B.A.R.G-489

su

GOVERNMENT OF INDIAATOMIC ENERGY COMMISSION

PROCEEDINGS OF THE SYMPOSIUM ONRADIATION CHEMISTRY

(TROMBAY, FEBRUARY 7, 1970)

BHABHA ATOMIC RESEARCH CENTRE

BOMBAY, INDIA

1970' 1

B . A . R . C . - 4 8 9

8QVEHHMQJT OF INDIAATOMIC ENEBJJY COMMISSIOH

PEOCEEBIKGS OF THE SYMPOSIUM OHRADIATION CHEMISTHT

(TROTBAT, FEBOTARY 7 . 1 9 7 0 )

ATOMIC HESEAHCH CENTBSBOMBAY, INDIA

1970

C O N T E N T S

Current Statue and Future Trends of Radiation Chemistry i 1E.J. Hart

Primary Processes in Radiation Chemistryi K.V.S. Rama Rao and 9P.N. Moorthy

Excitoha and Molecular Product Yields in Liquid Water - Part I i 28C» Gopinathan

Evidence for Triplet Excitons in the Radlolysis of Frozen 37Aqueous Systemsi P.N. Moorthy

On the Primary Species Involved in the Energy Transfer from 40Gamma Irradiated Alkali Halides and Other Solids to Water -Possible Involvement of Triplet State of Water: Jai ?• Mittal

Secondary Processes in Radiation Chemistry! Jai P» Mittal 41

Radiation Polymerisationi K.N. Rao, E.H. Sao and 71 jay Kumar 69

Role of Gas Phase Ionic Recombination Reactions in the 83Formation of Embryos of Condensation Nuclei t K.G. Vohra,P.V.N. Nair and K»N. Vasudevan

Radiation Chemistry of Systems of Biological Interests 100E.B. Singh

Experimental Techniques 1 R. Mahadeva Iyer 109

Radiolysis of Ferrous-Xylenol Ci'enge System in Aoidio 115Solutions! B.L. Gupta

RadiolysiB of P).utonium(in) in AoueouB Mediumi M.S, Nagar 126and P.R. Katarajan

Hadiation Chemistry of Nitric Acid Solution1 P.K. Bhatta- 131oharyya and R% Saini

Heaotion of 5ydrated iaeotrons with Pertechnetate 1 P.K. Uathuii 137and K.S. Venk&teewarlu

Electron Transfer Reaction in the Radiation Chemistry of Some 138Biologically Important Bisulphide Compounds 1 B. Lalitha and.Jai P. Mittal

Page

Effeota of Co Jf -Ray Irradiation In L-Threonlnei Ssrita V. 147Marathe and K.8. Korgaonkar

Oamma^Irradiation Studies with Synthetia Foly-L-Lyeine- 154BydrcbroBd.de Using Monolayer Technique 1 3.V. Joshi and K.S.Korgaonkar

Application of ESR for the Study of RadilJLysis of F;.*ozea 164Aquo-Oranio Systems 1 P.N. Moorthy, C Oopinathan and K.N. Sao

Energy Transfer Studies in Befisene Solutiona of Metal 168Acetylacetonatea1 Fluoreecanoe Quenching1 O.P. Kalantri andS«-B* Srlyaetara

Foreword

This i s the f i r at time that a symposium of thia type in radia-

tion chemistry ia being arranged in Trombeer. i t -sill be seen from the

programme that the emphasia i s on radic^ysr'.s of water and aqueous solu-

t ions . I t i s most appropriate that this should be ao i^x si-ireral reasonsi

(1) India- a programme of nuclear po~«»r production envisages, in the f i r s t

generation, nuclear power plants baaed on natural uranium as fuel and heavy

weter as moderator. (2) Water forms by far the largest proportion of a l l

terrestrial biological aysteos and the effect of radiation, mechanisms of

radiation injury, e t c . , are therefore of vital interest . (3) We have

amongst us today Dr. Edwin J.Hart of the Argonne National Laboratory of

the USAEG*, who i s in Trombay aa an IAEA expert in radiation chemistry.

Ed .Hart ia well known for hi a discovery and woric with tb» 'hydrated e lec-

trons • •

From the l i s t of contributors i t ia obvious that although the main

nucleus of work in radiation chemistry i s in the Chemistry Division,

researches are in progress in many other Divisions of BAHC and at the Indian

Cancer Research Centre. This i s therefore a unique opportunity to meet

here today to discuss the work of so many different groups.

This volume containing papers presented at the symposium ia also

indicative of the interests and the thinking of the research workers in

Trombay. This w i l l , I am sure* be a useful compilation to workers in the

f ie ld here as well as elsewhere in India, and will act as a window through

which thoae interested in the subject in other parts of the world can get

a brief idea of the progress made in this Centre.

* The symposium was formally inaugurated by Dr. Brahm Frakaah, Jirector,Metallurgy Group, Bhabha Atomic Research Centre, on February 7. iSffW*

• - 1 -

CURREHT STATUS AND FUTURE TREND? OF RADIATION CHEMISTRY

E . J . H a r t *

Chemistry Division,- Argonne National Laboratory, Argonne, I l l i n o i s , USA

INTRODUCTION

On studying the course of rad ia t ion chemistry in th is century

one l inds that the major developments followed the use of new

rad ia t ion sources and depended on the number of chemists involved

in t h i s area of research . Recently, not only have tht_ number of

s c i e n t i s t s and l abo ra to r i e s devoted to radia t ion chemistry increased

g r e a t l y , bat there has also been major improvements in radiat ion

sources and ana ly t i ca l instruments. My plan i s to discuss the sig-

nif icance of these t o p i c s , gaze in to a c rys ta l ba l l and predict some

future trends in r ad ia t ion research. In order to determine where we

are going, l e t us see from whence we came.

