, and theoretical studies of the heaviestelements will be reviewed. Recent reviews on the topic werepublished by Scha del18,19 and in a special edition ofRadiochimica Acta edited by Kratz.20−28 Books include TheChemistry of Superheavy Elements29 and book sections byMunzenberg et al.,30 Hoffman et al.,31 and Kratz.32 Previousreviews summarize the theoretical chemistry of the heaviestelements.25,33−41

2. HISTORY OF THE DISCOVERY OF THETRANSURANIUM ELEMENTS

2.1. Actinides

It is worthwhile to shed some light on the history of discoveryof new elements.42 Until World War II, the heaviest knownelements, Th, Pa, and U, were thought to be homologs of Hf,Ta, and W, respectively, and thus members of groups 4, 5, and6. With the discovery of neptunium (Np, Z = 93)43,44 in 1940and its chemical resemblance to U, and the subsequentdiscovery of plutonium (Pu, Z = 94),45,46 serious doubts aboutthe placement of these elements in the Periodic Table surfaced.The new elements were thought to be members of a new“uranide” series. A further significant change to the PeriodicTable came in 1944 with the recognition by Seaborg47 thatactually all the elements heavier than actinium were misplacedand that this series was the actinide series and not the uranideseries, resulting from the filling of the 5f shell. This concept hadgreat predictive value and was instrumental in the discovery ofmany new actinide and transactinide elements.



It is astounding that the rather innocent and misled (due tothe wrong placement of Th, Pa, and U as members of groups 4,5, and 6 in the Periodic Table) search for transuraniumelements by Fermi, and Hahn, Meitner, and Strassmann,ultimately started a spiral of monumental discoveries, whichhad a tremendous impact on humanity. Instead of identifying anew element, Hahn, Meitner, and Strassmann discovered thenuclear fission process late in 1938.48 The spiral took its firstturn with an experiment by McMillan, who studied the neutroninduced fission process of 238U. Unexpectedly, he observed aradionuclide with t1/2 = 2.3 days which did not recoil from thethin uranium target.43 This radioactivity turned out to be thenew element Np.44 Surprisingly, the chemical behavior of Npwas very similar to that of U and not at all similar to that of agroup-7 element.49 The discovery of Np led to the subsequentdiscovery of Pu. The β¯ decaying 238Np was produced by thebombardment of 238U with deuterons in 1941.45,46 Only a fewyears after the synthesis of a few atoms of Pu, kilograms of Puwere produced to build the first nuclear device using Pu asfissile material. The elements curium (Cm, Z = 96) andamericium (Am, Z = 95) (in this sequence) were discovered atthe Chicago Metallurgical Laboratory by irradiation of 239Puwith α-particles and neutrons, respectively.47 The bombard-ment with α-particles took place in Berkeley at the 60-in.cyclotron, after which the material was shipped to Chicago forchemical processing. Shortly after the end of World War II,berkelium (Bk, Z = 97)50 and californium (Cf, Z = 98)51 werethen discovered in Berkeley by irradiation of Am and Cm withα-particles. For the discovery of the actinide series and thesynthesis of all the transuranium elements up to Cf, Seaborgand McMillan were awarded the Nobel price in 1951.Again, very unexpectedly, the discovery of fission, which led

to nuclear weapons and to nuclear energy, led to the discoveryof rapid multiple neutron capture in the explosion of athermonuclear device and the isolation of the new elementseinsteinium (Es, Z = 99) and fermium (Fm, Z = 100) from thedebris in 1952.52 Transplutonium elements are being producednowadays in weighable quantities, from kilograms of 241Am to afew picograms of 257Fm in high flux reactors through successiveneutron capture reactions.42

The synthesis and subsequent identification of mendelevium(Md, Z = 101) in 1955 marked the beginning of a new era,53

since this was the first experiment where a new element wasproduced on a “one atom at a time” level. Also, the recoiltechnique was invented, which takes advantage of the fact that,in the fusion process of the heavy ion beam with the targetnucleus, enough momentum is transferred to the compoundnucleus to eject it from the layer of target atoms. This way, thereaction products are already separated from the target atoms.In the discovery experiment of Md, only 109 target atoms of253Es were bombarded with α-particles.53 In present dayexperiments, targets with thicknesses of typically 1018 targetatoms per cm2, such as 244Pu, 248Cm, 249Bk, and 249Cf, are used.The availability of even heavier target nuclides, such as 250Cf,251Cf, or 254Es, still is very limited. The high neutron flux from,for example, 252Cf presents enormous technical difficulties inhandling milligram quantities of this material. At Md, an eraended where chemical identification of a new element played adominant role. The position at which a newly created heavyactinide element was eluted from a cation-exchange (CIX)column was indicative of its ionic radius and thus also of itsatomic number. Analogous to the lanthanides, the radii of theM3+ and M4+ ions are decreasing with increasing positive charge

of the nucleus. This effect, generally referred to as lanthanide oractinide contraction, is a consequence of the addition ofsuccessive electrons to the inner f shell. The diminishingshielding of the increasing nuclear charge by the f electronscauses a contraction of the valence shells with each additionalproton in the nucleus and, hence, a decrease in the atomic orionic radius.The discovery of the last two actinide elements, nobelium

(No, Z = 102) and lawrencium (Lr, Z = 103), was not asstraightforward as the discovery of elements Np through Mdand involved scientists and institutions outside the UnitedStates. The name nobelium for element 102 was suggested by ateam of scientists working at the Nobel Institute in Stockholmwho claimed to have discovered the new element. This claim,however, has been proven wrong. Nevertheless, the elementname has been retained by the IUPAC and will remain in thePeriodic Table. The discovery of the last actinide elementsNo54,55 and Lr56 by the Berkeley Laboratory was notuncontested, and the discovery of new chemical elementsbecame part of the cold war between the United States and theSoviet Union. In this context, the name of G. N. Flerov at thenow named Flerov Laboratory of Nuclear Reactions (FLNR) atthe Joint Institute for Nuclear Research (JINR) in Dubna,Russia, has to be mentioned. A paper by Flerov et al. “Historyof the transfermium elements Z = 101, 102, 103” presents theview by the involved Dubna scientists about the priority ofdiscovery of elements 101, 102, and 103.57

2.2. Transactinides

All isotopes of the transactinide elements with atomic numberZ ≥ 104 have artificially been synthesized in heavy ion fusionreactions at accelerators and have only been studied in oneatom-at-a-time experiments. The discovery experimentsemployed newly developed methods that relied on physicsrather than chemistry to identify a new element. Thecontroversy on the priority of discovery between Berkeleyand Dubna also involved the first two transactinide elements,rutherfordium (Rf, Z = 104) and dubnium (Db, Z = 105). Adetailed analysis of all the experiments made in Dubna and inBerkeley was presented by Hyde et al.58 The authors concludedthat priority of discovery should be awarded to the work of theBerkeley team for both elements. A different view waspresented by Flerov and Ter-Akopyan from Dubna.59 Veryimportant points in the chain of evidence of the Russian claimwere the chemistry experiments conducted by Zvara et al.60−67

The fierce competition between Berkeley and Dubna,“frequently punctuated by acerbic comments” as noted bySeaborg,42 obscured to a great extent the advances that weremade in Berkeley and in Dubna in tackling the enormousdifficulties to produce and study few single atoms of very short-lived nuclides. In Berkeley, the development of the gas-jettechnique and of sophisticated counting equipment allowed theregistration of successive mother−daughter α-particle decaychains originating from the same sample, displaying the correctsequence of decay energies and time differences. Thus, thegenetic relationship could be established unequivocally. InDubna, the development of ultrafast gas chemical separationswas instrumental for later chemical studies of transactinideelements in the gas phase.Also the discovery of seaborgium (Sg, Z = 106) was not

uncontested. Almost simultaneously, scientists from Berkeleyand Dubna announced the discovery of element 106. InBerkeley the reaction 249Cf(18O, 4n)263Sg was used.68 The claim



from Berkeley was firmly established by the observation ofgenetically linked α−α correlations from the decay of 263Sg and259Rf. In the work of Oganessian et al.,69 the reaction 207Pb-(54Cr, 2n)259Sg was utilized. The observation of a spontaneous-fission (SF) radioactivity with t1/2 ≅ 4−10 ms could later not beconfirmed for 259Sg. However, the nuclide 260Sg decays byfission with about 50% branching and t1/2 = 3.6 ms. It istherefore conceivable that the 1n-channel was observed.Oganessian et al.69 realized that the so-called “hot” fusionreactions using light projectiles and heavy actinide targetswould at some point reach extremely low cross sections, sincethe produced compound nucleus was formed with a highexcitation energy of the order of 40−50 MeV. The cooling ofthe compound nucleus by the emission of neutrons alwayscompeted with prompt fission. Therefore, the Dubna teamfavored the so-called “cold” fusion reaction using 208Pb as targetmaterial. Due to the heavier projectiles, the fusion cross sectionwas diminished significantly, but due to the almost coldformation of the compound nucleus, only one or two neutronswere evaporated.The consequent exploitation of cold fusion reactions using

Pb and Bi targets and the detection of genetically linked decaychains by the group headed by Armbruster, Munzenberg, andHofmann led to the discovery of 6 new elements70 at theGesellschaft fur Schwerionenforschung, GSI, in Darmstadt,Germany. All new elements were discovered using the velocityfilter SHIP (Separator for Heavy Ion Reaction Products), whichseparates fusion reaction products in flight from the beam andfrom transfer reaction products. Since all identified newelements decayed rapidly by successive α-particle emissionsto lighter already known nuclei, no ambiguities concerning theidentification of the new nuclides surfaced. The number ofemitted α-particles in the chain was indicative of the atomicnumber of the new nuclide by adding two units in Z for eachemitted α-particle to the last securely identified decay product.The SHIP velocity filter can identify reaction products withproduction cross sections as low as ≈100 fb (1 fb = 10−39 cm2).With the reported discovery of elements up to Z = 109, the

time had come to authenticate the different claims and to cometo conclusions about priority of discovery so that officialelement names could finally be adopted. In 1985 IUPAP andIUPAC decided to establish a Transfermium Working Group(TWG) to consider questions of priority in the discovery ofelements with nuclear charge number Z > 100. The TWGissued its report16 in 1992. The contents of the report wereaccepted, in general, by the Dubna71 and Darmstadt72

laboratories, but heavily criticized by the Berkeley group.73 Aconcise summary of the developments concerning the discoveryof elements 101−111 was published by Greenwood.74 Finallyin 1997, a compromise was reached.75 The element names andsymbols adopted in August 1997 by the general assembly ofIUPAC for elements with atomic numbers 102 through 109 arelisted in Table 1. A new joint working party of IUPAP andIUPAC (JWP) took up its work in 1998 to assess the discoveryof elements 110−112. In its report,76 the JWP considered thework of Hofmann et al.77 as sufficient to claim discovery ofelement 110, while confirmation by further results wasrequested to assign priority of discovery of elements 111 and112. Priority of discovery of element 11178,79 and element11279,80 was assigned to Hofmann et al.81,82 in 2003 and 2009,respectively, especially since in the same reactions both nuclides272111 and 277112 were independently synthesized by Morita etal.83,84 at RIKEN in Japan. The names proposed by the

discoverers and accepted by the IUPAC are also included inTable 1. Three decay chains assigned to the nuclide 278113synthesized in the reaction 209Bi(70Zn, 1n) have been reportedby the same laboratory.85−87 The experiment required 553 daysof accelerator beam time, yielding a record low productioncross section of 22−13

+20 fb.87

All the isotopes of the newly discovered elements were veryshort-lived, and t1/2 decreased from ≈1 min for 261Rfa to <1 msfor 277Cn.80,88 Only a few atoms were produced in month longexperiments. With regard to chemical investigations, prospectsfor experimental chemical studies of elements beyond Sg wererather dim, with the exception of hassium, where, in the α-decay chain of the nuclide 277Cn, the isotope 269Hs with t1/2 =10 s was observed.80

Largely unnoticed by the community of chemists, the past 15years did bring another substantial extension of the PeriodicTable by six new elements through a series of physicsexperiments at the FLNR in Dubna, Russia. By using thevery tightly bound, doubly magic nucleus 48Ca and actinidetarget nuclei, Oganessian and co-workers89 were able tosynthesize single atoms of elements 113 through 118 andobserve their radioactive decay. So far, only elements 114 and116 have been authenticated by IUPAC,90,91 and the elementnames flerovium, Fl, and livermorium, Lv, suggested by theteam of discoverers, have recently been made official byIUPAC.4

The addition of six new elements in the past decade isremarkable in several ways. First, the maximum productioncross sections of elements 104−112 could be described ratherwell by an exponential decay law (see section 4, Figure 7),where the cross sections dropped by roughly a factor of 10when increasing the atomic number by 2 units. In the synthesisof 277Cn, production cross sections of less than 1 pb (10−36

cm2) were determined. However, by using 48Ca projectiles, thistrend was broken and rather constant maximum productioncross sections of several picobarns were measured for synthesisof elements 112−116, and even for elements 117 and 118,values near or slightly below 1 pb were observed. Nevertheless,even under optimum conditions, a production cross section of

Table 1. Element Names and Symbols Adopted by IUPACfor Transfermium Elements with Atomic Numbers 101−118

atomicno. element name symbol other names suggested or prev in use

101 mendelevium Md102 nobelium No flerovium joliotium, Jl103 lawrencium Lr104 rutherfordium Rf kurchatovium, Ku; dubnium, Db105 dubnium Db hahnium, Ha; nielsbohrium, Ns

joliotium, Jl106 seaborgium Sg rutherfordium, Rf107 bohrium Bh nielsbohrium, Ns108 hassium Hs hahnium, Hn109 meitnerium Mt110 darmstadtium Ds111 roentgenium Rg112 copernicium Cn113 ununtrium Uut114 flerovium Fl115 ununpentium Uup116 livermorium Lv117 ununseptium Uus118 ununoctium Uuo



1 pb translates into the synthesis of only 1 atom of a superheavynuclide every 36 h, on average. Second, of the more than 50new nuclides produced in these experiments, a number of themhave t1/2 > 1 s and, thus, live long enough for chemicalinvestigations. This result is in strong contrast to those of thepreviously known, more neutron deficient isotopes of Mt, Ds,Rg, and Cn in the range of a few milliseconds.Figure 2 shows the number of discovered transuranium

elements as a function of time. The synthesis of new elements

was not a continuous process but occurred in phases. A newtechnical development or a new concept allowed the discoveryof several elements before a limit was reached that could onlybe overcome by substantial improvements or a new concept.Such a period is evident between 1984 and 1994, where anumber of improvements at the SHIP separator and theUNILAC accelerator allowed an increase in sensitivity of about1 order of magnitude, resulting in the subsequent discovery ofanother three elements without abandoning the concept of coldfusion using Pb or Bi targets. A change of concept using 48Cabeams and actinide targets allowed the Dubna−Livermorecollaboration the discovery of six new elements. Since noheavier actinides than Cf are available in sufficient quantities astarget materials, this extremely successful path has beenexhausted and new ideas are required to push past element118 and open the eighth period of the Periodic Table.

3. NUCLEAR PROPERTIESThe synthesis and observation of long-lived heavy nuclides is atriumph of the nuclear shell model that has predicted theexistence of such nuclei since the 1950s. These hypotheticalnuclei were called “super-heavy”.92 However, the term hadappeared already in 1938 in a review by Quill93 ontransuranium elements and was later used also for newlydiscovered elements.94 The limit of existence of heavy elementsis determined by the balance between the repulsive Coulombforces of the many protons and the attractive nuclear forces.Immediately after the discovery of nuclear fission,48 Meitnerand Frisch95 located the limit of existence of heavy nuclei ataround Z ≈ 100, where the surface tension in the chargednuclear droplet model96 can no longer compensate therepulsive forces. Later, it was realized that at certain “magic

numbers” of nucleons, for example, 2, 8, 20, 28, 50, 82, and 126,the liquid-drop model underestimated the stability of thesenuclei significantly. The shell model accounts for these regionsof stability by showing that “magic numbers” of neutrons andprotons form stable shells similar to electrons in atoms andbased on the same quantum mechanical laws. Nuclear shelleffects can be so strong that nuclei exist beyond themacroscopic limit imposed by the liquid drop model of fission.By introducing the macroscopic−microscopic method, Strutin-sky97 was able to introduce shell correction energies to theliquid-drop model. The prediction of the existence of long-lived(i.e., t1/2 > 1 s) super-heavy nuclei has been credited to differentauthors.92,98 Myers and Swiatecki99 first predicted an island ofsuper-heavy elements around Z = 126 and N = 184, which waslater refined to mostly Z = 114 and N = 184.100−104

Modern theoretical descriptions of superheavy nuclei using apurely microscopic approach have been reviewed by Naza-rewicz et al.105 and Cwiok et al.106 These represent self-consistent Hartree−Fock-type calculations, which use effectivedensity-dependent interactions of both zero (Skyrme) andfinite (Gogny) range, and also the relativistic mean fieldapproach. These calculations tend to locate the center of theisland at Z = 114, Z = 120, or Z = 126 while confirming the N =184 neutron shell. The more traditional macroscopic−micro-scopic methods,21 which include higher orders of nucleardeformation are able to explain the increased stability observedaround Z = 108 and N = 162, a region that was much closer tothe region of then known nuclei. In Figure 3 the shell

correction energies calculated by Sobiczewski et al.107 areshown. For barrel shaped nuclei (hexadecapole deformation)around Z = 108 and N = 162, the shell-correction energyreached almost the same magnitude as for the traditional,spherical shell closure for superheavy elements around Z = 114.This meant that there exists not an isolated island far removedfrom the region of fairly well-known nuclides, but a subislandreaching out to the island of superheavies.108

Figure 2. Growth of the number of transuranium elements as afunction of time.

Figure 3. Contour map of calculated ground-state shell correctionenergies. Figure adapted from ref 107. Copyright 2001 by TheAmerican Physical Society. The locations of the doubly magic nuclei208Pb, 270Hs, and 298Fl are indicated. The nucleus in the center of thenew subisland, 270Hs, can be reached, for example, in the reaction248Cm(26Mg, 4n) and has recently been synthesized.109 48Ca-inducedreactions on actinide targets still fall short of reaching the predictedcenter of spherical shell closure for superheavy elements around Z =114.



This discovery had a tremendous impact on the synthesis ofnew elements, since now the region at or beyond Z = 114 couldbe accessed in a step by step procedure, without riskingplunging into the “sea of instability”. This is especiallyimportant, since 48Ca-induced reactions on actinide targets,which currently are the only experimentally feasible combina-tions to synthesize neutron-rich superheavy elements, still fallshort by a large margin of reaching the center of the predictedspherical shell closure around Z = 114 and N = 184.Due to the large number of nucleons in superheavy nuclides

and the resulting complicated nuclear structures and shapes,nuclear isomerism is a common phenomenon. This means thata nucleus may exist also in excited states, which are observableand have measurable half-lives. For example, the observeddecay properties of a nucleus may vary considerably dependingon whether it was produced directly in a nuclear reaction,usually with high spin, or as a decay product of a heaviernucleus. Isomeric states of nuclei are denoted with the suffix m(e.g., 211Pom). Often, it is not yet clear which one of theobserved states is the ground state and which is the excited one.In this case, the suffix a or b is used to denote these states (e.g.,261Rfa and 261Rfb).Figure 4 displays an updated transactinide cut-out of the

chart of nuclides. The color coding is that of the “Karlsruhe”chart of nuclides.110 One can see that the 48Ca inducedreactions on actinide targets have led to the discovery of sixnew elements and their associated α-decay chains, alwaysending in SF. This region of >50 new nuclides is detached fromthe older region of previously known nuclei. The fact that thereis no overlap of this new region with the region of previouslyknown nuclei created some problems in unambiguouslyassigning the atomic number of the synthesized nuclei, sinceall decay chains ended by SF in a previously uncharted region.

4. NUCLEAR REACTIONS OF COLLIDING NUCLEI

So far, transactinide elements can only be synthesized artificiallyin complete heavy ion fusion reactions. The fusion of twonuclei is a very complex process. Although there exist numerousarticles in the literature treating the collision and fusion of twonuclei, we restrict ourselves here to cite Zagrebaev et al.,111

which gives an in-depth analysis of the individual stepsinvolving the synthesis of a heavy nucleus. First the repulsiveCoulomb forces have to be overcome, so that the two collidingnuclei get into contact. When two nuclei are made to collide,several different reaction pathways open, depending on theimpact parameter of the approaching nuclei and the kineticenergy of the projectile. This is schematically shown in Figure5. If the impact parameter is too large or the kinetic energy toolow, the projectile is elastically scattered off the target nucleus,

Figure 4. Cut-out of a chart of nuclides showing all presently known transactinide nuclides (color coding according to the “Karlsruhe” chart ofnuclides110).

Figure 5. Schematic representation of projectile trajectories dependingon impact parameter. Reprinted with permission from ref 112.Copyright 1997 Clarendon Press.



leading to Coulomb excitation of the colliding nuclei. At lowerimpact parameters, the nuclei may get close enough so that theexchange of few nucleons can occur (reaction pathway a).These reactions are called quasi-elastic, since the contact time isvery short and the projectile is scattered off the target similarlyas in elastic scattering. At impact parameters significantlysmaller than the sum of both radii, the contact of both nuclei isdeeper and the attractive nuclear forces come into play to thepoint where the dinuclear system begins to fuse into a singlelarge nucleus. Exchange reactions of many nucleons are calleddeep inelastic transfer (reaction pathway b). In rare cases thetwo nuclei fuse (reaction pathway c). In most cases the fusedsystem decays again in two fragments in a fission-like process.Reaction pathways a−c can be observed experimentally. In

Figure 6 the total integrated mass yield of the reaction 288

MeV 40Ar + 238U (dash-dotted line) and its decomposition intoindividual contributions is shown.113,114 Quasielastic transferreactions (a) lead to two narrow distributions with highmaximum cross section near the masses of the projectile andthe target nucleus. Deep-inelastic transfer reactions lead towider distributions, where the identity of the projectile or thetarget is not completely lost (b). The dashed line above massnumber 240 indicates that most of these transfer reactionproducts will disintegrate by fission due to the relatively highexcitation energy and angular momentum they are formed with,giving rise to the distribution of fission fragments denoted with(d). Central collisions lead to complete fusion. The Q value ofthe fusion reaction is positive, meaning that, even if the fusionoccurs at the Coulomb barrier, the compound nucleus isformed with an excitation energy between 30 and 50 MeV inso-called “hot fusion” reactions, i.e. reactions with lightprojectiles (18O, 22Ne, 26Mg, 36S) and actinide targets, and10−20 MeV in “cold fusion” reactions, i.e. reactions with heavyprojectiles and 208Pb or 209Bi targets. In most cases, the formedcompound nucleus disintegrates by fission, giving rise to thebroad distribution denoted by (c). Only very rarely (≈10−9) isthe high excitation energy dissipated by the evaporation ofparticles, mostly neutrons, and by the emission of γ-rays. Eachevaporated neutron removes ≈10 MeV of excitation energy, buteach neutron evaporation process is in competition with

prompt fission. Production cross sections for transactinidenuclides range from nanobarns (10−33 cm2) to picobarns (10−36

cm2); these very low cross sections have to be compared withthe reaction channels a−c with cross sections of 1−100millibarns (10−27 cm2).In essence, a substantial part of the chart of nuclides (or the

Periodic Table) is produced in these collisions. Thus, at typicalbeam currents of 1012 projectiles/s and target thicknesses of1018 target atoms/cm2, quickly isolating one single atom fromthe plethora of reaction products is a formidable task.In Figure 7, experimental production cross sections of heavy

and superheavy nuclei for “cold fusion” reactions (1n emission)

and “hot fusion” reactions (5n emission) are shown as afunction of atomic number. The “abnormal” behavior of 48Cainduced production cross sections, which stay rather constant atthe level of a few pb, was associated with stabilizing shell effects,i.e. increasing fission barrier heights, when approaching theregion of Z = 114.89

5. NUCLEAR EXPERIMENTAL TECHNIQUESThe synthesis of transactinide elements requires intense heavyion beams of mostly neutron-rich, low-abundance isotopes andoften exotic, radioactive actinide target materials (see section7.1.1). The required projectile energies are those close to theCoulomb barrier, i.e. about 5 MeV/nucleon. Accelerators usedfor transactinide element synthesis are either cyclotrons such asthe U400 at FLNR (Russia), the 88-Inch at LBNL (UnitedStates), the K-130 at JYFL (Finland), or various cyclotrons atGANIL (France), or linear accelerators such as the UNILAC atGSI (Germany) or the RILAC at RIKEN (Japan). In order toprovide long-term stable beams for month-long experiments,highly efficient ion sources are required, which provide intensebeams with low material consumption (i.e., about 0.5 mg/h) ofexpensive enriched stable isotopes. These requirements are bestmet by modern electron cyclotron resonance (ECR) ionsources, which are installed at all the above-mentioned facilities.Since evaporation residues are recoiling out of the target

layer with the momentum of the beam, they can be separated inflight from the projectile beam but also from all other types ofreaction products (see section 4). Presently, kinematic

Figure 6. Total integrated mass yield of the reaction 288 MeV 40Ar +238U (dash-dotted line) and its decomposition into individualcontributions: (a) quasi-elastic processes, (b) deep-inelastic multi-nucleon transfer, (c) fusion followed by fission, and (d) fission oftarget-like reaction products.113,114. Reprinted with permission fromref 114. Copyright Wiley VCH Verlag GmbH.

Figure 7. Experimental cross sections for the formation of nuclei withZ ≥ 102 in (■) the 1n evaporation channel of cold fusion reactions,(○) the 5n channel of hot fusion reactions, and (△) the 3−4n channelof warm fusion reactions with 48Ca + actinide targets. The curves aredrawn to guide the eye.



separators such as velocity, mass, or energy filters and gas-filledseparators are in use.89,115,116 The flight time through theseseparators is of the order of microseconds. Extremely successfulseparators in the discovery of new elements were the velocityfilter SHIP at GSI and the Dubna gas-filled recoil separator(DGFRS) at FLNR. SHIP is a vacuum separator and consists oftwo symmetrically arranged Wien-filters with spatially separatedelectrostatic and magnetic dipoles. Quadrupole triplets areinstalled before and after the velocity filter. An additional 7.5°dipole bends the product beam out of a direct line of sight ofthe target position, which strongly reduces unwanted back-ground. For 48Ca induced reactions, the efficiency of theseparator was reported to be about 20%.117 A schematic view ofthe velocity filter SHIP is shown in Figure 8. SHIP is requiredto operate in vacuum, due to the high electric fields. In contrast,evaporation residues can spatially be separated in a gas-filleddipole magnet, making use of a charge focusing effect. Therecoiling fusion products change their charge state whentraveling through a dilute gas (usually H2 or He), which resultsin a trajectory through a dipole magnet that can be described byan average charge state, thus defining the magnetic rigidity. Themagnetic rigidities of the beam and other reaction products aredifferent so that a separation of evaporation residues can beaccomplished. Quadrupole magnets are added to the dipole tofocus the evaporation residues into a focal plane detector. Aschematic of the DGFRS is shown in Figure 9. Thetransmission through the separator for 48Ca-induced reactionswas estimated to be about 40%.89

One of the most important parts of the separator is the focalplane detector, where the decay of a transactinide nucleus isregistered. Typically, the rate of particles reaching the focalplane detector is still few hundred hertz. In order tounambiguously detect decay events of transactinide nuclei,this background must be significantly reduced. This isaccomplished in several ways. By adding so-called time-of-flight (TOF) transmission detectors, each particle transiting theseparator leaves a signal in the TOF detectors, which can thusbe separated from radioactive decay signals of implanted nuclei,which leave no TOF signal. Furthermore, the Si-detectors aredivided into position sensitive strips, so each signal(implantation, α-particle, and SF decay) is associated with aposition coordinate in the detector. This way, the observed

decay sequence of a heavy nucleus consists of an implantationsignal, followed by one or several α-particle decays. Often thesequence is terminated by SF decay. These events are not onlycorrelated in time but also in position in the Si-detector. Thisway, the probability that such a sequence is consisting of purelyrandomly correlated events is greatly diminished. Since theimplantation depth is rather shallow, α-particles and fissionfragments can escape the detector plane in the backward(upstream) direction. Such escaping events still leave adetectable signal in the focal plane detector, and often thefull energy of these escaping events can be recovered bydetecting them with side detectors. All these features are shownin the enlarged section of Figure 9. Since all events areregistered and stored in an event-by-event mode, month-longexperiments create rather large amounts of data that has to beanalyzed off-line to reveal the presence of few heavy nuclei. In

Figure 8. Schematic view of the velocity filter SHIP. Reproduced with permission from ref 22. Copyright 2011 Oldenbourg WissenschaftsverlagGmbH.

Figure 9. Schematic view of the DGFRS. The cut-out shows anenlarged view of the focal plane Si-detector box. Reprinted withpermission from ref 89. Copyright 2007 IOP Publishing Ltd.



earlier times, the analysis of the data required a computingcenter.New instruments, such as the gas-filled separator TASCA,

which are currently being used in the search for new elements119 and 120, have a transmission of about 60% for 48Ca-induced reactions on actinide targets.118 As focal planedetector, double-sided Si strip detectors are used, which allowfor a position resolution of 1 mm2. The focal plane consiststhus of more than 6900 pixels. In addition, new and fasterelectronics is required in order to resolve nuclides with t1/2 ≥ 1μs.

6. RELATIVISTIC EFFECTS ON CHEMICAL PROPERTIES

6.1. Relativistic Effects on Atomic Electronic Shells

With increasing Z of heavy elements, causing a strongerattraction to the core, an electron is moving faster, so that itsmass increase is

= −m m v c/[1 ( / ) ]02 1/2

(6.1.1)

where m0 is the rest mass of the electron, v is the velocity of theelectron, and c is the speed of light. The Bohr model for ahydrogen-like species gives the following expressions for thevelocity, energy, and orbital radius of an electron

π=v e nh Z(2 / )2(6.1.2)

π= −E e n h mZ(2 / )2 4 2 2 2(6.1.3)

=r Ze mv/2 2 (6.1.4)

where n is the principal quantum number, e is the charge of theelectron, and h is Planck’s constant. With increasing Z along thePeriodic Table, the m/m0 ratio gets larger. For H it is 1.000027.From the sixth period onward, this ratio is exceeded by 10%, sothat relativistic effects cannot be neglected anymore. Forexample, for Fl, m/m0 = 1.79, and for element 118, it is 1.95(see also Pyykko119 for other examples). The contraction (eq6.1.4) and stabilization (eq 6.1.3) of the hydrogen-like s andp1/2 electrons is a direct relativistic effect and was shown tooriginate from the inner K and L shell regions.120 This effectwas found to also be large for the valence region due to thedirect action of the relativistic perturbation operator on theinner part of the valence density.121 Figure 10 shows therelativistic contraction of the 7s atomic orbital (AO) of element105, Db, ΔR⟨r⟩ns = ⟨r⟩nr − ⟨r⟩rel/⟨r⟩nr = 21%. Figure 11 shows

the 25% relativistic contraction and 5.8 eV stabilization of the7s AO of Cn.The relativistic contraction and stabilization of the ns AO

reach their maximum in the seventh row of the Periodic Tableat Cn (Figure 12).37 The shift of the maximum to Cn in theseventh period in contrast to Au in the sixth period is due tothe fact that in Rg and Cn the ground state electronicconfiguration is d9s2 and d10s2, respectively, while thecorresponding electronic configurations in the sixth periodare Au(d10s1) and Hg(d10s2).The second (indirect) relativistic effect is the destabilization

and expansion of outer d and f orbitals. The relativisticcontraction of the s and p1/2 shells results in a more efficientscreening of the nuclear charge so that the outer orbitals, whichnever come close to the core, become more expanded andenergetically destabilized. As an example, the expansion anddestabilization of the (n−1)d AOs with Z are shown in Figure11 for group-12 elements. While the direct relativistic effectoriginates in the immediate vicinity of the nucleus, the indirectrelativistic effect is influenced by the outer core orbitals.The third relativistic effect is the well-known spin−orbit

(SO) splitting of levels with l > 0 (p, d, f, etc.) into j = l ± 1/2. Italso originates from the inner region in the vicinity of thenucleus. The SO splitting for the same l decreases withincreasing number of subshells; that is, it is much stronger forinner (core) shells than for outer shells. The SO splittingdecreases with increasing l for the same principal quantumnumber; that is, the np1/2−np3/2 splitting is larger than thend3/2−nd5/2 and both are larger than the nf5/2−nf7/2 one. This isexplained by the decreasing orbital densities in the vicinity ofthe nucleus with increasing l. In transactinide compounds, theSO coupling becomes similar or even larger in size compared totypical bond energies.All three effects change approximately as Z2 for the valence

shells down a column of the Periodic Table. It was suggestedthat relativistic effects depend even on higher powers of Z,especially for the heaviest elements.123

Breit effects (accounting for magnetostatic interactions andretardation effects to the order of 1/c2) on energies of valenceorbitals and IPs are usually small, e.g., 0.02 eV for element 121,but can be as large as 0.1 eV for transition energies between thestates including f orbitals.124 They can also reach a few percentfor the fine structure level splitting in the 7p elements and areof the order of correlation effects there.QED such as vacuum polarization and electron self-energy

are known to be very important for inner-shells,125,126 forexample, in accurate calculations of X-ray spectra127,128 for theheaviest elements. For highly charged few electron atoms, theywere found to be of similar size as the Breit correction to theelectron−electron interaction. It was shown that in the middlerange (Z = 30−80) both the Breit and Lamb-shift terms for thevalence shells behave similarly to the kinetic relativistic effects,scaling as Z2.129 For the higher Z, the increase is even larger.The nuclear volume effect grows even faster with Z.Consequently, for the superheavy elements, its contributionto the orbital energy will be the second most important oneafter the relativistic contribution. QED corrections for thevalence shells in heavy many-electron atoms of elements Rgthrough Fl, and 118 through 120 calculated using aperturbation theory are given in Thierfelder et al.130 Thus,for example, QED on the DCB IP of element 120 is −0.013 eV,while it is 0.023 eV for Cn. For element 118, QED effects onthe binding energy of the 8s electron cause a 9% reduction

Figure 10. Relativistic (solid line) and nonrelativistic (dashed line)radial distribution of the 7s valence electrons in Db. Reprinted withpermission from ref 39. Copyright 2003 Kluwer Academic Publishers.



(0.006 eV) of EA.131 Thus, the QED effects are not negligible:they are of the order of 1−2% of the kinetic relativistic effects,which means that the existing studies of relativistic effects areup to 99%129 (or 101%17) correct.6.2. Current Relativistic Quantum-Chemical Methods

The most appropriate quantum chemistry methods for theheaviest elements are those which treat both relativity andcorrelation at the highest level of theory.132−136 Presently, thehighest theoretical level for many-body methods for moleculesis the Dirac−Coulomb−Breit (DCB) Hamiltonian

∑ ∑= + +<

h h i r B( ) (1/ )DCBi

Di j

ij ij(6.2.1)

where the one-electron Dirac operator is

α β= + − +h i c p c V i( ) ( 1) ( )D i i in2

(6.2.2)

Here, α and β are the four-dimensional Dirac matrices, and Vn

is the nuclear attraction operator. The Breit term in the lowphoton frequency limit is

α α α α= − + − −B r r r r1/2[( ) ( )( ) ]ij i j ij i ij j ij ij1 3

(6.2.3)

The operators of the Dirac eq 6.2.1 are 4 × 4 matrix spinoroperators, and the corresponding wave function is therefore afour-component (4c) vector function. The Vn includes theeffect of the finite nuclear size, while some finer effects, such asQED, can be added to hDCB perturbatively. The DCBHamiltonian in this form contains all effects through thesecond order in α, the fine-structure constant. Correlationeffects are taken into account by the configuration interaction(CI), many-body perturbation theory (MBPT) and, presentlyat the highest level of theory, the coupled cluster single double(and perturbative triple) excitations (CCSD(T)) technique.The Fock−Space (FS) DCB CC method137,138 is presently

the most powerful method used for atomic calculations. It hasan accuracy of a few hundredths of an electronvolt forexcitation energies in heavy elements, since it takes intoaccount most of the dynamic correlation (states with high l).Due to the present limitation of the FS CCSD method intreating electronic configurations with no more than twoelectrons (holes) beyond the closed shell, further developmentsare underway to remove this limitation.138 Thus, the high-sectors FSCC code is under development, which will allow fortreating systems with up to six valence electrons/holes in anopen shell. The relativistic Hilbert space CC (HSCC) methodis also worked on, which could be used for systems with morethan a couple of electrons/holes in the active valence shell. Themixed sector (MS) CC method will be a generalization of theprevious two (FSCC and HSCC) and will combine theiradvantages. A further improvement is the introduction of theintermediate Hamiltonian (IH). It is a generalization of theeffective Hamiltonian (EH) method and serves as a core ofmost multiroot multireference approaches. The standardmultireference FSCC and HSCC methods (described above)are used in the effective Hamiltonian framework. The mostproblematic technical problem of the EH method is poor (orno) convergence of iterations due to the presence of so-calledintruder states. Recently, many groups developed differentforms of “intruder-free” intermediate Hamiltonian formulationsof FSCC and HSCC. These formulations substantially extend

Figure 11. Relativistic (solid line) and nonrelativistic (dashed line) energies and the maximum of the radial charge density, Rmax, of the valence nsand (n−1)d AOs of group-12 elements25 with data from Desclaux.122 Reprinted with permssion from ref 25. Copyright 2011 OldenbourgWissenschaftsverlag GmbH.

Figure 12. Relativistic stabilization of the 6s and 7s orbitals in the 6thand 7th rows of the Periodic Table.25 Redrawn from Schwerdtfeger etal.37 with Dirac−Fock (DF) data from Desclaux.122. Reprinted withpermission from ref 25. Copyright 2011 Oldenbourg Wissenschafts-verlag GmbH.



the scope and applicability of the multiroot multireference CCmethods. The XIH (extrapolated intermediate Hamiltonian)method is a specific (very efficient) form of IH developed bythe Eliav−Kaldor group.138For the heaviest elements, the electronic structures of Lr, Rf,

and Rg through element 122 were calculated with its use (seefurther). The DC FS CCSD method incorporated in theDIRAC package139 has a slightly lower accuracy than the DCBFSCC one. It was also applied to various heavy elementsystems.A practical instrument for many-electron open-shell systems

is the multiconfiguration Dirac−Fock (MCDF) method.140,141

Based on the CI technique, it accounts for most of thecorrelation effects while retaining a relatively small number ofconfigurations. It omits, however, dynamic correlation, whichmakes it less accurate than the DC(B) CCSD one. Elements Rfthrough Hs, Cn, and Fl were treated with its use (see further).In the past, predictions of atomic properties of the heaviestelements were made using the single-configuration DF methodand the Dirac−Slater (DS) one (see a review by Fricke,34 alsotables of Desclaux122). Atomic calculations for the heaviestelements were also performed using other approaches, such as,for example, the relativistic complete active space MCSCF(CASMCSCF) CI method.142,143

QED are presently included perturbatively in the atomiccalculations on top of the self-consistent-field (SCF)solutions.17,130,131 An attempt to bridge the gap betweenrelativistic quantum chemistry and QED has been undertakenby Liu.144

Ab initio DF molecular methods are still at the stage ofdevelopment.134,135,145 The problems of electron correlationand proper basis sets make the use of these methods for theheaviest elements very limited. Correlation effects are takeninto account there via the CI, MBPT (Møller−Plesset, MP2),or the CC techniques.146,147 One of the implementations of theDC method is a part of the DIRAC package.139 The methodsare still too computer time intensive and not sufficientlyeconomical to be applied to heaviest element systems in aroutine manner, especially to the complex systems studiedexperimentally.Due to the practical limitations of the 4c methods, the 2c

ones became very popular. In this approximation, the positronicand electronic solutions of the Dirac−Hartree−Fock (DHF)method are decoupled.135,145,148 This reduces the number ofinteractions in the Hamiltonian to solely those among electronsand nuclei and, therefore, the number of computations.Another method of decoupling the large and small componentsof the wave function is the Douglas−Kroll149 transformation, orthe Douglas−Kroll−Hess (DKH) one.150Effective core potentials (ECP) allow for more economical

calculations within the DHF schemes by replacing innerorbitals, which do not take part in the bond formation, by aspecial (effective core) potential. In this way, the number ofbasis functions and, therefore, two-electron integrals, isdrastically diminished.There are two main types of ECPs, as well as pseudo-

potentials (PP) and model potentials (MP).151 The energy-adjusted PPs, known as the Stuttgart ones, have been developedfor almost all elements of the Periodic Table,152,153 andrecently, also including QED effects, for the heaviest elementsup to Z = 118.154 The shape-consistent relativistic ECPs(RECP)155 have also been developed for the heaviestelements.156 Generalized RECPs accounting for Breit effects

were developed for some transactinides as well.157 Asmentioned, there are also ab initio model potentials(AIMP)158 that remove the drawbacks of the nodeless structureof the PPs but have not yet been used for the very heavysystems.PPs are also used for 1D, 2D, and 3D infinite systems

(polymers, surfaces, and the bulk). In the solid-statecalculations, PPs are constructed from Kohn−Sham ratherthat Hartree−Fock equations. An overview of this class of thePP methods is given elsewhere.151

Relativistic density functional theory (DFT) is very popularin the area of the heaviest elements calculations. Due to thehigh accuracy and efficiency, computational schemes based onthe DFT methods are among the most important in theoreticalchemistry, especially for extended systems, i.e., large molecules,liquids, or solids. The modern DFT theory is exact,159 and theaccuracy depends on the adequate choice of the exchange-correlation potential, Eex. The exact form of the latter is,however, unknown. In the past, the simplest local densityapproximation, LDA, was used, which laid the basis of theDirac−Slater Discrete Variational (DS-DV) method.160,161 Inmodern methods, the generalized gradient approximation(GGA), also in the relativistic version, RGGA, is used forEex.