THE PAST (FIRST THROUGH SEVENTH DECADES)

What f ac to r s have played the primary role in leading to

advances in r a d i a t i o n chemistry? Table I l i s t s the s t a tu s of

rad ia t ion sources, man power and ana ly t ica l too ls decade by decade

from the f i r s t through the eighth decades. In add i t ion , the pr inc ipa l

achievements, as I see them, in these decades are also l i s ted* Thus9

in the f i r s t and second decades of the twentieth century, natural

r ad i a t ion sources were used in the very few l abo ra to r i e s devoted to

rad ia t ion chemistry. Simple ana ly t i c a l tools only were avai lab le .

The pr inc ipal achievements was the discovery that chemical effects

a t t r i b u t a b l e to these rays e x i s t . Not only does water decompose in to

hydrogen and oxygen, but also c e r t a i n chemical reac t ions are i n i t i -

a ted . TJt^_2JjiaJLeJ_J^J_i>i^Xj-2^

* IAEA expert to the Radiation Chemistry Sect ion, Chemistiy Division„Bhabha Atomic Research Centre , Trombay, Bombay-85

the most important achievement here was the beginning of chemical

dosimetry.

The fourth decade was a banner one. X-rays were more extensively

used, and while man power was s t i l l insignificant and the analytical

tools simple, the achievements were impressive * The primary chemical

effects of aqueous solutions were unveiled• The indirect action of

ionizing radiations, the oxygen, pH and concentration effects were dis-

covered and chemical dosimetric techniques were perfected. In the fifth

decade, X-rays were s t i l l largely used, man power was small and analy-

tical tools simple. The major development here, as far as basic

research is concerned, was that the free E and OH radical theory was

propounded and molecular product yields were definitely established in

irradiated water.

Cobalt-60 y~rays were estensively used in the sixth decade,

and radiation research expanded from a few specialized laboratories

to many industrial and university ones. At the same time, analyti-

cal instruments were improved. During this period, ths free radical

yields in irradiated water, were determined, relative rate constants

measured an3 mechanisms of simple reactions established. In the

seventh decade, powerful accelerators were used and the number of

chemists devoted to research increased at a great rate. Furthermore,

simple analytical instruments of the previous decade were replaced by

new sophisticated ones. With these developments, the transient

species of short life-time were directly observed. The hydrated ele-

ctron „ one of the most important of these species, was identified,

and instead of relative rate constants, absolute rate constant could

now be measured, and the capability of studying individual steps in

-3=

reaction mechanisms became realized.

The Present

'He are in the early stages of the development of a new

branch of chemistry. For lack of a better term, I shall call i t

"transient species" chemistry. In the past, unstable species last-

ing of the order of milli-secdids were studied by stopped-flow

and flash photolysis methods* Now with the advent of micro-second9

nano-second and even pico-second radiolys^s techniques, the pro-

perties of transient species within these tiiue scales may be studied0

Although flash photolysis has been available to photochemists for

some thirty years* pulse radiolysis using extremely short and sharp

electron pulses enables the study of chemical species of sub-nano-

second time duration. Thus the properties of intermediate transient

species that heretofore could only be postulated are now open to

investigation.

Paralleling this development in accelerators has been the

invention of new anlytical tools. Through advances in electronics9

oscilloscopes and phototubes with nano-second responses are now

available for analyzing spectrophotometric and conductimetric signals

generated in electron pulse irradiated solutions. In addition, improve-

ments in gas and solution chromatography, in spectrophotometry, in

electron signal analyzing and averaging devices, in esr and emr, have

provided many new effective analytical tools for the radiation chemist.,

Thus, not only have transient species been revealed to a greater extent

than ever before, but the end reaction products megr also be more com-

pletely analyzed. During this period the primary chemical species in

irradiated water, namely, H, OH and e~ , have been positively identi-

fied by spectroscopy. Many of t h e i r r a t e constants have already

been measured and one can confidently p red ic t tha t ac t iv i ty w i l l

continue in t h i s important area of research.

These significant r e s u l t s in r ad i a t ion chemistry have a lso

sparked a c t i v i t y on the p a r t of the t heo re t i ca l chemist. In p a r t i c -

ular , much ac t i v i t y has recen t ly been shown in deriving the s t r u c -

ture of solvated electrons wi th special emphasis on the ammoniated

and hydrated e lectrons . In view of the complexity of the problem, a

remarkably good s t a r t in unconvering the s t r u c t u r e s of these species

has already been made. Theoretical work, t o e , has begun on the d e t a i l s

of energy los s in i r r ad ia ted mater ia ls . At t h i s s tage , diffusion

kinet ics has been p a r t i a l l y successful in explaining the quan t i t a t ive

and qua l i t a t ive aspects of rad ia t ion chemistry. Seldom has a branch

of science been in a more favourable pos i t ion for s ign i f ican t advances

than radia t ion chemistry i s a t the present t i n e ,

THE FDTTIRE (EIGHTH DECADE)

Accelerators will dominate a l l o the r rad ia t ion sources as a

research tool in the present decade. We wi l l a lso find an ever increa-

sing number of s c i e n t i s t s involved who wi l l have great ly improved

instruments a t t h e i r commands

At the present time micro-second techniques of pulse r ad io lys i s

ara rou t ine . In the future nano-and even pico-second techniques wi l l

f a l l in to t h i 3 category. And the data , now so d i f f i c u l t and tedious

to process, wi l l be d i g i t i l i z e d and computer analyzed. We w i l l havs

greatly improved acce lera tors and new de tec t ing techniques f o r obser-

ving t r ans i en t radicals w i l l be devised. On the horizon a re the use

- 5 -

of very short-pulsed lasers and chemical accelerators.

As far as experimental research is concerned, besides the

extensive study of the chemistry of free radicals and other inter»

mediates, studies on the nature of primary reactions will be con-

tinued. In this connection, the pre-hydration reactions of the elec-

tron will be under investigation. Even the excited and super-excited

states of free radicals and •'ons will be thoroughly studiedo

Paralleling the study of -Hie primary reactions will be

studies of the secondary reactions. In the past these have involved

relatively simple inorganic and organic molecules. And while complex

molecules are now under study, relatively simple measurements are being

made on than. In the future much more comprehensive studies on irradi-

ated complex proteins md the nucleic acids will be possible. While

these studies are important in radiobiology, with nano-second chemical

reactions under investigation, more and more attention will be paid to

the very rapid reactions in the radiation spur« I t is possible that

radiation dam^e to biological materials is largely a spur effect rather

than a free radical effect. And with nano-second capability9 the

radiation chemist should be able to study reactions within the spur.

While considerable progress has already been made in rate con-

stant measurements, we are only in the beginning stages of com-

piling data for condensed phases. In the radiation chemistry of

aqueous solutions, there will be greatly increased number of measure-

ments of rate constants of the H-atoms, OH radicals and hydrated elec-

trons with inorganic and organic molecules., Further, there will be

more investigations on the effect of temperature and pressure on

radiation induced reactions.

On the theoretical side, more attention will be paid to the

distribution and nature of primary species. The structure of ele-

- 6 -

mentary species, such as the bydrated electron, poses a challenge

to the theoretical chemist . But in view of the appreciable

progress made in the recent past there should be considerably more

in the future. The availability of many more rate constants may

lead to appreciable advances in reaction kinetic theory. Even the

role of the (hydrated) electron in electron transfer reactions

may give definitive information on tunnelling mechanisms. And

the use of the computer in the theoretical study of elementary

reactions wi l l be greatly expanded and become of considerable ass i s -

tance to the er^perimenial chemist <•

The profound knowledge gained through radiation chemistry

research applies to a l l branches of chemistry. In particular, e lec-

tron transfer reactions, oxidation reduction reactions, reaction •

mechanisms and the structure of molecules will be better understood

by these extensive investigations. Hadiobiology will benefit from a

greater understanding of free radical ef fects , spur effects and a

better knowledge of general mechanism of these complex reactions.

These results also apply to the radiation processing of foods and a

concerted attack of this problem \>T/ chemists and biologists wi l l lead

to an improved understanding of the steri l izat ion reactions involved.

The photo-synthetic process, while fa ir ly well understood, will

also benefit from the results emerging from our research. In particular

the study of chlorophyll and i t s role in the fixation of carbon dioxide

may be directly studied. Besides, research on the uti l ization of solar

energy may be helped by the more complete understanding of the radiation

and photochemistry processes taking place in water*

Finally, with the importance of environmental pollution coming

to the forefront, i t i s safe to predict that radiation chemistry will

- 7 -

expand into a study into the reactions involved in the pollution of

air and water* And since a great deal of radiation research has

already been carried out on air and water, their application to the

problems of pollution i s obvious*

TABLE I

Radiation Chemistry. Past and Future

7th

8th

20th CenturyDecade

1s t

2nd

3rd

4th

5th

6th

RadiationSources

naturalnaturalX-rays

X-rays

X-rays

Coy -rays

No. PapersPublished*

12

46

112

216

240

3752

AnalyticalTools

simplesimplesimple

simple

simple

improved

accelerators 12965

accelerators 38000

Principal Achievements

elegant

elegant

Chemical effects foundChemical effects foundChemical do sin*) try establishedIndirect, Og, pH effects discovered;dosimetry in ion

Hoi. product yields

Free Rad. Yields, relative rateconstants, Mech. of reaction

Transient species, e absoluterate constants, individual steps inreaction mechanisms.

Extensive study of transients, spurreactions, complex reaction mechanisms.

* ti Ross, Radiation Chemistry Data Center, Radiation Laboratory,U. of Notre Dame, Notre Damo, Indiana.

** Extropolated.

- 9 -

PRIMARY PROCESSES IN RADIATION CHEMISTRY

K.V.S. Rama Rao and P.N. MoorthyChemistry Divi sion

Bhfibha Atomic Research Cent re , Trombay, Bombay-85

In this paper we will briefly discuss some of the most recent

theoretical and experimental investigations which have greatly helped

our understanding of the primaiy processes in radiation chemistry.

When ionising radiation passes through a condensed molecular medium,

the primary ionisation and excitation events which ultimately lead

to the production of free radicals and solvated electrons can be

illustrated as follows i un —i

•gH,

Spurs

BulkH C H eivq

CHITIQUE OF IONIC PROCESSES

Although we have ^ v e n the example of water for which precise

estimates of the primary radiolytic y ie lds of molecular and free

r ad i ca l produce are known'1 , the general picture of the primary ion-

isat ion and excitation i s true for any other molecular system. Follow-

ing , on the one hand the discovery of ionic processes at low tempera-

turea in frozen aqueous ^sterns try Weiss et al and in liquid water

-10-

by the elegant characterisation of the hydrated electron spectrum

by Hart and Boag , and on the other, the conductivity experi-

ments of Allen and Hummel ' which suggested a low free ion yield

in organic liquids, the question arose as to what are the criteria

that determine the extentof ionic processes in different molecular

systems or, in other words, how far the electron escape and con-

sequent solvation predominates over geminate ion neutralisation. In

order to define this problem experimentally, the solvated electron

yield as a function of the medium dielectric constant was determined

in this laboratory^ . The solvents investigated included water,

water + dioxane, ethanol, 2-methyl-tetrahydrofuran, cyclohexane

and benzene* Figure 1 shows that in water-dioxane system, G -

decreases sharply as the dielectric constant decreases from 35 to

2 but remains fairly constant in the range 80 to 40.