162 There are quite a number of GGA potentials, and theirchoice is dependent on the system. Thus, PBE is usually usedfor physics applications, PBE0, BLYP, B3LYP, B88/P86, etc. forchemical, and LDA for the solid state. The most accuratenoncollinear 4c-DFT methods are that of Anton et al.163 andthe Beijing one (BDF).164 They differ by the basis settechnique, though they give similar results.The 2c-DFT methods are a cheaper alternative of the 4c-

ones.165 Quasi-relativistic methods such as the spin−orbitzeroth-order regular approximation (SO ZORA),166,167 alsoimplemented in the Amsterdam DFT code (ADF)168 and theDouglas−Kroll−Hess (DKH)169 one are also popular amongtheoretical chemists.

6.3. Relativistic Effects and the Future Periodic Table of theElements

The place of an element in the Periodic Table is determined byits atomic number Z and its electronic ground stateconfiguration. The most complete and systematic PeriodicTable up to Z = 172 is that of Mann, Fricke, Waber, andGreiner,170−172 which is based on DF calculations of atomicground states. DF calculations were also performed byDesclaux122 and Nefedov.173 Later on, MCDF174−182 andDCB CC137,183 calculations provided more accurate values ofthe energy terms, overall confirming the earlier DF predictions.According to these calculations, filling of the 6d shell takes

place in the first nine of the transactinide elements, Rf throughCn, followed by the 7p elements 113 through 118, withelement 118 falling into the group of noble gases. In elements119 and 120, the filling of the 8s shell takes place, so that theseelements will obviously be homologs of alkali and alkaline earthelements in group 1 and 2, respectively. The next element, Z =121, has a relativistically stabilized 8p electron in its groundstate electronic configuration124 in contrast to predictions basedon a simple extrapolation in the group. In the next element, Z =122, a 7d electron is added to the ground state, so that it has a8s27d8p ground state configuration in contrast to the 7s26d2

state of Th. This is the last element for which accurate DCBCC calculations exist.184



For heavier elements, calculations start to disagree on theground states. The situation there becomes more complicated:7d, 6f, and 5g levels, and further on 9s, 9p1/2, and 8p3/2 levelsare located energetically so close to each other that clearstructures of the pure p, d, f, and g blocks are no longerdistinguishable. The usual classification on the basis of a simpleelectronic configuration and the placement of elements in thispart of the Periodic Table becomes problematic. Thus, forexample according to Fricke et al.,33,34,172 the Periodic Tablehas a very long eighth period starting from Z = 119 andcounting 46 elements, so that the last element of this row is164, while elements 167 through 172 are 8p3/2 ones (due to thevery large SO effects on the 8p AOs) belonging to the ninthperiod. The 5g shell is being filled in elements 125 through 144.Elements 165 and 166 are then 9s ones belonging to group 1and 2, respectively.Seaborg and Keller have designed a different Periodic

Table,185 even though they used the same DF calculations ofWaber, Fricke, and Greiner.33,34,172 In their table, elements ofthe eighth period are from Z = 119 through 168, includingthose from Z = 122 through 153, called superactinides. Incontrast to the results of Fricke et al.,33,34,172 the 8p elementsare those from Z = 163 to 168, and the 9s elements are thosefrom Z = 169 to 170. However, such an arrangement of theelements is not reflecting the filling of the AOs obtained in theoriginal DF calculations.33,34,172

In a recent work based on MCDF (with average level, AL,energy functional) calculations of highly charged states of someelements of the eighth period, it was suggested that elements ofthe 5g series are those from Z = 121 to Z = 138.182 Elements139 and 140 are assigned then to groups 13 and 14,respectively, denoting that they are 8p1/2 elements, whilethose from Z = 141 to 155 are 6f elements. The eighth periodfinishes then at element 172. Thus, the Periodic Table ofPyykko182 looks quite different from that of Fricke et al.33,34,172

One should, however, note that it is difficult to decide aboutground states of the elements on the basis of their highlycharged states, so that those assignments are rather tentative.Attempts to define ground states of these elements on theexample of Z = 140 using the latest version of the MCDFmethod (with the optimal level, OL, energy functional) failed.17

The authors came to the conclusion that such calculations arepresently restricted by computer limitations. It was also statedthat, at the present level of the MCDF theory, which takes intoaccount QED effects, the Periodic Table ends at Z = 173, sincethe energy of the 1s electron dives below −2mec

2, and it is notaffected by the approximation used to evaluate the electron−electron interaction. Diving into the negative continuum is alsodiscussed by Greiner et al.186

Thus, at the modern level of relativistic electronic structuretheory, the problem of defining ground states of elementsheavier than 122 remains. Very accurate correlated calculationsof the ground states with inclusion of QED effects at the SCFlevel are needed in order to reliably predict the future shape ofthe Periodic Table. At the time of writing, a more logicalversion of the table is that shown in Figure 13, which is basedon the original DF calculations.33,34,172

7. CHEMICAL TECHNIQUES TO INVESTIGATETRANSACTINIDES

The difficulties involved in the production and rapid chemicalisolation of a few single atoms of a transactinide element fromnumerous other reaction products and the subsequent

detection of the nuclear decay require the development ofunique separation methods. As already discussed in section 2,the discovery of new elements up to Z = 101 was accomplishedby chemical means. Only from there on did physical methodsprevail. Nevertheless, rapid gas-phase chemistry played animportant role in the claim to discovery of elements 104 and105.58 In the following, a review of the instrumentation forchemical separation procedures for transactinide elements willbe given. Extensive reviews, covering instrumentation for singleatom chemistry, can be found in the literature.18,187

7.1. Prerequisites for a Chemical Isolation of HeaviestElements

Due to the very low production rates and short t1/2,transactinide nuclei must be chemically processed “one-atom-at-a-time”188 on a very short time scale. A chemistry experimentwith a transactinide element can be divided into three basicsteps: first, synthesis of the transactinide nuclide; second, rapidtransport of the synthesized nuclide to the chemical apparatus;third, fast chemical isolation of the desired nuclide, preparationof a sample suitable for nuclear spectroscopy, and detection ofthe nuclide via its characteristic nuclear decay properties.

7.1.1. Synthesis of Heaviest Elements. The synthesis ofheavy and superheavy elements requires technologicallyadvanced setups in order to allow the irradiation of exotic,highly radioactive target nuclides such as 244Pu, 243Am, 248Cm,249Bk, 249,250Cf, or even 254Es with high intensity heavy ionbeams such as 18O, 22Ne, 26Mg, 36S, 48Ca, or 50Ti. On the onehand, as intense beams as possible are to be used; on the otherhand, the destruction of the very valuable and highly radioactivetargets has to be avoided. Production rates are proportional toboth the target thickness and the beam intensity. The raretarget nuclides are produced at high flux nuclear reactors suchas the HIFR at Oak Ridge, United States, or the SM inDimitrovgrad, Russia, by successive neutron capture reactionson Cm and Am starting materials. The valuable target nuclidesare chemically isolated in heavily shielded hot cells and have acommercial value >100,000 USD/mg. From the isolated heavyactinide nuclides, targets suitable for heavy ion irradiations haveto be prepared. For nuclear reactions involving the lighterprojectiles, target thickness is limiting the compound nucleusrecoil ranges (<1 mg/cm2), because the separation techniquesused require the compound nucleus products to recoil out ofthe target. With the higher-Z projectiles, the recoil ranges arelarger, but because of the higher rate of projectile energy loss inthe target material and the narrow projectile energy range

Figure 13. Modern Periodic Table of the Elements.



effective for heavy element production, the effective targetthicknesses are once again limited to ∼1 mg/cm2. Thesethicknesses are too thin to allow the production of free-standing target foils. Therefore, thin backing foils are used onwhich the target materials are deposited as thin uniform layers.As backing materials, foils made of Be, Al, Ti, or Havar (a highstrength nonmagnetic alloy) have been employed. Passage ofthe beams through the targets produces large amounts of heat,which must be dissipated. In addition, the target material mustbe chemically stable in the highly ionizing environment createdby passage of the beam. In the majority of cases, the targets arebeing produced by the molecular plating technique.189 There,the actinide compound, typically the nitrate, is dissolved in asmall volume (5−10 μL) of nitric acid and the aqueous phase ismixed with a surplus of an organic solvent (∼14 mL), usuallyisopropanol or isobutanol. A high voltage is applied betweenthe solution and the target backing foil. Under these conditions,no electrolytic dissociation of the organic solvent occurs byapplying an electric current. After molecular plating, the foilsare heated to ∼500 °C to complete the conversion of theactinide materials to the oxides. An example of a freshlyprepared target segment of a rotating 249Bk target of ∼500 μg/cm2 thickness on 2.2 μm Ti backing foil is shown in Figure 14.The assembled target wheel consisting of four segments

showed a radioactive decay rate of nearly 1012 Bq(disintegrations per second).The projectile beam loses energy upon passing through the

target backing and the target, resulting in deposition of heat inthe target. The heat generated must be removed to preventdamage to the target. To allow the highest beam intensities,highly efficient target cooling is necessary. For stationarytargets, this is usually accomplished by double-window systemsand forced gas cooling (see upper part in Figure 19). In thedouble window system, a vacuum isolation foil is placedupstream of the target backing foil (with the target material onthe downstream side of the backing foil). Cooling gas atpressures near 1 bar is forced at high velocity through a narrowgap between the vacuum isolation foil and the target backingfoil, cooling both foils. Both the vacuum isolation foil and thetarget backing foil must hold a pressure difference of greaterthan 1 bar. Because of the mechanical stresses associated withthe pressure differences across the foils, target areas have beenlimited to <1 cm2. Heating of the target by passage of the beamis inversely proportional to the cross-sectional area of the beam;therefore, spreading the beam over a larger area decreases thethermal stress. This can be accomplished in two ways, eitherthe target foil is placed between two supporting honeycombgrid structures (see Figure 15), or the target is being rotated,

Figure 14. Target segment of freshly deposited 249Bk on Ti backing. The target was prepared at Mainz University for a search experiment for thesynthesis of element 119 in the reaction 50Ti + 249Bk at the gas-filled separator TASCA at GSI. The 249Bk target material was provided by Oak RidgeNational Laboratory (photograph courtesy of Mainz University).

Figure 15. Left-hand side: cartoon of the water-cooled collimator−target−recoil chamber−beam stop assembly as used for experiments atFLNR190−195 (red arrow: incoming beam). Right-hand side: Results of a finite element calculation of the thermal load in the Ti vacuum window.The scale indicates 2 cm in increments of 5 mm.



thus effectively spreading the beam heating over a larger targetarea.Rotating 248Cm targets have been used in heavy element

chemistry experiments at the GSI.189 The rotating target andvacuum window assembly ARTESIA196 has allowed the use ofmodern high-intensity accelerated beams for heavy elementchemistry experiments.197

7.1.2. Rapid Transport. When a compound nucleus isformed in the reaction of a projectile beam with a targetnucleus, the compound nucleus is formed with the momentumof the beam particle. If the target layer is thin enough, thisrecoil momentum is sufficient to eject the compound nucleusfrom the target. These recoiling reaction products can, inprinciple, be stopped in a metal foil placed directly downstreamof the target. However, with the recoil catcher foil technique,the time required for removal of the foil and dissolution andseparation of the foil material results in chemical separationtimes longer than a few minutes. Therefore, this technique isnot suitable for transactinide chemistry experiments. To achievefaster chemical separation times, the aerosol gas-jet transporttechnique has been used to deliver transactinide isotopes fromthe target chamber to various chemical separation devices.Transport times on the order of one second have beenachieved. The principles behind the aerosol gas-jet transporttechnique have been presented by Wollnik.198 Products ofnuclear reactions recoiling out of a thin target are stopped in agas at a pressure usually above 1 bar. To allow for relativelylong recoil ranges, He gas is usually used. The He gas is seededwith aerosol particles. After stopping in the gas, nonvolatilereaction products diffuse in the gas and attach themselves to thenext available surface, which is provided by the aerosol particles.By choosing appropriate particle sizes of about 100 nmdiameter, the aerosol particles can be transported rapidly andessentially without loss through long capillaries. In order tolimit the pressure in the recoil chamber, vacuum is applied tothe downstream end of the capillary. At the end of the capillary,the aerosol particles can be filtered off or collected on a foil byimpaction. The collected aerosol particles, containing thenuclear reaction products, can be made rapidly available forchemical separation. Many materials have been used togenerate aerosol particles. They can be specifically chosen tominimize interference with the chemical separation beingconducted. Widely used were KCl aerosols which can easily begenerated by sublimation of KCl from a porcelain boat within atube furnace. By choosing a temperature between 650 and 670°C, specially tailored aerosol particles with a mean mobilitydiameter of about 100 nm and number concentrations of a few

106 particles/cm3 could be generated. The same techniquecould be applied to produce MoO3 aerosols. Carbon aerosolparticles of similar dimensions were generated by sparkdischarge between two carbon electrodes. Transport efficienciesof 50%−80% have routinely been achieved for transportcapillary lengths over 20 m. However, the transport yields areheavily affected by the applied beam intensity.198 The passageof the beam through the gas in the recoil chamber creates aplasma, and the harsh ionizing conditions significantly affect themean mobility diameter and the number concentration ofparticles and thus negatively affect the transport efficiency.Furthermore, the transport by aerosols is unspecific. Trans-actinide atoms are produced at extremely low rates: atoms perminute for Rf and Db, down to atoms per day or week forheavier elements. They are produced among much largeramounts of “background” activities (see section 4), whichhinder the detection and identification of the decay oftransactinide atoms. For these reasons, there is a recognizedneed for a physical preseparation of the transactinide atomsbefore chemical separation. It was shown that kinematicseparators, such as the BGS, DFGRS, GARIS, or TASCA,could be coupled to a transactinide chemistry system with anaerosol gas-jet device.28,199−205 Except for the most asymmetricsynthesis reactions, the recoil energy of the fusion reactionproducts imparted by the momentum of the beam is sufficientto allow the separated nuclei to pass through a thin Mylarwindow, which separates the low pressure in the separator (i.e.,1 mbar or vacuum) from the stopping gas cell (i.e., 1 bar). Thethin windows are supported by honeycomb grids, which,however, diminish the transmission (see Figure 16). The use ofkinematic recoil separators as preseparator for chemistryexperiments undoubtedly offers a number of advantages, butalso some drawbacks have to be mentioned. Depending on thenuclear reaction, the transmission through the separator rangesfrom about 10% to 60%, not including the losses of the windowsupporting grid. The useful target thicknesses (approximately500 μg/cm2) in recoil separators are only about half thethickness of targets used without preseparation, which can becompensated to some extent with higher acceptable beamintensities.Obviously, transactinide elements that are volatile in their

elemental state or can be converted easily to volatile moleculesdo not need to be transported by the addition of aerosolparticles. For studies of Hs, Cn, and Fl, a new device namedIVO (in situ volatilization and online detection) wasdeveloped.206 By adding O2 to the He carrier gas, volatiletetroxides of group-8 elements were formed in situ in the recoil

Figure 16. Recoil transfer chamber installed at the BGS focal plane (left), and the support grid for the Mylar foil (right). Reprinted with permissionfrom ref 187. Copyright 2003 Kluwer Academic Publishers.



chamber. A quartz column containing a quartz wool plugheated to 600 °C was mounted as close as possible to the recoilchamber. The hot quartz wool served as a filter for aerosolparticles and provided a surface to complete the oxidationreaction. However, as with aerosol particles, the harsh ionizingconditions created by the passage of the beam through therecoil chamber, do not allow the in situ synthesis of fragilecompounds, such as organometallic compounds.28,204,207 Again,this drawback can be circumvented by coupling an IVO typedevice not directly to the target station but behind a kinematicrecoil separator (see section 5).7.1.3. Chemical Isolation, Sample Preparation, and

Detection. The choice of the chemical separation system hasto be based on a number of prerequisites that have to befulfilled simultaneously to reach the required sensitivity tochemically isolate a transactinide element.Speed: due to the short t1/2 of even the longest-lived

currently known transactinide nuclides, the time requiredbetween the production of a nuclide and the start of themeasurement is one of the main factors determining the overallyield. In contrast to physical separators, chemical instrumenta-tion currently only allows the investigation of transactinidenuclides with t1/2 ≈ 1 s or longer.Selectivity: due to the low production cross sections, ranging

from the level of a few nanobarns for the production of nuclidesof elements Rf and Db, down to the level of a few picobarns oreven femtobarns for the production of elements with Z > 108,the selectivity of the chemical procedure for the specificelement must be very high. Two groups of elements are ofmajor concern as contaminants: due to the fact that many Poisotopes have similar t1/2 and/or α-decay energies as trans-actinide elements, the separation from these nuclides must beparticularly good. Some short-lived Po isotopes are observed asdaughter nuclides of Pb or Bi precursors, also some At isotopesare of concern. All these elements are formed in multinucleontransfer reactions with Pb impurities in the target material and/or the target assembly. Here, a chemical purification of theactinide target materials and a careful selection of the materialsused in the target assembly can already reduce the productionof unwanted nuclides by orders of magnitude. A second groupof elements, which interfere with the detection of transactinidenuclei, are heavy actinides that decay by SF. These areinevitably produced with comparably large cross sections inmultinucleon transfer reactions. The separation from heavyactinides must be particularly good if SF is the only registereddecay mode and if no other information, such as t1/2 of thenuclide can be derived from the measurement. In order toenhance the quality of separation from undesired reactionproducts, increasingly, kinematic separator systems areemployed as preseparators in addition to chemical procedures.Single atom chemistry: due to the very low production rates,

transactinide nuclei must be chemically processed on a “one-atom-at-a-time” scale. Thus, the classical derivation of the lawof mass action is no longer valid. Guillaumont et al.208 havederived an expression equivalent of the law of mass action inwhich concentrations are replaced by probabilities of findingthe species in a given state and a given phase. The consequencefor single atom chemistry is that the studied atom must besubjected to a repetitive partition experiment to ensure astatistically significant behavior. Here, chromatography experi-ments are preferred.Repetition: Since the moment in time at which a single

transactinide atom is synthesized cannot be determined and

chemical procedures often work discontinuously, the chemicalseparation has to be repeated with a high repetition rate. Thus,thousands of experiments have to be performed. This inevitablyled to the construction of highly automated chemistry setups.Due to the fact that the studied transactinide elements as wellas the interfering contaminants are radioactive and decay with acertain t1/2, also continuously operating chromatographysystems were developed.Detection: the unambiguous detection of the separated atom

is the most essential part of the whole experiment. Even thoughsome techniques have reached the sensitivity to manipulatesingle atoms or molecules, the detection of the characteristicnuclear decay signature of transactinide nuclei remainscurrently the only possibility to unambiguously detect thepresence of a transactinide element after chemical separation.Thus, final samples must be suitable for high resolution α-particle and SF-spectroscopy (coincident detection of both SFfragments). Most transactinide nuclei show characteristic decaychains that involve the emission of α-particles or the SF ofdaughter nuclei. The detection of such correlated decay chainsrequires the event-by-event recording of the data. In experi-ments with physical separators, the use of position sensitivedetectors further enhanced the discrimination against randomlycorrelated events.Speciation: due to the fact that transactinide nuclei are

detected after chemical separation via their nuclear decay, thespeciation cannot be determined. Currently, the speciation inall transactinide chemistry experiments has to be inferred bycarefully studying the behavior of lighter homolog elements.The chemical system must be chosen in such a manner that acertain chemical state is probable and stabilized by the chemicalenvironment.The chemistry of the early transactinide elements has been

studied in the aqueous as well as in the gaseous phase.Techniques of varying complexity have been used: from the so-called “SRAFAP” technique209 (students running as fast aspossible), consisting of simple manual separations that wererepeated as often as possible to fully robotized setups allowingthousands of separations. Also, continuously working arrange-ments were highly successful.

7.2. Gas-Phase Chemistry: Isothermal Chromatographyand Thermochromatography

Despite the fact that only few inorganic compounds of thetransition elements exist, that are appreciably volatile below anexperimentally still easily manageable temperature of about1000 °C, gas-phase chemical separations played and still play animportant role in chemical investigations of transactinideelements. A number of prerequisites that need to be fulfilledsimultaneously to accomplish a successful chemical experimentwith a transactinide element are almost ideally met by gaschromatography of volatile inorganic compounds. Since thesynthesis of transactinide nuclei usually implies a thermalizationof the reaction products in a gas volume, a recoil chamber canbe connected with a capillary directly to a gas chromatographicsystem. Gas phase separation procedures are fast and efficientand can be performed continuously, which is highly desirable inorder to achieve high overall yields. Finally, nearly weightlesssamples can be prepared on thin foils, which allow α-particle-and SF-spectroscopy of the separated products with goodenergy resolution and in high, nearly 4π, detection geometry.For the experimental investigation of volatile transactinide

elements or compounds, two different types of gas chromato-



graphic separations have been developed: isothermal chroma-tography (IC) and thermochromatography (TC). Sometimes,also combinations of the two have been applied. The basicprinciples of IC and TC are explained in Figure 17.7.2.1. Isothermal Chromatography. In IC, a carrier gas is

flowing through a chromatography column of constant,isothermal temperature. Open or filled columns can beemployed. Depending on the temperature and on the enthalpyof adsorption, ΔHa

0(T), of the species on the column surface, thespecies travel slower through the length of the column than thecarrier gas. This retention time can be determined either byinjecting a short pulse of the species into the carrier gas andmeasuring the time at which it emerges through the exit of thecolumn210,211 or by continuously introducing a short-livednuclide into the column and detecting the fraction of nuclidesthat have decayed at the exit of the column.212−216 With thisrespect, IC is a variant of frontal chromatography (FC), wherethe sample is continuously injected into the column and abreakthrough profile is measured at the exit. A characteristicquantity is the temperature at which half of the introducednuclides are detected at the exit (T50%). In this case, theretention time in the column is equal to t1/2 of the introducednuclide, which is thus used as an internal clock of the system.

The T50% temperature depends on various experimentalparameters. It can be shown that, for similar gas flow ratesand column dimensions, Ta ≈ T50%. By varying the isothermaltemperature, an integral chromatogram is obtained. The yieldof the species at the exit of the column changes within a shortinterval of isothermal temperatures from zero to maximumyield. A variant of IC using long-lived radionuclides istemperature programmed chromatography. The yield ofdifferent species at the exit is measured as a function of thecontinuously, but isothermally, increasing tempera-ture.211,217−220 Online IC is ideally suited to rapidly andcontinuously separate short-lived radionuclides in the form ofvolatile species from less volatile ones. Since volatile speciesrapidly emerge at the exit of the column, they can be condensedand assayed with nuclear spectroscopic methods. Less volatilespecies are retained much longer, and the radionuclideseventually decay inside the column. A disadvantage of ICconcerns the determination of ΔHa

0(T) of transactinide nuclei onthe column surface. In order to determine the T50% temper-ature, a measurement sufficiently above and below thistemperature is required. Since for transactinide elements thistemperature is a priori unknown, several measurements atdifferent isothermal temperatures must be performed, which

Figure 17. Upper panel: temperature profiles employed in thermochromatography and isothermal chromatography. Lower panel: deposition peakand integral chromatogram resulting from thermochromatography and isothermal chromatography, respectively. Reprinted with permission from ref187. Copyright 2003 Kluwer Academic Publishers.

Figure 18. Schematic of the gas chromatography arrangement used to chemically isolate Rf in the form of volatile chlorides. Reprinted withpermission from ref 58. Copyright 1987 Oldenbourg Wissenschaftsverlag GmbH.



means that long measurements are required below the T50%temperature that demonstrate that the transactinide compoundis retained long enough that most of the nuclei decayed in thecolumn. Such an approach is very time-consuming. Further-more, it must be demonstrated that the experiment wasperforming as expected and the nonobservation of transactinidenuclei was not due to a malfunctioning of the apparatus.A schematic drawing of the chemical apparatus constructed

for the first chemical isolation of element 104 in Dubna isshown in Figure 18.62 In section I, a 242Pu target wasbombarded at the inner beam of the U-300 cyclotron atDubna with 22Ne ions. The target was held between two platesmade of an aluminum alloy, into which a number of closelyspaced holes were drilled. Both sides of the target were flushedwith nitrogen at 250 or 300 °C. In the outer housing of thetarget chamber, a number of closely spaced holes were drilled,which matched the holes of the target holder. An Al foil servedas vacuum window. The whole target block was heated with aheater. The space behind the target was limited with an Al foil.Reaction products recoiling from the target were thermalized ina rapidly flowing stream of N2 and transferred to section II,where the chlorination of the reaction products took place. Thetransfer efficiency was >75% and the transfer time only about10−2 s. In the reaction chamber of section II, vapors of NbCl5and ZrCl4 were continuously added as chlorinating agents and

as carriers.62 It was found that carriers with a vapor pressuresimilar to that of the investigated compound yielded optimumtransfer efficiencies.221 The addition of carriers was essential,since in this manner the most reactive adsorption sites could bepassivated. Volatile reaction products were flushed into thechromatographic section (section III), which consisted of a 4 mlong column with an inner diameter of 3.5 mm. This sectionconsisted of an outer steel column into which tubular inserts ofvarious materials (Teflon, glass) could be inserted.62 Section IVconsisted of a filter. This filter had a stainless steel jacket intowhich, as a rule, crushed column material from section III wasfilled.62 This filter was intended to trap large aggregateparticles.222 Aerosol particles were apparently formed by theinteraction of the chloride vapors with oxygen present in thenitrogen carrier gas. Volatile products passing the filter insection IV now entered the detector in section V. This detectorconsisted of a narrow channel of mica plates, a silicate solidstate detector for registering latent SF tracks of the SF decay ofa Rf nuclide. The mica plates were removed after completion ofthe experiment, etched, and analyzed for tracks. The filter insection VI served for the chemisorption of Hf nuclides inancillary work with long-lived nuclides. In section VII thechloride carriers were condensed.The whole apparatus has been built to chemically identify an

isotope of Rf decaying by SF with t1/2 = 0.3 s, that has

Figure 19. Schematic of OLGA(II) in combination with the tape detection system or the MG or ROMA wheel detection system.225 See text fordetails. Reprinted with permission from ref 225. Copyright 1992 Springer Science and Business Media.



previously been synthesized and identified by a team ofphysicists at Dubna. In a number of experiments, Zvara and co-workers identified multiple SF tracks in the mica detectorswhen they used glass surfaces and temperatures of 300 °C.62

They had shown in preparatory experiments with Hf thatindeed the transfer of Hf through the apparatus occurred withinless than 0.3 s, and, thus, that the experimental setup was suitedto study the short-lived Rf isotope.221 The distribution of SFtracks along the mica detectors appeared consistent with t1/2 =0.3 s.62 A number of possible sources of SF tracks in the micadetectors other than the SF decay of a Rf isotope werediscussed and ruled out. Further experiments with a slightlymodified apparatus63 were conducted immediately after theexperiments described here. A total of 63 SF events wereattributed to the decay of Rf nuclide. The team in Dubnaregarded these experiments as further proof of the claim ofdiscovery of element 104.One of the most successful approaches to the study of

volatile transactinide compounds is the so-called OLGA (onlinegas chromatography apparatus) technique. Contrary to thetechnique in Dubna, reaction products are rapidly transportedthrough a thin capillary to the chromatography setup with theaid of an aerosol gas-jet transport system. Transport times ofless than 10 s are easily achieved. This way, the chromatographysystem and also the detection equipment can be set up in a fullyequipped chemistry laboratory accessible during irradiation andclose to the shielded irradiation vault. A first version of OLGA(I) was developed and built by Gaggeler and co-workers214 forthe search of volatile superheavy elements, and it was testedwith 25 s 211Pom. Volatile elements were separated in a streamof He and hydrogen gas at 1000 °C from nonvolatile actinidesand other elements. The separated nuclei were condensed onthin metal foils mounted on a rotating wheel (ROMA, rotatingmultidetector apparatus223,224) and periodically moved in frontof solid state detectors, i.e. PIPS (passivated implanted planarsilicon) detectors, where α-particles and SF events wereregistered in an event-by-event mode. For the study of volatilehalides and/or oxyhalides of Rf225 and Db225,226 and theirlighter homologs,227 OLGA(II) was built.215 Instead ofcondensing the separated molecules on metal foils, they wereattached to new aerosol particles and transported through athin capillary to the detection system. This so-calledreclustering process was very effective and allowed collectionof the aerosol particles on thin (≈40 μg/cm2) polypropylenefoils in the counting system (ROMA or MG, merry-go-round228). Thus, samples could be assayed from both sides in a4π geometry, which doubled the counting efficiency. At thesame time, the PSI tape system was developed,215 whichallowed significant reduction of the background of long-livedSF activities, that accumulated on the wheel systems. However,only a 2π counting geometry could be realized. An improvedversion of OLGA(II) was built at Berkeley and nicknamedHEVI (heavy element volatility instrument).216 With bothinstruments, the time needed for separation and transport todetection was about 20 s, with the time-consuming processbeing the reclustering. A schematic of the applied experimentalequipment is shown in Figure 19.In order to improve the chromatographic resolution and

increase the speed of separation, OLGA(III) was developed.229

Using a commercial gas chromatography oven and a 2 m longquartz column, which ended in a much smaller, redesignedrecluster unit, the overall separation time could be reduced by 1order of magnitude, while the chromatographic resolution was

much better. OLGA(III) has very successfully been applied tostudy volatile halides and/or oxyhalides of Rf,230 Db,231

Sg,232−234 and Bh.235 In all these experiments the separatedtransactinide nuclides were unambiguously identified via theirnuclear decay properties. An improved version of OLGA(I),named HITGAS (high-temperature online gas chromatographyapparatus), has been developed at ForschungszentrumRossendorf, Germany, and successfully applied to study oxidehydroxides of group-6 elements including Sg.236−239

In order to further reduce the background of unwanted α-decaying nuclides, the so-called parent−daughter recoilcounting modus had to be implemented at the rotating wheeldetection systems. Since the investigated transactinide nucleidecay with characteristic decay sequences involving α-particledecay and/or SF of daughter nuclei, the significance of theobserved decay sequence can be enhanced by observing thedaughter decays in a nearly background free counting regime.This can be accomplished in the manner shown in Figure 20

on the example of 267Bh. In the parent mode, a 267Bh atom

attached to the aerosol transport material is deposited on thesurface of a thin foil. The wheel is double-stepped at presettime intervals to position the collected samples successivelybetween pairs of α-particle detectors. When the 267Bh α-decayis detected in the bottom of a detector pair, it is assumed thatthe 263Db daughter has recoiled into the face of the topdetector. The wheel is single-stepped to remove the sourcesfrom between the detector pairs, and a search for the 263Db and259Lr daughters is made for a second preset time interval, beforesingle-stepping the wheel again to resume the search for decaysof 267Bh. The measurement of daughter nuclides at significantlyreduced background conditions comes at the expense of areduced overall efficiency of the apparatus.

7.2.2. Thermochromatography. In TC,240 a carrier gas isflowing through a chromatography column, to which a negativelongitudinal temperature gradient has been applied. Open orfilled columns can be employed. Species that are volatile at thestarting point are transported downstream of the column by thecarrier gas flow. Due to the decreasing temperature in thecolumn, the time the species spend in the adsorbed stateincreases exponentially. Different species form distinctdeposition peaks, depending on their ΔHa

0(T) on the columnsurface, and are thus separated from each other. A characteristicquantity is the deposition temperature (Ta), which depends onvarious experimental parameters. The mixture of species to beseparated can be injected continuously into the col-

Figure 20. Parent−daughter mode for rotating wheel systems.187 Seetext for detailed description. Reprinted with permission from ref 187.Copyright 2003 Kluwer Academic Publishers.



umn,62,241−243 or the experiment can be performed discontin-uously by inserting the mixture of species through the hot endof the chromatography column and removing the columnthrough the cold end after completion of the separation. Thetwo variants (continuous or discontinuous) result in slightlydifferent peak shapes. The chromatographic resolution issomewhat worse for the continuous variant. Thermochromato-graphic separations are the method of choice to investigatespecies containing long-lived nuclides that decay either by γ-emission, by EC- or β+ decay, or by the emission of highlyenergetic β− particles.244−249 Thus, the emitted radiation caneasily be detected by scanning the length of the column with adetector. The detection of nuclides decaying by α-particleemission or SF decay is more complicated. By inserting SFtrack detectors into the column, SF decays of short- and long-lived nuclides can be registered throughout the duration of theexperiment. After completion of the experiment, the trackdetectors are removed and etched to reveal latent SF tracks.Columns made from fused silica have also been used as SFtrack detectors.250 However, the temperature range for whichSF track detectors can be applied is limited, due to theannealing of tracks with time. It should also be noted that inTC all information about t1/2 of the deposited nuclide is lost,which is a serious disadvantage in experiments with trans-actinide nuclides, since SF is a nonspecific decay mode of manyactinide and transactinide nuclides. However, TC experimentswith transactinides decaying by SF have an unsurpassedsensitivity (provided that the chromatographic separationfrom actinides is sufficient), since all species are eventuallyadsorbed in the column and the decay of each nuclide isregistered. Thus, the position of each single decay in thecolumn contributes chemical information about ΔHa

0(T) of theinvestigated species. TC, as a nonanalytical tool to study thebehavior of compounds at a tracer scale, was developed andapplied to the gas−solid chromatographic separation oftransactinide elements mainly by the group of Zvara and co-workers at FLNR Dubna, Russia. Zvara et al. reported thechemical identification of elements Rf,61−64 Db,65−67 andSg,250−253 whereas experiments to chemically identify Bh254

and Hs255−257 yielded negative results. However, due to the factthat in all these experiments the separated nuclides wereidentified by the noncharacteristic SF decay, and no furtherinformation such as t1/2 of the investigated nuclide could bemeasured, most of the experiments fell short of fully convincingthe scientific community that indeed a transactinide elementwas chemically isolated.32,58,258,259

TC experienced a renaissance in the chemical investigationsof Hs, as volatile HsO4,

197 Cn,192,194 and Fl195,260 in theelemental state, which all are volatile at room temperature.Since it was no longer necessary to introduce highly corrosivehalogenating chemicals to synthesize volatile compounds, thesimple quartz tubes that served as chromatography columnscould be replaced by narrow channels formed by silicondetectors, which are able to record high resolution α-particlespectra and register SF fragments in a time-resolved mode.Along this channel a longitudinal negative temperature gradientis established. Due to the close proximity of the silicon diodesfacing each other, the probability to register a complete decaychain of a superheavy nucleus, consisting of a series of α-decaysand often terminated by SF correlated in time and position, israther high. A position resolution of 1−3 cm was sufficient toalso extract chemical information. Usually, the channels areformed by silicon detectors of 1 × 1 cm2 dimension, which are

facing each other at a distance between 0.5 and 1.5 mm. A firstcryo-thermochromatographic separator (CTS) was constructedat LBNL.201 In the Hs experiment,197 the cryo online detector(COLD) was used, which was constructed at PSI. A schematicof the experimental setup used in the first successful chemicalidentification of Hs as volatile tetroxide is shown in Figure 21.

The geometrical efficiency for detecting a single α-particleemitted by a species adsorbed inside the detector array was77%. The detectors of the COLD array were calibrated onlinewith α-decaying 219Rn and its daughters 215Po and 211Bi using a227Ac source. A further improved TC detector nicknamedCOMPACT (cryo online multidetector for physics andchemistry of transactinides) was constructed at TechnicalUniversity Munich (TUM). By reducing the gap between thedetectors to only 0.5 mm and integrating four detectors on onechip, the active area inside the detector channel could beincreased to 93%. The COMPACT detector was used inexperiments to discover new Hs isotopes.109,261 Furthermore,the detectors were of the PIPS type, which have a more ruggedsurface and also allow surface modifications, e.g. the depositionof a thin layer of noble metals such as Au or Pd. This becamevery important in experiments where the interaction of a singleatom of a superheavy element such as Cn or Fl was investigatedwith a Au surface.7.3. Liquid-Phase Chemistry

Liquid-phase chemical separations are standard; thus, theirutility for separation and isolation of the chemical elements hasbeen demonstrated. Usually, liquid−liquid extractions orcolumn based separations are performed. Adaptations ofthese well-understood separation techniques have been

Figure 21. The 26Mg-beam passed through the rotating vacuumwindow and 248Cm-target assembly. In the fusion reaction, 269,270Hsnuclei were formed which recoiled out of the target into a gas volumeand were flushed with a He/O2 mixture to a quartz column containinga quartz wool plug heated to 600 °C by an oven. There, Hs wasconverted to HsO4, which is volatile at room temperature andtransported with the gas flow through a perfluoroalkoxy (PFA)capillary to the thermochromatography detector array registering thenuclear decay (α-decay and SF) of the Hs nuclides. The arrayconsisted of 36 detectors arranged in 12 pairs, with each detector pairconsisting of 3 PIN (positive implanted N-type silicon) diodesandwiches. Always, 3 individual PIN diodes (top and bottom) wereelectrically coupled. A thermostat kept the entrance of the array at −20°C; the exit was cooled to −170 °C by means of liquid nitrogen.Depending on the volatility of HsO4, the molecules adsorbed at acharacteristic temperature.262



extensively applied to the transactinide elements. Theseadaptations have been developed to overcome the single-atom and short t1/2 limitations inherent in the study oftransactinide element chemical properties.7.3.1. Manual Liquid−Liquid Extractions and Column

Chromatography. Manually performed liquid−liquid extrac-tions have been used for the study of chemical properties ofRf263−266 and Db.267 The microscale liquid−liquid extractiontechnique used in these studies allowed minimizing theseparation and sample preparation times; phase volumes werekept to ∼20 μL. Usually, a KCl aerosol gas-jet system was usedfor rapid transport and samples were collected on a specialturntable by impaction on a Teflon disk. At the end ofcollection, the sample was dissolved with a few microliters ofliquid. The time from end of collection to the beginning ofcounting of the transactinide chemical fractions was as short as50 s, and the collection-separation-counting cycle could berepeated every 60−90 s.Because of interference from the radioactive decay of other

nuclides (which are typically formed with much higher yields),extraction systems with relatively high decontamination factorsfrom actinides, Bi, and Po must be chosen. The presence of thetransactinide in the selectively extracting organic phase wasdetermined by evaporating the fraction on a hot Ta foil andplacing the sample in a α-particle spectroscopy system. Withthis technique, the measurement of distribution coefficients issomewhat difficult. By comparing the Rf or Db detection rateunder a certain set of chemical conditions to the rate observedunder chemical control conditions known to give near 100%yield, distribution coefficients between about 0.2 and 5 can bedetermined. If the control experiments are performed nearlyconcurrently, many systematic errors, such as gas-jet efficiencyand experimenter technique, are canceled out. However, it mustbe ensured that during the chemical operations the trans-actinide element is not adsorbed to surfaces of the usedequipment. For example, it was observed that Hf did adsorb onthe used Teflon surfaces265 and that Pt-foils and polypropylenevials and pipet tips were better suited to conduct theexperiments. Additionally, extraction systems which come toequilibrium in the 5−10 s phase contact time must be chosen.

The first liquid-phase transactinide chemical separations weremanually performed Rf cation exchange separations performedby Silva et al.268 in 1970 using α-hydroxyisobutyrate (α-HIB) aseluent. The then newly discovered 65-s 261Rfa was produced inthe 248Cm (18O,5n)261Rfa reaction, and the recoils were stoppedon NH4Cl-coated Pt foils which were transported to thechemical separation area with a rabbit system. The 261Rfa andother products from the nuclear reaction, along with theNH4Cl, were collected from the Pt disk in a small volume of α-HIB and were run through a small cation exchange resincolumn. Under these conditions, all cations with charge statesof 4+ or higher were complexed with the α-HIB and elutedfrom the column. These experiments showed that Rf had acharge state of 4+ (or higher) and that its chemical propertiesare distinctly different from those of the actinides.

7.3.2. Automated Column Separations. With expecteddetection rates of only a few atoms per day or week, manuallyperformed chemical separations become impractical. Withautomated liquid-phase chemical separation systems, fasterchemical separation and sample preparation times can beachieved and the precision and reproducibility of the chemicalseparations has been improved over that obtainable via manualseparations. An early, already rather sophisticated apparatus forautomated extraction chromatographic studies of Rf−chloridecomplexes was described by Hulet et al. in 1980.269 Theexperimental apparatus and the data storage were fullyautomated and computer controlled. A total of six decaysattributable to 261Rfa and its daughter 257No were registered.Later, to improve the speed and reduce cross-contamination,the ARCA II (automated rapid chemistry apparatus) was builtby the GSI-Mainz collaboration, featuring two magazines of 20miniaturized chromatography columns.270 With the largenumber of columns, cross-contamination between samplescan be prevented by using each column only once. Byminiaturizing the columns, the elution volume, and therefore,the time needed to dry the final sample to produce a source forα-particle spectroscopy, is much reduced. The radioactivity isdelivered from the site of production at the accelerator toARCA by an aerosol gas-jet system. Aerosol particles arecollected on a frit or by impaction on a small spot on a slider(seen at the center of Figure 22). At the end of the collection

Figure 22. Schematic drawing of AIDA. Reprinted with permission from ref 280. Copyright 2005 Oldenbourg Wissenschaftsverlag GmbH.



time, the slider is moved to position the collection site aboveone of the miniature chromatography columns. A suitableaqueous solution is used to dissolve the aerosol particles andload the activities onto the column below. Selective elutions oftransactinide elements are carried out by passing appropriatesolutions through the column. A slider below the ion exchangecolumn is moved at the appropriate time to collect the chemicalfraction of interest on a hot Ta disk. A sample suitable for α-particle spectroscopy is prepared by rapid evaporation of thechemical fraction on the Ta disk, which is heated from below bya hot-plate and from above by a flow of hot He gas and a high-intensity infrared lamp. The final samples are then manuallyplaced in a detector chamber. The two magazines ofchromatography columns can be moved independently. Duringa chemical separation on the left column, the right column isconditioned by passing an appropriate solution through it. Afterthe separation on the left column is finished, the magazine ismoved forward; placing a new column in the left position, thenext separation is performed on the right column while the leftcolumn is being prepared for the subsequent separation. In thisway, up to 40 separations can be carried out, at time intervals ofless than 1 min, with each separation performed on a freshlyprepared, unused column. Although the column separations arefully microprocessor controlled and performed automatically,still several people were required to operate the ARCA IIsystem for a transactinide chemistry experiment. Transactinidechemical separations with ARCA II have been performed withRf,271,272 Db,273−277 and Sg.278,279

Building upon the design of ARCA II, an automated columnseparation apparatus, AIDA (automated ion-exchange separa-tion apparatus coupled with the detection system for α-spectroscopy), has been developed at JAERI (now JAEA).280

Even though the apparatus for collection of aerosol particlesand performing multiple chemical separations on magazines ofminiaturized ion-exchange chromatography columns is verysimilar to that in ARCA II, AIDA has automated the tasks ofsample preparation and placing the samples in the detectorchambers. Using robotic technology, the selected fractions aredried on metal Ta disks and are then placed in vacuumchambers containing large-area PIPS α-particle detectors. Aschematic drawing of AIDA is shown in Figure 22.In the ion-exchange process as shown in Figure 23, two

different paths to supply solutions are available; the first eluentgoes through the collection site to the microcolumn, while thesecond strip solution is directed to the column after one-step

forward movement of the column magazine to avoid cross-contamination at the collection site.The robotic sample preparation and counting technology,

together with mechanical improvements in the chemicalseparation system, resulted in an automated columnchromatography system that runs almost autonomously. Eachseparation in columns is accomplished within 20 s and the α-particle measurement can be started within 80 s after thecollection of the products at the AIDA collection site. Toshorten the time for the sample preparation of α sources, thenewly developed rapid ion-exchange apparatus AIDA-II wasintroduced; the apparatus is based on continuous samplecollection and evaporation of effluents, and successive α-particle measurement. The ion-exchange part is the same asthat of AIDA. The AIDA-II was successfully applied for thechemical experiments with Db.282 The effluent is collected asfraction 1 on a 15 mm × 300 mm tantalum sheet which wascontinuously moved toward an α-particle detection chamber at2.0 cm s−1. The sample on the sheet is automatically evaporatedto dryness with a halogen heat lamp and then subjected to α-particle spectroscopy in a chamber equipped with an array of 12silicon PIN photodiode detectors.282 Products remaining onthe resin were eluted with a strip solution. The eluate wascollected on a second Ta sheet as fraction 2, followed by thesame procedures for sample preparation and measurement. Themeasurements were started 14 and 38 s after the end of productcollection.

7.3.3. Automated Liquid−Liquid Extractions. Liquid−liquid extractions allow for very fast, continuously workingarrangements, especially if detection of the separated nuclidesoccurs with highly efficient, continuous liquid scintillationcounting (LSC). A system that has successfully been applied totransactinide chemistry is the so-called SISAK (short-livedisotopes studied by the AKUFVE-technique, where AKUFVE isa Swedish acronym for an arrangement of continuousinvestigations of distribution ratios in liquid extraction) system.This system performs continuous liquid−liquid extractionsusing small-volume separator centrifuges.283 Nuclear reactionproducts are delivered to the apparatus by an aerosol gas-jet.The gas-jet is mixed with the aqueous solution to dissolve theradioactivity-bearing aerosol particles, and the carrier gas isremoved in a degasser centrifuge. The aqueous solution is thenmixed with an organic solution, and the two liquid phases areseparated in a separator centrifuge. A scintillation cocktail isthen mixed with the organic solution, and this is passed through

Figure 23. Schematic of the ion-exchange part in AIDA. Reprinted with permission from ref 281. Copyright 2004 Elsevier.



a detector system to perform liquid scintillation α pulse-heightspectroscopy on the flowing solution. An electronic suppressionof signals originating from β-decays is applied.This modular separation and detection system allows the use

of well-understood liquid−liquid extraction separations on timescales of a few seconds, with detection efficiencies near 100%,and it has been used to study the subsecond α-active nuclide224Pa.284−286 However, the detection of α-active transactinideisotopes failed despite pulse-height discrimination againstinterfering β-activities, which are produced with much largeryield. Preseparation with a kinematic recoil separator, asdescribed in section 7.1.2, allowed chemical separation anddetection of 4-s 257Rf using the SISAK technique.200 Aschematic of the BGS-RTC-SISAK apparatus is presented inFigure 24. These proof-of-principle experiments have paved theway for detailed liquid−liquid extraction experiments withshort-lived transactinide element isotopes. By isolating acounting cell from the stream of flowing liquid as soon as apotentially interesting signal occurred, genetically correlateddecay chains could be detected.

8. METHODS TO PREDICT EXPERIMENTALLYMEASURABLE PROPERTIES OF TRANSACTINIDES

Due to the very small production rates on the “one atom at atime” level, only very few physicochemical quantities oftransactinide elements and their compounds can be determinedexperimentally. A further difficulty arises from the fact that,even in experiments with lighter homolog elements at the tracerscale (i.e., 106 to 109 atoms), there currently exist no analyticalinstruments with the required sensitivity that would allowdetermining the speciation of the investigated elements.Therefore, the measured microscopic properties have to bedirectly predicted by theory. Those theoretical predictionsshould be based on the best possible relativistic electronicstructure calculations, since relativistic effects influence not onlyproperties of volatile atoms and molecules but also theadsorption phenomenon. Some empirical correlation can alsobe useful. Such correlations between micro- and macroamountsmay be valid for selected groups of elements and types ofcompounds, but they cannot necessarily be extrapolated totransactinide elements.Several constraints have to be fulfilled simultaneously to

serve the purpose of a meaningful chemical study oftransactinide compounds. First, a class of compounds shouldbe studied, where relativistic effects are expressed in anexperimentally detectable parameter. Second, fast, simple, andefficient procedures must be available to isolate the trans-actinide compound from interfering components. Third, thestudied class of compounds should be thermodynamicallystable and allow multiple interactions of the transactinide

molecule with its surroundings in order to ensure a statisticallysignificant behavior. Here, chromatographic methods arepreferred.

8.1. Volatility

In experiments with single molecules, the behavior of a speciesin a gas adsorption chromatographic experiment is determinedby its molecular properties and the nature of the interactionwith the adsorbent. The state of the adsorbent (columnmaterial) should be known, if possible. It is assumed that theinvestigated molecules are rather stable and/or stabilized by thechemical environment, which usually is characterized byextreme surpluses of the reaction partners in the carrier gas.Also, the state of zero surface coverage can be assumed for thestationary phase.As described in section 7, in gas-phase chromatography

experiments, a measure of volatility is either the depositiontemperature in a TC column, Ta, or the temperature at which50% of the investigated species emerge from an isothermalcolumn, T50%. From these temperatures, the enthalpy ofadsorption, ΔHa

0(T), is deduced using adsorption models,287

or Monte Carlo simulations are applied using a microscopicmodel developed by Zvara.288,289 The adsorption enthalpy,ΔHa

0(T), can be empirically related to the sublimation enthalpy,ΔHS

0(298), of the macroamount. However, the usage of acorrelation between ΔHa

0(T) and ΔHS0(298) is restricted to some

groups and types of compounds, while not generallypermissible (see below). In macrochemistry, a measure ofvolatility is an equilibrium vapor pressure over a substance, Pmm.Boiling points, Tb, and enthalpy of evaporation, ΔHevap,basically correlate with Pmm.Besides the low statistics of single events, a difficulty arises

with respect to the interpretation of results, since the surface ofthe chromatographic column is not well-defined. Usually, it ismodified by the evaporated aerosol transport particles and/orhalogenating agents, so that the mechanisms of adsorption and,associated with it, the nature of chemical or physicalinteractions can only be assumed.240 Thus, available exper-imental data are often difficult to interpret and do not correlatewith a single property or electronic structure parameter of theadsorbate.

8.1.1. Physisorption and Chemisorption Models.Quantum-mechanical calculations of adsorptionboth phys-isorption and chemisorptionof atoms and molecules on(poly)crystalline and even amorphous substrates are nowadaysperformed using modern periodic DFT codes. Within this slab/supercell-approach, the surface of usually a single crystal ismodeled by a slab of finite thickness due to the application ofperiodic boundary conditions, which introduce the semi-infinitecharacter of the system. While in these codes the relativistic

Figure 24. Schematic of the SISAK liquid−liquid extraction system using the BGS as a preseparator. Reprinted with permission from ref 200.Copyright 2002 The Japan Society of Nuclear and Radiochemical Sciences.



effects of the core electrons are treated on the (relativistic)core-potential or pseudopotential level, the valence electronsare treated in nonrelativistic or outermost in a scalar-relativisticmanner. Unfortunately, there are no full-relativistic, all-electronperiodic implementations available yet.For adsorption of atoms or small molecules on metal

surfaces, straightforward fully relativistic calculations of theadsorption energy are possible using relativistic 4c-DFTmethods and the cluster approach. In this case, a surface ofan adsorbent is modeled by a cluster Mn of a specific number nof metal atoms. The size of the cluster is then steadily enlargeduntil convergence of the binding energy of the ad-atom-metalcluster is reached with cluster size n. Such direct calculations arenow possible up to more than a hundred atoms of thecluster.290a A possibility to treat an even larger number of atomseconomically is foreseen via an embedded cluster procedure(see schematic in Figure 25).290b

As was mentioned, to determine ΔHa0(T) of a heavy molecule

on a complex surface is still a formidable task for quantumchemical calculations. Especially difficult is the prediction ofphysisorption phenomena caused by weak interactions, wherethe DFT generally fails.In the past, DS-DV calculations were helpful in establishing

some correlations between electronic structure parameters andvolatilities of halides, oxyhalides, and oxides known frommacrochemistry.35,39 For example, it was established thatcovalent compounds having high overlap populations (OP)are more volatile than ionic ones, that molecules with dipolemoments (μ) interact more strongly with surfaces than thosewithout, and that the sequence in the adsorption energy isdefined by the sequence in μ.Lately, predictions of interaction energies of heaviest element

molecules with inert surfaces (quartz, silicon nitride, alsomodified) were made with the use of physisorptionmodels.291−294 These models are based on the principle ofintermolecular interactions subdivided into usual types for long-range forces: dipole−dipole, dipole−polarizability, and van derWaals (dispersion) ones. The molecular properties required bythose models are then calculated using the most accuraterelativistic methods.Thus, for a molecule with zero dipole moment adsorbed on a

dielectric surface by van der Waals forces, the following modelof the molecule-slab interaction294 is used:

εε

α= − −

+ +⎜ ⎟⎛⎝

⎞⎠( )

E xx

( )3

1612

mol

IP IP1 1 3slab mol (8.1.1)

where IPslab and IPmol are ionization potentials of the moleculeand slab, respectively, ε is the dielectric constant of the surfacematerial, and x is the molecule−surface distance. In acomparative study, x for a lighter element is deduced fromthe known ΔHa

0(T), while that for a heaviest element isestimated using the difference in their molecular size.Thermodynamic equations to predict Ta of a heaviest

element with respect to Ta of a homolog in a comparativestudy using the knowledge of the electronic structure of theadsorbate are also presented.295 One of those equations is givenbelow for the case of mobile adsorption of molecules with onerotational degree of freedom:

=−Δ −Δet r d T m

et r d T m

1 1E RTA

A A A A

E RTB

B B B B

/

1/21/2 1/2

/

1/21/2 1/2

A B

(8.1.2)

where t1/2 is the half-life of the central nuclide, r the molecularradius, d the metal−ligand distance, R the gas constant, T theadsorption temperature, m the mass, and ΔE the adsorptionenergy of a heaviest molecule A and of its lighter homolog B. Inthe same work, various measures of volatility were criticallycompared. The most adequate one in a comparative study (inmacrochemistry) was shown to be the ratio of adsorption/desorption constants, Kads/Kdes.For predictions of adsorption of molecules with nonzero

dipole moments, equations taking into account long-rangeinteractions, such as molecular dipole−surface charge, dipole−induced dipole, and van der Waals one, were used. Thus, forexample, the interaction energy of a molecule with a surfacehaving a charge is as follows:291

μ α α α= − − −

+( )E x

Qe

xQ e

x( )

2

232

mol mol mol slab

IP IP

2

2

2 2

4 1 1

mol slab

(8.1.3)

where μmol, IPmol, and αmol belong to the molecule and thosewith index “slab” to the surface; Q is the charge of the surfaceatom and x is the molecule−surface distance.With the use of those models (eqs 8.1.1−8.1.3), adsorption

of various group-4 through group-8 species, including theheaviest ones, on various (nonmetal) surfaces was predicted(see below).

8.1.2. Empirical Correlations and Extrapolations. Theconcept that bond energies between identical molecules oratoms in the crystal lattice should be proportional to theadsorptive bond energies between the same single molecules oratoms and a surface was empirically demonstrated by plottingΔHa

0(T) versus ΔHS0(298) for certain gas chemical systems, for

example, for chlorides and oxychlorides in Cl2, HCl, and CCl4(O2) on quartz, and for oxides and oxyhydroxides in O2 (H2O)on quartz.296 In other words, it is assumed that the molarbinding energy of an adsorbed single molecule to the surfaceapproximately equals its partial molar adsorption enthalpy atzero surface coverage. As an example, the empirical correlationobserved for chlorides and oxychlorides between −ΔHa

0(T)

measured for single molecules on the chromatographic surface(quartz, statically or dynamically modified by the chlorinatingreagents) and the macroscopic ΔHS

0(298) is shown in Figure 26.

Figure 25. Embedded M′−Mn system. Reprinted with permissionfrom ref 25. Copyright 2011 Oldenbourg Wissenschaftsverlag GmbH.



Unfortunately, there are no generally valid correlations thatrelate ΔHS

0(298) to other molecular properties. Attempts weremade to correlate the inverse boiling point (Tb) with thegeometric structure of molecules. For the halides of the metalsof the type MXn (n = 4, 5, 6; X = F, Cl, Br, I), Zvara empiricallyderived a formula which, for a given stoichiometry (atmaximum symmetry), correlated Tb with the ionic radius ofthe central metal ion.289,297 For certain classes of compounds,such as the tetrachlorides, an empirical correlation existsbetween ΔHS

0(298) and the orbital radii or crystal ionic radii ofthe group-4 elements Ti, Zr, and Hf, but also U and Th.298 Itshould be noted that due to the influence of relativistic effectsthe extrapolative power of such empirical relationships islimited.A simple and very early extrapolation, which will be referred

to later, is a prediction of the standard enthalpies of monatomicgaseous elements, ΔH*298(E(g)), to the heavy transactinides Cnthrough element 120 by Eichler299 by extrapolating over theatomic number Z (Figure 27). The standard enthalpies ofmonatomic gases are mostly equal to the standard sublimation

enthalpy ΔHS0(298) of these elements. Those linear extrapola-

tions should be used with care, as there is no solid theoreticalbasis for them.8.2. Complex Formation in Aqueous Solutions

In liquid-phase chemistry, single atoms of transactinideelements (or of their lighter homologs) are transported by anaerosol gas-jet technique from the site of synthesis behind thetarget or behind a kinematic recoil separator to a filter orimpaction device, where samples are collected and dissolved inan aqueous solution after an appropriate collection time.Usually, salt aerosol particles, such as KCl, are used that areeasily generated by heating the salt in a gas stream to atemperature slightly below the melting point. The KCl particlesreadily dissolve in contact with an aqueous solution. Theaqueous solution contains suitable ligands for complexformation. The complexes are then chemically investigatedusing a partition method by studying, for example, theirextraction into an organic solvent, or by anion- or cation-exchange chromatography, or by reversed-phase extractionchromatography. The ultimate goal of partition experiments isto determine the distribution coefficient Kd as a function ofligand concentration.For a simple complex MLn, the cumulative complex

formation constant

β = − −[ML ][M] [L]n nn1

(8.2.1)

is a measure of its stability. For stepwise processes, consecutiveconstants Ki are used. If various MLn

z−n complexes exist in theaqueous phase, but only one MLi

p− complex in the organicphase, the distribution coefficient, Kd, for the anion exchange isgiven by the following equation300

β

β=

∑

+ − − −

−KK [RB L ] [L ]

[L ]d

DM orgp

ii p

Nn

n0 (8.2.2)

where KDM is the association constant with the organic cation.Thus, a sequence in the Kd values for a studied series, forexample, for elements of one group, reflects the sequence in thestability of their complexes.Complex formation is known to increase in the transition

element groups. In aqueous solutions, it is, however, competingwith hydrolysis. This may change trends in the stabilities ofcomplexes and, finally, in their extraction into an organic phase.One should distinguish between hydrolysis of cations and

hydrolysis of complexes.301 The former is described as aprocess of a successive loss of protons

⇄ ++−

− + +M(H O) MOH(H O) Hnz

nz

2 2 1( 1)

(8.2.3)

In acidic solutions, hydrolysis involves either the cation,anion, or both, and is competing with complex formationdescribed by the following equilibrium

+ +

⇄

+ ° + −

°+ − −

−− − +

x y i

xw u w

M(H O) OH L

M O (OH) (H O) L

( )H O

wz

x u z u w ixz y i

2

2 2( )

2 (8.2.4)

In order to predict a sequence in Kd (eq 8.2.4), one shouldpredict a sequence in the formation constants of a series ofspecies of interest. For a reaction such as, for example, eq 8.2.3or 8.2.4,

= − ΔK G RTlog /2.3ir

(8.2.5)

Figure 26. Empirical correlation observed for chlorides and oxy-chlorides between −ΔHa

0(T) measured for single molecules on thechromatographic surface (quartz, statically or dynamically modified bythe chlorinating reagents) and the macroscopic ΔHS

0(298).234

Figure 27. Extrapolation of the standard enthalpies of monatomicgaseous elements ΔH*298(E(g)) along the groups of the Periodic Tablebased on the atomic number Z.299



where ΔGr is the free energy change of the complex formationreaction. To obtain it in a straightforward way, binding or totalenergies of species in the left-hand and right-hand parts of thereaction should be calculated. Such relativistic calculations, withfull geometry optimization for the heaviest elements, areextremely computer time intensive. In addition, the obtainedaccuracy might be insufficient to predict stabilities of similarspecies of homologs. Therefore, the following economicalmodel was suggested by Pershina.In a fashion analogous to that of Kossiakoff and Harker,302

the following expression for the free energy of formation of theMxOu(OH)v(H2O)w

(xz−2u−v)+ species from the elements wasadopted

∑ ∑−Δ

= + + − ! ! !

+ + +

G u v w RT

a a P u v w

u v

( , , )/2.3

log log( 2 )

(2 1)log 55.5

f

i ijw

(8.2.6)

The first term on the right-hand side of eq 8.2.6, ∑ai, is thenonelectrostatic contribution from M, O, OH, and H2O, whichis related to the overlap population. For a reaction,

∑Δ = Δ = Δa E k OPiOP

(8.2.7)

where k is an empirical coefficient. The next term, ∑aij, is asum of each pairwise electrostatic (Coulomb) interaction:

∑ ∑= = −E a B Q Q d/Cij

iji j ij

(8.2.8)

where dij is the distance between moieties i and j; Qi and Qj aretheir effective charges, and B = 2.3RTe2/ε, where ε is adielectric constant. For a reaction, ΔEC is the difference in EC

for the species in the left and right parts. P in eq 8.2.6 is thepartition function representing the contribution of structuralisomers if there are any. The last two terms are statistical: one isa correction for the indistinguishable configurations of thespecies, and the other is a conversion to the molar scale ofconcentration for the entropy. ∑aij and ∑ai for eachcompound are then calculated directly via a Mulliken analysisimplemented in most of the quantum-chemical methods (e.g.,4c-DFT163). To predict log Ki or log βi for transactinidecomplexes, coefficients k and B should then be defined byfitting log Ki to experimental values for the lighter homologs.Using this model, hydrolysis and complex formation constantswere predicted for a large number of aqueous compounds ofgroup-4 through group-6 elements303−311 in very goodagreement with experimental results. The results of thesecalculations and a comparison with experimental data revealthat a change in the electrostatic metal−ligand interactionenergy (ΔEC) of a complex formation reaction contributespredominantly in the change in ΔGf, i.e., in ΔGr. Thus, only bycalculating ΔEC can trends in the complex formation be reliablypredicted.

9. RUTHERFORDIUM (Z = 104)

9.1. Theoretical Predictions

The chemistry of Rf is expected to be similar to that of Zr andHf and defined by the four valence electrons, thus favoring thestable 4+ oxidation state. The MCDF calculations175,176 havegiven 7s27p6d (3D2) as ground state for this element (Thestabilization of the 7s2 pair was found for the entire seventhperiod as a result of the relativistic stabilization of the 7s AO, in

difference to some elements of the sixth row.). The relativisticstabilization of the 7p1/2 electron was explained to beresponsible for the unusual ground state in comparison withthe (n−1)d2ns2 (3F2) state of the lighter homologs Zr and Hf.More accurate DCB FSCC calculations,312 however, correctedthe MCDF result,175,176 giving 7s26d2 (3F2) as ground stateconfiguration. A very high level of correlation with l = 6 wasrequired to reach this accuracy.The IP of Rf of 6.01 eV was calculated at best using the DCB

FSCC method.312 It is smaller than the IP of Hf (6.83 eV313)due to the relativistic destabilization of the 6d AOs. The firstionized electron in Rf is the 6d one, like the 5d electron in Hf.MCDF multiple IPs of M through M4+ states were alsocalculated.176 There is a steady decrease in the IPs(M → M4+)in group 4 due to the same reason, i.e., the destabilization of the(n−1)d AOs with increasing Z (Figure 28).

This results in an increase in the stability of the maximumoxidation state in this group, which was shown by the values ofthe respective redox potentials.314,315 The atomic radius (AR)defined by the outer 7s AO of Rf is 1.49 Å, as derived from theearlier DF atomic calculations.35 It is smaller than the AR(Hf)of 1.55 Å316 due to the relativistic contraction of the 7s(Rf)AO.Ionic radii (IR) obtained via a correlation with the outer (n−

1)p AOs of the M4+ ions show an increase in the group, so thatthe IR(Rf4+) is 0.79 Å,176 which is larger than the IR(Hf4+) of0.71 Å317 (Figure 28). A better value of the IR(Rf4+) derivedfrom molecular calculations is, however, 0.76 Å.318−320 A set ofatomic single and triple bond covalent radii (CR) for most ofthe elements of the Periodic Table, including the heaviest onestill Z = 118 and Cn, respectively, was suggested.319,320 They arededuced from the calculated molecular (equilibrium) bondlengths (Re) of various covalent compounds. The CR of thegroup-4 to group-8 6d elements are about 0.5−0.8 Å largerthan those of the 5d elements. An important finding of theseworks is a decrease in the R6d − R5d difference starting fromgroup 9, reaching negative values in groups 11 and 12, as aresult of the relativistic bond contraction (Figure 29). This iscalled a “transactinide break”.There are several other calculations of molecular compounds

of Rf. The hydrides MH4 (M = Ti through Rf) were calculatedusing the DF one-center expansion method.321−323 Relativisticeffects were shown to decrease the Re in RfH4, so that it is only0.03 Å larger than Re(HfH4). The relativistic contractions of

Figure 28. Ionization potentials to the maximum oxidation state(IPmax) and ionic radii (IR) for Rf through Hs obtained from theMCDF calculations.175−179 Reprinted with permission from ref 39.Copyright 2003 Kluwer Academic Publishers.



orbitals and bond lengths were shown to be two parallel butlargely independent effects. The calculations showed a decreasein the atomization energy, De, of RfH4 as compared to that ofHfH4. Results of ab initio noncorrelated DF calculations werereported for RfCl4.

324

DS-DV calculations were performed for MCl4.315,325 The

calculations agreed on the fact that the electronic structure ofRfCl4 is similar to that of ZrCl4 and HfCl4 and that RfCl4 is atypical d-element compound. Covalency judged by theMulliken OP was shown to increase down group 4.315 This isdue to an increase in an overlap of the relativistic ns and(n−1)d AOs of the central ion with AOs of the ligand.Nonrelativistically, the trend is just opposite. Due to its highestcovalency, RfCl4 is therefore expected to be the most volatileamong group-4 MCl4.

315 This is in contrast to simpleextrapolation procedures229,298,326 using a radius−volatilitycorrelation of tetrachlorides that predict the opposite (Figure30). So experimentally measuring the volatility of RfCl4 incomparison to its lighter homologs HfCl4 and ZrCl4 providesan ideal test case.

The binding energy of RfCl4 and homologs was calculatedusing the 4c-DFT method.318 The compound was shown to bestable with De of 19.5 eV, though less stable than ZrCl4 (21.7eV) and HfCl4 (21.1 eV). The lower stability was explained viaa smaller contribution of the ionic contribution to the bindingenergy due to a decreasing effective metal charge, QM.

315 TheRECP calculations327 gave 18.8 eV for De(RfCl4), in overallagreement with ref 318. A decrease in De(RfCl4) in comparisonwith De(HfCl4) was shown to be due to the larger SO effects onthe 6d AOs. Re(RfCl4) was shown to be larger by about 0.05 Åthan Re of HfCl4,

327 in agreement with other data.319,320

Solid-state calculations were performed on Rf metal.329 Thestructural and electronic properties were evaluated by firstprinciples DFT in the scalar relativistic formalism with andwithout SO coupling and were compared with those of its 5dhomolog, Hf. It is found that Rf should crystallize in thehexagonal close packed structure, as does Hf. However, underpressure, it should have a different sequence of phasetransitions than Hf: hcp → bcc instead of hcp → ω → bcc.An explanation is offered for this difference in terms of thecompetition between the band structure and the Ewald energycontributions.The aqueous chemistry of Rf has also been studied

theoretically.307,310 As do other group-4 elements, Rf undergoeshydrolysis and complex formation in acidic solutions. Thefollowing reactions are important:

the first hydrolysis step

⇄+ +M(H O) MOH(H O)2 84

2 73

(9.1.1)

the stepwise fluorination

⇄ ⇄ ⇄+ + +M(H O) MF(H O) ... ... MF (H O) ...2 84

2 73

3 2 5(9.1.2)

⇄ ⇄− −MF (H O) ... MF (H O) MF4 2 2 5 2 62

(9.1.3)

and the total chlorination

+ ⇄+ −M(H O) 6HCl MCl2 84

62

(9.1.4)

The free energy change of reactions 9.1.1−9.1.4 wascalculated using the model described in section 8.2 and 4c-DFT calculations of the electronic structure of the com-plexes.307 The obtained data suggest the following trend inhydrolysis of the group-4 elements Zr > Hf > Rf. For cationexchange separations (CIX) performed at <0.1 M HF (nohydrolysis), that is, for extraction of the positively chargedcomplexes, the Kd values will change in the following way ingroup-4: Zr ≤ Hf < Rf. This is caused by the decreasing trendin the formation of positively charged complexes according toeq 9.1.2: Zr ≥ Hf > Rf. In the case of formation of complexeswith a lower positive charge from complexes with a higherpositive charge, the sequence in the Kd values is opposite to thesequence in complex formation, since complexes with a highercharge are better sorbed on the CIX resin than those with alower charge.For the formation of anionic complexes sorbed by anion-

exchange (AIX) resins, the trend becomes more complicateddepending on pH, that is, depending on whether thefluorination starts from hydrated or hydrolyzed species. Thus,for experiments conducted in 10−3 to 10−1 M HF (where somehydrolyzed or partially fluorinated species are present), thetrend for the formation of MF6

2− (eq 9.1.3) should be reversedin group 4: Rf ≥ Zr > Hf.

Figure 29. The difference in the lengths of the single (open circles)and triple (filled triangles) bonds between the 6d and 5d metals.319,320

Figure 30. Vapor pressure of group-4 chlorides over their respectivesolids as a function of temperature. Literature data for ZrCl4 and HfCl4from Knacke et al.328 The vapor pressure curve labeled “RfCl4relativistic” resulted from using the QM data from Pershina et al.315

and applying the procedure outlined in section 8.1.1, whereas theshaded area labeled “RfCl4 extrapolated” indicates the predicted vaporpressure of RfCl4 using two different extrapolation procedures.326



For the AIX separations at 4−8 M HCl, where no hydrolysisshould occur at such high acidities, the theoretical data suggestthat the trend in the complex formation and Kd values shouldbe continued with Rf: Zr > Hf > Rf.Complex formation of group-4 elements in H2SO4 solutions

was also studied theoretically.310 In this work, relative valuesof the free energy change of the M(SO4)2(H2O)4,M(SO4)3(H2O)2

2−, and M(SO4)44− (M = Zr, Hf, and Rf)

formation reactions from hydrated and partially hydrolyzedcations were calculated using the 4c-DFT method. Geometricalconfigurations of two of the considered complexes are shown inFigure 31.The results indicate the following trend in complex

formation, Zr > Hf ≫ Rf. The obtained log Kd values for theextraction of Zr, Hf, and Rf from H2SO4 solutions by aminesare shown in Figure 32.

9.2. Experimental Results

Early on, two different chemical strategies were chosen todemonstrate that the chemical properties of Rf are distinctlydifferent from those of the actinide elements. In Dubna, the fact

that group-4 elements form rather volatile halides wasexploited. A good measure for the volatility of a molecule isits vapor pressure over its respective solid (see Figure 33) (It

was, therefore, originally suggested by Pershina et al.330,331 touse those vapor pressure curves also in the chemical studies onthe transactinides in comparison with their lighter homologs).The volatility decreases according to MCl4 > MBr4 > MI4 >MF4 with M = Zr, Hf. While the iodides show poor thermalstability and the fluorides are the least volatile, chlorides andbromides are a good choice for a transactinide chemistryexperiment. Clearly, the heavy actinides do not form volatilechlorides or bromides. Therefore, an experiment that isolates Rfin the form of volatile chlorides or bromides also demonstratesthat this element does not belong to the actinide series.Immediately after the discovery of a short-lived spontaneouslyfissioning radioactivity at Dubna in a physics experiment, aseries of gas-phase chemistry experiments were conducted byZvara et al.60−67 to prove that this radioactivity was indeed notdue to an actinide element. Later, the volatility of group-4chlorides and bromides was reinvestigated by Turler,225,230

Kadkhodayan,332 and Sylwester333 using online IC withidentification of 261Rfa through α-particle spectroscopy, α−αcorrelations, and determinations of t1/2 of 261Rfa and itsdaughter 257No. Enthalpies of adsorption of the investigatedvolatile compounds on the chromatographic surface (−ΔHa

0(T))were deduced.In Berkeley, aqueous phase chemistry experiments88 were

conducted with the same isotope 261Rfa. Tetravalent Rf (Rf4+)

Figure 31. M(SO4)2(H2O)4 and M(SO4)44− complexes of Zr, Hf, and Rf. Reprinted with permission from ref 310. Copyright 2006 Oldenbourg

Wissenschaftsverlag GmbH.

Figure 32. Predicted log Kd for the extraction of Hf and Rf fromH2SO4 solutions by amines with respect to the ones measured for Zr.Reprinted with permission from ref 310. Copyright 2006 OldenbourgWissenschaftsverlag GmbH.

Figure 33. Vapor pressure curves for Zr and Hf halides over theirrespective solids.334. Reprinted with permission from ref 334.Copyright 1994 Springer Science and Business Media.



was eluted with the complexing agent ammonium α-hydroxyisobutyrate (α-HIB) from a cation exchange column,way ahead of the trivalent actinides or divalent No. Bypreparing samples suitable for α-particle spectroscopy, Silva etal.268 could unambiguously identify 261Rfa in the eluatecontaining the tetravalent elements by observing timecorrelated α−α mother daughter correlations attributed tothe decay of 261Rfa followed within 1 min by the α-particledecay of the daughter 257No.9.2.1. Gas-Phase Chemistry of Rutherfordium. When it

became obvious that t1/2(260Rf) < 0.3 s and a newly detected

4.5-s SF radioactivity335 was due to a possible SF (or EC,leading to SF in 259Lr) branch336,337 in 259Rf (which decaysprimarily by α-emission with t1/2 = 3.2 s338), Zvara et al.64

repeated their experiments with a new apparatus, whichcombined IC with TC. A schematic of the apparatus and theobtained results is shown in Figure 34.

As in previous experiments, the nuclides recoiling from thetarget were stopped in a stream of N2 gas and flushed into thechromatography section. Due to the longer t1/2(

259Rf), the flowrate could be reduced by about 1 order of magnitude, allowinghigher separation efficiency from short-lived SF-isomers in theactinides and better chromatographic resolution. SF trackdetectors were inserted into sections II and III. As reactiveagents a mixture of SOCl2 and TiCl4 (which also served ascarrier) was used. In order to simultaneously produce Hfnuclides, the 242Pu target contained an admixture of Sm. Thisway, the deposition peak of long-lived 170,171Hf and 259Rf couldbe measured simultaneously in the same experiment. Indeed,after completion of the experiment, the isothermal section IIcontained only a few fission tracks at the very beginning andone SF track further downstream, whereas, in the gradientsection, 15 SF tracks were observed at about the same positionas long-lived Hf was deposited. The results of this experiment

were interpreted that the SF-tracks observed were due to SF of259Rf behaving like the lighter homolog element Hf, and thatalso in earlier experiments the SF decay of this Rf isotope hasbeen observed.This work and also the earlier experiments by Zvara et al.

were criticized by the members of the Berkeley group.258 Thethree points of criticism concerned the unknown magnitude ofthe SF-branch in 259Rf, the inability to determine t1/2 of thesource of SF, which would have allowed excluding SF fromactinide nuclei such as 256Fm, and the absence of many SFtracks in the isothermal section due to the decay of shorter-lived 260Rf.Concerning the measurement of the magnitude of the SF-

branch of 259Rf not too much progress has been accomplished.The only direct measurement was performed by Bemis et al.,336

who reported a SF branching ratio of 6.3 ± 3.7%. In the workof Bemis et al.,336 a total of 22 SF events were observed. Ofthese, 8 ± 2 events were ascribed to long-lived 256Md/256Fm.Another source of SF was identified in the presence of 256No.Here, a SF branch of 0.25% was used to estimate an additionalcontribution of 4.8 ± 1.2 SF events not related to 259Rf, leaving9.2 ± 5.2 SF events. However, in a recent experiment, the SF/αratio in 256No was measured339 as 0.0053−0.0003

+0.0006 . With this newvalue, the additional contribution due to 256No increases to 10.2SF events, leaving now only 3.8 of the observed 22 SF eventsattributable to 259Rf. Thus, the SF branch in 259Rf reduces to2.6%. Gates et al.337 have observed an EC branch of 15 ± 4% in259Rf, leading to 259Lr, which decays by SF with 25 ± 3%branching. This would result in an apparent SF-branch of 259Rfof 3.8 ± 1.1%. Ascribing some of the observed SF events to259Lr after EC decay of 259Rf would not alter the conclusionsdrawn on the chemical properties of Rf, since, under theconditions of the experiment by Zvara et al.,61,63,64 Lr isexpected to form nonvolatile compounds which would depositessentially at the same position in the column as Rf. In addition,a SF-branch of >2% (or an EC-branch >8%) in 259Rf would besufficient to explain the number of SF events by Zvara etal.61,63,64

Interestingly, the chemical aspects of the experiment werenot discussed. The fact that Rf nuclides with t1/2 = 3 s depositedin the column at the same position as did 170,171Hf (t1/2 = 16.0and 12.2 h, respectively) does not at all mean that HfCl4 andRfCl4 exhibit the same volatility. With the development of amicroscopic description of the chromatographic process,288 themigration of a molecule through the column can be simulatedusing a Monte Carlo technique. With this new technique, aquantitative analysis of this experiment is possible and willallow a comparison of the obtained ΔHa

0(T)(RfCl4) with data ofnewer, IC experiments. The results of a Monte Carlosimulation with the microscopic model of Zvara288 are shownin Figure 35. In the upper panel, the experimental data areshown together with the simulated deposition zone profiles for259RfCl4 and 170,171HfCl4. The only adjustable parameter wasΔHa

0(T); all other parameters were fixed at their experimentalvalues. In the lower panel the integrated yields are shown incomparison with the simulation. The deposition zone of HfCl4can be reproduced very accurately. The experimentaldistribution of SF events of 259Rf is somewhat narrow comparedto the simulated zone profile, but in the light of the poorstatistics, the two distributions are in reasonable agreement.The determined −ΔHa

0(T)(RfCl4) is 110 kJ·mol−1. For thelighter homolog Hf, −ΔHa

0(T)(HfCl4) = 146 kJ·mol−1 resulted.