I t i s , therefore, clear that ionic processes predominate in

polar solvents and have diminished role as the medium becomes non-

polar. This is not to suggest that ionic processes have no signifi-

cance in organic non-polar liquids. I t is just that even efficient

ion scavenging solutes will find i t difficult to compete with the gemi-

nate ion recombination step and as such require much larger concentra-

tions to attain appreciable ion scavening comparable with the polar

solvents.

(BH+, a")

IBH* G

U HH + e~ , G w,o .1 t o 0 .3solute

Ionic products

The difference between the polar and non-polar cases can be

understood by considering the electron escape probability as given by

0n8ager"s equation (6),

P ( <?Q ) = exp (- r e / r )

-11-

wh.-re the escape dislance rg is given by rg

tronic charge and D the dielectric constant of the medium. Thus the

escape probability for an ion-pair depends on D and the distance r

between the components of the ion-pair. The la t ter can be taken as the

one pertaining to t ;e situation when the electron is thermalised. This

thermal is ation length does not seem to have a unique value but ranges

from r~vw/ 20A in waterv ' to -"- BOA in non-polar hexane^7. For a

dipolar medium, the static and high frequency values of D are greatly

different aid the choice of one or the other is not unambiguous. I t

is also apparent that the dielectric relaxation in the time scale of

interest to us may continuously weaken the Coulomb attraction.

(7}The early theoretical approaches by Samuel and Magee , and(9)about the same time by Platzmanv , have either excluded electron

solvation as unimpo tant even in polar water, or required very largee

thennalisation lengths of 50 - 100A for the electron to escape theCoulomb field of the positive ion and slowly undergo solvation in a

-11time scale of, say 10 sec. required for the dielectric relaxation

of water molecules. The intervening time between 10 sec for ther-

malisation and 10 sec. for solvation is simply too large to be without

chemistry. Hamill's recent dry electron hypothesis^ attempts to

bridge this gap and his concept of three entities as a model for the

radiolysis of water are siiimnarised in Table I . While this model

apparently explains the anamolous electron scavenging ability of N«O

from vitreous 5MHOSCV by t^e implicit assumption that dry e does

not react with H , the requirement that 7\~7 *2g to give meaningful

G - from entity II s=eems unequivocal if the ion-pair sees only the

hign^frequency dielectric constant and the dielectric does not relax

t i l l 10" sec. Nevertheless, the model takes a fresh look at the

possible participation of H20+ in chemical reactions other than the simple

H O++ Ho0 > H,O+ + OHand deserves further investigation. I t i s

pertinent to point out tnat Moorthy and Weissv ' have established

- 1 2 -

typical hole-solute reactions in frozen aqueous systenm and alao

suggested the existence of correlated hole-electron pairs, or ex-

citons, in these systems. Also, recently Rama Rao, Shastri and

Shankar' suggested the existence of correlated hole-electron pairs

in liquid water radiolysis.

An obvious difficulty in completely ignoring the static

dielectric constant is that Onsager's equation predicts about the

same low Gegolv ^ o r either polar or non-polar case since the ^0

varies only from 4.9 to 2. For a polar medium such as water, sinceo

the escape distance r for D = 80, is V-*7A and for D = 4«9» i t is

^-^100 A, a significant formation of persistent ion-pairs undergoing

geminate neutralisation would be erroneously predicted if only D is

applied. Thus, i t is clear that D alone leads to an untenable

result and does not explain the high hydrated electron yield in polar

lA significant development in the theory of ion-pair diffusion

in a Coulomb field is the recent paper by Mozunider . In the case of

a radiation induced perturbation of a dielectric, the appropriate time-

dependent dielectric constant, as shown by Mozumder, has the form

D"1 - D8"1 + (DQp-

1 - Ds~1) exp ( - t / T O

since the dielectric sees a constant charge as i t relaxes unlike the

conventional case of a core tant external electric field force where

D = Vg - (D8 - D ) exp ( - t / ^ ) . The time constant Y" i s t h e revised

dielectric relaxation time for the condition of constant charge and is

related to the ordinary Debye relaxation time "£" by ~T = T .o p t

Mozumder has established that the dielectric relaxes much faster when

the charge is held constant and requires only 10~ sec for water.

Thus, an apparent obstacle for the electron having to wait 10~11 aec

for solvation after a rapid thermalisation is now overcome. Using

this time dependent dielectric constant in the method of prescribed

- 1 3 -

diffusion in the solution of Smoluchowski's equation, Mozumder has

further derived, as an infinite time limit that the escape probability

is given by

2

^ 5 ) C ^ +(D;J - IT1) exp (

Comparison of this equation with the Onsager formula clearly indicates

that the relaxing dielectric behaves towards an ion-pair with respect

to escape as if i t had an effective dielectric constant driven by

In as much as this theory takes note of the none too insignificant

influence of J> , i t successfully explains the high solvated electron

yield in polar media while the tr ivial Onsager1s formula is sufficient

to understand the low free ion yield in non-polar media.