Figure 34. (a) Schematic of the experimental apparatus to investigatethe volatility of 259Rf and 170,171Hf chlorides; (b) temperature profile inthe column (isothermal combined with a gradient); (c) distribution ofSF tracks (open and closed circles) for 44mSc and 170,171Hf. Reprintedwith permission from ref 58. Copyright 1987 OldenbourgWissenschaftsverlag GmbH.



Further TC experiments with the nuclide 259Rf involving alsothe study of the tetrabromides were conducted much later(1991) and reported only in the form of a contribution to theJoint Institute for Nuclear Research, Laboratory of NuclearReactions annual report.340 A quantitative analysis of theseexperiments was reported.229 For the adsorption on quartzsurfaces, ΔHa

0(T)(RfBr4) = −68 kJ·mol−1 and ΔHa0(T)(HfBr4) =

−86 kJ·mol−1 were obtained. Again, RfBr4 seemed to be morevolatile than the homologous HfBr4.A first experiment using IC of group-4 chlorides and

bromides with direct identification of the reaction productswith α-particle and SF-spectrometry was published in 1992225

using the OLGA(II) setup in combination with the MGrotating wheel detector. For these studies, the nuclide 261Rfa

(t1/2 ≈ 1 m) was used,88 and altogether 14 α−α correlationsattributed to the decay sequence

→ →α α

Rf No Fma261 257 253

were detected, yielding the correct t1/2 of the mother and thedaughter nuclides. So, this experiment yielded unambiguousproof that volatile complexes of Rf were isolated using gas-phase chromatography. In addition, the behavior of Rf could bedirectly compared with that of 162Hf (t1/2 = 41 s), a nuclide witha very similar t1/2 compared to that of 261Rfa. It was shown thatgroup-4 chlorides RfCl4 and HfCl4 were more volatile thantheir respective bromides. In addition, RfBr4 was more volatilethan HfBr4 in the same experiment. Note that in earlier TCexperiments the behavior of 3.1-s 259Rf was compared with thatof 16-h 170Hf. Although the isothermal temperature profileswere far from being ideal and the column was too short in thisfirst experiment, it was possible to extract thermochemicalinformation326 by applying the microscopic model by Zvara.288

A further drawback was the usage of an aerosol particle gas-jet,which lead to visible deposits of KCl in the chromatographycolumn, and thus the adsorption behavior was not studied on a

pure quartz surface but on a surface covered at least partiallywith KCl. These first experiments were repeated with thesignificantly improved HEVI setup.216,332,333 Instead of KCl,MoO3 was used as aerosol particle forming material, which isconverted to the volatile MoO2Cl2 in the chloride experi-ments.332 In the bromide experiments, using KBr aerosolparticles, significantly smaller deposits inside the chromatog-raphy column were observed compared to the cases of thechloride experiments with KCl aerosol particles.333 As in earlierexperiments, RfBr4 was more volatile than its lighter homologHfBr4. The volatility of ZrBr4 was in-between that of HfBr4 andRfBr4. A similar picture emerged for the chlorides. Again RfCl4was more volatile than HfCl4, while ZrCl4 turned out to besimilarly volatile as RfCl4. The volatility of RfCl4 in comparisonto its lighter homologs was once again investigated using theOLGA(III) setup.230 In this work, carbon aerosol particles wereused, which were converted to CO2 in the hot reaction oven. Inthese experiments it was shown that the concentration of O2 isan important parameter, which has to be carefully controlled.The addition of O2 led to a significant shift of the volatility ofboth Hf and Rf to higher temperatures by about 100 to 200 °C,respectively, as shown in Figure 36, but this confirmed the

higher volatility of RfCl4 compared to HfCl4. In a more recentpublication,341 the volatility of Zr, Hf, and Rf chlorides wasinvestigated simultaneously. In this work, no significantdifferences in the volatility of the three group-4 chlorideswere observed. Nevertheless, the T50% temperature of RfCl4 wasabout 25 °C lower compared to that of the longer-lived Hf atthe same isothermal temperature, yielding some indications of amore volatile RfCl4.In the following, the available results on group-4 halides,

including Rf, have been analyzed in a meta analysis using themicroscopic model of Zvara288 and a standard set ofparameters. This was necessary since, in the originalpublications, differing parameters were used and thus apparentdeviations occurred. In Table 2, the published and theevaluated ΔHa

0(T) values measured for group-4 tetrachloridesand tetrabromides of Zr, Hf, and Rf in various experiments aresummarized. The table shows ΔHa

0(T) values evaluated with aconsistent characteristic period of oscillation of SiO2 in the

Figure 35. Measured and simulated deposition peaks of 259RfCl4 and170,171HfCl4. The modeled deposition peaks were obtained using themicroscopic model of Zvara288 and a Monte Carlo simulationtechnique, with the only adjustable parameter being the adsorptionenthalpy (for details see text).326

Figure 36. Yields of 261Rfa and 165Hf as a function of isothermaltemperature. Reactive gases were 200−300 mL/min HCl purified fromtraces of O2 (□, ●) and 150 mL/min Cl2 with SOCl2 and 20 mL/minO2 (△, ◆), respectively.230. Reprinted with permission from ref 230.Copyright 1998 Elsevier.



Monte Carlo model of τ0 = 2 × 10−13 s. Also shown are theΔHa

0(T) values originally published in the literature or theevaluated ΔHa

0(T) values of Zvara.297 After analyzing theexperiments of Kadkhodayan et al.332 and Turler et al.230

using a consistent τ0, both experiments are in very goodagreement concerning the ΔHa

0(T) values of HfCl4 and RfCl4,while there are differences in the ΔHa

0(T) values for ZrCl4. Theunpublished value of −ΔHa

0(T)(ZrCl4) = 97 kJ·mol−1, which wasevaluated in measurements with the OLGA(III) setup in testexperiments with 98Zr under very similar conditions, is in verygood agreement with off-line TC experiments, where −ΔHa

0(T)

(ZrCl4) = 98 kJ·mol−1 was determined.342 The use of MoO3aerosol particles (Kadkhodayan et al.332) or C aerosols (Turleret al.230) did not influence the adsorption characteristics ofgroup-4 chlorides on the quartz surface. In the first ICexperiments with Rf of Turler et al.,225 only upper limits couldbe established for −ΔHa

0(T)(HfCl4) and −ΔHa0(T) (RfCl4). Due

to the very short isothermal length of the chromatographycolumns, no decrease of the yield at low temperatures wasobserved. While the upper limit deduced for −ΔHa

0(T)(RfCl4) isin agreement with the values obtained in later experiments,HfCl4 was found to be much more volatile. The reason for theobserved behavior is not clear, and the experiment was notrepeated at the time. The ΔHa

0(T) values for HfCl4 and RfCl4evaluated from a TC experiment of Zvara et al.340 in a quartzchromatography column are in very good agreement with theresults of IC experiments. The ΔHa

0(T) values of RfCl4 evaluatedfrom the very early experiments at Dubna are in goodagreement with each other, but compared to later experiments,Rf-chlorides were about 25 kJ·mol−1 less volatile. However,these experiments were performed on different chromato-graphic surfaces. The columns were made from glass into whichmica sheets were inserted. Also, the ΔHa

0(T) values for HfCl4 arequite different in the two experiments, which may point to avarying content of residual O2 in the two experiments. Theaddition of O2 reduced the volatility of both Hf- and Rf-chlorides considerably in isothermal experiments of Turler etal.230 (see Figure 36).The introduction of a consistent characteristic period of

oscillation τ0 has reduced the apparent differences in theΔHa

0(T) values significantly. The reason for the disagreeingΔHa

0(T) values originally published in the literature is thereforenot mainly due to the varying degree of the chemicalmodification of the column surface by the different

halogenating agents, as was speculated by Zvara,297 but onlydue to differences in the analysis procedures. Actually, theagreement between all experiments conducted on quartzsurfaces230,332,340 is extraordinary. In all experiments, RfCl4was found to be more volatile than HfCl4. The volatility ofRfCl4 seems to be more similar to that of ZrCl4, so that thesequence in volatility is ZrCl4 ≈ RfCl4 > HfCl4. This result issurprising and was interpreted as a manifestation of relativisticeffects.230 Extrapolation procedures that rely on the regularitiesthat govern the trends in the physicochemical properties withinthe groups and periods of the Periodic Table clearly indicatedthat RfCl4 should be less volatile than HfCl4. A high volatility ofRfCl4 was only discussed after relativistic MO calculations315,325

pointed to a low ionic and a high covalent contribution to thebonding in RfCl4.For group-4 bromides, the agreement of the evaluated

ΔHa0(T) values of different experiments is poor (see Table 2).

While all experiments consistently found RfBr4 to be morevolatile than HfBr4, the evaluated −ΔHa

0(T) values range from68 to 111 kJ·mol−1. The fact that in OLGA(II) experimentsabout 15 kJ·mol−1 less volatile Hf- and Rf-bromides wereobserved compared to experiments with HEVI could beattributed to the use of a KCl aerosol gas-jet to transport thereaction products to the chemistry setup. Thus, the quartzcolumn was at least partially covered with KCl. Generally, lowervolatilities have been observed on KCl surfaces compared topure quartz.246 The two experiments using IC found RfBr4 andHfBr4 to be less volatile than the corresponding tetrachlorides,while in the TC experiment RfBr4 and HfBr4 were considerablymore volatile. Considering macroscopic properties such as thevapor pressure over the respective solid or the boiling point,ZrBr4 and HfBr4 are expected to be slightly less volatile thantheir corresponding tetrachlorides, as observed in both ICexperiments. In the TC experiment the possibility of two closeand thus unresolved deposition peaks was discussed, since thedistribution of SF tracks was much wider than that of the Hfdeposition peak.340 However, the Monte Carlo analysis of theexperiment clearly showed229 that almost all data points arecontained in a 3σ error interval if only one deposition peak isassumed. The presence of two deposition zones for Rf-bromides is possible but is not corroborated by the measureddata due to the low statistics. In conclusion, the amount of dataaccumulated for RfCl4 and RfBr4 is impressive. In Figure 37 theevaluated adsorption enthalpy values for group-4 tetrachlorides

Table 2. Comparison of Published and Evaluated ΔHa0(T) Values Measured for Group-4 Tetrachlorides and Tetrabromides in

Various Experiments

−ΔHa0(T) publ (kJ·mol−1) −ΔHa

0(T) eval (kJ·mol−1)

technique ref year aerosol or carrier gas ZrCl4 HfCl4 RfCl4 ZrCl4 HfCl4 RfCl4

FC 62 1969 N2 84 84 84−96 84 84 100−109a

TC 64 1971 N2 155c 104c 146b 110b

TC 340 1991 Ar 105c 83c c c cIC (OLGA II) 225 1992 He/KCl ≤70d ≤80d ≤75 ≤85IC (HEVI) 332 1996 He/MoO3 74 ± 5 96 ± 5 77 ± 5 79 ± 5 103 ± 5 82 ± 5IC (OLGA III) 230 1998 He/C 110c 92c 97e 103 87

ZrBr4 HfBr4 RfBr4 ZrBr4 HfBr4 RfBr4TC 340 1991 Ar 82c 63c 86f 68f

IC (OLGA II) 225 1992 He/KCl 125d 105d 130 111IC (HEVI) 333 1996 He/KBr 91 ± 6 113 ± 5g 89 ± 5g 95 ± 6 117 ± 5 93 ± 5

aData from ref 343. bSee Figure 35. cData from ref 297; evaluation not possible due to missing experimental details. dData from ref 344.eUnpublished data by Turler et al. fSee also ref 229. gData from ref 345.



and tetrabromides are shown.332,333 It is evident that theadsorption properties of Rf halides do not follow simple lineartrends, which seem to be valid for the lighter homologs.Whether the surprisingly high volatility of RfCl4 and RfBr4 isdefinitely due to relativistic effects requires detailed calculationsof the interaction of the molecule with the adsorbent. This isnot an easy task as modifications of the surface by the reactiveagents occur and the structure of real surfaces is very hard tointegrate into the model calculations. Also, it is not clear if theadsorption process on real surfaces is sufficiently approximatedby the model of mobile adsorption. Since a detailed discussionof all these aspects is beyond the scope of this review, we referhere to the book by Zvara.240

9.2.2. Liquid-Phase Chemistry of Rutherfordium. Thefirst investigations of Rf in the liquid phase were performed asearly as in 1970 by Silva et al.268 The technique employed builton the very successful application of cation-exchangechromatography using the chelating agent α-HIB in discoveringactinide elements. The elution position was indicative of theionic radius of the heavy actinides that were all present in the+3 oxidation state (except for No). The largest hydrated ionsare eluted first. This effect can be enhanced by using

appropriate oxycarboxylates, such as α-HIB, which appears tobe the most selective.In the experiment by Silva et al.,268 261Rfa was produced by

irradiation of only 47 μg of 248Cm with 92 MeV 18O ions. Rfatoms recoiling from the target were swept with He from therecoil chamber through a nozzle and deposited on the surfaceof a rabbit which was coated with thin layer of NH4Cl.Periodically, the rabbit was transported to the chemistryapparatus, where the surface was washed with 50 μL of α-HIBsolution (0.1 M, pH 4.0) on the top of a small, heated (80 °C)Dowex 50-X12 cation exchange column (2 mm i.d., 20 mmlong). The solution was forced into the resin, and more eluentwas added. The first two drops corresponding to the freecolumn volume contained no or little radioactivity and werediscarded. The next four drops (in 2 drop fractions) werecollected on Pt disks, evaporated to dryness, and heated to 500°C. The disks were assayed by α-particle spectroscopy. Group-4elements Zr, Hf, and Rf were strongly complexed with α-HIBand were thus not retained on the column and eluted far aheadof the actinides in the +3 or +2 oxidation state. A large part ofthe chemical system was automated so that the average timefrom beam off to counting was reduced to about 60 s. Severalhundred separations were performed, but only 17 α-particles inthe energy range from 8.2−8.4 MeV were registered; about halfof these events are due to the decay of the 26-s 257No daughter.In two experiments, a correlated α−α pair was registered whichwas attributed to the decay of 261Rfa and its 257No daughter.Due to the limited 2π detector geometry, this number ofcorrelated events is consistent with the total number ofregistered α-particles in the energy range 8.2−8.4 MeV. Silvaand co-workers concluded268 that “for the particular cationexchange conditions used in these experiments, the behavior ofthe radioactivity assigned to element 104 with mass 261 isentirely different from trivalent and divalent actinide elementsbut is similar to Hf and Zr as one would predict for the nextmember of the Periodic System following the actinide series ofelements”. However, the experiment was not yet efficientenough to determine Kd values.

9.2.2.1. Fluoride Complexation of Group-4 Elements Zr,Hf, and Rf. The fluoride complexation of group-4 elements wasstudied by various authors.266,272,346−350 HF is one of the bestmedia on an anion-exchange resin, because hydrolysis need not

Figure 37. Evaluated adsorption enthalpies (ΔHa0(T)) of group-4

tetrachlorides and tetrabromides from isothermal gas adsorptionchromatographic experiments with HEVI.

Figure 38. (a) Sorption of Zr, Hf, Th, and Rf on the cation exchange resin Aminex A6 in 0.1 M HNO3 at various HF concentrations. Off-line dataare shown for Zr, Hf, and Th (open symbols) and online data for Hf and Rf (closed symbols), re-evaluated data from Strub et al.272 Reprinted withpermission from ref 26. Copyright 2011 Oldenbourg Wissenschaftsverlag GmbH. (b) Sorption of Zr, Hf, Th, and Rf on the anion-exchange resin(Riedel-de Haen) in 0.1 M HNO3 at various HF concentrations. Off-line data are indicated by lines, online data for Hf and Rf by symbols. Reprintedwith permission from ref 272. Copyright 2000 Oldenbourg Wissenschaftsverlag GmbH.



be taken into account due to the high complexing strength ofthe fluoride ion. In early studies,347,351 Rf nuclides were notdirectly detected. In the so-called multicolumn technique(MCT), the reaction products, including 261Rfa, were trans-ported to the chemistry setup with the aid of a KCl aerosol gas-jet and continuously dissolved in 0.2 M HF. The solution waspassed through 3 ion-exchange columns. In the first cation-exchange column, heavy actinide elements were adsorbed while261Rfa in the form of anionic RfF6

2− passed through and wasadsorbed on an anion-exchange column. A third cation-exchange column sorbed the descendants 253Fm and 253Es,which were eluted at the end of each experiment and measuredoff-line by α-particle spectroscopy. Detection of the descend-ants was regarded as proof of the formation of anionic fluoridecomplexes. Pfrepper et al.351,352 refined the method and studiedgroup-4 elements fluoride complexation in mixed HF/HNO3solutions and determined first Kd values. These wereindistinguishable for Hf and Rf. However, these results are atvariance with recent results of Toyoshima et al.350 that will bediscussed later.The fluoride complexation of group-4 elements Zr, Hf, and

Rf, and of the pseudohomolog Th, in mixed 0.1 M HNO3/HFsolutions was investigated by Strub et al.272 studying Kd valueson both cation exchange resins and anion exchange resins usingARCA. As expected, in cation exchange experiments below10−3 M HF, the Kd values for Zr, Hf, and Th are >103,indicating the presence of cations. At higher concentrations,between 10−3 M and 10−2 M HF, neutral or anionic fluoridespecies are formed and the Kd values are decreasing. Thebehaviors of Zr and Hf were very similar. Also, off-line andonline data for Hf were consistent. Later, the Kd valuesmeasured for 261Rfa were slightly adjusted26,29,32 compared tothe original publication272 by applying a more accurate analysisprocedure. The separation of Zr, Hf, Th, and Rf on the cation-exchange resin Aminex A6 in 0.1 M HNO3 at various HFconcentrations is shown in Figure 38a. Much later, Ishii etal.353,354 reexamined Kd values for Zr, Hf, Th, and Rf on acation-exchange resin in mixed HF/0.1 M HNO3 solutions asfunction of the fluoride ion concentration, fully validating theresults of Strub et al.272 The observed behavior wastheoretically predicted (see section 9.1).310 The Kd valuesshould change in the following way in group-4: Zr ≤ Hf < Rf.This trend was indeed observed in the experiments.272,353

Investigations of the anion-exchange behavior showed somesurprises. While the off-line data, as expected, clearly indicatedthe formation of anionic fluoride complexes for Zr and Hfabove 10−3 M HF, the online data for Hf as well as Rf weredifferent. There was almost no sorption of Rf on the resinbetween 10−2 and 1 M HF, and the sorption of Hf was lowercompared to the case of off-line experiments, as displayed inFigure 38b.The unusual result concerning the anion-exchange behavior

of Rf-fluorides, which would be in contradiction to earlierresults, called for a detailed investigation of the influence of thecounterion NO3

−. The trend for the formation of MF62− (eq

9.1.3) should be reversed in group 4: Rf ≥ Zr > Hf310 (see alsodiscussion at the end of this section). Such a trend was, indeed,observed in the experiments on the AIX separations of group-4elements from 0.02 M HF.355 A similar result was obtainedlater,350 where the formation constant of RfF6

2− was reportedto be at least 1 order of magnitude smaller than those of ZrF6

2−

and HfF62−. When the HNO3 concentration was lowered to

0.01 M and the HF concentration was chosen as 0.05 M, the Kd

values of Hf and Rf were rising with decreasing NO3−

concentration. Apparently, the counterion NO3− is much

more effective in competing for binding sites on the anionexchanger to remove Rf compared to Zr and Hf.272

In order to better understand the anion-exchange behavior ofgroup-4 elements in mixed HF/HNO3 solutions, first thebehavior in pure HF solutions of varying concentrations of1.9−13.9 M HF was investigated by Haba et al.348 As it wasobserved that NO3

− can compete as counterion for bindingsites for Rf on the anion-exchanger and HF is a weak acid, theequilibrium among HF, H+, F−, and HF2

− in aqueous solutionhas to be considered:

+ ⇄+ −H F HF (9.2.1)

+ ⇄− −HF F HF2 (9.2.2)

At an initial concentration of 0.4 M [HF]ini, theconcentration of the HF2

− anion starts to dominate comparedto that of F− and becomes by far the dominating anionic speciesat concentrations >1 M [HF]ini. Indeed, decreasing Kd valuesare observed with increasing [HF]ini, which can be explained asthe displacement of the metal complex from the binding sites ofthe resin by HF2

−. Ignoring the knowledge of the activities ofthe chemical species involved, the adsorption equilibrium of ananionic complex An− with the charge state n− to the counterionHF2

− between the resin phase R and the solution can berepresented by the equation

+ ⇄ +− −n nRHF A R A HFnn2 2 (9.2.3)

The equilibrium constant of the exchange reaction Dn can beexpressed as follows:

= −

‐D

[R A][A ]

[HF ][RHF ]n

nn

n

n2

2 (9.2.4)

For anionic complexes being present as single chemicalspecies, the Kd value can be written as follows:

= =− −K D[R A][A ]

[RHF ][HF ]d

nn n

n

n2

2 (9.2.5)

Taking the logarithm on both sides yields:

= −−

K D nlog log log[HF ][RHF ]d n

2

2 (9.2.6)

The concentration of the counterion in the resin phase canbe regarded as a constant value that is equal to the exchangecapacity of the anion-exchange resin. Since log [HF2

−] ≈ log[HF]ini − 1.3, the charge of the anionic complex n can thus beevaluated from a plot of log [HF]ini versus log Kd as shown inFigure 39. The adsorption of Rf−fluoride complexes on theanion-exchanger is clearly weaker compared to that of thelighter homologs Zr and Hf. Also, the slope of the curve of −2.0± 0.3 for Rf clearly differs from the slope of −3.0 ± 0.1 for Zrand Hf. The associated anionic fluoride complexes are RfF6

2−

and ZrF73− or HfF7

3−, respectively. For the first time, asignificant deviation of the chemical behavior of a transactinideelement compared to its lighter homologs was observed.Building on the work of Strub et al.,272 Toyoshima et al.350

have investigated the fluoride complexation of group-4elements in mixed HF/HNO3 solutions in the concentrationranges of 0.0054−0.74 M HF and of 0.010−0.030 M HNO3. Inoff-line static distribution experiments, the behavior of Zr andHf was studied as a function of the equilibrated concentration



of free F− ([F−]eq) at concentrations of [NO3−]eq of 0.01, 0.03,

0.1, and 0.3 M. In column experiments using AIDA, Kd valuesfor Rf were measured at [NO3

−]eq of 0.01 and 0.015 M. It wasshown that, at concentrations of [F−]eq < 5 × 10−3 M, thecontribution of the additional counterions F− and HF2

− presentin the HF/HNO3 solutions is negligible. A slope analysis in thelog [NO3

−]eq versus log Kd plot at a fixed [F−]eq = 3 × 10−3 Mconsistently resulted in an anionic charge of −2 for Zr, Hf, andRf, indicating the presence of MF6

2− complexes (see Figure40a). However, as already observed in the pure fluoride system,the complexation of Rf was again much weaker compared tothat of Zr and Hf, as the Kd values differed by almost 3 ordersof magnitude. This result is in contrast to the experiments byPfrepper et al.351,352 The difference was attributed to incorrectassumptions about the breakthrough volume of trivalentactinides,351,352 which was probably a factor of 10 lower thananticipated,26,32 based on newer experimental data.356 Ob-viously, at low fluoride concentrations, Zr and Hf form MF6

2−

complexes, whereas, at higher concentrations of HFini, whereHF2

− becomes the dominant species, MF73− complexes are

formed.348 In Figure 40b, Kd values of Zr, Hf, and Rf are shownas a function of [F−]eq. The increasing Kd values of Rf, Zr, andHf with increasing [F−]eq under constant [NO3

−]eq indicatethat the anionic fluoride complexes are increasingly formed.The formation reactions are represented as follows:

+ ⇄ = −−− − − − nMF F MF ( 1 6)n

nn

n1

4 ( 1) 4(9.2.7)

where M indicates Zr or Hf and n denotes the coordinationnumber in the products. In anion exchange, only anionicfluoride complexes of MF5

− and MF62− participate in the

adsorption−desorption process, with MF62− being the domi-

nant species, as confirmed by the slope analysis. The suddendrop of the Kd values above [F

−]eq > 5 × 10−3 M was associatedwith the abrupt increase of [HF2

−]eq. The solid, dashed, anddotted curves in Figure 40b are results of calculations where Kdis related to the consecutive formation of the fluoridecomplexes of Zr and Hf in mixed HF/HNO3 solutions andthe onset of the additional counterion HF2

−.As was mentioned in section 9.1, the trend in the extraction

of group-4 species becomes complicated depending on pH andother experimental conditions. For very dilute HF solutions,the trend in the extraction of MF6

− was predicted as Rf ≥ Zr >Hf.310 Such a trend was observed in the experiments on theAIX separations of group-4 elements from 0.02 M HF.355 Theassumed formation and extraction of the ZrF7

3− or HfF73

complexes at higher HF concentrations in comparison withRfF6

− studied experimentally353 have not yet been consideredtheoretically. However, the suggested lower coordinationnumber (CN) of Rf equal to 6 in the fluorine complexes incomparison with CN of 7 in complexes of Zr and Hf can hardlybe expected, since the Rf ion is larger than those of Hf and Zr:that is, the IR (CN = 6) of Rf4+ (0.76 Å321−323 or 0.79 Å176) islarger than the IR of Zr4+ (0.72 Å) and Hf 4+ (0.71 Å)317 (seesection 9.1). Also, the competition between the NO3

− and F−

ions in the complex formation has not been treatedtheoretically. Thus, this complex aqueous and extractionbehavior of group-4 species at various experimental conditionsneeds further theoretical considerations.

9.2.2.2. Chloride Complexation of Group-4 Elements Zr,Hf, and Rf. The chloride complexation and hydrolysis of group-4 elements including Rf was studied with a variety of methods.The formation of group-4 anionic-chloride species of the formMCl6

2− (M = Zr, Hf, Rf) was observed on the one hand by the

Figure 39. Variation of the distribution coefficient, Kd, of Rf, Zr, andHf on the anion-exchange resin CA08Y as a function of the initial HFconcentration, [HF]ini. The Kd values of Rf, Zr, and Hf are shown bydiamonds, squares, and circles, respectively. Open and closed symbolsdepict different column dimensions. Linear relationships with slopes of−2.0 ± 0.3 for Rf and −3.0 ± 0.1 for Zr and Hf in the log [HF]iniversus log Kd are indicated by solid (−) and dotted (···) lines,respectively.348

Figure 40. (a) Distribution coefficients, Kd, of Zr and Hf under static conditions (open symbols) and those of Zr, Hf, and Rf from columnchromatography (closed symbols) as a function of [NO3

−]eq and [F−]eq = 3.0 × 10−3 M. Reproduced with permission from ref 350. Copyright 2008

Oldenbourg Wissenschaftsverlag GmbH. (b) Variation of the Kd values of Zr and Hf under static conditions and of Rf in column chromatography asa function of [F−]eq. Values for Zr and Hf are shown for [NO3

−] = 0.01, 0.03, 0.1, and 0.3 M. Values for Rf are shown for [NO3−] = 0.01 and 0.015

M. The solid, dashed, and dotted curves are theoretical calculations of Kd values. Reproduced with permission from ref 350. Copyright 2008Oldenbourg Wissenschaftsverlag GmbH.



adsorption on anion-exchange chromatography columns from>7 M HCl solutions357 and by their extraction into a quaternaryamine in reversed phase chromatography269 and into TiOA(triisooctylamine)263 in liquid−liquid extractions.Using a fully automated solvent extraction chromatography

apparatus, Hulet et al.269 investigated the chloride complexationof Rf in comparison to Hf, Cm, and Fm. On the basis of theirtendency to form strong anionic-chloride complexes in 12 MHCl, group-4 elements were separated from group-1, 2, 3, andactinide elements by extraction chromatography into aquaternary amine (Aliquat-336 on an inert support). Anionic-chloride complexes were thus extracted and retained on thecolumn while i.e. actinide elements passed through. Sub-sequently, group-4 elements were eluted with 6 M HCl.Altogether, in 44 chemistry runs, only 6 α-particle decays of261Rfa and its daughter 257No were registered, including onecorrelated mother−daughter pair. Therefore, Hulet et al.269

concluded that “in 12 M HCI solutions the chloridecomplexation of element 104 is clearly stronger than that ofthe trivalent actinides and is quite similar to that of Hf, which isexpected to be its homolog in the Periodic Table”.More than 10 years later, liquid-phase chemical separations

of Rf were picked up again at LBNL.263,265,358 These weremanually performed separations, as described in section 7.3.1.In the beginning only the organic phase was assayed, whichturned out to be a source of systematical errors.26,32,265,359

Liquid−liquid extractions from 12 M HCl into TiOA263

corroborated earlier results of Hulet et al.269 Cationic specieswere extracted into TTA (thenoyltrifluoroacetone).360 Themeasured distribution coefficient for Rf in comparison to thoseof the pseudohomologs Th and Pu indicated that Rf is lessaffected by hydrolysis than Zr, Hf, and Pu.360

The first hydrolysis constant log K11(Rf) ≈ −4 was predictedtheoretically on the basis of the 4c-DFT calculations for thegroup-4 hydrated and hydrolyzed complexes,307 in goodagreement with the experimental value of −2.6 ± 0.7.360 Thepredicted trend Zr > Hf > Rf is also in agreement with theexperimental data for Zr and Hf, giving log K11(Zr) = 0.3 andlog K11(Hf) = −0.25.301 One should note here that a simplemodel of hydrolysis301 based on the ratio of a cation charge toits size would give an opposite and, hence, a wrong trend fromZr to Hf, since IR(Zr4+) > IR(Hf4+).317

Bilewicz et al.358 studied the onset of hydrolysis at decreasingHCl concentrations by sorption of Zr, Hf, Th, and Rf on cobaltferrocyanide surfaces, which are known to be selective sorbentsfor singly charged cations such as Rb+ or Cs+, but also fortetravalent metal ions such as Zr4+, Hf4+, and Th4+. This resultwas in contrast to the findings of Czerwinski et al.360 andattributed by Bilewicz et al.358 to relativistic effects whichpredict that Rf4+ would be more prone to having a CN of 6rather than 8 in aqueous solutions due to a destabilization ofthe 6d5/2 shell. The latter supposition can, however, not besupported by the relativistic calculations that show that boththe 6d3/2 and 6d5/2 AOs take part in the bond formation of theRf compounds.The chloride complexation and hydrolysis of group-4

elements using anion-exchange chromatography was studiedin much more detail by Haba et al.357 First, in batchexperiments Kd values of carrier-free radiotracers of 88Zr and175Hf and the pseudohomolog 234Th were determined on theanion-exchange resin CA08Y in 1.0−11.5 M HCl. Second, inonline experiments using the fully automated AIDA apparatus,the anion-exchange behavior of 85Zr, 169Hf, and 261Rfa was

investigated in 4.0−11.5 M HCl. The results of theseexperiments clearly showed that at concentrations below 8 MHCl the Kd values of Zr and Hf are similar, indicating theformation of cationic or neutral species of the typeM(OH)2Cl

+, M(OH)2Cl2, and M(OH)Cl3 (M = Zr, Hf). Atconcentrations above 8 M HCl, the Kd values of both Zr and Hfincrease steeply with increasing HCl concentration, indicatingthat anionic species such as M(OH)Cl5

2− and MCl62− are

formed. The behavior of the pseudohomolog Th is similar tothat of Zr and Hf at HCl concentrations below 8 M; at higherconcentrations, the Kd values strongly deviate, as Th does notform anionic chloride species. In online experiments at 6different HCl concentrations between 4.0 and 11.5 M HCl, theextraction behavior of Rf was studied. At each measuredconcentration, up to 400 separations were performed, resultingin a total of 186 α-particles attributed to 261Rfa and its daughter257No including 35 α−α correlations. The resulting adsorptioncurve as a function of HCl concentration is shown in Figure 41.

The %adsorption values of Rf increase rapidly with increasingHCl concentration from 7.0 to 9.5 M, very similar to Zr and Hf,indicating that anionic chloride complexes such asRf(OH)Cl5

2− or RfCl62− are formed. Between 7 and 9 M

HCl, the extraction sequence is Rf > Zr > Hf, probablyindicating a decreasing chloride complexing strength in thesame order.357 This result cannot find its theoreticalexplanation, as the theory claims that the complex formationat 4−8 M HCl should be continued with Rf, e.g. having thetrend Rf: Zr > Hf > Rf.307

In order to elucidate possible differences in the chemistry ofRf compared to its lighter homologs, a series of liquid−liquidextractions into TBP (tributylphosphate) in benzene to studythe effect of HCl, Cl−, and H+ concentration between 8 and 12M on the extraction of Zr4+, Hf4+, and Rf4+, as well as thepseudohomologs Pu4+ and Th4+, was conducted.264 TBP is oneof the most important organic extractants and widely used inreprocessing of spent nuclear fuel, and it extracts the metal inthe form of a neutral species. The phosphoryl oxygencoordinates with the metal ion by forming an adduct. TheTBP extraction process of group-4 elements in HCl can beexpressed as follows:

Figure 41. Variation of %ads values of carrier-free radionuclides of Zr,Hf, and Rf on the anion-exchanger CA08Y as a function of HClconcentration. Reproduced with permission from ref 357. Copyright2002 The Japan Society of Nuclear and Radiochemical Sciences.



+ ⇄ ++ −M(H O) 4Cl MCl 8H O2 84

4(aq) 2 (9.2.8)

+ ⇄MCl 2TBP MCl (TBP)4(aq) (aq) 4 2(org) (9.2.9)

The extraction of group-4 elements into TBP depends on thestrength of the chloride complexation and the stability of theTBP complex. In the experiments by Czerwinski et al.,264 anextraction sequence Zr > Rf > Hf for the group-4 chlorides wasestablished. In additional experiments by Kacher et al.,265 it wasobserved that significant adsorption of Hf on the Teflonsurfaces occurred. Subsequently, all equipment was changed topolypropylene equipment. The extraction sequence from 8 MHCl into TBP/benzene was revised to Zr > Hf > Rf > Ti.A critical evaluation of these experiments is discussed in

detail by Kratz.26,32,359 On the one hand, the observed sorptionof group-4 elements on Teflon that went unnoticed in the workof Czerwinski et al.264 and the suspected slow kinetics in thework of Bilewicz et al.358 may have contributed to partlyconflicting results and warranted improved experiments todetermine Kd values of group-4 elements by measuring thedistribution in both liquid and organic phases.Gunther et al.271 have investigated the Kd values of the

liquid−liquid extraction of group-4 elements, including Rf, intoundiluted TBP using reversed phase column chromatographyand the ARCA II apparatus. Group-4 elements were fed ontoundiluted TBP/Voltalef (an inert support) columns in 12 MHCl and quantitatively adsorbed. While 75% Hf and no Zrwere eluted in a first fraction with 8 M HCl, the remaining Hfand 93% Zr were stripped in a second elution in 2 M HCl. Thedistribution of Rf in both fractions was measured. In the Hffraction, two correlated α−α pairs of 261Rfa were observed, andthree were observed in the Zr fraction, indicating an extractionof Rf in-between Hf and Zr. The extraction trend of group-4elements from undiluted TBP was established as Zr > Rf > Hfat 8 M HCl. The determined Kd values and the extractionsequence were at variance with earlier results and conclusionsconcerning the extraction of Hf and Rf into TBP.264,265 Such aninversion of the trend is consistent with the theoretical trendfor the formation of the MCl4 species.

307

Recent, very detailed experiments by Haba et al.361

concerning the extraction behavior of group-4 elements intoTBP at different HCl concentrations using the AIDA apparatuslargely confirmed the results obtained by Gunther et al.271 The%extraction values (%Ext) of Zr, Hf, and Rf are plotted inFigure 42 as a function of HCl concentration.Within the error limits, the extraction sequence between 7.2

and 8.0 M HCl into TBP is Zr > Hf ≈ Rf. Although Gunter etal.271 determined at 8 M HCl an extraction sequence Zr > Rf >Hf with Kd values of 1180, 150−46

+64 , and 64, respectively, also inthis work Rf and Hf are much closer compared to Zr. Moreinteresting than these small differences is the fact that Rf is notextracted ahead of Zr. The fact that Zr is extracted at lower HClconcentrations than Hf is corroborated by the larger complexformation strength of Zr as observed in anion-exchangeexperiments357 and also EXAFS studies.281 Some furthercalculations for the MCl4(TBP)2 complexes should beperformed to study this case in more detail.9.2.2.3. Sulfate Complexation of Group-4 Elements Zr, Hf,

and Rf. The sulfate ion SO42− is a strong complexing ligand for

group-4 elements. Its strength to form complexes with Zr andHf is intermediate between those of F− and Cl− ions, i.e. F− >SO4

2− > Cl− ≥ NO3−. Taking into account the experimentally

studied complexing behavior of Rf with F−, Cl−, and NO3−

ions, and the sequence of complexing strength, one wouldexpect that also in the sulfate system Rf forms significantlyweaker complexes compared to those of Zr and Hf underidentical conditions. First, distribution coefficients, Kd, of Zr,Hf, and Th between a cation-exchange resin and a 0.0018−0.69 M H2SO4/HNO3 mixed solution ([H+] = 1.0 M) by abatch method using the long-lived carrier-free radiotracers 88Zr,175Hf, and 234Th were measured. The online chromatographicbehavior of Zr and Hf was then studied to obtain theadsorption probability on the resin as well as elution curves ofthese elements with the simultaneously produced, short-livedisotopes 85Zr and 169Hf. Finally, cation-exchange experimentswith 261Rfa were performed together with 169Hf in 0.15−0.69 MH2SO4/HNO3 mixed solutions ([H+] = 1.0 M) using AIDA.362

As can be seen in Figure 43, adsorption of Rf, Zr, and Hf on thecation-exchange resin decreases with an increase of [HSO4

−],representing the successive formation of sulfate complexes ofthese elements. While Rf shows the same functional shape asHf, the decrease for Rf is shifted to about 0.15−0.20 M higher[HSO4

−]. This indicates that the sulfate complex formation ofRf is significantly weaker than that of the lighter homolog

Figure 42. Percent extractions (%Ext) of Rf, Zr, and Hf on the 20 wt% TBP/CHP20Y resin as a function of HCl concentration.Reproduced with permission from ref 361. Copyright 2007 Old-enbourg Wissenschaftsverlag GmbH.

Figure 43. Variation of Kd values of261Rfa on a cation-exchange resin

derived from its adsorption probabilities as a function of [HSO4−] in

0.0018−0.69 M H2SO4/HNO3 mixed solutions ([H+] = 1.0 M),together with those of 88Zr, 175Hf, and 234Th obtained in batchexperiments. Reproduced with permission from ref 362. Copyright2012 Oldenbourg Wissenschaftsverlag GmbH.



Hf,362 very much similar to the fluoride complexationbehavior272,353,354 (see also Figure 38a). The observed trendis in good agreement with the predicted one from the 4c-DFTcalculations of various sulfate complexes (see section 9.1 andFigure 32).310

SISAK experiments on liquid−liquid extractions of Zr, Hf,and Rf from H2SO4 solutions by trioctylamines (TOA)363

confirmed the predicted trend in the complex formation, Zr >Hf > Rf, and have given Kd(Rf) values close to the predictedones.310

10. DUBNIUM (Z = 105)


The predicted ground state electronic configuration of Db is6d37s2 based on the results of the DF122 and MCDF177

calculations. This suggests that the element will have themaximum oxidation state 5+ as its lighter homologs Nb and Ta.The MCDF calculations have also given the first IP as 7.37 eV(corrected value obtained by an extrapolation procedure177),which is smaller than the IP of Ta (7.89 eV)313 with the 6sionized electron. However, the energies of the 6d(Db) and6s(Ta) AO are very similar.122 All multiple IPs(M → MZ+)were also calculated there.177 The IPs(M → M5+) were shownto decrease from V to Db, due to the destabilization of the(n−1)d AOs, so that the stability of the maximum oxidationstate should increase toward Db (see Figure 28). The IR(M5+)increase in group 5 with Z,317 as was shown earlier by theMCDF calculations (0.74 Å for Db5+ in comparison with 0.64 Åfor Nb5+ and Ta5+), because Rmax values

177 of the outer (n−1)pAOs in M5+ increase in this direction (see Figure 28). Later on,molecular calculations confirmed such an increase in IR andgave a more accurate value for IR of Db5+ of 0.69 Å.327,364 Suchan increase is also in line with an increase in CR.319,320

On the basis of the MCDF calculated multiple IPs, redoxpotentials were estimated for Db and its homologs in group5.177,365 It was indeed shown that the stability of thepentavalent state increases from V to Db, while that of thetetra- and trivalent states should decrease. Thus, for example,the stability of the M3+ should decrease from V3+ to Db3+,because the ground state of the Db3+ ion is not 7s2, as wasassumed earlier, but 6d2.MO calculations were performed mostly for chemically

interesting stable compounds of Db and its group-5 homologsin the highest oxidation states. Thus, the electronic structures ofthe following molecular compounds were studied with the useof the relativistic DFT method: MCl5, MOCl3, MBr5, andMOBr3 (M = Nb, Ta, and Db).330,331,364,366,367 RECPs wereapplied to TaBr5 and DbBr5.

327 Various properties, such asoptimal geometries, bonding, IPs, polarizabilities, dipolemoments, and charge density distribution, were predicted.According to the calculations, Db should be a typical d-elementwhere bonding is defined by the participation of the 6d AO and7s AO. There is also a slight admixture of the relativisticallystabilized 7p1/2 AO.The influence of relativistic effects on the electronic structure

of group-5 d-element compounds was studied on the exampleof MCl5 (M = Nb, Ta, and Db).367 Relativistic effects wereshown to increase the HOMO-LUMO gap, ΔE, due to therelativistic destabilization of the (n−1)d AOs. This results in adecrease in the EAs, defined by the LUMO, and an increase inthe energies of the electron charge-transfer transitions,E[3p(Cl)→(n−1)d(M)]. The latter is associated with the

reduction of the metal, so that an increase in this energy meansan increase in the stability of the 5+ oxidation state in thegroup. This is also shown by the reduction potentials E°(V−IV) for MCl5 (M = V, Nb, Ta, and Db) estimated using acorrelation with the energies of the charge-transfer transitions(Figure 44).38,366 Thus, nonrelativistically, Db5+ would havebeen even less stable than Nb5+. (Similar correlations can beshown for compounds of group-4 to group-8 elements).

Relativistic effects were also shown to be responsible for atrend to a decrease in ionicity (QM) and an increase incovalency (OP) (Figure 45). A partial OP analysis (Figure 46)

shows that such an increase in covalency (total OP) is due tothe increasing contribution of the relativistic ns1/2, np1/2, and(n−1)d AOs. Thus, relativistic effects are responsible for thecontinuation of trends in IP, EA, and stabilities of oxidationstates in the groups in going over to the 6d elements. (The SOeffects are, however, responsible for a trend reversal in De; seebelow.) The nonrelativistic description of these propertieswould give opposite and, therefore, wrong trends.

Figure 44. Correlation between reduction potentials E°(V−IV) andenergies of the lowest charge transfer transitions E[3p(Cl)→(n−1)d(M)] in MCl5 (M = V, Nb, Ta, and Db). The nonrelativisticvalue for Db is shown as a filled circle. Reprinted with permission fromref 38. Copyright 1999 World Scientific.

Figure 45. Relativistic (rel) and nonrelativistic (nr) effective charges,QM, and overlap populations, OP, in MCl5 (M = V, Nb, Ta, andDb).367 L denotes the ligand. Reprinted with permission from ref 367.Copyright 1993 American Institute of Physics.



The De in the Db molecules turned out to be lower than Deof the Ta homologs due to a decrease of the ionic contributionto bonding, although the covalent contribution increases with Zfrom Ta to Db.366 Also, SO effects decrease De, as was shownby the RECP calculations for TaBr5 and DbBr5.

327 The bondlengths of the Db molecules are typically about 0.05 Å largerthan the Re of Nb and Ta homologs,327,364 in agreement withthe IR and CR.319,320 Dipole moments should increase from theNb to Db oxyhalides due to the increase in the interatomicdistances.331

Using the calculated properties of the group-5 molecules,predictions of the volatility of halides and oxyhalides wereattempted.315,330,331,364,366,367 Volatility of the oxyhalides, aseither a sublimation process or adsorption on a surface, shoulddecrease in group 5 with increasing Z due to an increase in thedipole moments of MOCl3 causing their stronger attraction tothe surface.331 Volatility of the pure pentahalides is, however, amore complex process and depends on the definition of thisphenomenon and the nature of the interactions. Thus, forexample, as a sublimation process, volatility should increase ingroup 5, as was shown by the calculations of intermolecularinteractions.330 It was shown that DbBr5 should have a higherPmm over the solid than its lighter homologs.330 The samehigher volatility of DbBr5 in comparison with the homologs waspredicted on the basis of calculations of the molecule−inertsurface interaction energies (eq 8.1.1).364 Such a decrease in themolecule−surface binding energy is caused by the increasingmolecular size related to the separation distance x with Z (eventhough polarizabilities are about the same for TaBr5 andDbBr5). This trend is in agreement with ΔHS

0(298) ofmacroamounts of NbBr5 and TaBr5, so that ΔHS

0(298) ofDbBr5 obtained on the basis of the correlation with E(x) shouldbe smaller than those of the lighter homologs.364

The trend in volatility of MBr5 should, however, be differentif adsorption takes place via a chemical bond formation on asurface, for example, on quartz modified with KCl or KBr. The4c-DFT calculations364 have shown that, in this case, complexesof the type MBr5L

− (L = Br and Cl) are formed on the KCl/KBr surface (Figure 47) and their strength changes as Nb < Ta< Db. This means that volatility should change in an oppositeway in group 5, e.g., Nb > Ta > Db.A number of studies were devoted to predict the extraction

and ion exchange behavior of Db as well as other group-5homologs from HF, HCl, and HBr solutions.304,305 Like group-4 cations, also group-5 ones undergo hydrolysis according tothe reaction

⇄ ++ − +M(H O) M(OH) 6H2 65

6 (10.1.1)

Hydrolysis of the Nb, Ta, Db, and Pa (for comparisonpurposes) cations was studied theoretically on the basis of 4c-DFT calculations of the components of reaction 10.1.1.303 Thecalculated relative ΔEC values for this reaction are indicative ofthe following trend in hydrolysis of group-5 cations: Nb > Ta >Db ≫ Pa. This sequence is in agreement with experiments onhydrolysis of Nb, Ta, and Pa.301 A simple model ofhydrolysis301 does not reproduce the difference between Nband Ta having equal IR. The present model (section 8.2) basedon the real (relativistic) distribution of the electronic densitycorrectly describes the experimental observations.In solution, for example HCl, a large variety of complexes,

such as M(OH)2Cl4−, MOCl4

−, MOCl52−, and MCl6

− (M =Nb, Ta, Db, and Pa) can be formed with different degrees ofhydrolysis according to the following equilibrium

+ ⇄− −−

− −iM(OH) L MO (OH) Lu u ii

6 6 2(6 )

(10.1.2)

Their stability and energy change of the complex formationreactions were predicted theoretically on the basis of the 4c-DFT calculations.304,305 The obtained data suggest thefollowing trend in complex formation in group 5: Pa ≫ Nb> Db > Ta. Taking into account the association with an organiccation, the following trend was predicted for the sorption ofgroup-5 complexes by an anion exchanger

Figure 46. Relativistic (rel) and nonrelativistic (nr) partial overlap populations in MCl5 (M = Nb, Ta, and Db). L denotes valence orbitals of theligand. Reprinted with permission from ref 367. Copyright 1993 American Institute of Physics.

Figure 47. Formation of MBr6− on the KBr surface (M = group-5

elements). Reprinted with permission from ref 364. Copyright 2012American Institute of Physics.



≫ ≥ >Pa Nb Db Ta (10.1.3)

Thus, complexes of Pa should be formed in much moredilute HCl solutions, while much higher acid concentrations areneeded to form those of Ta. The calculations also predicted thefollowing sequence in the formation of various types ofcomplexes as a function of the acid concentration:M(OH)2Cl4

− > MOCl4− > MCl6

−, in agreement withexperimental data for Nb, Ta, and Pa. The calculations alsoreproduced the MF6

− > MCl6− > MBr6

− sequence as a functionof the type of ligand.Theoretical investigations304,305 have shown that the trend in

complex formation and extraction (sequence 10.1.3) known forNb, Ta, and Pa turned out to be reversed in going to Db. Thiscould not have been predicted by any extrapolation of thisproperty within the group, which would have given acontinuous and, hence, wrong trend, but this came as a resultof the relativistic electronic structure calculations for the realchemical equilibrium. As will be shown below, the firstexperiments on the AIX separation of the group-5 elementsfrom mixed HF/HCl solutions have given a different trend thantheoretically predicted, where Db was found in the Nb/Pafraction.368 In view of this disagreement, a recommendationwas made304 to repeat the AIX separations in either pure HClor HF solutions. Accordingly, amine separations of the group-5elements were systematically redone by Paulus et al.277 Areversed extraction sequence Pa > Nb ≥ Db > Ta, as thatpredicted theoretically (sequence 10.1.3), was then observed(see below).


As homolog of the group-5 elements Nb and Ta, also Db isexpected to behave chemically like a typical transition element.Early chemical investigations of single atoms of Db wererestricted to rapid gas-phase chemical investigations due to therelatively short t1/2(

261Db) = 1.8 s that was accessible in the22Ne + 243Am reaction. Only when sufficient amounts of thevery rare and short-lived target material 249Bk (t1/2 = 320 d)became available was the longer-lived 34-s 262Db synthesized inthe reaction 249Bk(18O, 5n).369,370 Therefore, the first aqueousphase chemistry experiments were only conducted in 1988.Indeed, 262Db can also be produced in the reaction 248Cm(19F,5n), however with lower production rates.371,372

10.2.1. Gas-Phase Chemistry. A similar apparatus asshown in Figure 34 was used as early as 1970 to study volatilechlorides of element 105, Db.65 Isothermal section II wasmissing: the chromatography column consisted of sections I(30 cm) and a longer temperature gradient section III (120−130 cm). Section I was heated to 300 °C, whereas in section III

the temperature gradually decreased to 50 °C. The reaction22Ne + 243Am was used to produce 261Db, a nuclide decayingmainly by α-particle emission with a t1/2 = 1.8 s. The branchingratio for SF was determined to be <18%.373 A total of fourindividual experiments were performed. In the first experimentat 114 MeV beam energy using SOCl2 as chlorinating agent, noSF tracks were observed in the column. In the next threeexperiments the beam energy was raised to 118−119 MeV, andin addition to SOCl2 also TiCl4 was added as reactive agent. Inthese experiments a total of 18 SF tracks were observed in thecolumn, which were attributed to the decay of a Db isotope(Figure 48). Later studies were conducted in a brominating gasmedium and again yielded evidence that Db bromide is lessvolatile compared to the homolog compound with Nb.67

A quantitative analysis of these experiments was not made atthe time. The location of the SF tracks coincided roughly withthe position where 170Hf was deposited, whereas 90Nb wasdeposited further downstream, at the very end of the column.Applying the microscopic model of Zvara,288 the distribution ofSF tracks can best be described with −ΔHa

0(T)(Db-chloride) =100 kJ·mol−1. For Nb resulted −ΔHa

0(T)(Nb-chloride) = 96kJ·mol−1. The influence of the different t1/2 values on thedeposition temperature is clearly apparent. Since both thepentachlorides as well as the oxytrichlorides of group-5elements Nb and Ta are thermodynamically stable compounds,no conclusions about the speciation of the observedcompounds can be drawn.The fact that nearly identical ΔHa

0(T) values were derived forcompounds of Nb and, presumably, Db cannot be interpretedas positive proof that indeed a Db isotope was chemicallyisolated. Due to the absence of a long isothermal section, a verysimilar distribution of SF tracks can, in principle, result from thedeposition of a long-lived SF radioactivity such as 256Fm.Indeed, the separation from long-lived actinides was notparticularly good; between 2.2 and 8.5% of 246Cf was found inthe gradient section (detectors 1−27).65 That the separationfrom actinide nuclides could have been insufficient is alsoapparent from later experiments with 262,263Db produced in thereaction 18O + 249Bk.374,375 In these experiments, the depositionpeaks of 262,263Db were observed on top of a background ofhundreds of SF tracks in the column. Again 5−10% of all Fmnuclei were flushed into the gradient section of the column.Isothermal experiments with Db-halides were conducted

with the longer-lived Db-nuclides 262Db and 263Db (t1/2 = 34and 27 s, respectively). In a first experiment, employing the 18O+ 249Bk synthesis reaction and the OLGA(II) setup inconjunction with the moving tape detection system,225,226

only SF decays could be used as signal for the decay of a Db

Figure 48. Distribution of SF tracks attributed to the SF decay of a Db isotope. These events were registered in three separate experiments (a, b, andc). The deposition zones of other elements are also indicated. Reprinted with permission from ref 58. Copyright 1987 OldenbourgWissenschaftsverlag GmbH. Figure adapted from Zvara et al.65



nuclide, since these experiments were severely hampered bycomparably large α-particle emitting Po-activities, whichobscured the interesting energy range 8.40−8.60 MeV, whereα-decays of 262,263Db and their 258,259Lr daughters wereexpected. The volatility of group-5 bromides decreased in thesequence Nb ≈ Ta < Db. Later, the HEVI setup and the MG-rotating wheel detector were used to investigate the volatility ofgroup-5 chlorides.376 This part of the PhD-thesis ofKadkhodayan is unpublished and has only appeared as aninternal Lawrence Berkeley Laboratory report. In this work thenuclide 262Db was unambiguously identified after chemicalseparation by observing seven correlated α−α correlations. Theα-particle energies as well as the deduced t1/2 for

262Db and itsdaughter 258Lr were in good agreement with literature values.The observed high volatility of Db-chlorides was comparable tothat of Nb under similar conditions. For Ta-chlorides only asignificantly less volatile compound was observed, which wasattributed to the compound TaOCl3. Group-5 chlorides werereinvestigated using the OLGA(III) setup.231 Again, 262,263Dbwere identified via α−α correlations and SF decays. This time,two species were observed for Nb- and Db-chlorides, whichwere attributed to the pentachlorides and the oxytrichlorides.No data was measured for Ta-chlorides.In Table 3 the evaluated ΔHa

0(T) values measured for group-5halides and oxyhalides of Nb, Ta, and Db in variousexperiments are summarized. In cases where the assignmentto either the pure pentahalide or the oxytrihalide was uncertain,the obtained ΔHa

0(T) value was listed in a separate columnbetween the two species. In the very first TC experiments withgroup-5 chlorides on glass and/or mica surfaces,65 similarΔHa

0(T) values were measured for Nb- and Db-chlorides, but theassignment to either the pentachloride or the oxytrichloridecannot be made. In experiments with Nb- and Db-chlorides ofTurler et al.,231 two species of different volatility were observedand attributed to MCl5 and MOCl3 (M = Nb, Db). WhileDbOCl3 is less volatile than NbOCl3, only an upper limit couldbe established for −ΔHa

0(T)(DbCl5), which allowed noconclusions about the relative volatility of NbCl5 and DbCl5.Also, experiments investigating the volatility of TaCl5 are stillmissing. The strong tendency of group-5 elements to formoxyhalides, even with traces of oxygen, requires experimentswith a very low oxygen partial pressure, which is hard to realizein online gas chromatography experiments, where typically 1 L/min of, for example, He is used as carrier gas. Earlier relativisticcalculations predicted an increasing tendency down group 5 topreferentially form the oxyhalide.331 However, in a recenttheoretical publication concerning group-5 bromides, it was

concluded that while the pure bromides are observedexperimentally, also DbBr5 should be formed, since Db showedthe lowest affinity toward oxygen.364 The ΔHa

0(T) values for Nb-and Db-chlorides evaluated from TC experiments on a quartzsurface374 are in very good agreement with the ΔHa

0(T) valuesmeasured for oxytrichlorides in OLGA experiments.231 Also,ΔHa

0(T)(NbCl5) evaluated from HEVI experiments of Kadkho-dayan376 is in agreement with the value measured by Turler etal.231 In experiments with Ta, only TaOCl3 was obtained. Thenuclide 262Db was positively identified after chemical separationin experiments with HEVI376 at isothermal temperatures as lowas 250 °C. A measurement at 100 °C isothermal temperatureresulted in a significantly lower yield compared to 250 °C andhigher temperatures. If this data point is included into theanalysis, −ΔHa

0(T)(DbCl5) = 76 ± 10 kJ·mol−1 (in Kadkho-dayan376 −ΔHa

0(T)(DbCl5) = 73 ± 10 kJ·mol−1) results.A similar situation as for group-5 chlorides was observed for

the bromides. Here, Db-bromides seemed to be less volatilethan NbBr5 or TaBr5 in experiments by Gaggeler et al.226

However, since the measured ΔHa0(T) value for Db-bromide is

very similar to the one determined for DbOCl3, it is likely thatthe volatility of DbOBr3 was measured. Interestingly, TaBr5could only be formed when a mixture of HBr and BBr3 wasused as brominating agent, whereas NbBr5 was formed withHBr only. Again, TC experiments on quartz surface374 yieldedsimilar results. Nb- and Db-bromides are about 10 kJ·mol−1

more volatile as compared to the data of Gaggeler et al.226 Thiscould be attributed to the fact that, in experiments of Gaggeleret al.226, a KCl aerosol was used, which may have covered thequartz surface to some extent. Generally, a lower volatility wasobserved on KCl surfaces compared to a pure quartz surface. Ina meta analysis by Zvara,297 Db was listed as DbBr5; however,also DbOBr3 must be considered, as in the experiments ofGaggeler et al.226 More recently, the experimental study of Db-bromide was repeated377 with an improved purification of theHe carrier gas. Under these conditions, Db-bromide is morevolatile than observed previously. The volatility sequencededuced from this experiment together with independentstudies on the behavior of Nb and Ta under identical chemicalconditions was Db > Nb > Ta.377 This experimentalobservation is in conflict with previous studies, but onlyconcerning the behavior of Db (see Figure 49). The deducedadsorption enthalpies for NbBr5 and TaBr5 were in excellentagreement. It was concluded that indeed the previousstudies67,226 were performed with DbOBr3 rather than DbBr5.A different explanation is offered by Pershina and Anton,364

where the formation of MBr5L− (L = Br or Cl) complexes is

Table 3. Comparison of −ΔHa0(T) Values Evaluated for Group-5 Halides and Oxyhalides in Various Experiments

−ΔHa0(T) (kJ·mol−1)

technique ref year aerosol material NbCl5 NbOCl3 TaCl5 TaOCl3 DbCl5 DbOCl3

TC 378 1973 85a 88a

TC 374 1991 95a 118a

IC (HEVI) 376 1993 MoO3 75 ± 5b 157 ± 12b 76 ± 10b

IC (OLGA III) 231 1996 C 80 ± 1 99 ± 1 ≤97 117 ± 3

NbBr5 NbOBr3 TaBr5 TaOBr3 DbBr5 DbOBr3TC 378 1973 87a 82a

TC 374 1991 83a 108a

IC (OLGA II) 226 1992 KCl 93 ± 4c 101 ± 4c 121 ± 11c

IC (OLGA III) 377 2012 KBr 89 ± 5 155 ± 5 103 ± 5 71 ± 5aEvaluated data.297 bReanalyzed data with τ0 = 2 × 10−13 s.376 cData from Gaggeler et al.226 reanalyzed with τ0 = 2 × 10−13 s.231



discussed on the KCl or KBr covered surface, which wouldindeed suggest a less volatile Db compared to Nb and Ta.10.2.2. Liquid-Phase Chemistry of Dubnium. The first

studies of aqueous phase properties of Db were performed byGregorich et al.267 in a manually performed experiment makinguse of the typical property of group-5 elements to stronglyadsorb on glass surfaces from strong nitric acid solutions. Therelatively long-lived 34-s 262Db was produced in the reaction249Bk(18O, 5n) and transported with the aid of a KCl seededaerosol gas-jet to the chemistry laboratory. The KCl aerosolparticles were collected by impaction on a glass plate. After theend of collection, the glass plate was fumed twice with 15 MHNO3 and then rinsed with 1.5 M HNO3 and acetone. Afterdrying in hot air, the plate was assayed by α-particlespectrometry. In 801 collection and separation cycles, 26 α-particles in the pertinent energy range 8.42−8.70 MeV, ofwhich 5 were time correlated mother−daughter pairs, as well as26 SF events were observed. Therefore, Db was shown tobehave like a typical group-5 element. In a second experiment,the extraction of anionic fluoride species into methyl isobutylketone (MIBK) was investigated. The KCl aerosol particleswere collected on Pt disks and dissolved in 3.8 M HNO3/1.1 MHF. This solution was transferred to a centrifuge conecontaining the MBIK. After mixing and phase separation, theMIBK phase was dried on a Ni foil and subsequently assayed byα-particle spectroscopy. Under the conditions of the experi-ment, Ta is extracted nearly quantitatively, while Nb isextracted only to a small extent. In 335 extraction experiments,neither α-particles nor SF events of Db were observed,indicating a non Ta-like behavior of Db.Using the ARCA II setup, the extraction of group-5 elements

into TiOA from HCl solutions typically containing 0.02 M ofHF to prevent hydrolysis was studied in reversed phaseextraction chromatography.368 From 12 M HCl/0.02 M HFand from 10 M HCl, 262Db was extracted into TiOA, like thegroup-5 elements Nb and Ta and the pseudohomolog Pa andseparated from actinide elements. In elutions of a Pa−Nbfraction with 4 M HCl/0.02 M HF and a Ta fraction with 6 MHNO3/0.015 M HF, 262Db was found in the Pa−Nb fraction.In separate elutions with 10 M HCl/0.025 M HF (Pa-fraction)and 6 M HNO3/0.015 M HF (Nb-fraction), 262Db was equallydistributed among the Pa- and Nb-fractions. Kratz and co-workers368 concluded that the non Ta-like halide complexation

behavior of Db was indicative of the formation of oxo-halide orhydroxohalide complexes such as NbOCl4

− and PaOCl4− or

Pa(OH)2Cl4− at intermediate HCl concentrations, in contrast

to the pure halide complexes of Ta, like TaCl6−.

The observation that Db might show some chemicalresemblance with Pa prompted experiments where theextraction of Db into 2,6-dimethyl-4-heptanol (diisobutylcarbinol, DIBC), a secondary alcohol, was studied.275 DIBCis a specific extractant for Pa. In online experiments with Db,the KBr aerosol particles delivering the reaction products fromthe production site were dissolved in concentrated HBr and fedonto a reversed phase chromatography column (DIBC/Voltalef). The extraction was followed by the elution of anNb fraction in 6 M HCl/0.0002 M HF and a Pa fraction in 0.5M HCl. The number of 262Db decays observed in the Nbfraction indicated that <45% of the Db was extracted intoDIBC, in qualitative agreement with experiments where thecolumns where striped directly with acetone after the extractionstep. An extraction sequence Db < Nb < Pa was established.275

A possible explanation might be the increasing tendency toform nonextractable polynegative complexes in the sequence Pa< Nb < Db.In elutions from cation-exchange columns with α-HIB, the

elution sequence strongly depends on the charge of the metalion. As was shown in experiments by Schadel et al.274 Db iseluted rapidly from a cation-exchange column with unbuffered0.05 M α-HIB together with group-5 elements Nb and Ta andthe pseudohomolog Pa, while group-4 elements and trivalentactinides are strongly retained on the resin. These experimentsdemonstrated that the pentavalent state is the most stable onein aqueous solution and also provided the cleanest separation ofDb so far and allowed discovery of the new isotope 263Dbproduced in the reaction 249Bk(18O, 4n).273,274

The competition between hydrolysis and halide complexformation of group-5 elements was systematically investigatedby Paulus et al.277 by studying the extraction from pure HF,HCl, and HBr solutions by the quaternary aliphatic amine(Aliquat-336 on an inert support). In the pure chloride system,the following separation procedure was established for ARCAII. The activities were fed onto the reversed-phase chromato-graphy column (Aliquat-336/Voltalef) in 10 M HCl, wherebythe trivalent actinides passed through. Subsequently, a Ta-fraction was eluted with 6 M HCl, and a combined Nb, Pa-fraction was stripped with 6 M HNO3/0.015 M HF.Theoretical considerations involving complex formation andhydrolysis predicted an extraction sequence Pa ≫ Nb ≥ Db >Ta above 4 M HCl.304 As can be seen in Figure 50, theexperimental extraction sequence is in perfect agreement withpredictions concerning the behavior in a pure chloride system.Interestingly, the extraction sequence from 4 M HCl/0.03 MHF into TiOA, also an aliphatic amine, was exactly reversed, i.e.Ta > Db ≥ Nb ≥ Pa, which in retrospect must probably beattributed to the influence of the fluoride ions. This iscorroborated by extraction studies from pure HF solutionsinto Aliquat-336.277 Even in dilute 0.5 M HF solutions,extractable fluoride complexes are formed by group-5 elementsand Pa. In 4 M HF, the Kd value of Db is high, of the order ofthe ones of Nb and Ta, but it differs markedly from that of Pa.In pure HCl and HBr, extractable chloride and bromidecomplexes are only formed above 3 M HCl and above 6 MHBr, respectively.277

Investigations of the fluoride complexation and the anion-exchange behavior of Db have just been initiated. Using the

Figure 49. Evaluated adsorption enthalpies (ΔHa0(T)) of group-5

halides and oxyhalides from isothermal gas adsorption chromato-graphic experiments.



2 4 8 C m ( 1 9 F ,5n)262Db reaction, Tsukada et al.379 investigated the anion-exchange behavior of group-5 elements Nb and Ta and thepseudohomolog Pa. The Kd value of Db was determined only atone [HF]ini concentration of 13.9 M. The sorption of Db onthe resin was significantly different from that of the homologsNb and Ta. Sorption on the resin decreases in the sequence Ta≈ Nb > Db > Pa. Theoretical investigations have not beenoffered for complex formation at such high HF concentrations.It is clear only that the lowest sorption of Pa by the AIX resinfound in the experiments379 is due to the PaF7

2− or PaF83−

formation. Further determinations of Kd values at different HFconcentrations are, therefore, required to determine the chargeof the anionic species.In solutions with more dilute fluoride ion concentration

[F−], Nb is known to form fluoro−oxo complexes NbOF4−,

whereas Ta forms pure fluoro complexes TaF6−, which was

confirmed in anion-exchange experiments in mixed HF/HNO3solutions.282 The Kd value of Db was measured at 0.31 M HF/0.10 M HNO3, corresponding to a concentration of [F−]eq =0.003 M. The measured Kd value of Db was similar to that ofNb but also to that of Pa. Therefore, it was speculated that Dbforms either DbOF4

− complexes, as does Nb, or even DbOF52−

or DbF72− complexes, as does, Pa. However, theory says that

the latter is less probable, as Db5+ has a much smaller IR (0.69Å) than Pa5+ (0.78 Å) and is not its homolog. Again, furtherexperiments are required to elucidate the speciation of Db.

11. SEABORGIUM (Z = 106)


The ground state of Sg is 6d47s2, and it is a homolog of Mo andW. The first IP is 7.85 eV according to MCDF calculations178

(corrected value obtained by an extrapolation procedure, whilethe calculated one is 7.03 eV), where the first ionized electron is6d. This is almost equal to the IP(W) of 7.864 eV, where thefirst ionized electron is 6s.313 The energies of the 6d(Sg) AOand 6s(W) AO are indeed very close to each other.122 MultipleIPs(M → MZ+) were also calculated within the MCDFapproach.178 The IPs(M → M6+) of group-6 elements revealthe same decreasing trend as the IPs in the maximum oxidationstate for group-4 and group-5 elements (see Figure 28). Thus,as in groups 4 and 5, the stability of the maximum oxidationstate increases in group 6, while those of the lower oxidation

states decrease. The calculations have shown that the stepwiseionization process results in the 6d2 state of Sg4+ and not in the7s2 one.380 Since the 6d AOs of the 6d elements are moredestabilized than the (n−1)d AOs of the 4d and 5d elements,the Sg4+ will even be less stable than W4+. Nonrelativistically,the trend would be opposite. Using IPs, redox potentials for Sgand its homologs in acidic solutions were estimated.380 Theycan be used as a guiding tool for reduction experiments on Sgprobing the stability of the lower oxidation states.IR(M6+) values of group-6 elements were estimated using a

correlation with Rmax of the outer (n−1)p AOs obtained in theMCDF calculations.178 They also show a typical increase of0.05 Å from W (0.60 Å) to Sg (0.65 Å). This is in agreementwith the increase in CR.319,320

Like Mo and W, Sg should form volatile halides, oxyhalides,oxide hydroxides, or carbonyls. In order to predict the stabilityof these complexes for Sg, electronic structures of MCl6,MOCl4, MO2Cl2, and MO3 (M = Mo, W, and Sg) werecalculated using the 4c-DFT and RECP CCSD(T) meth-ods.315,327,381,382 Optimized geometries (Re and bond angles),De, IP, α, and μ were predicted for the Sg compounds. Trendsin these properties in group 6 turned out to be similar to thoseof group-4 and group-5 halides and oxyhalides.Both DFT and RECP calculations predicted an increase in

the stability of compounds of the 6d elements with increasingnumber of oxygen atoms, e.g., from SgCl6 to SgOCl4 and toSgO2Cl2, as is experimentally known for the lighter homologsMo and W. Thus, SgO2Cl2 was recommended

382 as the moststable type of oxychloride for high-temperature gas phaseexperiments. SgCl6 was shown to be unstable with respect tothe loss of Cl transforming into compounds of Sg5+.315,381

RECP CCSD(T) calculations for the group-6 oxyhalides,with and without SO coupling,327 have shown that larger SOeffects on the 6d AOs result in a decrease in De of the 6dcompounds by 1−1.5 eV in comparison with the 5d ones. Theeffects are larger for the Sg compounds than for the Rf ones dueto an increasing 6d3/2−6d5/2 splitting.As in groups 4 and 5, covalency increases down group 6. The

dipole moments of the oxyhalides also increase due to theincrease in the metal−ligand separation. Such an increase in μof the MO2Cl2 molecules (1.04 D for Mo, 1.35 D for W and1.83 D for Sg) should result in a decrease in the volatility ofthese compounds, so that the trend is MoO2Cl2 > WO2Cl2 >SgO2Cl2.

382

The importance of electron correlation for QM, OP, μ, andDe was demonstrated on the example of group-6 MO2Cl2.

327

Correlation effects were shown to significantly decrease QM andμ, and increase De, accounting, for example, for about 65% inDe(SgO2Cl2). The effects on De were found to be larger in theW compounds than in the Sg ones, and they become moresignificant as the number of oxygen atoms increases.DS-DV calculations were performed for M(CO)6 (M = Mo,

W, Sg, and U).383 Sg(CO)6 was found to be very similar toW(CO)6 and different from U(CO)6. A typical bond lengthincrease was found for the Sg compound in comparison withthat of W, as for the other types of species.Group-6 hydrides, MH6 (M = Cr, Mo, W, and Sg) were used

to study the influence of relativistic effects on the molecularproperties of Sg. The DF one-center expansion calcula-tions321−323 showed relativistic effects to decrease the bondlength of SgH6, so that Re(SgH6) is 0.06 Å larger thanRe(WH6). The calculations revealed a slight increase in De ofSgH6 as compared to that of WH6.

Figure 50. Kd values of Pa, Nb, and Ta in the system Aliquat-336/HCl(left-hand side) and Aliquat-336/HF (right-hand side). The Kd valuedetermined for Db in 6 M HCl (right-hand side) and 4 M HF (left-hand side) is also indicated. Reproduced with permission from ref 277.Copyright 1999 Oldenbourg Wissenschaftsverlag GmbH.



The aqueous chemistry of Sg and its homologs has alsoreceived detailed theoretical consideration. In aqueoussolutions, Sg should undergo hydrolysis similarly to its lighterhomologs Mo and W. To predict hydrolysis of Sg at various pHof solutions, the free energy change of the followingprotonation reactions was calculated using the 4c-DFTmethod:306

⇄ ⇄

⇄ ⇄ ⇄

⇄

− −

+ +

+

MO MO (OH) MO (OH) (H O)

MO(OH) (H O) M(OH) (H O) ...

M(H O)

42

3 2 2 2 2

3 2 2 4 2 22

2 66

(11.1.1)

The results indicate that for the first two protonation steps,the trend in group 6 is reversed: Mo < Sg < W. For the furtherprotonation process, the trend is continued with Sg: Mo < W <Sg. Thus, the same reversal of the trend is predicted for theprotonation of oxyanions of the group-6 elements as that forthe complex formation of the group-4 and group-5 elements.The predicted trends in the complex formation are inagreement with experiments for Mo and W at various pHvalues.301 log K values were determined for Sg, as given inTable 4.306

Complex formation of Mo, W, and Sg in HF solutions wasalso studied theoretically on the basis of 4c-DFT calculations308

of the following stepwise fluorination process

+ ⇄

⇄ ⇄ ⇄

− − −

− −

MO [or MO (OH) ] HF MO F

MO F (H O) MO F (H O) MOF4

23 3

2 2 2 2 2 3 2 5(11.1.2)

The obtained ΔEC values indicate a very complicateddependence of the complex formation of these elements andtrends on pH and HF concentration (Figure 51). Thus, at thelowest HF concentrations (≲0.1 M HF), a reversal of thetrends in Kd should occur in the group, while, at higher HFmolarities (≲0.1 M HF), the trend should be continued withSg: Mo < W < Sg. At the range of these HF concentrations,separation between the homologs is the best.The obtained sequences are in agreement with experiments

for Mo and W.384,385 Future experiments on the AIXseparations of group-6 elements from HF solutions shouldclarify the position of Sg in this group.11.2. Experimental Results

11.2.1. Gas-Phase Chemistry. The first chemical identi-fication of Sg as volatile oxychloride in TC experiments wasreported in a preliminary report by Timokhin et al.251 in 1993and again in 1996.253 Later, Yakushev et al.252 reported aboutancillary experiments with Mo and W nuclides and about afurther experiment of the same type with Sg. A full paper giving

a detailed account of all Sg TC experiments was published byZvara et al.250 In these experiments the nuclide 263Sg (t1/2 = 0.9s) was produced in the reaction 249Cf(18O, 4n). A very similarsetup, as already shown in Figure 33, was used in theseexperiments. Reaction products were thermalized behind thetarget setup in a rapidly flowing stream of Ar gas and flushed tothe adjoining TC column. Volatile oxychlorides weresynthesized by adding air saturated with SOCl2 as reactiveagent. The formed oxychloride species migrated downstream inthe fused silica chromatography column, to which alongitudinal, negative temperature gradient was applied, andfinally deposited according to their volatility. In contrast toearlier experiments, no mica plates were inserted, but the fusedsilica column itself served as SF track detector. The depositionof Sg was registered after completion of the experiment bysearching for latent SF tracks left by the SF decay of 263Sg.Indeed, in several experiments, a number of SF tracks werefound in the column in the temperature region 150−250 °C,which were attributed to the decay of a Sg nuclide. The SFtracks were only found when the quartz wool plug, which wasinserted as a filter for aerosol particles, was absent. This wasattributed to the increased surface and thus a much longerretention time. In ancillary experiments, the production crosssections of transfer reaction products, notably 256Md/256Fm,were measured. The measured cross sections agreed well withprevious values measured at Dubna, but they differed by almost2 orders of magnitude from a measurement at Berkeley.386 Thelower cross sections were attributed to the thick target and thecollimation technique employed in Dubna. The adsorptionbehavior of long-lived 176W was simultaneously studied in theSg experiments. The results of these experiments are shown inFigure 52. The number of SF tracks has been corrected forlosses due to annealing of tracks throughout the experiment.These corrections are substantial. At temperatures above 400°C, the correction increases the number of observed SF tracksby a factor of 5. No adsorption enthalpies were deduced fromthe experimental data. Nevertheless, different chemical statesare discussed for Sg and W.250

Table 4. log K Values for the Stepwise Protonation of MO42−

(M = Mo, W, and Sg)306

log Kn

reaction Mo W Sg

MO42− + H+ ⇆ MO3(OH)

− 3.7 3.8 3.74MO3(OH)

− + H+ + 2H2O ⇆MO2(OH)2(H2O)2

3.8 4.3 4.1 ± 0.2

MO42− + 2H+ + 2H2O ⇆ MO2(OH)2(H2O)2 7.50 8.1 8.9 ± 0.1

MO2(OH)2(H2O)2 + H+ ⇆MO(OH)3(H2O)2

+0.93 0.98 1.02

Figure 51. Predicted relative values of log Kd of W (triangles) and Sg(squares) with respect to those of Mo (rhomboids) by AIX separationsfrom HF solutions as a function of the acid concentration. Points 1through 5 correspond to the following extracted complexes: MO3F

−,MO2F2(H2O)2, MO2F3(H2O)

−, MO2F42‑, and MOF5

−. Reproducedwith permission from ref 308. Copyright 2004 OldenbourgWissenschaftsverlag GmbH.