TRIPLET H20

A consequence of these notions of ion recombination is the

formation of excited molecules. As early as 1958, Magee, Burton and

Hamill^ ' had predicted that ion neutralisation should lead to a

preponderance of tr iplet excitation. While the recent pulse radio-(16)

lysis work in organic systems suggested this to be indeed the casev ,

the evidence for tr iplet water is very scarce at least until quite

recently. The earliest experimental evidence suggesting a chemical

role for tr iplet water is +he observation several months ago by

Prasad and Rama Rao ' that water undergoes photochemical decom-e

position to H? and E O,, when exposed to 2537 A light or to a lesser

extend at 3100 A. Typical experiments, especially designed to mini-

mise the photo-electric wall effects, if any, clearly suggested

genuine photolysis of water. These results would have been t r iv ia l

but for the fact that the photon energy exactly corresponds with the

triplet EgO cnetered around 4.1 eV, ae established by Christophorou's

-14-

(16)

electron impact experiments with water vapour^ . A further incent-

ive to continue these experiments was the unassigned optical absorp-(19)

tion band of low extinction in this region by Larzulv '. The natural

next step is, therefore, to establish the triplet HpO unequivocally^

Figure 3 shows the emission spectrum from water when it is excited byo o

( i ) 2700 A and ( i i ) 2400 A l igh t as recently obtained by Rama Rao and(20) *

Sapre^ . The emission spectrum peaks around 3400 A and t a i l s off .e

around 2900 A and is apparantly quenched by air. Further experiments

to establish whether or not triplet H?O is responsible for this emission

are now under active pursuit both by steady and flash illumination

techniques. Supporting experiments with ESR are also planned. Since i t

is much easier to understand triplet formation by resonance energy elec-

tron impact, pulse radiolysis of water vapour vi.ll be done in the uear

future.

While 1he foregoing results strongly suggest triplet H?0 excited

by optical means, i t is only appropriate to ask how the forbidden

transition from a singlet ground state can be induced by photons of only

requisite energy, A possibility is a low lying triplet near the ground

singlet*• Other possibilities ares (i) free electrons exist in liquid

water and under optical illumination they induce triplet H^O, (ii) the

air-water, glass-water interface has definite dipolar orientation and

releases electrons of appropriate energy into the bulk and, ( i i i ) the

excess and defect proton sites ( D and L defects) in the liquid water

are responsible for triplet excitation by optical means.

Electron Solvation

We now consider the. mechanism of hy drat ion or solvation of the

thermalized electron - another aspect of the theory of radiation

chemistry which has been the object of great theoretical attention.

The classical model of electron solvation i s essentially the one based

on the 'polaron1 model developed by Davidov, Diegen and Pekar and more

- 1 5 -

recently by Jortner. According to this model, the presence of the

(thermal) electron in the (polar) medium causes the reorientation

(or relaxation) of the otherwise randomly oriented dipoles around

the electron in such a manner that the positive ends of the dipoles

point towards the electron and the negative ends away from i t , che

whole process taking about 10~ sec. (in the case of water), the

relaxation time for dipolar orientation. In terms of energies, this

reorientation is equivalent to an orientations! polarisation expre-

ssible as a function of distance r from 1he electron by the formulas

ep

o .-«• r 2 Dop

where e is the electronic charge and D and D are respectively

the static and optical dielectric constants of the medium, the latter

being given by the square of the refractive index. This polarisation

creates a trapping potential y>o for the electron given by

The potential aiergy of the electron in thiB potential field is then

2e 2 f 1 _ - _ 1 , . _ 1" r L % \ -i

As in the classical H-atom problem, the energies for the bound states

of the electron in this potential field are given byt

where n can have the values 1,2,3 and n is the rest mass of the

electron. The trnasition of the electron from the n « 1 to the n - 2

state iseosidered. to be responsible for i ts optical absorption spectrum,

so that the position of the maximum is given by

-16-

Moorthy and Shanksr ^ ' have recently reconsidered both the kinetic

as well as the energetic aspects of the solvated electron on the basis

of the poiaron model. In what follows we adopt their approach.

One can look at the kinetic aspect in two ways. The mean dis-

placement ( X * J °f tke thermal electron during the relaxation time

of the matrix dipoles i s given by Einstein's formula for Brownian

motion 8

where k i s the Boltzman constant, T the temperature (*Z) of the

medium, °) i t s viscosity and r the radius of 1he electron, which can

be computed to be 2.8 x 10 cm on the basis of the assumption that

the mass of the electron is entirely of electronuagnetic origin. For

water at T = 298eK, "I - 0.894 x 10"2 dyne sec. cm2 and "£> 10~11

s e c , so that (Xnf turns out to be 420 A, a distance spanning about

170 water molecules. In the alternative approach one can equate

kinetic energy of tie thermal electron with the average kinetic energy

of the medium molecules: •§• me*- = ~ kT so that the average d is -

placement of the electron during the period of dipolar orientation is

given by (c5-) X = (^h"r/'r^) T &*& t n i s works out to be

11630 A for water at 298°K. Thu3, by the time the matrix dipoles have

oriented around the 'electron1 the latter has moved away from i t s

original ate several hundred molecular distances away. The situa-

tion worsens as one goes to lower temperatures so much so that in

frozen systems, for which the dipolar relaxation times are virtually

infinity the thermal electron would have drifted out of 1he medium

altogether. On the basis of either of the above formulae, one can

compute that, in order to confine the electron during the period of

dipolar orientation within one molecular distance from the origin, the

relaxation time muat be much smaller, v i z . 10 - 10 sec. This time

i s much too short for molecular motion, but only electronic displacemats

-17-

in the moclecules are possible. This implies that the polarisation

which can bind a thermal electron in a potential well is the elec-

tronic (or optical) polarisation and not the orientational polari-

sation as conceived in the original polaron theory.