As with most of the earlier TC experiments with trans-actinide elements, the Sg experiments have not unanimouslybeen accepted as the first positive identification of Sg afterchemical separation. Serious criticism has been voiced byKratz32,259 concerning mostly the magnitude of a SF-branch in263Sg but also the assignment of the volatile species.In the reaction 18O + 249Cf, at a beam energy of 94 MeV, the

nuclides 264Sg, 263Sg, and 262Sg could, in principle, be producedin the 3n, 4n, and 5n evaporation channels. The nuclide 264Sgdecays by SF with t1/2 = 37−11

+27 ms.387 However, according toHIVAP calculations, the 3n evaporation channel is expected tohave a 1 order of magnitude lower production cross sectionthan the 4n channel at 94 MeV. The 4n deexcitation channelleads to 263Sg, for which two isomeric states are known: 263Sga

and 263Sgb with t1/2 = 0.9 and 0.3 s, respectively. The nuclide263Sgb is populated by the decay of

→ →α α

Ds Hs Sg271 267 b263

(refs 70 and 388) while 263Sga is predominantly populated inthe direct synthesis reaction. For 263Sga a SF branch of 13 ± 8%was measured,387 in contrast to an earlier indirect determi-nation of 70% by Druin et al.389 based on cross sectionarguments. The 5n deexcitation channel leads to 262Sg, anuclide decaying by SF and by α-particle emission with a t1/2 =15−3

+5 ms. The calculated cross section at 94 MeV is again 1order of magnitude smaller than that for 263Sg. Considering theshort t1/2 and low production cross sections of 262,264Sg, themost likely candidate responsible for the observed SF tracksmust be 0.9-s 263Sga. Using the efficiencies given by Zvara etal.,250 a SF branch of 13%,387 and a production cross section of0.3 nb as measured independently by Ghiorso et al.68 andGregorich et al.,390 detection of about 21 SF events in the

column can be expected, compared to about 64 detected events(after applying the annealing correction). Therefore, contribu-tions other than SF of 263Sga have to be considered. Acontribution of about nine SF events can be estimated due toSF in 259Rf (or 259Lr after EC in 259Rf), the α-decay daughter of263Sg. A possible EC decay branch in 263Sg leading to SF in263Db or in 259Lr after α-decay must also be considered.273,391

Assuming a similar EC-branch as measured for 259Rf (15 ±3%), a contribution of about 18 SF events must be assumed,which brings the number of expected SF events to roughly 50,not far from the 64 observed events. It is therefore of crucialimportance that, under the conditions of the experiment,actinides, as well as the transactinide elements Rf and Db, aremuch less volatile than the studied element 106 compound.Even though this possibility was dismissed in the work of Zvaraet al.,250 it must be noted that Db forms volatile DbOCl3 attemperatures above 250 °C231 (see section 10.2.1), whereas Rfcan also be transported in an oxygen containing carrier gas via atransport reaction mechanism230 (see section 9.2.1). Theconclusion by Zvara et al.250 that the observed distribution ofSF events exclusively reflects the chemical behavior of Sg onlycan, therefore, not be substantiated.In the upper panel of Figure 52, the registered SF tracks per

5 cm column length (gray histograms) attributed to the SFdecay of 263Sga and the corrected number of SF events due tothe annealing of tracks (white histograms) are displayed. In thelower panel, the integrated yield along the length of the columnis shown. The influence of the vastly different t1/2 values on thedeposition temperature was discussed, and the authors came tothe conclusion that “this factor can hardly account for so large adifference”.250 The distribution of SF tracks can best bedescribed with ΔHa

0(T) (Sg-oxychloride) = −110 kJ·mol−1. ForW, ΔHa

0(T)(W-oxychloride) = −102 kJ·mol−1 resulted. Bothvalues are very similar. Actually, the ΔHa

0(T) value deduced forW-oxychloride is in perfect agreement with the data of Gartneret al.392 However, Gartner et al.392 attributed the observedvolatile species to WO2Cl2, based on thermodynamicconsiderations, while, in the work of Zvara et al.,250 differentchemical species are discussed for Sg- and W-oxychlorides. Thequantitative analysis performed here shows that the observeddifferences in the deposition temperature are mainly due to thelarge differences in t1/2 of the two nuclides and that indeed theΔHa

0(T) values of both compounds are very similar. In TCexperiments by Yakushev et al.252 with short- and longer-livedMo and W isotopes, it was observed that, depending on t1/2,two different chemical species of W with different volatilitywere formed. The less volatile compound was deposited atabout 250 °C, whereas the more volatile one was deposited atabout 160 °C. It was concluded that first the less volatileMO2Cl2 (M = Mo, W) was formed, which was then slowlyconverted to the more volatile MOCl4 (M = Mo, W). Thus,short-lived Sg was deposited in the column as SgO2Cl2, whereaslonger-lived W was observed as WOCl4. Again, theseexperiments were only analyzed qualitatively. Due to thepossibility of different chemical states of Sg and W, the TCexperiments of Zvara et al.250 allow no conclusions aboutchemical similarities of Sg to either W or Mo. Furthermore, theuncertainties about the origin of the observed SF tracks, whichmight be partly due to lighter transactinide elements, renderconclusions about the chemical speciation and properties of Sgoxychlorides questionable.The announcement of the synthesis of longer-lived 265,266Sg

in the reaction 248Cm(22Ne, 4−5n) by Lazarev et al.393 laid the

Figure 52. Measured250,253 and simulated deposition peaks of 263Sga-and 176W-oxychlorides. The modeled deposition peaks were obtainedusing the microscopic model of Zvara288 and a Monte Carlosimulation technique, with the only adjustable parameter being theadsorption enthalpy (for details see text).



foundation for further chemical experiments with Sg in theliquid- as well as in the gas-phase.232,234,239,279,394 Theseexperiments were all successful; however, due to an erroneousassignment of mass numbers and decay properties in thephysics discovery experiment,393 which served as a basis for thesearch for Sg isotope decays in all subsequent Sg chemistryexperiments, it was believed that also in the chemistryexperiments two different isotopes of Sg, namely 265Sg and266Sg, were observed.232−234,239 Much later, after the discoveryof 270Hs (the α-decay mother of 266Sg)109, it became evidentthat all decay chains observed in the Sg chemistry experimentswere due to 265Sg only.395,396 There is now conclusive evidencefor two isomeric nuclear states in 265Sg.397 The nuclide 265Sga

decays with t1/2 ≈ 9 s preferentially to 261Rfa, which furtherdecays by α-particle emission and t1/2 = 68 s to 257No, whereas265Sgb with t1/2 ≈ 14 s decays preferentially to 261Rfb, whichundergoes spontaneous fission with t1/2 ≈ 3 s.397,398 Both statesare formed in the direct synthesis reaction 248Cm(22Ne,5n)265Sga,b but also in the α-decay of 269Hs.395

Following the considerations discussed in section 11.1, theobvious choice was to study Sg as the volatile SgO2Cl2compound. For online gaschromatographic studies of Sg, theOLGA(III) system in conjunction with the ROMA detectionsystem was employed (see section 7.2.1). Preparatory experi-ments with short-lived Mo-, W-, and U-isotopes were describedby Gartner et al.392 Experimental results with Sg werecommunicated in a short account by Schadel et al.232 Later,further experiments involving the measurement of a break-through curve were published in more detail by Turler etal.233,234 Typically, the reaction products recoiling from thetarget were transported to the OLGA(III) setup with the aid ofa C-aerosol gas-jet. As chlorinating agents, Cl2 saturated withSOCl2 and traces of O2 were introduced to the reaction oven,where the formation of oxychlorides occurred. Volatile speciestraveled through the colder, isothermal section of the columnand were reattached to KCl aerosol particles while exiting intothe recluster chamber. This second aerosol gas-jet transportedthe separated activities to the ROMA detection system. Theregistered spectra were dominated by α lines originating fromisotopes of Po and Bi. Presumably, these nuclides wereproduced in multinucleon transfer reactions from Pb impuritiesin the Cm target. These elements were not or only partlyretained in the chromatographic column. Except for 211Pom and212Pom, all very short-lived Po activities were due to longer-livedBi and/or Pb precursors. Therefore, the mother−daughterrecoil technique had to be implemented in ROMA. In two runsat isothermal temperatures between 300 and 400 °C, ten decaychains attributed to 265Sg were registered, which weresummarized in one data point of 350 ± 50 °C. One of thedecay chains consisted of a complete chain

→ → →α α α

Sg Rf No Fm265 a261 257 253

which unambiguously demonstrated that Sg was chemicallyisolated. At 250 °C, an additional three decay chains wereobserved. Due to the complicated detection technique, not onlythe chromatographic transport through the column but also thedecay and detection of Sg nuclei was modeled with a MonteCarlo procedure. This way, the most probable number ofinitially produced 265Sg nuclei and the chemical yield could bedetermined, which allowed extracting a probability densitydistribution of −ΔHa

0(T)(SgO2Cl2) = 98−5+2 kJ·mol−1 (68%

error interval). For 168WO2Cl2, −ΔHa0(T)(WO2Cl2) = 96 ± 1

kJ ·mol−1 was deduced, whereas, for 104MoO2Cl2 ,ΔHa

0(T)(MoO2Cl2) = 90 ± 3 kJ·mol−1 resulted, in goodagreement with theoretical predictions.382 However, it shouldbe noted that, due to the very limited number of observedevents, it was not possible to, for example, provide 95% or 99%error intervals, and there remains an about 15% chance thatSgO2Cl2 is more volatile than MoO2Cl2 compared to an 85%chance that it is less volatile, as reflected by the deduced−ΔHa

0(T) value. The observed yield curves for 104MoO2Cl2,168WO2Cl2, and

265SgO2Cl2 are displayed in Figure 53. Thus,

both relativistic theory and experiment have shown that thevolatility of group-6 dioxidichlorides should decrease withincreasing Z due tofrom the theoretical point of viewanincrease in the molecular dipole moments.Hubener et al.239 investigated the behavior of group-6 oxide

hydroxides including Sg in the system O2−H2O(g)/SiO2(s). It iswell-known that in an O2/H2O atmosphere the solid MO3 (M= Mo, W) are in equilibrium with gaseous MO2(OH)2. Underthe assumption that, in one-atom-at-the-time experiments, thegaseous MO2(OH)2 undergoes a dissociative adsorptionprocess, the process can be described as follows:

⇄ + =MO (OH) MO H O (M Mo, W, Sg)2 2(g) 3(ads) 2 (g)

(11.2.1)

The gas chromatographic investigation of Sg oxidehydroxides in quartz-glass columns with He/O2/H2O as carriergas must be highly characteristic, since neither actinides nor thelighter transactinides should form volatile species that wouldobscure the unequivocal identification of Sg. Preparatoryexperiments involving TC and high-temperature online ICwere carried out, which demonstrated that Mo and W oxidehydroxides are not transported by simple reversible adsorptionof MO2(OH)2 (M = Mo, W) but can be best described by amicroscopic description of the dissociative adsorption process.The relative yields of 104Mo and 168W oxide hydroxides as afunction of isothermal temperature are shown in Figure 54.In the actual experiment with Sg, the synthesis reaction 22Ne

+ 248Cm was employed. Reaction products recoiling from thetarget were stopped in He seeded with MoO3 aerosol particlesand transported to the HITGAS237 setup. At the entrance tothe chromatography column, moist O2 was added to the gas-jet.The temperature of the quartz chromatography column was1325 K in the reaction and 1300 K in the isothermal zone.Loosely packed quartz wool in the reaction zone served as filter

Figure 53. Relative yield of 104 MoO2Cl2,168WO2Cl2, and

265SgO2Cl2as a function of isothermal temperature in the chromatographycolumn.234



for aerosol particles. Retention times of about 8 to 9 s weredetermined from measurements with short-lived Mo and Wnuclides at isothermal temperatures above 1270 K. Bycondensing the separated volatile species directly on metalfoils mounted on the circumference of the rotating wheel of theROMA detection system, the time-consuming reclustering stepcould be avoided, however, at the expense of a reduceddetection efficiency for α-particles. Due to the thickness of themetal foils, final samples could be assayed only in a 2πgeometry. The search for genetically linked decay chainsrevealed two candidate events, which must be attributed to thesequence

→ →α

Sg Rfsfb265 b261

(ref 395). In the original publication, these events wereerroneously attributed to the decay of 266Sg. The probabilitythat both of these events were entirely random was only 2%.239

Since Sg appeared to be volatile under the conditions of theexperiment, it showed the typical behavior of a group-6 elementand was transported presumably as Sg oxide hydroxide. Underthe given conditions, this coincides also with the behavior ofthe pseudohomolog U(VI). U is also known to form a volatileoxide hydroxide. In order to answer the question about thesequence of volatility of oxide hydroxides within group 6,further experiments have to be conducted at lower isothermaltemperatures.11.2.2. Liquid-Phase Chemistry. As described in section

11.1, group-6 elements form MO42−, MO3F

−, MO2F2,MO2F4

2−, MO2F3−, and MOF5

− depending on the HFconcentration. In test experiments,278 optimum conditions foran isolation of group-6 elements were established. In 0.1 MHNO3/5 × 10−4 M HF, Mo and W are rapidly eluted from acation-exchange column while di- and trivalent actinides, as wellas group-4 elements or UO2

2+, are retained on the column. Thegroup-6 elements form anions of the type MO4

2−, MO3F−, or

MO2F3− (M = Mo, W). Also, the formation of a neutral

compound such as MO2F2 cannot be excluded. In experiments

with 265Sg, produced in the reaction 248Cm(22Ne, 5n), in 3900collection and separation cycles with ARCA, three decay chainsattributed to the decay chain

→ →α α

Rf No Fma,b261 257 253

were observed.232,278 On average, the measurement of a samplestarted 38 s after the end of collection. As the experiment wasdesigned to retain the group-4 element Rf, the decay chains aredue to chemically isolated 265Sga,b. Thus, decay of the Sgnuclides occurred after the Rf/Sg separation, which, on average,occurred within 5 s. Since the operation of ARCA still involvedalso manual labor, the experiment was a tour de force, whichalso demonstrated that the method had reached its limits.Nevertheless, the experiment signified the first chemicalseparation of Sg in aqueous solution and demonstrated that,presumably, either SgO3F

−, SgO2F3−, or SgO2F4

2− complexeswere formed. Also, a neutral complex such as SgO2F2 cannot beexcluded. However, Sg did not behave similarly to thepseudohomolog U, which forms UO2

2+ under the presentconditions (which is natural, in view of 5f contributions to thebonding of uranyl). Due to the low fluoride concentration used,the anionic SgO4

2− (“seaborgate” in analogy to molybdate,MoO4

2−, or tungstate, WO42−) could not be excluded.26

A new series of Sg experiments279 was performed to studythe possible formation of the seaborgate anion, SgO4

2−. It wasattempted to elute Sg from a cation-exchange column with pure0.1 M HNO3 (without the addition of HF). If Sg had beeneluted from the cation-exchange column, this would havedemonstrated the formation of the seaborgate anion SgO4

2−, inanalogy to WO4

2−. In 4575 experiments, only one candidateα−α correlation was observed, with an expected background of0.49 correlations. If Sg would have been eluted with the samechemical yield as W, then 4.7−2.5

+3.7 (68% confidence interval)α−α correlations from 261Rfa,b and 257No would have beendetected.279 Therefore, the non-W like behavior of Sg wasattributed to a weaker tendency to hydrolyze,26 in agreementwith the theoretical predictions.306

Thus, in the presence of fluoride ions,278 which have a strongtendency to replace water or OH− ligands, the formation ofneutral or anionic fluoride complexes seems to be favored,whereas, in the absence of fluoride ions, the elution of SgO4

2−

anions is rather unlikely.279

12. BOHRIUM (Z = 107)


The ground state of Bh is 6d57s2, so that it is a homolog of Tcand Re in group 7. MCDF calculations179 have given the first IPof 7.7 eV (corrected value obtained by an extrapolationprocedure, while the calculated one is 6.82 eV), which is slightlylower than IP(Re) of 7.83 eV. The first ionized electron both inBh and Re is the (n−1)d one, which is more bound in Re thanin Bh. Multiple IPs(M → MZ+) for group-7 elements179 revealthe same decreasing trend in the group with increasing Z as thatfound for group-4, group-5, and group-6 elements (see Figure28). IR values obtained via a linear correlation with Rmax[(n−1)p] AO in the group are also shown in Figure 28,179 followingthe established trends for group-4 through group-6 elements.The estimated IR(Bh7+) of 0.58 Å is also typically larger thanIR(Re7+) of 0.53 Å. This is also in line with the CR of theseelements.319,320

Among gas-phase compounds of Bh in the highest oxidationstate, oxyhalides should be most stable. As 4c-DFT calculations

Figure 54. Relative yields in isothermal gas chromatography of 104Mo(○) and 168W (●) oxide hydroxides in quartz columns using humidoxygen as reactive carrier gas component. Sg was observed at anisothermal temperature of 1300 K. The solid lines are the result of aMonte Carlo model based on a microscopic description of thedissociative adsorption process236 with ΔH0

diss.ads(MoO2(OH)2) =−54 kJ·mol−1 and ΔH0

diss.ads(WO2(OH)2) = −56 kJ·mol−1. Thedashed line represents a hypothetical yield curve assuming that group-6 oxide hydroxides are transported by simple reversible adsorptionwith ΔHa

0 = −220 kJ·mol−1.239 Copyright 2011 OldenbourgWissenschafts-verlag GmbH.



have shown, there is a trend of decreasing metal−ligand bondstrength of the group-4 to group-6 halides with increasinggroup number. In addition, the metal−ligand bond strengthdecreases also from the 5d to the 6d compounds within thesame group.315 Thus, SgCl6 was shown to be unstable.Consequently, BhCl7 should not exist. This is also connectedwith a decrease in the relative stability of the maximumoxidation state along the transactinide series (see Figure 5 inPershina et al.315). Oxyhalides of Bh should be rather stable,like those of the lighter homologs. The 4c-DFT calculations ofthe spectroscopic properties of MO3Cl (M = Tc, Re, andBh)291 showed that the electronic structure of BhO3Cl is verysimilar to that of TcO3Cl and ReO3Cl. The Bh molecule shouldbe stable with De of 22.3 eV, only slightly less stable thanReO3Cl, with De of 24.3 eV. Re(BhO3Cl) values are alsotypically larger than Re(ReO3Cl). μ and α of MO3Cl shouldincrease in group 7, which is connected with an increase in themolecular size. Increasing dipole moments and electric dipolepolarizabilities in the group suggest a decreasing volatility in thesequence TcO3Cl > ReO3Cl > BhO3Cl.Using calculated molecular properties, the volatility of group-

7 trioxychlorides as adsorption on a surface of a chromatog-raphy column was predicted via a physisorption model for long-range interactions.291 Since the surface of the quartz column isobviously modified with chlorine in the gas-phase experiments,the adsorption model takes into account the following types ofthe molecule−surface interactions: the dipole-effective surface(Cl) charge, the polarizability-surface (Cl) charge, and thedispersion one (see section 8.1.1 and eq 8.1.3). All thosecontributions were evaluated, and they all were shown toincrease in the group with increasing Z. As their sum, thefollowing adsorption energies ΔHa

0(T)(BhO3Cl) = −78.5kJ·mol−1 and ΔHa

0(T)(TcO3Cl) = −48.2 kJ·mol−1 were thendetermined with respect to the experimentally measuredΔHa

0(T)(ReO3Cl) = −61 kJ·mol−1.399 Thus, the sequence involatility was predicted as TcO3Cl > ReO3Cl > BhO3Cl. Thistrend is caused by an increasing μ in this group.The aqueous chemistry of Bh has not yet been studied

theoretically. Predictions of other chemical properties of Bhbased on earlier DF calculations are given in refs 33 and 34.


12.2.1. Gas-Phase Chemistry of Bohrium. Similar togroup-6 elements, group-7 elements form a number ofmononuclear compounds, of which some are appreciablyvolatile and can thus be utilized for gas chromatographicinvestigations. Oxides and oxide hydroxides of Tc and Re aretypically formed in an O2/H2O containing gas phase. Theywere extensively studied, mostly using the method ofTC.244,400−410 The technique has also been applied to developTc and Re generator systems for nuclear medical applica-tions.411,412 Schadel et al.224 and Eichler et al.413 studied theoxide and the oxide hydroxide chemistry of trace amounts of Rein an O2/H2O-containing system with respect to its suitabilityfor a first gas chemical identification of Bh. In TC experiments,the formation of ReO3 and HReO4 was observed. However,three transport processes probably took place simultaneously,depending also on the pretreatment of the quartz columns:

mobile adsorption of ReO3:

⇄ReO ReO3(g) 3(ads) (12.2.1)

transport reaction of ReO3:

⇄ + +HReO ReO 14O 1

2H Og4( ) 3(ads) 2(g) 2 (g) (12.2.2)

mobile adsorption of HReO4:

⇄HReO HReO4(g) 4(ads) (12.2.3)

While formation of volatile 169RemO3 (t1/2 = 16 s) was observedin online isothermal experiments at temperatures above 950 K,the significantly more volatile H169RemO4 could not besynthesized online.413 This is probably the reason why twoearly attempts to chemically identify Bh in the form of volatileoxides or oxide hydroxides failed.254,414 It is interesting to notethat in both attempts the utilized production reactions led toisotopes that were unknown at the time.Two developments were instrumental in leading to the first

successful chemical identification of Bh. First, the nuclides266Bh and 267Bh were synthesized in the reaction 249Bk(22Ne,4−5n) and their decay properties and production cross sectionsdetermined.415 Especially interesting for chemical investigationsis the relatively long-lived 267Bh with t1/2 = 17−6

+14 s. Second, dueto the not well suited properties of the oxide-hydroxide systemto rapidly isolate group-7 elements, chlorides and oxychlorideswere investigated as potential candidate compounds for anonline gas chemical isolation of Bh.399 This approach hadalready been successful in studies of volatile Db and Sgoxychlorides. The first TC experiments with Tc and Re using amixture of He(g)/O2(g)/HCl(g) revealed only one singledeposition zone for each element despite the large number ofknown chloride and oxychloride compounds of group-7elements. The formed compound was identified as MO3Cl(M = Tc, Re) as the most likely one and was also formed inonline isothermal gaschromatography experiments.399 Interest-ingly, TcO3Cl was so volatile that it could no longer bereclustered in the OLGA(III) setup with CsCl aerosol particles,while these worked fine for ReO3Cl. Aerosol particles with areducing surface such as FeCl2 increased the yield of Tcsignificantly. This property allowed the distinction between a“Tc-like” and a “Re-like” behavior in future experiments withBh.The first successful chemical isolation and identification of

Bh was accomplished in an experiment of four weeks durationat the PSI Philips cyclotron employing the OLGA(III) setupand the ROMA detection system.235 A mixed 249Bk/159Tbtarget was irradiated with 22Ne ions, producing simultaneously17-s 267Bh and 5.3-m 176Re. Nuclear reaction products recoilingfrom the target were attached to carbon aerosol clusters andtransported with the He carrier gas through a capillary to theOLGA(III) setup. As reactive gas, a mixture of HCl and O2 wasadded. After chemical separation, the final products wereattached to CsCl aerosol particles and transported to thedetection system, where α-particle and SF decays wereregistered in an event by event mode. The yield of Re andBh was determined at isothermal temperatures of 180 °C, 150°C, and 75 °C. Altogether nearly 180,000 samples werecollected and measured. A total of six genetically correlateddecay chains of 267Bh were observed, four at 180 °C, two at 150°C, and zero at 75 °C. One of the decay chains at 180 °C wascomplete and consisted of the sequence

→ → →α α α

Bh Db Lr Md267 263 259 255

Due to a non-negligible background created by the presence ofPo and Bi nuclides, 1.3 of the 4 decay chains at 180 °C and 0.1



of the 2 decay chains at 150 °C had to be subtracted. While, intest experiments, 16-s 169RemO3Cl was still observed with 80%yield at an isothermal temperature of 75 °C, no decay chain of17-s 267BhO3Cl was observed at this temperature, indicating aless volatile BhO3Cl compared to ReO3Cl. The fact that

267Bhwas identified after chemical separation already excludes a “Tc-like” behavior of Bh, since CsCl was used as the reclusteraerosol material. The relative yields of compounds 108TcO3Cl,169RemO3Cl, and (most likely) 267BhO3Cl as a function ofisothermal temperature are shown in Figure 55. The deduced

enthalpies of adsorption on the column surface were−ΔHa

0(T)(TcO3Cl) = 51 ± 3 kJ·mol−1, −ΔHa0(T)(ReO3Cl) =

61 ± 3 kJ·mol−1, and −ΔHa0(T)(BhO3Cl) = 75−9

+6 kJ·mol−1 (68%confidence interval). Therefore, the sequence in volatility isTcO3Cl > ReO3Cl > BhO3Cl. The probability that BhO3Cl isequally or more volatile than ReO3Cl was estimated to be lessthan 10%. This sequence in volatility agrees well withpredictions from fully relativistic density-functional calculations

for group-7 oxychlorides by V. Pershina et al.291 The results ofthese calculations showed that he electronic structure ofBhO3Cl is very similar to that of TcO3Cl or ReO3Cl. Increasingdipole moments and electric dipole polarizabilities in the groupsuggest a decreasing volatility in the sequence TcO3Cl >ReO3Cl > BhO3Cl. Also, the calculated ΔHa

0(T)(BhO3Cl) =−78.5 kJ·mol−1 is in perfect agreement with the experimentalresult. Despite the success of the experiment, it should be notedthat the overall efficiency to detect a correlated α−α pair wasonly about 4%. Significant improvements of the overallefficiency were mandatory to continue chemical investigationsof even heavier elements.

13. HASSIUM (Z = 108)


The ground state of Hs is 6d67s2, so that it is a homolog of Ruand Os in group 8. MCDF calculations179 have given the firstIP of 7.6 eV (corrected value obtained by an extrapolationprocedure, while the calculated one is 6.69 eV), which is lowerthan the IP(Os) of 8.44 eV. The first ionized electron in Hs isthe 6d one, while in Os it is the 6s.313 The 6d(Hs) electron isslightly more bound than the 6s(Os),122 so that a larger IP ofHs than that of Os is expected. The opposite trend obtained inMCDF calculations179 should therefore be revisited on thebasis of more accurate calculations. All multiple IPs(M → MZ+)are also given in ref 179 and those to the 8+ oxidation state arealso shown in Figure 28, revealing the same decreasing trendwith Z in the group in IPs(M → MZmax+), as for group-4through group-7 elements. The IR values obtained via a linearcorrelation with Rmax of the (n−1)p AOs are also shown inFigure 28,179 following the trend of increasing IR for group-4through group-7 elements. The IR(Hs8+) of 0.45 Å is alsotypically larger than the IR(Os8+) of 0.39 Å. This is inagreement with the CR of these elements.319,320

As with other group-8 elements, Hs should form volatiletetroxides, whose volatility was studied experimentally (seebelow). The group-8 elements Ru and Os are the only elementswhich can form an 8+ oxidation state (with the exception of Xe,which is known to form tetrahedral XeO4

416), the highestoxidation state known for a transition metal.417 The perfect

Figure 55. Relative yields of the compounds 108TcO3Cl (blue circles),169RemO3Cl (green circles), and (most likely) 267BhO3Cl (red squares)as a function of isothermal temperature. The error bars indicate a 68%confidence interval. The solid lines indicate the results of simulationswith the microscopic model of Zvara288 with the adsorption enthalpiesgiven in the text. The dashed lines represent the calculated relativeyield concerning the 68% confidence interval of ΔHa

0(T) (BhO3Cl)from −66 to −81 kJ·mol−1.235

Figure 56. Relativistic (rel.) and nonrelativistic (nonrel.) bond lengths, Re, ionization potentials, IP, and polarizabilities, α, of MO4 (M = Ru, Os, andHs). Reproduced with permission from ref 293. Copyright 2008 by The American Physical Society.



tetrahedral symmetry of the neutral OsO4 makes this moleculenonpolar and highly volatile (Tm = 40.25 °C). This outstandingproperty distinguishes Os from any other transition metal oxideand allows, thus, a clear differentiation of the chemistry of Hsfrom that of the actinide and lighter transactinide elements. Topredict their properties and volatility, very accurate 4c-DFTcalculations with maximum large basis sets were performed.293

The results have shown that group-8 MO4 molecules should allbe very similar and stable, with the same trend reversal in De,RuO4 < OsO4 > HsO4, as for the earlier 6d elements withrespect to the 5d ones. SO effects on the 6d AO are responsiblefor such a trend reversal. Re(HsO4) should also be larger thanRe of RuO4 and OsO4, as in compounds of group-4 throughgroup-7 6d elements with respect to the 5d ones. Thecalculations have also revealed an inversion of the trend in αand IPs values beyond Os (Figure 56). This trend reversal isexplained by the behavior of the valence (n−1)d AOcontributing predominantly to bonding.For the MO4 (M = Ru, Os, and Hs) molecules, the influence

of relativistic effects on properties important for gas-phaseexperimental investigations was studied in detail.292,293 Figure56 shows the relativistic and nonrelativistic IP, α, and Re valuesof these molecules. One can see that relativistic effects decreaseRe, increase IPs (with the strongest effect on HsO4), anddecrease α. However, they do not change trends in theseproperties in the group, since those for the relativistic andnonrelativistic (n−1)d AOs are the same.There are also ab initio DF418 and infinite-order regular

approximation with modified metric method (IORAmm/HF)419 theoretical studies of the electronic structures of MO4(M = Os and Hs). These works, however, revealed somedeficiency of the calculations that resulted in predicting a wrongtrend in properties from Os to Hs, as compared to a moreaccurate calculation.293 (See a critical analysis in ref 292.)Using a model of dispersion interaction (eq 8.1.1), ΔHa

0(T)

values of group-8 tetroxides on a silicon nitride surface of thedetectors of the chromatography column were predicted.293

The inversion of the trend in α and IPs beyond Os (Figure 56)resulted in a trend reversal in −ΔHa

0(T): RuO4 > OsO4 < HsO4.This prediction turned out to be in agreement with theexperimentally observed trend in −ΔHa

0(T): OsO4 < HsO4.197

Also, the calculated −ΔHa0(T)(HsO4) = 45.1 kJ·mol−1 proved to

be in excellent agreement with the measured −ΔHa0(T) (HsO4)

= 46 ± 2 kJ·mol−1.197 Relativistic effects were shown to have noinfluence on the trend in ΔHa

0(T), as they have no influence onthe trends in the molecular properties (Figure 56), sincerelativistic and nonrelativistic (n−1)d AOs change in the sameway with increasing Z in group 8.293

In a work of Pershina,295 thermodynamic equations topredict Ta of a heaviest element with respect to Ta of a homologin a comparative study are given. As an example, the adsorptionof group-8 MO4 (M = Os, Hs) was considered. Accordingly,Ta(HsO4) with respect to Ta(OsO4) was predicted. In the samework, various measures of volatility were critically compared.Volatile tetroxides of Hs, like other group-8 elements, should

react with a moisturized NaOH surface, forming the sodiumhassate (VIII), Na2[HsO4(OH)2], by analogy withNa2[OsO4(OH)2], according to the reaction:

+ →2NaOH HsO Na [HsO (OH) ]4 2 4 2 (13.1.1)

The reactivity of RuO4, OsO4, and HsO4 with NaOH wasstudied on the basis of 4c-DFT calculations of the componentsof the reaction of eq 13.1.1.309 The calculated free energy

change of the reaction is indicative of the following trend in thecomplex formation in group 8: Os > Hs ≫ Ru. The predictedlower reactivity of HsO4 with NaOH as compared to that ofOsO4 has so far not clearly been revealed experimentally.420


13.2.1. Gas-Phase Chemistry. The experimental chemicalinvestigation of the transactinide element hassium (Hs, Z =108), an expected member of group 8 and homolog of Os andRu, presented a number of challenges. For obvious reasonsdescribed above, from the very beginning, all attemptsconcentrated only on the isolation of Hs as a very volatileHsO4, probably very similar in volatility to OsO4.The first synthesis of Hs was reported by Munzenberg et

al.421 in 1984, which identified the nuclide 265Hs with t1/2 = 1.5ms, far too short for any currently available chemical separatorsystem. The more neutron-rich nuclide 269Hs with t1/2 ≈ 10 ssuitable for chemical investigations was observed much later inthe α-particle decay chain originating from 277Cn.80 However,the production cross section of only about 1 pb (10−36 cm2) forthe reaction 208Pb(70Zn, 1n)277Cn was discouragingly small. Asomewhat larger production cross section of about 7 pb wascalculated for the direct production of 269Hs in the reaction248Cm(26Mg, 5n).422 This production cross section is 1 order ofmagnitude smaller than that for the synthesis of 267Bh, thenuclide that was used for chemical investigations of the nextlighter transactinide element. Therefore, new techniques had tobe introduced for irradiation, separation, and detection in orderto accomplish the required sensitivity of chemically investigat-ing the element Hs.Three early attempts to chemically identify Hs as volatile

HsO4 demonstrated the high chemical selectivity of the chosenapproach; however, the experiments lacked the overallefficiency and the long-term stability to reach the requiredsensitivity. In experiments conducted at Dubna, Zhuikov etal.255,256 employed the reaction 40Ar + 235U to produce short-lived α-decaying isotopes of element 110 and their Hs daughternuclides. The technique employed fission track detectors,assuming that the produced Hs nuclides would decay mainly bySF. No SF decays were registered, resulting in a productioncross section limit of 10 pb for nuclides with t1/2 > 150 ms. In asecond attempt,255,256 the reaction 249Cf(22Ne, 4n)267Hs wasused to search also for short-lived α-particle emitting isotopesof Hs. The decontamination from actinides (separation factor>106) as well as that from Po (>103) was excellent.Nevertheless, no α-particles in the energy range above 8.5MeV and no SF events were registered. An upper limitproduction cross section of 100 pb for α-decaying nuclides with50 ms ≤ t1/2 ≤ 12 h and of 50 pb for spontaneously fissioningnuclides was established. A similar experiment was reported byDougan et al.423 by applying the so-called On-line Separationand Condensation AppaRatus (OSCAR), which was installed atthe LBNL 88-Inch Cyclotron. The OSCAR setup was used tosearch for α-decaying 272Hs, the expected EC decay daughter of272Mt (estimated t1/2(EC) ≈ 25 m), produced in the254Es(22Ne, 4n) reaction. However, no α-decays between8.7−11.0 MeV were observed and an upper limit for theproduction cross section of 1 nb was derived.The first successful Hs chemistry experiment was conducted

in the spring of 2001 in the framework of an internationalcollaboration at the GSI, Darmstadt. In order to significantlyincrease the level of sensitivity, all features of earlierexperiments were either improved or replaced by new



techniques. The stationary target was replaced by the rotatingwindow and target system ARTESIA196 (see section 7.1.1),which could accept significantly higher beam intensities fromthe accelerator. The reaction 248Cm(26Mg, 5n)269Hs was chosenas the most promising one, also due to the availability ofsufficient quantities of the 248Cm target material for thepreparation of three segments of the target wheel and its stillmanageable radioactive decay properties. With the rotatingtarget wheel, synthesis of about 3 atoms of 269Hs per day couldbe expected. The transport of reaction products with the aid ofan aerosol gas-jet was abandoned. Instead it was observed thatOs tetroxides were formed directly in the recoil chamber in testreactions when adding O2 to the He stopping gas.206 Theaddition of an oven heated to 600 °C at the exit of the recoilchamber helped to complete the oxidation reaction andincreased the yield. Volatile tetroxide species could then betransported essentially without loss through Teflon capillariesto the chemistry and detection setup. Instead of IC, TC wasapplied where the column was replaced with a narrow channelformed by silicon particle detectors;201 see also Figure 21(section 7.2.2). The efficiency of detecting an α-particle or a SFevent was about 80%. In addition, the deposition temperatureof each detected single Hs atom also provided chemicalinformation. The required gain in sensitivity of 1 order ofmagnitude compared to the OLGA setup used in experimentswith Bh was thus accomplished. In an experiment conducted atthe GSI, valid data was collected during 64.2 h. During thistime, 1.0 × 1018 26Mg beam particles passed through the 248Cmtarget. Only α-lines originating from 211At, 219,220Rn, and theirdecay products were identified. While 211At and its decaydaughter 211Po were deposited mainly in the first two detectors,219,220Rn and their decay products accumulated in the last threedetectors, where the temperature was sufficiently low to partlyadsorb Rn. During the experiment, seven correlated decaychains were observed in detectors 2 through 4 and wereassigned to the decay of either 269Hs or the yet unknown270Hs.197 This assignment was based on an erroneousassignment of mass numbers and decay properties of Sgisotopes in the physics discovery experiment.393 The excellentseparation from unwanted activities resulted in a backgroundcount-rate of α-particles with energies between 8.0−9.5 MeV of0.6 per hour and detector, leading to very low probabilitiesbetween 2 × 10−3 and 7 × 10−5 for any of the chains being ofrandom origin. One of the observed decay chains was completeconsisting of four consecutive α-particles attributed to thedecay sequence

→ → → →α α α α

Hs Sg Rf No Fm269 265 261 257 253

The efficiency and selectivity of the system were so superior,that it was also used later on for the search and discovery of theHs-isotopes 270Hs and 271Hs.109,261 With the correct identi-fication of 270Hs and its α-decay daughter 266Sg, it becameobvious that all 7 observed chains in the first Hs experimentwere due to 269Hs.109 The thermochromatogram of these 7events along with the distribution of Os is shown in Figure 54.The maximum of the Hs distribution was found at atemperature of −44 ± 6 °C. The distribution of 172OsO4(t1/2 = 19.2 s) measured before and after the experimentshowed a maximum in detector 6 at a deposition temperatureof −82 ± 7 °C. As in experiments with lighter transactinideelements, the Monte Carlo model of Zvara,288 that describesthe microscopic migration of a molecule in a gas chromato-

graphic column, was used to evaluate the adsorption enthalpyof HsO4 and OsO4 on the silicon nitride detector surface. Themodeled distributions with −ΔHa

0(T)(HsO4) = 46 ± 2 kJ·mol−1

(68% confidence interval) and −ΔHa0(T)(OsO4) = 39 ± 1

kJ·mol−1 are shown as solid lines in Figure 57. The obtained−ΔHa

0(T) value for HsO4 and the trend from OsO4 to HsO4 arein excellent agreement with the 4c-DFT theory.293

13.2.2. Liquid-Phase Chemistry. In an independentexperiment, von Zweidorf et al.420 used the CALLISTO(continuously working arrangement for cluster-less transportof in situ produced volatile oxides) setup to demonstrate thatthe volatile Hs compound formed in situ with oxygencontaining carrier gases reacts readily with a thin layer ofhydroxide in the presence of water. This behavior is well-knownfor OsO4, which behaves as an acid anhydride, forming withaqueous NaOH sodium osmate(VIII) of the stoichiometryNa2[OsO4(OH)2] (see section 13.1). In an experiment similarto the one of Dullmann et al.,197 the reaction 248Cm(26Mg,5n)269Hs was employed to form volatile 269HsO4 by stoppingthe fusion reaction products in a mixture of He/O2 and passingthe gas stream through a hot quartz wool filter. The addition ofwater vapors significantly improved the deposition yield oftetroxides on freshly prepared NaOH surfaces. Therefore, theHe/O2 gas stream (1 L/min He, 0.1 L/min O2) containingHsO4 or OsO4 was mixed with 0.1 L/min He saturated withwater at 30 °C. The detection system consisted of fourdetection arrays, each containing four silicon PIN-diodes of 10mm × 8 mm active area facing a stainless steel plate, coatedwith NaOH at a distance of about 1 mm.The gas stream was passing through three of these arrays,

whereas the fourth one was in so-called “service mode”. Every60 min, one of the stainless steel plates had to be replaced witha freshly coated one, since the reactive surfaces were loosingdeposition efficiency with time, probably due to theneutralization of the alkaline surface with CO2, which wasprobably formed under the influence of the heavy ion beam bya reaction of the carbon beam dump with the oxygen of the jetgas. The flow of the gas stream was controlled by fourcomputer controlled valves. The working principle of thedeposition and detection system is illustrated in Figure 58. Adisadvantage of the one-sided detection system is the reduced

Figure 57. Relative yields of HsO4 and OsO4 for each of the 12detector pairs. Measured values are represented by bars: 269HsO4, darkblue; 172OsO4, light blue. The dashed line indicates the temperatureprofile (right-hand scale). The maxima of the deposition distributionswere evaluated as −44 ± 6 °C for HsO4 and −82 ± 7 °C for OsO4.Solid lines represent results of a simulation of the adsorption processwith −ΔHa

0(T)(HsO4) = 46.0 kJ·mol−1 and −ΔHa0(T)(OsO4) = 39.0

kJ·mol−1.



detection geometry compared to a two-sided geometry in acryo TC detector, which significantly lowers the probability todetect complete nuclear decay chains. In total, five nucleardecay chains attributed to the decay of Hs isotopes wereregistered.420 The distribution of the five Hs events in relationto the lighter homolog Os on the 3 times 4 detectors, that is, 12positions, is depicted in Figure 59. The gas stream alwaysentered the detection setup before detector position 1 and leftafter passing detector 12.The authors420 concluded that, for the first time, an acid−

base reaction was performed with the tetroxide of hassium,leading to the formation of a hassate(VIII). Evidence for alower reactivity of HsO4 with respect to moisturized NaOH ascompared to OsO4, as tentatively suggested by the maximum ofthe Hs distribution on detector 3 and predicted by the 4c-DFTtheory,309 was not judged as significant due to the few detectedevents.

14. MEITNERIUM (Z = 109), DARMSTADTIUM (Z =110), ROENTGENIUM (Z = 111)


Mt, Ds, and Rg have 6d77s2, 6d87s2, and 6d97s2 ground statesand belong to groups 9, 10, and 11 of the Periodic Table,respectively. The ground states for Ds and Rg are different fromthose of their lighter homologs Pt (5d96s) and Au (5d106s),which is explained by the relativistic stabilization of the 7s AOin the heavier elements. The M+ configurations Ds+(6d77s2)and Rg+(6d87s2) are also different from those of their homologsPt+(5d9) and Au+(5d10), which is also a relativistic effect.Belonging to groups 9, 10, and 11, Mt, Ds, and Rg are expectedto exhibit a behavior similar to that of their lighter homologs inthese groups, the noble metals Ir, Pt, and Au. Strong relativisticeffects should influence the properties of Mt, Ds, and Rg andtheir compounds to a larger extent than those of the homologsof the sixth period. The chemistry of these elements predictedon the basis of atomic DF calculations is described else-where.33,34

These early DF calculations33,34 have given an IP of 8.7 eVfor Mt, which is obviously too low (it is lower than the IP of Irof 8.967 eV313), while it should probably be larger, because the6d(Bh) AO is more stabilized than the 6s(Ir) AO.122 For Ds,the DF calculated33,34 IP of 9.6 eV is larger than that of Pt(8.959 eV313), because the 6d(Ds) AO is more stabilized thanthe 6s(Pt) AO. In any case, more accurate calculations areneeded for these two heaviest elements. For Rg, the best DCBFSCC calculated IP is 10.6 eV,425 which is also larger than theIP(Au) of 9.2255 eV313 because the 6d(Rg) is more stabilizedthan the 6s(Au) AO. According to the higher IPs of theheaviest elements in groups 9 through 11, they should be evenmore inert and noble than their homologs of the sixth period.For Mt, the Kα1 transition energies for different ionization

states were predicted using the DHF theory, taking intoaccount QED and nuclear-size effects. The results werecompared with recent experiments in the α-decay of 272Rg.128

The main oxidation states for Mt and Ds are 3+ and 2+,respectively, though some other oxidation states are alsoforeseen.426 In Rg, the most stable state should be 3+,33,34 whilethe 1+ state should be very unstable. Due to the relativisticdestabilization, that is, the 6d AOs that start to be chemicallyactive at the end of the 6d series, the 5+ state of Rg is alsoexpected. The −1 state is also expected due to the EA of Rg of

Figure 58. Comparison of two different states of the deposition anddetection system of CALLISTO. In the upper part of the figure,detection array 4 is in “service mode”; in the lower part of the figure,detection array 1 is in “service mode”, allowing replacement of thesteel plate of array 1 with a freshly prepared NaOH surface.424

Figure 59. Deposition pattern of OsO4 (blue) and269HsO4 (red) on a

NaOH surface from a moist gas stream. The Os α-radioactivity wasmostly due to the decay of 19.2-s 172Os and 22.4-s 173Os.424



1.56 eV, which is, however, much lower compared to Au (2.31eV).425

The AR of Rg (1.2 Å) is smaller than the AR of Au (1.35 Å)due to the stronger relativistic contraction of the 7s AO,33,34 aswas also mentioned earlier. From group 9 onward, there is adecrease in the difference in the single and triple bond CRbetween the 6d and 5d compounds, reaching negative values ingroups 11 and 12 (Figure 29).319,320 The relativistic bondcontraction is caused mainly by the relativistic stabilization ofthe ns AO.On the MO level of theory, Mt and Ds have received little

attention so far. It was suggested that volatile hexafluorides andoctafluorides might be produced and used for chemicalseparation experiments. DS-DV calculations for DsF6 indicatethat DsF6 should be very similar to PtF6, with very close valuesof IPs.427 Relativistic effects were shown to be as large as ligand-field splitting.427

Bond lengths in MtH3, MtC− and DsH2, and DsH3 werecalculated using the ADF ZORA method.319,320 Using the sameapproach, electronic structures of DsC and DsCO werecalculated,428 suggesting that these compounds are chemicallysimilar to the corresponding 5d homologs.In contrast, Rg received much attention from theory. A

special interest in the chemistry of Rg is explained by theexpectation of unusual properties of its compounds due to themaximum of relativistic contraction and stabilization of the 7sAO in this group.122 The electronic structure of the simplestmolecule RgH, a sort of a test system such as AuH, was studiedat various levels of theory, REPP, DHF, and 4c-DFT.429−434

Because the obtained Re(RgH) between 1.503 Å and 1.546 Å(the DHF CCSD(T) value is 1.523 Å432) is so close toRe(AuH) of 1.5236 Å,435 a very high accuracy is needed topredict the correct trend. At the best present level of accuracy,the bond lengths of these molecules are about the same.A comparison of relativistic (DF or ARPP) with non-

relativistic (HF or NRPP) calculations shows bonding to beconsiderably increased by relativistic effects doubling thedissociation energy, though the SO splitting diminishes it by0.7 eV.429 The trend to an increase in De from AgH to AuHshould be reversed from AuH to RgH, as was also shown by theBDF calculations.436 The PP and BDF calculations disagree,however, for the trend in Re(MH) in group 11 from Au to Rg.The trend to an increase in force constant, ke, was found to becontinued, with RgH having the largest value of all knowndiatomic molecules.429 The μ was shown to be relativisticallydecreased from AgH to AuH and to RgH, indicating that RgHis more covalent and element Rg(I) is more electronegativethan Au(I).429,436

Results of 4c-BDF436 and 4c-DFT calculations433 for AuXand RgX (X = F, Cl, Br, O, Au, Rg), indicate that relativisticeffects follow a similar pattern to that for RgH, except for RgFand RgO, where SO splitting increases De. The singlet state wasfound to be the ground state for Rg2, in comparison with thetriplet state.433 The dissociation energy was found to change inthe following order: Au2 > RgAu > Rg2.To study the stability of higher oxidation states, energies of

the MF6− → MF4

− + F2 and MF4− → MF2

− + F2 (M = Cu, Ag,Au, and Rg) decomposition reactions were calculated at the PP,MP2, and CCSD levels of theory.437 Relativistic effects wereshown to stabilize higher oxidation states in the high-coordination compounds of Rg due to the destabilization ofthe 6d AOs and their larger involvement in bonding. RgF6

− wasshown to be the most stable in this group. SO coupling

stabilizes the molecules in the following order: RgF6− > RgF4

−

> RgF2−. This order is consistent with the relative involvement

of the 6d electrons in bonding for each type of molecule.The aqueous-phase chemistry of Rg received some

theoretical attention on the basis of DFT theory. It wasshown that Rg(I) is the softest metal ion.438 No chemistryexperiments have been conducted with elements Mt, Ds, andRg so far.

15. COPERNICIUM (Z = 112)


Heavy group 12 elements all have a closed shell d10s2 groundstate and should therefore be rather inert. With increasingrelativistic stabilization and contraction of the ns AO in group12, elements become even more inert. Thus, bulk Hg is knownto be a liquid; however, it is very different from the condensednoble gases. In the case of Cn, relativistic effects are expected tobe further amplified. Pitzer439 in 1975 found that the very highexcitation energy of 8.6 eV from the s2 closed shell into the spvalence state of Cn will not be compensated by the energy gainof the chemical bond formation. Thus, Cn should reveal anoble-gas character.The atomic properties of Cn are very well studied (for older

predictions, see refs 33 and 34). The DCB FSCC calculationshave given the first IP of Cn of 11.97 eV and the second one of22.49 eV.440 It is the highest in group 12 (Figure 60) and in theseventh row of the Periodic Table, evidencing a very highinertness of Cn. However, the first ionized electron is the 6d5/2,in difference to Hg (6s) (Figure 11). The DCB FSCCcalculations of EA found no bound anion for Cn.440 Excitationenergies, IPs, and oscillator strengths for neutral and up to 5+ionized states of Cn and Zn, Cd, and Hg were calculated usingthe MCDF method.441 The calculated MCDF IPs441 are,however, less accurate than the DCB FSCC ones.440 IPs ofinternal conversion electrons, that is, of K-shell (1s) and L-shell(2s), of Cn are predicted to an accuracy of a few tens ofelectronvolts using DHF theory taking into account QED andnuclear-size effects.127 This data can be used for experimentalstudies of Cn involving K conversion electron spectroscopy.The main oxidation state of Cn is expected to be 0 due to its

closed shell structure and strongest relativistic effects. However,the 2+ state could also be foreseen. As in the case of Rg, the 6delectrons start to be chemically active in Cn. As a consequence,an increase in the stability of the higher oxidation states, 4+, isexpected also for this element. Due to the maximum relativisticcontraction of the 7s AO in group 12 and in the seventh period(Figure 12), the AR of Cn should be the smallest in group 12(1.71 Å)442 (see also Figure 60). This also results in theshortest bond lengths of Cn compounds in group 12, with thepredominant contribution of the 7s AOs (see also CR in Figure29319,320).The static dipole polarizability of Cn of 27.64 au was

calculated most accurately at the DC CCSD(T) level oftheory.442 Because of the relativistic 7s AO contraction, it is thesmallest in group 12 (Figure 60) and the smallest in the seventhperiod. Due to the smallest α, Cn should be very volatile overinert surfaces, which guarantees its transport from the targetchamber to the chemistry setup through Teflon capillaries inchemical experiments. Deposition of Cn on ice and quartz athigher temperatures than those for Hg was also predicted.442

The influence of relativistic effects on the atomic propertiesof group-12 elements, as the most interesting case, was



investigated.294 Figure 60 shows relativistic and nonrelativisticvalues of IPs, α and AR, which change in an opposite way fromHg to Cn. One can see that, exceptionally due to relativisticeffects, Cn should have the largest IP and smallest α and ARand, therefore, be chemically rather inert, much more than thelighter homologs in the group.Taking into account the strong relativistic effects on the AOs

of Cn, the following questions were of high interest, especiallyfor gas-phase chemical experiments: Is Cn metallic in the solidstate, or is it more like a solid noble gas? How volatile andreactive toward Au is the Cn atom in comparison with Hg andRn?Since bonding in the solid state in the first approximation is

described by bonding in a homonuclear dimer, De(M2) valueswere calculated to estimate ΔHS

0(298) of the Cn metal.Moreover, Hg2 and Cn2 have been of special interest inchemical theory in testing the accuracy of quantum-chemicalmethods in treating closed-shell interactions. Accordingly, theelectronic structures of these dimers were calculated using avariety of methods, such as 4c-BDF, ECP CCSD(T), QP-PPCCSD(T),443 and 4c-DFT.294,433 The calculations have shownthat even though bonding in both Hg2 and Cn2 is preferentiallyof the van der Waals type, a partial overlap occurs. Both theDFT and PP calculations agree on an increase in De of about0.04 eV from Hg2 to Cn2 with the corresponding bondshortening. Thus, due to the relativistic 7s AO contraction, Cn2should be more stable than Hg2. This is in agreement with theLDA DFT solid-state calculations for solid Cn.444 A cohesiveenergy of 1.13 eV was obtained for Cn at the SR-level of theory,which is larger than that of Hg (0.64 eV) and is an order ofmagnitude larger than those of the solid noble gases. It was alsoconcluded that Cn is not a metal, but rather a semiconductorwith a band gap of at least 0.2 eV. In this sense, Cn resemblesthe group-12 metals more closely than it does the noble gases.Predictions of the interaction of Cn with Au were also very

important to compare the volatility of Cn with that of Hg asadsorption on noble metal (Au) surfaces of detectors in gas-phase chromatography experiments. With this aim in view,electronic structure calculations were performed for HgM andCnM, where M = Ag, Au, Pt, Pd, and Cu using the 4c-DFTmethod.445,446 It was demonstrated that Cn forms a chemicalbond with Au primarily due to the overlap between the doubly

occupied 7s(Cn) AO and singly occupied 6s(Au) AO, as well asbetween the 6d5/2(Cn) AO and 5d5/2(Au) AO. Thus, CnAushould be chemically bound, having a σ2σ*1 2Σ+ ground stateconfiguration with two electrons in the bonding and one in theantibonding MOs (Figure 61).

Overall, Cn should be about 0.1−0.2 eV more weakly boundwith a transition metal atom M than Hg, due to the strongerrelativistic contraction of the 7s(AO) in comparison with the6s(Hg) AO, while the bonds should be longer, because of themore extended 6d(Cn) than the 5d(Hg). Among the group-11and group-12 metals, bonding with Ag was found to be theweakest while that with Pt the strongest.The influence of relativistic effects on properties of MAu (M

= Hg and Cn) was studied as well.294 Relativity was shown toincrease De(HgAu) by 0.13 eV but to decrease it by about thesame amount (0.12 eV) in CnAu due to the contraction of the7s(Cn) AO. This makes trends in nonrelativistic vs relativisticDe values opposite from HgAu to CnAu, so that De

nr(CnAu) >

Figure 60. Relativistic (solid lines) and nonrelativistic (dashed lines) ionization potentials, IP, polarizabilities, α, and atomic radii, AR, of group-12elements. Reprinted with permission from ref 294. Copyright 2005 Elsevier.

Figure 61. Bond formation (principal MOs) of the CnAu and FlAumolecules. Reproduced with permission from ref 25. Copyright 2011Oldenbourg Wissenschaftsverlag GmbH.



Denr(HgAu), while De

rel(CnAu) < Derel(HgAu). Re is decreased

by relativity in both systems, and the trends are the same forboth the nonrelativistic and relativistic Re.Further on, 4c-DFT calculations were performed for Hg and

Cn interacting with Aun clusters simulating Au (111) and (100)surfaces.290a,446,447 Convergence of Eb with cluster size wasreached for n = 95 (top position), n = 94 (bridge), n = 120(hollow1), and n = 107 (hollow2) on the Au(111) surface. Theobtained M−Aun binding energies, Eb, evidence that anAu(111) surface is a good approximation of the real one,which is usually unknown in the experiments. The bridgeadsorption position was found to be preferential for Hg, while ahollow2 one was preferential for Cn (Figure 62).

The Eb(Cn-Aun) for the hollow2 position was calculated as0.46 eV.290a This is significantly lower than −ΔHa

0(T)(Hg) of1.0 eV on Au.449 The reason for that, as in the case of the golddimers of these elements, is the relativistic stabilization of the7s(Cn) AO. Thus, a lower volatility as adsorption on a goldsurface was expected for Cn in comparison with Hg.Works on RECP and 2c-DFT (SO corrected) calculations

for Hg and Cn interacting with small Au clusters (n = 1−4 and10) arrived at the same conclusion, namely that Eb(Cn-Aun) isabout 0.2 eV smaller than Eb(Hg−Aun).450−453The influence of relativistic effects on the adsorption process

of Hg and Cn on metal surfaces was investigated on theexample of small M−Aun clusters.294 Relativistic effects wereshown to define a decreasing trend in Eb(M−Aun) from Hg toCn, even though they increase Eb(M−Aun) in these systems,especially at the hollow position due to the involvement of the6d(Cn) AOs in bonding. This makes the difference in Eb(M−Aun) between Hg and Cn very small. Relativistic effects wereshown to decrease Re, the distance of the adatom to the surface,in all the cases.The relativistic contraction of the 7s AO is expected to

manifest itself also in properties of other Cn compounds, e.g.,in shortening Re in CnH and CnH+.434,454−458 Anotherinteresting point is that, in contrast to the group-11 hydrides,the trend in the dissociation energies from Cd to Hg iscontinued with Cn, i.e. De(CdH

+) < De(HgH+) < De(CnH

+),but De(AgH) < De(AuH) > De(RgH).

454,456,457 The reasons for

this difference are greater relativistic effects in CnH+ than inRgH.Due to the larger involvement of the 6d AOs in bonding,

higher oxidation states should be observed for high-coordination compounds of Cn, as was proven by the PPCCSD(T) calculations.455 No definite conclusion, however,about the existence of CnF4 can be drawn from itsdecomposition MF4 → MF2 + F2 energy between 100 kJ·mol−1

and 200 kJ·mol−1. Nonrelativistically, CnF4 would be definitelyunstable. It was also found that the addition of F− ions to HgF2and to HgF4 is energetically favorable.459,460 By analogy, it isassumed that, in combination with an appropriate polar solvent,CnF5

− and/or CnF3− may be formed.455 The small energy of

the decomposition reaction MF2 → M + F2 confirms theprediction that Cn should be more inert than Hg, though thedifference to Hg is not that large. A comparison withnonrelativistic results shows that this is a pure relativistic effect.15.2. Experimental Results

15.2.1. Gas-Phase Chemistry. With the synthesis of partlylong-lived isotopes of Cn through element 118 in 48Ca inducedreactions on actinide targets,2,89,461−463 the focus of chemistsshifted to the chemical exploration of these superheavyelements. Of special interest are chemical investigations ofCn with its 6d107s2 closed shell and strong relativisticstabilization of the 7s AO that should result in a high inertness,higher than that of Hg. Experimental investigations of Cn holdthe promise to study this element as the first transactinide inthe elemental state.Suitable isotopes of Cn with sufficiently long half-lives are

283Cn (t1/2 = 3.8 s) and 285Cn (t1/2 = 29 s). The nuclide 283Cncan be synthesized either directly in the reaction 238U(48Ca, 3n)or indirectly as a decay product of 287Fl in the reaction

→α( )Pu Ca, 3n Fl Cn242 48 287 283

with the latter reaction having the higher production crosssection. The heavier, but longer-lived 285Cn can only besynthesized indirectly in the

→α( )Pu Ca, 3n Fl Cn244 48 289 285

reaction. From the viewpoint of identification, the decay chainof 283Cn is preferred since the α-decay (Eα = 9.54 MeV) isfollowed shortly in time by SF of 279Ds (t1/2 = 0.18 s). Thisdecay chain constitutes quite a unique signature and allows thesafe identification of 283Cn after chemical isolation. In thisrespect, the decay of 285Cn (Eα = 9.16 MeV), which is followedby SF of 281Ds (t1/2 = 13 s), is less favorable. For the ensuingdiscussions, it is important to note that, in the first attempts tochemically identify Cn, it was believed that 283Cn decays by SFwith t1/2 ≈ 3 m.461,462

A first attempt to chemically identify Cn in the elementalstate was made by Yakushev et al.464 in Dubna. The isotope283Cn was produced by bombarding a U target of naturalisotopic composition with 48Ca ions. In test experiments, short-lived Hg isotopes could be isolated in the elemental form fromother reaction products, transported in He quantitativelythrough a 30 m long Teflon capillary, and adsorbedquantitatively on Au, Pt, or Pd coated silicon detectors atroom temperature. If Cn behaved chemically like Hg and allefficiencies measured for Hg were also valid for Cn, detectionof 3.4−22

+4.3 SF events could be expected assuming the crosssection value for the production of 283Cn measured in ref 461.

Figure 62. 4c-DFT calculated binding energies of Pb, Hg, Cn, and Flwith the Aun clusters in comparison with experimental −ΔHa

0(T) of Pb,Hg, and Cn on Au.194,448,449 Reproduced with permission from ref290a. Copyright 2009 American Institute of Physics.



However, no SF events were observed. Therefore, nounambiguous answer as to the chemical and physical propertiesof Cn was obtained.464 In a next experiment, the questionwhether Cn remained in the gas phase and passed over the Auand Pd surfaces was addressed.190 Therefore, a specialionization chamber to measure SF fragments of nucleiremaining in the gas was added at the exit of the Au or Pdcoated silicon detector array. Again, zero SF events wereregistered on the Au and Pd coated silicon detectors,confirming the result of the first experiment. However, eighthigh energy events were detected in the ionization chamberascribed to SF decays because they were accompanied byneutrons registered in the surrounding neutron counters, whileonly one background count was expected.190 The majority ofthe events were attributed to the decay of an isotope of Cn,since there are no other known volatile nuclides decaying bySF. From this experiment it appeared that the interaction of Cnwith an Au or Pd surface is much weaker than that for Hg.190

These results were seemingly confirmed in a TC experimentusing the COLD detector where one side of the channel wasreplaced by an Au covered surface, allowing only measurementsin a 2π geometry, i.e. no coincident detection of both SFfragments. The temperature in the gradient started at +35 °Cand reached down to −185 °C, the adsorption temperature ofRn. Using the same 48Ca + 238U synthesis reaction, reactionproducts were stopped in He carrier gas and transportedthrough a 25 m long capillary within about 25 s to the detectorarray. Seven events were detected that were attributed to fissionfragments from the SF decay of 283Cn. The position of most ofthese events was identical to the deposition peak of Rn.However, the fragment energies were lower than expected,which was attributed to a thin layer of ice that has formed onthe detectors at such low temperatures. It was concluded thatall three chemical studies on Cn yielded a consistent picture,namely that Cn is not interacting with Au and is more similar toRn.465 However, since later physics studies could not confirmthe SF-decay of 3-min 283Cn but rather showed that this isotopedecays via α-emission with t1/2 ≈ 4 s to 279Ds, the transporttime for nuclei produced at the accelerator to the detector arrayin all chemistry experiments performed so far was too long foridentification of a 4-s isotope. Therefore, without safeidentification of the isolated nuclide, all conclusions concerningthe chemical properties of Cn were questionable and called forsignificantly improved experiments. At present it remainsunclear what the source of the signals was that were measuredin these early experiments.A new attempt to investigate the chemical properties of Cn

took advantage of the higher production cross section of the

→α( )Pu Ca, 3n Fl Cn242 48 287 283

synthesis reaction. A prerequisite of this approach is, however,that first an Fl isotope is formed that has a too short t1/2 forchemical study, followed by an isotope of Cn with a sufficientlylong t1/2, which is fulfilled in this case as t1/2(

287Fl) = 0.48 s.Again the COLD setup was used for this study, now with a 4πdetection geometry where one side of the channel wasequipped with Au-covered silicon detectors. Moreover, thesetup was operated in a closed loop mode to reduce the watervapor content of the carrier gas. This was decisive to reduceformation of ice layers at low temperatures. Still it wasimpossible to exclude ice formation at temperatures belowabout −100 °C. With a transport time of 2.2 s, the yield of 287Fl

is reduced to 5% while for 283Cn it is still 68%. A total of fivedecay chains was detected in two experiments. They startedwith the α-decay of 283Cn followed by SF-decays within lessthan one second.192,194 The positions of the five atoms inside ofthe detector array are depicted in Figure 63. They represent the

outcome of three different experiments with varying values ofthe temperature range inside the detector array and velocity ofthe carrier gas, respectively. Also shown are the depositionpatterns of α-decaying isotopes of Hg and Rn formed in nuclearreactions in the same experiment with a minor admixture of Ndto the 242Pu target (for Hg) or formed in transfer reactions withthe target (for Rn). In the first experiment, the temperaturerange inside the detector array was −24 to −185 °C. Under thiscondition, the first decay chain of 283Cn was observed in thesecond detector, at a position where also major Hg depositionwas found. To search for a possible difference in chemicalbehavior between Hg and Cn, the temperature at the beginningof the detector array was increased to the maximum value atwhich a semiconductor detector is still operational (+35 °C).Indeed, in the second experiment, one decay chain of 283Cn wasobserved at −5 °C, which is at the edge of Hg deposition.Therefore, a third experiment was conducted with increased gasflow rate (increase from about 1 L/min to 2 L/min). Under

Figure 63. Deposition of the five detected atoms (indicated by arrows)assigned to 283Cn in 48Ca + 242Pu experiments. The dotted linesindicate the temperature gradient inside the detector array (right axisin °C). Three different regimes in terms of temperature range insidethe detector array and gas flow rates were applied (see text). The solidred lines depict results of a Monte Carlo model prediction (left axis inrel. units), including the given experimental parameters and assumingthe deposited atoms to have always −ΔHa

Au(Cn) = 52 kJ·mol−1.27,194

The vertical dashed lines at detectors 17, 19, and 21, respectively,indicate the start of ice layer formation toward lower temperaturescorroborated by reduced resolutions in the α-spectra.



this experimental condition, two atoms of 283Cn were detectedon the Au covered detector array at −29 and −39 °C,respectively. One further atom was found on the ice coveredpart at −124 °C. All three atoms were observed where onlylittle if any Hg deposition occurred. The adsorption enthalpy ofCn on Au surfaces was evaluated from the depositiontemperature of the four atoms adsorbed on Au as −ΔHa

Au(Cn)= 52−3

+4 kJ·mol−1 (68% confidence interval). For Hg,−ΔHa

Au(Hg) > 65 kJ·mol−1 was evaluated, in agreement withliterature data,449 where −ΔHa

Au(Hg) = 98 ± 3 kJ·mol−1 wasdetermined. For the noble gas Rn on an ice surface,−ΔHa

ice(Rn) = 19 ± 2 kJ·mol−1 was measured, in excellentagreement with −ΔHa

ice(Rn) = 20 ± 2 kJ·mol−1 from literaturedata.466 In later experiments, one decay of 283Cn and one decayof 285Cn were observed, the latter one in experiments where theonline thermochromatographic setup was attached to the exitof a gas-filled separator.467,468 The adsorption temperatures ofthe additional events were in full agreement with −ΔHa

Au(Cn)= 52−3

+4 kJ·mol−1. The theoretically predicted −ΔHaAu(Cn) of

0.46 eV290a on Au turned out to be in good agreement with theexperimental −ΔHa

Au(Cn) = 0.54−0.030.04 eV.194 This value,

reflecting the fact that Cn was observed in a range ofadsorption temperatures that are significantly higher than thoseof Rn, is indicative of an Eb(Cn−Aun) that is larger than that forpure van der Waals interaction. Thus, in agreement with theory,chemical bond formation obviously takes place in the case ofCn with Au, similar to, but weaker than for Hg with Au. Thus,Cn is a d-metal, and not an inert gas, and its place in group 12of the Periodic Table is legitimate.

16. ELEMENT 113


In elements 113 through 118, filling of the 7p shell takes place.The 7p AOs experience a very strong influence of relativisticeffects: a large SO splitting and a strong contraction andstabilization of the 7p1/2 AO, as well as an expansion anddestabilization of the 7p3/2 AO, that all increase along the 7pseries. The 7s2 stabilization is so large that it practicallybecomes an inert pair in these elements. Early predictionsindicated that these elements should be very volatile.33,34

Extrapolations from lighter homologs in the chemical groupshave, indeed, shown that elements 113 through 117 shouldhave smaller ΔHS

0(298) or formation enthalpies of gaseous atoms,ΔH*298(E(g)), than their lighter homologs (Figure 27).299

In element 113, the SO splitting is 3.1 eV, and the relativisticstabilization of the 7p1/2 AO is 2.2. eV.37 The DCB FSCCcalculations confirmed the 7s27p1/2 ground state of element 113and have given the first IP of 7.306 eV.469 An EA of 0.68(5) eVwas calculated in the same work, which is larger than EA(Tl)due to the relativistic stabilization of the 7p1/2 AO. Thepolarizability of element 113 was calculated as 29.85 au via theDC FSCC approach.470 A trend reversal is observed in group13 in IPs (an increase), α (a decrease), and AR (a decrease)from In on. This is connected with a trend reversal in theenergies (an increase) and Rmax (a decrease) of the np1/2 AOs,that define those atomic properties, at In. Using the calculatedatomic properties, ΔHa

0(T) of element 113 on Teflon andpolyethylen were estimated via an adsorption model (eq 8.1.1)as 14.0 and 15.8 kJ·mol−1, respectively, which guarantees itstransport through Teflon capillaries in chemical experiments. Ingroup 13, ΔHa

0(T) values were shown to exhibit a trend reversalbeyond In, due to the trend reversal in the atomic IP, AR, and

α. The extremely small α of element 113, caused by thecontraction of the 7p1/2 AO, is the main reason for the very low−ΔHa

0(T) on inert surfaces.The main oxidation state of element 113 is expected to be

1+, though the 3+ state should also appear.33,34

There are quite a number of molecular calculations forelement 113. The influence of relativistic effects on the 7pcompounds was studied on the example of the hydrides MH(M = 113 − 118) using various methods: DFC, RECP, 2c- and4c-DFT.166,433,434,456,471−479 In 113H, the 6d and 7s AOs ofelement 113 participate little in bonding and all the effects aredefined by the 7p1/2 shell. A large relativistic contraction of the7p1/2 AO results in a large contraction of the 113-H bond. TheSO effects on the bond lengths, ΔRe(SO), are about −0.2 Å.Such a large bond contraction is not found in the other 7p MH.In moist oxygen atmosphere, element 113 should react with

OH, forming 113OH by analogy with TlOH. The compound ispredicted to be stable, with De of 2.42 eV in comparison withDe(TlOH) of 3.68 eV, according to 4c-DFT calculations.480

The lower binding energy is due to the relativistic stabilizationof the 7p1/2 AO. If element 113 adsorbs on a Au surface in theform of 113OH, −ΔHa

0(T) should be smaller than that ofTlOH.480

Element 113 should also form 113F, as other group-13elements. Results of PP, DCB, RECP, and 4c-DFTcalculations433,473,474 reveal increasing Re and μe from TlF to113F, in contrast to decreasing values from TlH to 113H.These different trends in Re and μe for the MF compounds ascompared to MH are explained by a more ionic nature of theMF molecules. DF calculations481 have shown that in the(113)(117) molecule there is a reversal of the dipole directionand a change of the sign of μ in comparison with other group-17 homologs. The reason for that is the energetically higher-lying 7p3/2 of the element 117 shell that donates into the low-lying 7p1/2 shell of the element 113 atom, while, in lighterhomologs, the single electron of the group-13 atom usuallycompletes the valence p shell of the group-17 atom.481

Like in Cn, the relativistic destabilization of the 6d AOs isexpected to influence properties of high-coordination com-pounds of element 113, as was confirmed by PP and RECPcalculations for 113X3 (X = H, F, Cl, Br, and I).473,482 As aconsequence of the involvement of the 6d AOs, a T-shapedrather than trigonal planar geometric configuration waspredicted for these molecules, showing that the valence shellelectron pair repulsion (VSEPR) theory is not applicable to theheaviest elements. Vest et al.483 have shown that thedecomposition energy of 113H3 into 113H and H2 becomesmore favorable in going down group 13. The reason for that isenhanced relativistic effects on the 7p1/2 AO.A stable high-coordination compound of element 113,

113F6−, with the metal in the 5+ oxidation state is also

foreseen.474 113F5 will probably be unstable, since the energy ofthe 113F5 → 113F3 + F2 decomposition reaction is less than−100 kJ·mol−1. The calculated energies of the reaction MX3 →MX + X2 (from M = B through element 113) suggest adecrease in the stability of the 3+ oxidation state in this group.In order to predict ΔHS

0(298) of the 7p metals and ΔHa0(T) of

the 7p elements on a Au surface that might be measured infuture gas-phase experiments, 4c-DFT calculations of M2 andMAu (M = 113 through 118) were performed.484,485,490 Someother calculations were also performed for these species: abinitio DF for (113)2,

486 4c-BDF, and 2c-SO ZORA; DC/MP2-DFT for the element 113−117 dimers165,166,471,472,487−489 and



RECP ones for Fl- and 118 dimers.456 The obtained 4c-DFTDe(M2) and De(MAu) are shown in Figure 64.484,490

According to these results, the (113)2 dimer should beweakly bound because the 7p1/2 electron yields a weak bondhaving 2/3π bonding and 1/3σ antibonding character (or 2/3πantibonding and 1/3σ bonding).486

ΔHS0(298) = ΔH*298(E(g)) values were estimated then for the

7p metals via a correlation with De(M2) in the respectivechemical groups.484 They are given in Table 5 along withΔH*298(E(g)) predicted via a linear extrapolation in the groups(Figure 27),299 in good agreement with each other. One can seethat ΔH*298(E(g)) changes almost linearly with Z in thesegroups. For element 113, ΔHS

0(298) is lower than that of Tl dueto the bonding preferentially made by the 7p1/2 AO.To study the interaction of element 113 with Au, 4c-DFT

calculations were performed for TlAu and 113Au.480 In 113Au,bonding should be weaker than in TlAu due to the relativisticstabilization of the 7p1/2 AO.

490 One can therefore expect thatelement 113 will adsorb on Au at much lower temperaturesthan Tl. An −ΔHa

0(T)(113) = 159 ± 5 kJ·mol−1 on Au wasestimated with respect to −ΔHa

0(T)(Tl) = 240 ± 5 kJ·mol−1

using the difference in De(MAu), where M = Tl and element113.480 Adsorption of an element 113 atom and Tl on Au(111)and Au(100) surfaces was also modeled by M−Aun (n = 20)clusters and 2c-DFT calculations.491 The results show that thedifference in binding energy, Eb(M−Aun), between Tl andelement 113 stays within ±15 kJ·mol−1 of 82 kJ·mol−1 obtainedin ref 480. Thus, the cluster calculations performed on a largerscale confirmed the estimate of Pershina et al.,480 so that−ΔHa

0(T)(113) can be given as 159 ± 15 kJ·mol−1.


16.2.1. Gas-Phase Chemistry. The first experiments wereconducted by Dmitriev et al. at FLNR Dubna, studying theadsorption of element 113 on Au surfaces. Only preliminaryresults have been communicated.492

17. FLEROVIUM (Z = 114)


Element 114, Fl, has a quasi-closed shell 7s27p1/22 ground state

caused by the large SO splitting of 4.7 eV122 and relativisticstabilization of the 7p1/2 AO. A high excitation energy into thevalence state configuration, 7p1/2

2 → 7p2, of 6.2 eV calculatedby Pitzer439 was a reason for him to believe that Fl, like Cn,would be an inert-gas like element.The most accurate DCB IHFSCC value of the IP(Fl) is

8.626 eV, which is, indeed, indicative of a very high inertness ofFl, though less than that of Cn.493 Other IPs up to the M3+ statefor Fl and Pb were also calculated.493 The MCDF IPs and IR ofthe neutral to the 4+ ionized state of Fl and other group-14homologs are also given,494 though being less accurate than theDCB values.493 The first IP of Fl and energies of several excitedstates were also calculated with the use of the relativisticcomplete active space MC CI method.142,143 The IPs of group-14 elements show a decreasing trend from C to Sn and anincreasing trend from Sn to Fl, so that the IP of the latter iseven higher than the IP of Si. The reason is the relativisticstabilization of the np1/2 AO with Z. Due to the quasi-closed7p1/2

2 shell, Fl will have a zero EA, as was shown by DCBFSCC calculations.495 The main oxidation state should be2+.33,34

IPs of internal conversion electrons, that is, of K-shell (1s)and L-shell (2s), of Fl are predicted to an accuracy of a few 10eV using DHF theory, taking into account QED and nuclear-size effects.127 This data can be used for experimental studies ofFl involving K conversion electron spectroscopy.A polarizability of Fl of 30.6 au (as well as of Pb of 46.96 au)

was calculated using the DC CCSD(T) method.442 Polar-izabilities of group-14 elements including Fl were alsocalculated using the DK and DC methods, though with aslightly smaller basis set496 (without h-functions taken intoaccount in ref 442). The DC CCSD(T) value of 31.49 au isvery close to that of ref 442. A Gaunt contribution wasestimated in this work as 0.38 au, being rather significant.Finally, the recommended value of 31.0 au is that of ref 442,corrected for the Gaunt term.As in group 13, α shows a reversal of the increasing trend in

group 14 at Sn, so that α(Fl) is smaller than that of Ge. This isdue to the relativistic contraction of the np1/2 AO, which isdocumented by a correlation between polarizabilities and themean radius of the np1/2 AO in group 14.122

The AR of Fl was estimated as 1.75 Å, while the van derWaals radius, RvdW, of 2.08 Å,442 which both are relativisticallycontracted. The trend in AR and RvdW is also reversed at Sn dueto the same reason, the contraction of the np1/2 AO withincreasing Z. Using the α and RvdW values, −ΔHa

0(T)(Fl) = 10.4kJ·mol−1 on Teflon was predicted via a model ofphysisorption.442 As do α and radii, −ΔHa

0(T) shows also areversal of the increasing trend at Sn, so that −ΔHa

0(T)(Fl) issmaller than that of Ge. The very low value of −ΔHa

0(T)

indicates that Fl should be easily delivered to chemicalexperiments through Teflon capillaries.Due to the very strong stabilization and contraction of the

7p1/22(Fl) shell and, therefore, expected van der Waals nature

of the M−M bonding, the homonuclear dimer Fl2 was ofparticular interest for theory. Also, the knowledge of the Fl−Flbinding energy was important to estimate its sublimationenthalpy. The ECP and 2c- and 4c-DFT calcula-tions166,471,484,485 agree on the fact that Fl2 is more strongly

Figure 64. Calculated atomization energies of MAu and M2 (M areelements Hg/Cn through Rn/118). Filled and open squares areDe(MAu) and De(M2) of the 6p elements, respectively, while filled andopen rhomboids are De(MAu) and De(M2) of the 7p elements,respectively.484,490. Reprinted with permission from ref 490. Copyright2010 American Institute of Physics.



bound than a typical van der Waals system. At the 4c-DFT levelof theory, it is slightly more strongly bound than Cn2, but muchmore weakly bound than Pb2. A Mulliken population analysisindicates that both the 7p1/2 and 7p3/2 AOs of Fl take part inthe bond formation.484,485 The participation of the moreextended 7p3/2(Fl) AO in bonding in comparison with the6p3/2(Pb) AO explains an increase in Re from Pb2 to Fl2. SOeffects were shown to decrease De, but increase Re in bothsystems.471

ΔHS0(298) of Fl was estimated using a correlation with the

calculated De(Fl2) in group 14 (see Table 5). The obtained

value is in very good agreement with that obtained via a linearextrapolation in the group (Figure 27).299 Ecoh of Fl waspredicted from the SR and SO-GGA-DFT solid-statecalculations.497 The obtained SO-PW91 value of 48.2 kJ·mol−1

is in reasonable agreement with other estimates.299,484 SOeffects were shown to lower Ecoh and lead to structural phasetransitions for solid Fl (the hcp structure in contrast to the fccfor Pb). In a nonrelativistic world, all group-14 elements wouldadopt a diamond structure.To estimate the interaction strength of Fl with noble metals,

particularly with Au, 4c-DFT calculations were performed forMM′, where M are group-14 elements and M′ are group-10and 11 metals.485,490 The results have shown that Fl interactsmost strongly with Pt, while least strongly with Ag. De(FlAu) issignificantly (1.42 eV) smaller than De(PbAu) (Figure 64) dueto the very large 7p1/2 AO stabilization. However, the 7p3/2 AOalso takes part in the bond formation, which results in anincrease in Re from the Pb to Fl dimers. The Fl−Au bondshould, however, be stronger than the Cn−Au one.290a,490 Thisis due to the fact that in FlAueven though both FlAu andCnAu are open shell systems with one antibonding σ*electronelectron density is donated from the 7p1/2(Fl) AO,lying higher in energy, to the 6s(Au) AO, while, in CnAu, someexcitation energy is needed to transfer electron density from theclosed 7s2(Cn) to the open 6s(Au) shell (Figure 61).Large-scale 4c-DFT calculations were also performed for M

= Pb and Fl interacting with large Aun (n > 90) clusterssimulating a Au(111) surface.290a Both Pb and Fl were found toprefer the bridge adsorption position. The calculated Eb valuefor Pb (2.40 eV) is in very good agreement with the

experimental −ΔHa0(T)(Pb) = 2.43 eV on Au,448 so that

Eb(Fl−Aun) (n = 94) was given as 0.71 eV (Figure 62). Theobtained −ΔHa

0(T)(Fl) = 68.5 kJ·mol−1 is indicative offormation of a chemical bond with Au. A comparison withgroup-12 Hg (where, however, Hg dissolves into Au) and Cnshows that the trend in −ΔHa

0(T) should be Cn < Fl < Hg≪ Pb(Figure 62).290a Calculations for the C−Aun and Fl-Aun systemsusing other relativistic DFT methods450−453 came to the sameconclusion: Fl should form a rather strong chemical bond withAu, stronger than that with Cn. A comparison of results ofvarious calculations is given in Table 6.Many other, mostly simple, compounds of Fl were treated

theoretically using various approaches. In the 7p MH, from Flon, both the relativistically contracted 7p1/2 and expanded 7p3/2AOs take part in the bond formation, so that Re(FlH) is longerthan Re(PbH). De(MH) (M = 113 through 117) is reduced bylarge SO effects, with the lowest value at FlH. Trends in thestability of hydrides were predicted as follows: RnH ≪ HgH <PbH and 118H ≪ FlH < CnH. RECP and DC CCSD(T)calculations for PbH+ and FlH+ have also given a 50% weakerbond and a shorter Re in the latter due to the contraction of the7p1/2 AO.