Another approach is to consider the energetics of the polaron

on the basis of both electronic and orientational polarisations, as

well as total polarisation. This has led to even more interesting

results. Without going into details, which are similar to those of the

classical polaron baaed on orientational polarisation only, the opti-

cal transition ensrgy on the basis of electronic polarisation and total

polarisation would be given bys

(4E) - 10

(^E) t = 10

(^E) and (

.21

.21

<iE

[•-

[•

1

"op

1Bs

i) t co

•]

-]impIn Table I I , (^E),. and (AE) , (^E). computed for a number of

similar polar systems are compared with the experimental values, from

which it is seen that -the ( 1E) 'a agree with experimental values atleast as well as, if net better than, ihe (^lE)e's whereas the (i lE) t ' s

are far too high. For water and ice fcr which static dielectric con-

stant and refractive index data are available over a wide range of tem-

perature, Moorthy and Shankar have also calculated the (^E) 's and

(<£E) 'S at different temperatures in order to compare with the experi-

mentally observed variation of ^ E with temperature which is now available

for both these phases from pulse radiolysis studies. This comparison 1B

shown in Figs. 3 & 4. One can see from these figures that the optical

transition energy of the 'hydrated1 electron calculated on the basis of

electronic polarisation only varies with temperature in the same direction

as the experimertal values, the slopes being -1.1 x 10~ eV/eC and

-2.9 x 10~5 eV/°C respectively in water and -0.3 x 10~5 e¥/°C and

-18-

-0.93 x 10 eV/°C respectively in ice, whereas (^E)e exhibits an

opposite trend (slope «= %-~ 0-49 x 10 eV/°C in water-, positive,

but widely varying over the temperature range studied, in ice) .

The consideration of both the kinetics as well as the energe-

tics ©f the solvated electron ca the basis of the 'polaron1 model

thus leads to the conclusion that only the electronic polarisa-

tion, and not the orientational polarisation as used to be believed

t i l l now, i s of significance in the process of 'solvation1 of the

electron and this conclusion i s of even greater importance in the case

of non-polar media and frozen systems, wherein also there i s evidence

for the formation of solvated electrons. Thus, in non-polar media,

there are no permanent molecular dipoles, whereas in frozen systems

the relaxation times are so very long that the dipoles are, so to say,

frozen in and cannot orient at all around an election introduced into

the mediums Hence, the experimentally observed bound states must

arise through electron trapping in potential wells created by elec-

tronic polarisation only.

Recently, Jortner has carried out calculations based on a

self-consistent field polaron model in which the additional electron and

the medium electrons are treated on an equal basis. In th limit of

zero cavity radius, this model predicts an optical transition energy of

1.35 eV for the hydrated electron at 25°C, whereas the experimental

value is 1.72 eV, and the agreement at larger cavity radii i s poorer

s t i l l .

A physical picture -of the hydrated electron has been given by

Hatoari and Watanabe ' in their tetrahedral model. This i s based

on .the fact that liquid water has micro crystalline regions where the

tetrahedral structure of ice is retained, a central water molecule

linked by hydrogen bonds to four water molecules at the corners of a

regular tetrahedron (Pig.5). Waen an electron enters a defect site

-19-

wherein the water molecule in the centrer of the tetrahedron is

missing, the electron causes orientation of two of the corner

water mole cute in such a way that the positive (H) ends (one of

each) of these water dipoles also point towards the electron. The

electron finds itself trapped in this state. The optical transition

energy for such a species has been calculated to be 0.8 eV whereas

the experimental value is 1.72 eV, at 25°C.

In a more recent calculation Natoari takes the distance

(\) from the electron at the center of the tetrahedron to the oxygen

atom at the corner as a variable parameter and also considers the

possibility that the presence of the electron instead of the water

molecule at the centre of the tetrahedron would lengthen the O-H

bonds of the corner molecules because of the higher effective charge

on the e (-e ) as compared to that on the 0 atom in the H_0 molecule

(-0.652 e). They find the best agreement (calculated and experimental

£ E values about the same) with 0-H bond length of 1.1 A, H-O-H angle

of 116° and 1 =2.62 A. Temperature dependence o f ^ E has not been

worked out on this model, but could be attributed to possible expansion

of the tetrahedron with increasing temperature as Natoari finds that E

decreases with increasing \ . This structural model would also suffer

from the same draw-back as the classical polaron model, viz., that by

the time the water molecules have oriented, the electron would have moved

considerable distance away, unless the attractive potential between the

electron and two of the corner water molecules which, even prior to the

introduction cf the electron, are oriented with their positive ends

pointing to the center of the tetrahedron, is sufficient to localise

the electron, the orientation of the water molecules at the other two

corners taking place only subsequently.

EEFERENCES

1. A.O. A l l e n ; Bad. Research Suppl . 4 , 54 (1964)

2« L. Kevan, P.N. Moorthy and J . J . Weiss; J . Am. Chem. Soc. 86,771 (1964)

-20-

2b. P.S. Rao, J.H. Hash, J .P . Guarino, M.R. Rouayne and W.H. Hamill;J . Am. Chem. Soc. 84,500 (1962) of also. W.H. Handll in "Radicalions" ed. by E.T. Kaiser and L. Kevan, Interscience (1968);J.E. Willard in "Fundamental processes in radiat ion chemist-Ly"Ed. by P. Ausloos, Interscience (19^8)

3. E.J. Hart and J.W. Boag; J . Am. Chem. Soy. 84, 4090 (1962)

4. A. Hummel and A.0. Allen; J . Chem. Phys. 44., 3426 (1966)

5. K.V.S. Rama Rao and A.V. Sapre (Paper under preparation);K.V.S. Rama Rao and A.V. Sapre; Proc. Chem. Symp., Chandigarh (1969)