434,456 CAS-SCF/SOCI RECP calculations for FlH2demonstrated breakdown of the conventional singlet (X1A1)and triplet (3B1) states due to large SO-effects.

498 SO-effects areshown to destabilize FlH2 by almost 2.6 eV.Electronic structures of FlX (X = F, Cl, Br, I, O) and FlO2

were calculated using 2c-RECP CCSD(T), 2c-DFT SO ZORA,and 4c-BDF methods.166,471 In contrast to PbO2 (De = 5.60eV), FlO2 (De = 1.64 eV) was predicted to be thermodynami-cally unstable with respect to the decomposition into the metalatom and O2. According to results of these calculations, Flshould not react with O2 under typical experimental conditions,as was discussed in ref 442.Ab initio DF and PP calculations481,499 for the decomposition

reactions MX4 → MX2 + X2 and MX2 → M + X2 (M = Si, Ge,Sn, Pb, and Fl; X = H, F, and Cl) also predicted a decrease inthe stability of the 4+ oxidation state in group 14. Theinstability was shown to be a relativistic effect. The neutral statewas found to be more stable for Fl than for Pb. As aconsequence, Fl is expected to be less reactive than Pb, butabout as reactive as Hg. The possibility of the existence of FlF6

2‑

was also suggested in ref 499.


17.2.1. Gas-Phase Chemistry. The heaviest elementinvestigated experimentally to date is Fl, which would beplaced as eka-Pb into group 14 of the Periodic Table. Suitablenuclides for chemical experiments are 287Fl (t1/2 = 0.48−0.09

+0.16 s)89

or 288Fl (t1/2 = 0.69−0.11+0.17s) and 289Fl (t1/2 = 2.1−0.4

+0.8s),500 which

Table 5. Standard enthalpies of monotaomic gaseouselements, ΔH*298(E(g)) (in kJ·mol−1), of the 7p Elements

method E113 E114 E115 E116 E117

extrapolation299 138.1 70.3 146.4 92.1 83.7correlation484 144.7 70.4 152 ± 12 101.3 91.7

Table 6. Cn-Aun and Fl-Aun Binding Energies (in eV) Simulating Interactions of Cn and Fl with Au(100) and Au(111) Surfaces(Bold Values Are for the Preferential Positions)

method n position surface Cn Fl ref

4c-DFT 1 top 0.51 0.73 290a2c-DFT 1 top 0.47 0.72 450−452SO DFT 3 top, bridge 0.47 0.77 450−4522c-DFT 26 bridge Au(100) 0.33 0.55 4522c-DFT 37 hollow-2 Au(111) 0.49 4534c-DFT 95 top Au(111) 0.30 0.47 290a4c-DFT 94 bridge Au(111) 0.42 0.71 290a4c-DFT 107 hollow-2 Au(111) 0.46 0.59 290a−ΔHa

Au(exp) ∞ unknown unknown 0.54−0.030.04 0.35−0.1

0.6 , or ≥ Cn 192, 195, 260



can be synthesized in the reaction 48Ca + 242Pu or 244Pu,respectively.89 In the experiment where three decay chains of283Cn were registered in the COLD setup (see Figure 63),194

also a three member decay chain was registered in detector pair19 held at −88 °C that was attributed to an atom of 287Flreaching the detector despite its short t1/2 of only 0.5 s. Thetransport efficiency was estimated to be only 5%.195

An additional experiment conducted by the PSI group in2007 at FLNR employing a 244Pu target in combination withthe COLD setup revealed two further decay chains that wereattributed to 288Fl.195 The transport efficiencies of 288Fl and289Fl can be estimated using the currently best t1/2 of

288Fl and289Fl to be 11% and 48%, respectively, due to their longer t1/2compared to that of 287Fl. Indeed, two events attributed to thedecay of 288Fl were detected. One α-decay event of 9.95 MeVoccurred on the bottom detector of detector pair 18, followed0.109 s later by one SF fragment in the neighboring topdetector 19 held at −90 °C. The deposition of 288Fl occurredmost likely on the Au covered detector 19 top, where the SFfragment was observed, whereas the α-particle was emittedacross the gap and registered in the detector 18 bottom. Thesecond decay chain started with an α-particle of 9.81 MeV inthe top detector of pair 3 and ended 0.104 s later with a singlefission fragment observed in bottom detector 6 at −4 °C. Theobservation that the decay of a daughter atom is displaced fromthat of the mother atom was explained by the recoil of thedaughter atom out of the detector during the α-particleemission, followed by a transport with the carrier gas.195 Abackground of α-particle events in the region where decays of289Fl and its daughter 285Cn were expected prevented thepositive identification of the rather long decay chain from289Fl.195

The location of the three detected events attributed todeposition of Fl atoms in relation to the elements Cn, Hg, andRn is shown in Figure 65, panels 1 through 4. The dashed line(right-hand axis) always indicates the temperature gradientestablished during the experiments. The left-hand axis indicatesrelative yields per detector pair in percent of a given element.

The vertical dash-dotted line indicates the temperature at whichthe dew point in the gas was reached. Left of the dash-dottedline, the surface of the COLD detector was either Au (topdetectors) or SiO2 (bottom detectors); right of the dash dottedline, the surfaces were covered by a thin layer of ice. In panel 1the deposition of the nuclide 185Hg (t1/2 = 49 s) (gray bars),produced from an admixture of Nd of natural isotopiccomposition to the target material, is shown. Single atoms ofHg show the expected diffusion controlled deposition patternfrom irreversible adsorption on the Au surface of the topdetectors. Due to the high flow rates and since only one side ofthe detector channel was Au covered, the distribution of Hgextends far into the COLD detector. The nuclide 219Rn (whitebars) being produced in transfer reactions was deposited on theice surface only at very low temperatures close to the exit ofCOLD, as expected for the noble gas Rn. The depositionpatterns of both 185Hg and 219Rn could be satisfactorilydescribed by a microscopic model of the adsorption chromato-graphic process based on a Monte Carlo approach288 (solidlines) with −ΔHa

0(T)(Hg) ≥ 50 kJ·mol−1 on a Au surface and−ΔHa

0(T)(Rn) = 19 kJ·mol−1 on an ice surface, respectively, ingood agreement with literature data. In panel 2, the location ofthree decays of 283Cn in COLD (see Figure 63) is depicted thatwas observed in the same experiment as the decay chainattributed to 287Fl (panel 3). In panels 3 and 4, the locations ofdeposition of one atom of 287Fl and two atoms of 288Fl,respectively, are shown. From these three events, a mostprobable adsorption enthalpy of −ΔHa

0(T)(Fl) = 34−3+20 kJ·mol−1

(68% c.i.) on a Au surface was deduced.195 The correspondingcalculated model distributions are truncated at the dash-dottedline indicating the dew point.An adsorption enthalpy of −ΔHa

0(T)(Fl) = 34−3+20 kJ·mol−1 on

a Au surface is surprisingly low, since Fl is expected to be morereactive than Cn and should thus deposit at higher temper-atures than Cn and not a lower temperatures, as observed.Theoretical calculations290a predicting −ΔHa

0(T)(Cn) = 45kJ·mol−1 on Au, in good agreement with experiment, predicted−ΔHa

0(T)(Fl) = 68 kJ·mol−1 on Au, corresponding to asomewhat less volatile Fl compared to Cn. The DFT solid-state calculations give a higher Ecoh of the Cn solid incomparison with the Hg and Fl ones,444,497 (though a directcomparison of the calculations performed in differentapproximations is not straightforward). Because in group 12there is no correlation between ΔHS

0(298) and −ΔHa0(T) on Au

ΔHS0(298)(Hg) < ΔHS

0(298)(Cn), while, on a Au surface,ΔHa

0(T)(Hg) > ΔH a0(T)(Cn). On the contrary, in group 14

there is a correlation between ΔHS0(298) of metals and −ΔHa

0(T)

on Au, so that ΔHS0(298)(Pb) > ΔHS

0(298)(Fl) and −ΔHa0(T)(Pb)

> −ΔHa0(T)(Fl) on Au. This case shows clearly that there is no

general correlation between ΔHS0(298) on metals and −ΔHa

0(T)

on Au. In groups 15 through 17, this is another obvious case, aswill be shown later.The observation that Fl exhibits an unexpected high

volatility, together with the fact that a background in thedetector array made the positive identification of 289Fl eventsimpossible, caused some skepticism.501

In order to remove the background of undesired reactionproducts, the chemical techniques developed so far werecoupled to a physical preseparator (see section 7.1.2). A firstchemical experiment with Fl using a combination of IC and TCon Au surfaces was conducted behind the TASCA separator inthe fall of 2009. Two events attributed to the decay of Fl were

Figure 65. Deposition patterns of the elements Hg, Rn, Cn, and Fl inthe COLD detector as observed in experiments by Eichler et al.195

Reproduced with permission from ref 195. Copyright 2010 Old-enbourg Wissenschaftsverlag GmbH. For a detailed discussion see text.



observed.260 Both α-particle decays of Fl isotopes occurred inthe isothermal section at room temperature.A detailed comparison with the experiment of the PSI-FLNR

collaboration195 cannot be made, especially since no analysis ofthe experiment behind TASCA has been published so far.Despite this, it is quite remarkable that two independentexperiments were able to separate and detect Fl in a chemistryexperiment, demonstrating the power of gas chemicalinvestigations.

18. ELEMENT 115, LIVERMORIUM (Z = 116), ELEMENT117, AND ELEMENT 118


In elements 115 through 118, filling of the 7p3/2 shell takesplace that will determine their chemical properties. For element115, DCB FSCC calculations give an IP of 5.579 eV.502 For Lvand element 117, older DF calculations give IPs of 6.6 and 7.7eV, respectively,33,34 while the DCF FSCC calculations are inprogress.503,504 For element 118, the DC FSCC value of the IPis 8.914 eV.505 Since 7p3/2 AOs are relativistically moredestabilized than the np3/2 AOs of the lighter homologs, IPs ofelements 115 through 118 are smaller than those of the lighterhomologs in groups 15 through 118. This means that the 7p3/2elements should be chemically more reactive than the 6p3/2ones.Due to the relativistic destabilization of the np3/2 AOs, EAs of

the elements 115 (with a DCB FSCC value of 0.383 eV502), Lv(with a DCB FSCC value of 0.905 eV504), and 117 (with a DCCCSD(T) value of 1.589 eV504) will also be smaller than thoseof the 6p elements. For element 118, DCB FSCC+QEDcalculations have given a positive EA of 0.056 eV.131 Thereason for that is the relativistic stabilization of the 8s AO.IPs of internal conversion electrons, that is, of K-shell (1s)

and L-shell (2s), of Lv and element 118 are predicted to anaccuracy of a few 10 eV using DHF theory, taking into accountQED and nuclear-size effects.127 This data can be used forexperimental studies of Fl involving K conversion electronspectroscopy.For elements 115 through 118, lower oxidation states should

be more stable in comparison with those of the lighterhomologs due to the inaccessibility of the relativisticallystabilized 7p1/2 AO for bonding and the SO destabilized7p3/2 electrons. Thus, for element 115, the 1+ state should bethe most important one. The 3+ state should also be possible,while 5+ should not. For Lv, a decrease in the stability of the 4+oxidation state is expected, and the 2+ state should beimportant. For element 117, the 1+ and 3+ oxidation statesshould be the most important ones, while the 5+ and 7+ statesare less important. The 1− state of element 117, having oneelectron hole on the 7p3/2 AO, should, therefore, be lessimportant (its EA is the smallest in the group). For element118, the 2+ and 4+ states are possible, while the 6+ one will beless important, since the 7p1/2

2 pair is very much stabilized.The AR of the 7p3/2 elements should be larger than those of

their 6p3/2 homologs due to the spatially more expanded 7p3/2AOs. Accordingly, the polarizability of these elements shouldalso be larger. For element 118, the polarizability of 167.4 au, aDCB FSCC result, is also the largest in the 18th group.505

Using the calculated atomic properties of element 118, itsΔHa

0(T) values on noble metals (Au and Ag) and nonmetals(quartz, ice, Teflon, and graphite) were predicted using aphysisorption model.505 A very low value of −ΔHa

0(T)(118) of

10.8 kJ·mol−1 on Teflon should allow for its transport throughTeflon capillaries to the chemistry setup. The obtained almostequal values of −ΔHa

0(T) of element 118 and Rn on varioussurfaces are indicative, however, that experimental distinctionbetween these elements by using these surfaces will beimpossible. A possible material could be activated charcoal;however, further studies are needed to test this assumption.To estimate the strength of the M−M bonding in the solid

state of elements 115 through 118, 4c-DFT calculations wereperformed for their homonuclear dimers.484 Results shown inFigure 64 indicate that the M−M bonding is weaker in (115)2through (117)2 compared to Bi2 through At2, respectively, dueto the availability of practically only 7p3/2 AOs for bonding.This means that ΔH*298(E(g)) of elements 115 through 117should be the smallest in the groups. Their estimates madeusing the calculated De(M2) values are in good agreement withthose obtained via linear extrapolations in groups 15 through17 (Figure 27), meaning that these elements should be rathervolatile. Bonding of (118)2 is, however, stronger than that ofRn2, since it is of van der Waals type and caused by the largestpolarizability of the 118 atom in group 18.To predict ΔHa

0(T) of elements 115 through 118 on a Ausurface, 4c-DFT calculations were performed for the MAudimers and their 6p homologs.490 Results are shown in Figure64. One can see that, in groups 15 through 17, De(MAu) valuesare about the same for the 7p and 6p elements. This is incontrast to the trends in De(M2) in these groups, where De(Bi2)≫ De[(115)2], De(Po2) ≫ De(Lv2), and De(At2) > De[(117)2].The relatively strong M−Au bonding of elements 115 through117 with Au is explained by the relativistic destabilization of the7p3/2 AOs fitting energetically better to the 6s(Au) AO, thusmakingtogether with the 7p1/2 AOa full σ-bond in MAu, indifference to M2, where only the 7p3/2 AOs are involved inbonding.484 In group 18, a reversal of the trend takes place, sothat De(118Au) > De(RnAu), in agreement with the trend inDe(M2). This is due to the relativistically more destabilized7p3/2(118) AO than the 6p3/2(Rn) AO, thus better overlappingwith the valence AOs of Au. Hence, for elements 115 through118, temperatures as high as for their 6p homologs will beneeded to detect their equilibrium adsorption position on Au.The calculations have also revealed that the M−Au bondstrength does not decrease linearly with Z in groups 15, 16, and17, which means that −ΔHa

0(T) on Au will not correlate withΔHS

0(298) in these groups.Formation enthalpies of MX2 and MX4 (X = F, Cl, Br, I,

SO42−, CO3

2−, NO3−, and PO4

3−) for Po and Lv were estimatedon the basis of the MCDF atomic calculations.508 Theyconfirmed the instability of the 4+ oxidation state of Lv.The influence of SO effects on the molecular structure of

MX2 (X = F, Cl, Br, I, At, and element 117) of Lv and its lighterhomologs was studied with the use of the 2c-HF and DFT ECPmethods.509 The results have shown that while the moleculesare bent at a scalar relativistic level, SO coupling is so strongthat only 7p3/2 AOs of Lv are involved in bonding, which favorslinear molecular geometries for MX2 with heavy terminalhalogen atoms.The influence of relativistic effects on the electronic structure

of 117H was investigated on the basis of the HF and DCCCSD(T) calculations.434 Relativistic effects, including SOones, were shown to significantly decrease De(117H) andincrease Re(117H).Electronic structures of IF, AtF, and 117F were considered at

the DC and RECP levels of theory.482 De(117F) was shown to



be the largest among the group-17 fluorides. For 117F3, theRECP calculations have shown that the D3h geometry is not theproper one, in difference from the sixth period compound ofAt, thus again indicating that the VSEPR theory is notapplicable to the heaviest elements.482,510 117Cl is predicted tobe bound by a single π bond and have a relativistically (SO)increased Re.

511

Due to the relativistic destabilization of the 7p3/2 AO inelement 118, it is predicted to be rather reactive. It should formthe 118-Cl bond.512 The RECP calculations for the reactions M+ F2 → MF2 and MF2 + F2 → MF4, where M = Xe, Rn, andelement 118, confirmed the increasing stability of the fluoridesin the group as a result of the increasing polarizability of thecentral atom.482,513 Also, the following trends in the stability ofthe fluorides were established: RnF2 < HgF2 < PbF2, whileCnF2 < FlF2 < 118F2. The influence of relativistic effects on theelectronic structure of 118H+ was investigated on the basis ofHF and DC CCSD(T) calculations.434 Relativity (including SOeffects) was shown to significantly decrease De(118H

+) andincrease Re(118H

+).The influence of the SO interaction on the geometry of

group-18 MF4 was investigated by the RECP-SOCI/CCSDcalculations.478,513,514 It was shown that in 118F4, a Tdconfiguration becomes more stable than the D4h one knownfor the lighter homologs. The reason for this unusual geometryis the availability of only the stereochemically active four 7p3/2electrons for bonding. This is another example of theinapplicability of the VSEPR theory for the heaviest elements.An important observation was made that the fluorides ofelement 118 will most probably be ionic rather than covalent,as in the case of Xe. This prediction might be useful for futuregas-phase chromatography experiments.

19. ELEMENT 119 AND ELEMENT 120


At the time being, the chemistry of elements heavier than Z =118 rests on a purely theoretical basis. The synthesis ofelements beyond Z = 118 appears to be difficult, and several

attempts have failed.515,516 The currently best options517 basedon the parameters reaction asymmetry and Q value appear tobe the reactions 50Ti + 249Bk and 50Ti + 249Cf to synthesizeelements 119 and 120, which are currently attempted atTASCA at GSI. However, the t1/2 values of the synthesizednuclei are expected to be of the order of milliseconds or evenmicroseconds. Provided suitable long-lived isotopes of theseelements are found, the volatility of their atoms might bestudied in the long term using some advanced chromatography(e.g., vacuum) techniques that can cope with extremely shortlifetimes of their isotopes. To foresee the outcome of suchexperiments, as well as to study the influence of relativisticeffects on the 8s AOs of these heaviest elements and theircompounds, some theoretical studies have been under-taken.506,507,518,519 For earlier predictions of properties ofthese elements based on the atomic calculations, see refs 33 and34. Properties that are of interest for gas chromatographicstudies, i.e., ΔHS

0(298) and ΔHa0(T) of elements 119 and 120 on

noble metals were predicted on the basis of 4c-DFTcalculations for intermetallic M2 and MAu (M are group-1and group-2 elements) compounds.518,519

Figure 66 demonstrates the obtained De(M2) and theirtrends in groups 1 and 2. One can see that, in these groups,there is a reversal of the trends in De(M2) at Cs and Ba,respectively, though in an opposite way. The reason for thedifferent behavior is a different type of M−M bonding in thesegroups: a covalent one in group 1, while a van der Waals one ingroup 2, even though both are defined by the behavior of the nsAOs.Thus, (119)2, having a σg

2 ground state, should be bound thestrongest by covalent forces among the homologs and have ashort bond length (about that of Rb2) caused by thecontraction of the 8s AO. On the contrary, (120)2 with aσ2gσ*

2u ground state should be the most weakly bound, only by

van der Waals forces, among the homologs (the number ofbonding and antibonding electrons is the same), and the bondshould be the longest.

Figure 66. Dissociation energies, De of group-1 and group-2 M2 (filled rhomboids are 4c-DFT calculations;518 open squares are experimentalvalues), as well as ΔHS

0(298) (filled triangles are experimental values; open ones are estimates). Reprinted with permission from refs 518 and 519.Copyright 2012 Elsevier and 2012 American Institute of Physics, respectively.



ΔHS0(298)(119) = 94 kJ·mol−1 and ΔHS

0(298)(120) = 150kJ·mol−1 values were obtained via a correlation with thecalculated De(M2) in groups 1 and 2, respectively. The ΔHS

0(298)

values show the same reversal in the groups as the De(M2)values do. According to these results, element 119 metal shouldbe as strongly bound as K, while element 120 metal should bethe most weakly bound in group 2, though it is more stronglybound than element 119 (Figure 66).Binding energies of MAu of group-1 and group-2 elements

are shown in Figure 67. ΔHa0(T) values, estimated using a

correlation with the calculated De(MAu) in these groups, arealso depicted there. According to the data, elements 119 and120 should form stable compounds with gold. The De(MAu)values reveal a reversal of the increasing trend at Cs and Ba ingroups 1 and 2, respectively, so that both 119Au and 120Aushould be the most weakly bound among the consideredhomologs in these groups. The trend is defined by the behaviorof the ns AOs, whose relativistic stabilization in the groupsstarts to dominate over the orbital expansion beyond Cs andBa, respectively.The −ΔHa

0(T)(119) = 106 kJ·mol−1 and −ΔHa0(T)(120) = 172

kJ·mol−1 were determined via a correlation with De(MAu) inthese groups. Using correlations with −ΔHa

0(T)(M) on othernoble metals, −ΔHa

0(T) of these elements on Ag and Pt werealso predicted (Figure 67). The very moderate −ΔHa

0(T) valuesof elements 119 and 120, the lowest in groups 1 and 2,especially on Ag (63 kJ·mol−1 and 50 kJ·mol−1, respectively),would allow adsorption chromatographic measurements ofthese elements.The obtained ΔHS

0(298) and −ΔHa0(T) values show that there

is no correlation between these quantities in group 1, as theychange in the opposite way with Z. In group 2, there is acorrelation between ΔHS

0(298) and −ΔHa0(T).

Thermodynamic properties of metals of elements 113through 120 were also predicted521 using atomic calculationsand mathematical models.Hydrides and fluorides of elements 119 and 120 were

considered within the PP and ab initio DF approxima-tions.434,522,523 It was shown that bond distances decreasefrom the seventh to the eighth period for group-1 and group-2

elements due to the relativistic ns AO contraction. The 119Fwas found to be less ionic than lighter alkaline fluoridehomologs, in contrast to expectations based on periodic trends.

20. ELEMENTS BEYOND Z = 120


The chemistry of these elements will be defined by many openshells and their mixing.33,34 Due to very strong relativisticeffects, things will be much more different than anything knownbefore. However, without relativistic effects, it would also bevery different due to the very large orbital effects.Very few molecular calculations exist in this superheavy

domain. Properties of elements heavier than 120 predicted onthe basis of atomic calculations were discussed.33,34,185,524,525

More recent considerations of their chemistry can be found inrefs 526 and 527.A list of possible molecules of elements in the range Z =

121−164 was suggested,527 though their verification should beleft to future theoretical studies. Interesting examples are thosewhere the elements are in unusual valence states orcoordination, such as, for example, 144F8 (an analog ofPuF8) or 148O6 (an analog of UO6).Quasi-relativistic multiple-scattering calculations on 125F6

have found that bonding is defined by the 5g1 electron, with thesituation being analogous to NpF6 with the 5f1 electron.528

There are noncorrelated DF calculations for fluorides ofelement 126.529,530

Accurate predictions of properties of specific compounds willbe quite a challenging task in this area. This may need inclusionof QED effects to reach the required accuracy.

21. CONCLUSIONS AND OUTLOOKDespite the fact that only single atoms of transactinide elementscan be studied “one-atom-at-the-time” and the short t1/2 are ofthe order of seconds, the experimental body of data is alreadysubstantial and impressive. It was shown that the heaviestelements are basically homologs of their lighter congeners inthe chemical groups, though their properties may be ratherdifferent due to very large relativistic effects on their electronshells. Relativistic effects were found to be predominant over

Figure 67. 4c-DFT dissociation energies, De, of group-1 and group-2 MAu, as well as adsorption enthalpies −ΔHa0(T) (filled symbols are

semiempirical calculations,520 while open ones were obtained via correlations with De(MAu), where M = Au, Pt, and Ag). Reprinted with permissionfrom refs 518 and 519. Copyright 2012 Elsevier and 2012 American Institute of Physics, respectively.



the orbital ones in the electronic structures of the elements ofthe seventh period and heavier. They are responsible for trendsin the chemical groups (a continuation, or a reversal) withincreasing Z from the elements of the sixth period. Thus, forelements of the seventh period and heavier, the use ofrelativistic methods is mandatory. Straightforward extrapola-tions of properties from lighter congeners may, therefore, resultin erroneous predictions.Spectacular developments in relativistic quantum theory,

computational algorithms, and computer techniques allowedfor accurate calculations of properties of the heaviest elementsand their compounds. Nowadays, most accurate atomic DC(B)correlated calculations including QED effects are available forthe heaviest elements, reaching an accuracy of a fewmillielectronvolts for electronic transitions and ionizationpotentials. These calculations allow predictions of electronicconfigurations of the heaviest elements up to Z = 122. Forheavier elements, as well as for the midst of the 6d-elementseries, MCDF calculations are still the source of usefulinformation.Most of the molecular calculations were performed with the

use of relativistic DFT and RECP methods that turned out tobe complementary both conceptionally and quantitatively.Their combination is presently the best way to study propertiesof complex systems of the heaviest elements. DC ab initiomolecular methods are in the phase of development, and their

routine application to the heaviest systems lies still in thefuture.Using the methods described in this review, reliable

predictions of properties of the heaviest element and theircompounds became available. Theoretical calculations permit-ted establishment of important trends in spectroscopicproperties, chemical bonding, stabilities of oxidation states,ligand-field effects, complexing ability, and others in the groupsof the Periodic Table including the heaviest elements, as well asassessment of the role and magnitude of relativistic effects.Detailed studies were offered for elements Rf through 120, aswell as for some species of even heavier elements. A highaccuracy of total energy calculations allowed for predictions ofstability of species, of their geometry and energies of chemicalreactions in the gas and aqueous phases, as well as of adsorptionon surfaces of metals. However, fully relativistic descriptions ofadsorption processes on complicated or inert surfaces are stillproblematic. Therefore, some models were used in practicalapplications. Also, physicochemical models were helpful inpredicting some other properties that are difficult to handle in astraightforward way, such as, for example, extraction fromaqueous solutions or ion exchange separations. Such studieswere performed for elements Rf through Hs, Cn, Fl, andelement 113. Some estimates of adsorption enthalpies of evenheavier elements, up to Z = 120, on noble metals are alsoavailable.

Table 7. Trends in Volatility of the Heaviest Element Compounds and Their Lighter Homologs in the Chemical Groups

group species theor pred ref exp obsd ref

4 MCl4, MBr4 Hf < Rf 38 Hf < Rf 332, 3335 ML5 (L = Cl, Br) Nb < Ta < Db 331 NbCl5 ≈ DbCl5 376

DbCl5 > DbOCl3 331 DbCl5 > DbOCl3 229MBr5 → MBr6

¯ Nb > Ta > Db 364 Nb > Ta > Db 226Db > Nb > Ta 377

6 MO2Cl2 Mo > W > Sg 382 Mo > W > Sg 232, 2347 MO3Cl Tc > Re > Bh 291 Tc > Re > Bh 2358 MO4 Ru < Os > Hs 293 Os > Hs 19712 M Hg < Cn 290a, 445, 447, 450 Hg < Cn 192, 19414 M Pb ≪ Fl < Cn 290a, 452, 453, 485 Fl ≥ Cn 195

Fl ≤ Cn 260

Table 8. Trends in Hydrolysis and Complex Formation of the Heaviest Element Compounds and Their Lighter Homologs inthe Chemical Groups

group extracted complex theor pred ref exp obsd ref

4 hydrolysis of M4+ Zr > Hf > Rf 307 Zr > Hf > Rf 360MFx(H2O)

z−x8−x (x ≤ 4) Zr > Hf > Rf 307 Zr > Hf > Rf 272, 354

MF62− Rf ≥ Zr > Hf 307 Rf ≥ Zr > Hf 272, 350, 355

Zr > Hf ≫ RfMCl6

2− Zr > Hf > Rf 307 Rf > Zr > Hf 357MCl4 Rf > Hf > Zr 307 Zr > Rf > Hf 271

Zr > Hf ≈ Rf 361M(SO4)4

4− Zr > Hf ≫ Rf 310 Zr > Hf ≫ Rf 362, 3635 hydrolysis of M5+ Nb > Ta > Db 303 Nb > Ta 360

MOCl4−, MCl6

− Nb ≥ Db > Ta 304 Nb ≥ Db >Ta 277MF6

−, MBr6− Nb > Db > Ta 305 Nb > Db > Ta 277

6 hydrolysis of M6+ Mo > W > Sg 306 Mo > W > Sg 279hydrolysis of MO2(OH)2 Mo > Sg > W 306 Mo > W 279MO2F2(H2O)2 Mo > Sg > W 308 Mo > W 385, 531, 532MOF5

− Mo < W < Sg 308 Mo < W 3858 MO4(OH)2

2− Os > Hs ≫ Ru 309 Os ≥ Hs 420



21.1. Summary of Volatility Studies of the HeaviestElements and Their Compounds

Predicted trends in volatility of the heaviest elements and theircompounds compared to the experimental observations aresummarized in Table 7. One can see that almost all thepredictions for group 4 through group 8, as well as for group 12were confirmed by the experiments. In addition, the calculatedabsolute values of ΔHa

0(T) were in very good agreement with theexperimental ones, as discussed above. Thus, due to theirhighest covalency, the pure halides of Rf and Db were expectedto be more volatile than their next lighter homologs in theirrespective groups. This was clearly observed experimentally forRf in group 4 (see Figure 37). Open questions remain in theinterpretation of the volatility of group-5 pure halides, whichmight need further experimental or/and theoretical consid-erations. In groups 6 and 7, the trend to a decrease in volatilityis clear as defined by decreasing dipole moments of theoxyhalides. In group 8, hassium tetroxide should be less volatilethan the Os homolog because of its larger polarizability. Cn ismore volatile than Hg due to its high inertness. It should alsobe more volatile than Fl. While predictions of the adsorptionproperties of Cn on a Au surface were in line with experimentalobservations, predictions for Fl are awaiting further exper-imental verifications.21.2. Summary of Aqueous Chemistry Studies of theHeaviest Elements

A summary of the predicted trends in hydrolysis, complexformation, and extraction of the heaviest element complexesand their homologs as compared to the experimental results isgiven in Table 8. As one can see, most of the predictions wereconfirmed by experiments for the heaviest elements and theirhomologs, while some of them are still awaiting verification, asin the case of Sg in HF solutions.The calculations have shown that the theory of hydrolysis301

based on the relation between the cation size and charge didnot explain all the experimental behavior, such as, for example,the difference between Nb and Ta, or Mo and W. Only byperforming relativistic calculations for real chemical equilibriumin solutions can complex formation and hydrolysis constants, aswell as distribution coefficients between aqueous and organicphases (or sorption coefficients), and their order in thechemical groups be correctly predicted.Being often conducted in a close link to the experiment,

those theoretical works helped design chemical experimentsand interpret their outcome. In turn, experimental results puttheoretical predictions (and hence models used for makingthese predictions) to the test, and in this way, they help toimprove the models. The synergism between the theoreticaland experimental research in this field led to better under-standing of the chemistry of these exotic species.21.3. Future Developments

In the near future, first results concerning the chemistry ofelement 113 can be expected. Also, element 115 may comewithin reach with present day technologies, provided a constantsupply of 249Bk target material. The knowledge of lightertransactinides will grow, and new classes of compounds, such asvolatile carbonyls, open new perspectives,533 especially to firstchemical studies of elements such as Mt. For the not yetstudied elements, such as Mt, Ds, Rg, and element 115,isotopes with t1/2 suitable for chemical studies have alreadybeen identified. For elements 116 through 118, new isotopessuitable for chemical studies must first be discovered. Their

separation will also need new technological developments tocope with the very low production rates and short t1/2.In this area, theoretical chemistry will have a number of

exciting tasks to predict the experimental behavior in chemicalseparation experiments. Even though some basic properties ofthese elements have been theoretically outlined, more detailedstudies should follow, taking into account experimental details.Some further methodical developments in the relativisticquantum theory, such as, for example, fully relativistic ab initiomolecular, cluster, and solid state codes, also with inclusion ofQED effects on a SCF basis, will be needed to achieve arequired accuracy of the predicted quantities for those very highZ numbers. These future calculations will also need powerfulsupercomputers.In the long run, new accelerators delivering higher beam

intensities and even more exotic target materials, such as 250Cm,251,252Cf, and 254Es, will allow production of nuclides closer tothe line of β-stability534 in superheavy element factories.Possible electron-capture branches in the members of the α-particle decay chains may lead to the formation of longer-livednuclides of Mt, Ds, Rg, and Cn, that could be stored andstudied in traps.The theoretical and experimental investigation of trans-

actinide elements continues to be a fascinating branch ofchemistry that, by its nature, is pure fundamental research.

AUTHOR INFORMATION

Corresponding Author

*Electronic address: [email protected].

Notes

The authors declare no competing financial interest.

Biographies

Andreas Turler was born in Winterthur, Switzerland. He received hisDiploma and Ph.D. in chemistry from University of Bern, Switzerland,having carried out research on nucleon transfer reactions under thesupervision of Prof. Hans-Rudolph von Gunten. He then joined thegroup of Prof. Darleane C. Hoffman at Lawrence Berkeley NationalLaboratory, Berkeley, United States, as postdoctoral fellow. In 1992, hemoved back to Switzerland and in the following years worked as staffscientist at Paul Scherrer Institute, Villigen, with Prof. Heinz Gaggeler.In 1994 he was awarded the “Fritz-Strassmann-Preis” of theGesellschaft Deutscher Chemiker. The habilitation in 2000 atUniversity of Bern was followed with an appointment as full professorand director of the Institute of Radiochemistry at Technical Universityof Munich, Germany. In 2009 he returned to Switzerland as head ofthe Laboratory of Radiochemistry and Environmental Chemistry at



mailto:[email protected]

Paul Scherrer Institute and University of Bern. His scientific interestsare nuclear and radiochemistry in general, with one focus on thephysics and chemistry of transactinide elements.

Valeria G. Pershina was born in Cheliabinsk, Russia. She received herDiploma from Mendeleev University of Chemistry and Technology(Moscow) and her Ph.D. degree (with Professors G. Ionova and V.Spitsyn) from Institute of Physical Chemistry, USSR Academy ofSciences (Moscow). In the following years she worked as a researcher,group leader, and deputy head of the quantum chemistry lab at thesame Institute of Physical Chemistry. Since 1990, she has been at theUniversity of Kassel (with Prof. B. Fricke), and from 1996 on, at theGSI, Darmstadt, as a senior researcher. In 1994 she habilitated at theInstitute of Physical Chemistry, Russian Academy of Sciences,Moscow, and received a degree of a professor of the Russian Academyof Sciences. Her scientific interests are in general relativistic quantumchemistry, inorganic chemistry, and physical chemistry, with special-ization in the chemistry of the heavy and heaviest elements.

ACKNOWLEDGMENTS

One of the authors gratefully acknowledges the financialsupport of this work through the Swiss National ScienceFoundation Grant No. 200020_126639, Paul Scherrer Institute,and University of Bern.

ABBREVIATIONS AND QUANTITIES

4c four component vector functionA mass numberADF Amsterdam DFTAIDA automated ion-exchange separation apparatus

coupled with the detection system for α-spectroscopy

AIMP ab initio model potentialAL average levelAO atomic orbitalAR atomic radiusARCA automated rapid chemistry apparatusARTESIA a rotating target wheel for experiments with

superheavy-element isotopes at GSI usingactinides as target material

BGS Berkeley gas-filled separatorCALLISTO continuously working arrangement for cluster-

less transport of in situ produced volatile oxidesCASMCSCF complete active space multiconfiguration self-

consistent fieldCC coupled clusterCCSD(T) coupled cluster single double (and perturbative

triple) excitations

CI configuration interactionCIX cation exchangeCOLD cryo online detectorCOMPACT cryo online multidetector for physics and

chemistry of transactinidesCR covalent radiusCTS cryo thermochromatographic separatorDCB Dirac−Coulomb−BreitDe atomization energyDF Dirac−FockDFT density functional theoryDGFRS Dubna gas-filled recoil separatorΔHa

0(T) enthalpy of adsorption at standard conditions atTa or T50%

ΔHevap enthalpy of evaporationΔHS

0(298) sublimation enthalpyΔH*298(E(g)) standard enthalpy of monatomic gaseous ele-

mentsDHF Dirac−Hartree−FockDKH Douglas−Kroll−HessDS Dirac−SlaterDS-DV Dirac−Slater discrete variational methodE(x) energy of the charge transfer transitionEA electron affinityEcoh cohesive energyECP effective core potentialECR electron cyclotron resonanceEH effective Hamiltonianfb femtobarn (10−39 cm2)FC frontal chromatographyFLNR Flerov laboratory of nuclear reactionsFS Fock-spaceGANIL grand accelerateur national d’ions lourdsGARIS gas-filled recoil isotope separatorGGA generalized gradient approximationGSI Helmholtzzentrum fur Schwerionenforschung

GmbH, DarmstadtIC isothermal chromatographyHEVI heavy element volatility instrumentHITGAS high-temperature online gas chromatography

apparatusIH intermediate HamiltonianIP ionization potentialIR ionic radiusHSCC Hilbert space coupled clusterIUPAC International Union of Pure and Applied

ChemistryIUPAP International Union of Pure and Applied PhysicsIVO in situ volatilizationJAEA Japan atomic energy agencyJYFL Jyvaskylan Yliopiston Fysiikan LaitosKads adsorption constantKdes desorption constantLBNL Lawrence Berkeley National LaboratoryLDA local density approximationLSC liquid scintillation countingmb millibarn (10−27 cm2)MBPT many-body perturbation theoryMCDF multiconfiguration Dirac−FockMCSCF multiconfiguration self-consistent fieldMCT multicolumn techniqueMeV mega electronvoltMG merry-go-round



MIBK methyl isobutyl ketoneMP model potentialMSCC mixed sector coupled clusterN neutron numbernb nanobarnOL optimal levelOLGA online gas chromatography apparatusOSCAR online separation and condensation apparatuspb picobarn (10−36 cm2)PBE Perdew−Burke−ErnzerhofPIPS passivated implanted planar siliconPP pseudopotentialPSI Paul Scherrer InstituteQED Quantum Electro DynamicsQM effective charge on the central metal atomRe molecular bond lengthRECP relativistic effective core potentialRGGA relativistic generalized gradient approximationRIKEN rikagaku kenkyushoRILAC RIKEN linear acceleratorRmax maximum of the radial charge densityROMA rotating multidetector apparatusRTC recoil transfer chamberRvdW van der Waals radiusSCF self-consistent fieldSHIP separator for heavy ion productsSISAK short-lived isotopes studied by the AKUFVE-

techniqueSO spin−orbitSO ZORA spin−orbit zeroth-order regular approximationSRAFAP students running as fast as possibleT50% temperature of 50% relative yield in ICTa adsorption temperature in TCTASCA transactinide separator and chemistry apparatusTb boiling pointTC thermochromatographyTiOA triisooctylamineTm melting pointTUM Technical University MunichTWG transfermium working groupUNILAC universal linear acceleratorXIH extrapolated intermediate HamiltonianZ atomic number

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