6. L. Onsager; Phys. Rev. 9_2, 1152 (1953)

7. A.H. Samuel and J . I . Kagee; J . Chem. Phys. 21., 1080 (1953)

8. A. Mozumdsr and J.L. Magee; J . Chem. Phys. 42., 939 (1967)

9> R.L. Platzman, "Basic mechanisms in radiobiology", U.S. Nat. Ac ad.Sci. Pub. No.505, p.34 (1953)

10. W.H. Hamill; J . Chem. Phys. 49» 2446 (1968),- J , Phys. Chem. 7_3_, 1341(1969)

11. P.S. Dainton and P.T. Jones; Trans. Faraday Soc. 6i_, 1681 (19^5)

12. P.K. Moorthy and J . J . Weiss; Adv. Chem. Series 50,185' (1965)

13* K.V.S. Rama Rao, L.V. Shastr i and J . Shankar; Radiation Effects2, 193 (1970)

14* A. Mozumder; J . Chem. Phys. 50_, 3153 (1969)

15« M. fa r ton , J.L. Magee and W.H. Hamill; in "Peaceful Uses of AtomicEnergy" 2nd In t . Conf. Geneva (1958)

16. G.R.A. Johnson and M. C Sauer J r . ; J . Chem. Phys. ^1_, 496 (1969)

17« K.V.S. Rama Rao and D. Prasad (Unpublished resu l t s )

18. R.N. Compton, R.H. Hueber, P.W. Reinhardt and L.G. ChriBtopnorou;J . Chem. Phys. 48, 901 (1968)

19* H. Larzul, P. (Jelebart and A- Johannin - Gi l l e s ; Compt. Rend. 2614701 (1965)

20. K.V.S. Rama Rao and A.V. Sapre (Unpublished work)

21. P.N. Moorthy and J . Shankar; Radiation Effects £ , 91 (1970)

22. H. Natoari and T. Watanabe; J . Phys. Soc. Japan 21_, 1573 (1966)

-21-

Questions by Dr. H.J. Arnikar

1» In discussing the time-variations of dielectric constant of

water two conditions were considered: i ) condition of con-

stant field, and i i ) condition of constant charge. The latter

condition of constant charge may be explained„ What exactly is

considered constant-only the number of electrons and ions as a

whole, or their time and spatial distribution as well?

2o In mixed solvents as dioxan and water, are 2 or 3 3pecies of

solvated electron (e . e". e~ , , . ) envisaged?v water, dioxan, water + dioxany =-&•=«•

A single G value for 'solvated1 electron was plotted for each com-

position of the medium. Was this G value the sum of all the species

present?

3« On the cavity model, each e i s envisaged as being surrounded byaq

about 4 water molecules each oriented with positive dipole pointing

towards the centre of the cavity. How is this model compatible with

the high rate constant for the reaction.

2 eaq - * 20H" + H2

which requires the e~ to come out of the shielded cavity?

4* In one of the papers the reaction

H_0 + OH~ —*. Water

was considered of less importances How is this compatible with the

well known very high value of equilibrium' constant for the above

reaction?

Answers to questions by H.J. Arnikar

by K.V.S. Rama Rao & P.N. Meorthy.

1.' The condition of constant charge implies the constancy only of the

number of electrons and positive ions. The time and spatial d is t r i -

- 2 2 -

bution of the electrons and positive ions follows from considera-

tions which among otfter things include the concept of time dependent

dielectric constant as appropriate to the case of constant charge.

2. Only one species, viz. e"* , . , (with the solvent s ^vater + dicxan

of varying composition) is envisaged, and i t is only this that is

plotted in the figure.

3. In the 'polaron1 or 'cavity' model the bi-electron state, viz,

e* also turns out to be a bound state, and i t is believed that

ths reaction under question passes through e as an intermediate

state t

Answer to question

by C. Gopinathan

4

2 eaq —-> e —*

^ aq

Arnikar

2 + 20H

This question probably refers to the 3 possible ways of exciton

formation during gamma radiolysis which I had considered. I had

thought that this reaction was unlikely to give rise to an exciton

since the energy liberated would not be sufficient to raise HO even

to the lowest triplet level* However, I did not say this reaction

does not occur. In fact, this is probably how the returning electrons

get neutralised, e.g.

+ + OH) + e~ —> (H50+ + 0H°)

H20

Entity

Entity

-23-

TABEE I (From ref

i i ZTH5oH

H,

I I I /~H,0H

k , OH, e "

H , OH

* , OH, e

1

.10)

7

, OH

Diffuses

Entity I I I . £H 0* , OH , e" J

H20, H, OH,

TABLE I I . (Prom r e f . 21)

Compound I

Ethylene Glycol 1.43 58.7 2.19 2.67

Methanol 1.33 33.6 2.93 1.93

Hthanol 1.36 25.1 2.56 2.15

n-Propanol 1.38 "0.8 2.32 2.5O

Ieo-Propanol 1.38 19 2.28 2.30

Water 1.33 80 3»12 1.93

Ammonia 1.33 22 2.76 1.93

eV e7 eV eV

9.699.61

9.419.259.16

9.959.30

2.141.97

1.771.68

1.51

1.72

O.84

-24-

Figures

1. Dependence of G( - ) on the static dielectric constant £ insolv

the radiolysis of water-dioxan mixtures. (From ref.5)

2. Luminescence spectrum of water irradiated in the 270 mk. band (Frqm Ref.20)

3. Variation of the optical transition energy of the solvated electron

in water with temperature (From ref.21)

4. Variation of the optical transition energy of the 'solvated electron'

in ice with temperature (From ref»2i).

5> 'Structural model' of the solvated electron (From ref.22)

-25-

FIG.1

Luminescence of water

(I) In vpcuum excited at 275 nm(II) In vacuum excited at 240 nm(0) In air excited at 275 nm

\minco SPF 8202 BRange 0 03, Sen 40

- 2 6 -

600 640- -600

-400 240-

Zi 40Temperature it)

PIG.3

2200-

60 60

-18000

-200 -160 -120 -80Temperatune(t:)

-40

PIG.4

-27-

Normal structrure Defect structure

Hydrated electron

FIG.5

-28-

EXGITOFS ATT) MOLECULAR PRODUCT YIELDS IF LIQUID "'ATER - PART I

C. Gopinathan

The deficiencies of the diffusion kinetics :aodc-l 'rur.ve been

excellently reviewed recently and hence, need not be elaborated Ixere.

The present work is an attempt to explain molecular product yields and

solute effects in liquid v/ater through a mechanism not mainly based on

diffusion of intermediates. A preliminary description of the present(2)

model has already been presentedv

An excxton can be broadly defined as a mobile quantum of

excitation energy in an ordered matrix . It is probably because of

the requirement of an ordered matrix that the exciton has generally

been ignored in liquid water. However, a considerable amount of the

strongly hydrogen-bonded ice structure is known to persist in liquid

water at room temperature and a number of models of liquid water have(A)

made use of this fact. In the present work, Nemethy and Sheraga's

extention of Frank and Wen's 'flickering cluster1 model has been used

to deduce quantitatively the molecular product yields.

In the above model, liquid water consists essentially of

clusters of hydrogen-bonded molecules, probably with a t.ridyraite like

structure immersed in a cloud of non-hydrogen bonded water. The

average number of water molecules per cluster is 57 at 20°C. The

life-time of clusters is about 10~ to 10~ sec. A cluster as a

v/hole dissolves rather than individual molecules breaking off from

i t . The core of a cluster consists of quatruply and t r ip ly bonded

(both regular and hydrogen bonds) water molecules. On the surface

doubly bonded molecules will be present. Also, on the surface of the

cluster singly bonded species will be present as extensions of the

cluster.

In the present work, an exciton is expected to be created

in the fast proton transfer reaction

-29-

0 +H20+ + H2° ^ ~ * H3

0 + + °H • • • (1)

An amount of energy, equal to the sum of the ionisation potential of

water plus the proton affinity for water and minus the energy necessary

to remove a proton from H20 will be available as a result of this

reaction. In fact, this is one of the fastest known protcn transfer

reactions , needing only about 10~ see. S/hile there may be some

doubt about how fast this reaction takes place in liquid water, i t will

certainly take place very much faster than relaxation in liquid water.

While this reaction is taking place, the cluster structure will be

stable, resulting in the energy of the reaction being transferred to

the lattice in a Tianner similar to the Mossbauer effect. According to

Frenknel , an exciton can be described in terms of a wave function

which can have N possible values, where N is the number of identical

units in the l a t t i ce . Or in other words, the exciton propagates

through the lat t ice like a wave.

In the present model, excitons react in pairs to give the

molecular products H_0 and H?.

.**exciton + exciton » (SLO + TT o) > K + iLO,.

This is expected to take place only in the clusters as the require-

ment of an ordered matrix is satisfied only ir. a elusher. Even in a

cluster, it would be logical to discard the singly hydrogen bonded

molecules on the surface, as they are not a part of the lattice and

are too different from the regular members of the cluster. An exciton

generated in a cluster moves around the lattice randomly until it en-

counters another exciton. The quantum mechanical movement of an

exciton is likely to be very fast, but the encounter of two excitons

would be like the meeting of two wave crests on a stretched string,

resulting in a combined and enlarged crest. The resultant localisa-

tion of energy is sufficient to break two 0-H bonds.

This process can be described in ano+her way also. If the

two excitons can be imagined as two wave fronts moving towards each

other, then obviously, the interaction of these two waves, even before

-30 -

they meet would produce higher excitational levels in the molecules in

between. The net effect i s a localisation of energy.

If one considers the probable tridymite structure of the

clusters, i t is clear that the breaking of two 0-H bonds in neigh-

bouring molecules will leave two hydroxyl radicals in close proximity.

Bond formation between the oxygen atoms will then give hydrogen peroxide.

However, 1iie hydrogen atoms are farther apart and the hydrogen molecule

is probably formed only when the cluster dissolves.

Obviously, exciton reactionscannot be dealt with in terms of

diffusion kinetics, since the movement takes place along specific path-

ways . The mathematical concept of the random walk is more appropriate

for this situation. The random walk of a number of species in a space

lattice is an extremely complicated problem. However, in the present

case there are two simplifying factors: (i) The lattice is f ini te , and

(ii) whenever two excitons meet, reaction takes place. Since in a

small and finite lattice two fast moving speciee are bound to meet each

other within a short and finite time, the approximation can be made

that all excitons created inside a cluster undergo bimolecular reaction.

This enables a quantitative evaluation of molecular product

yields according to this model and comparison with experimental values.

It is inevitable that some value for W in liquid water for gamma

radiation has to be chosen. A value of 30 ev has been chosen for the

succeeding calculation. Actually, the product yields obtained are

not very sensitive to small changes in this figure.

Taking values from Hemethy and She rags !s work, the mole

fraction of quadruply, triply and doubly hydrogen bonded molecules

taking part in cluster formation is 0.48 at 20°C. The energy depo-

sited in this fraction will therefore be 48 ev for every 100 ev of

actual energy deposited. The ion pairs produced in clusters will

therefore be 1.6 for every 100 ev. Assuming that each of these gives

one exciton through the proton transfer reaction and all of them

undergo bimolecular combination followed by ILO formation, we get

- 3 1 -

G_ n = 0.8, very close to the accepted value of 0.7.*2°2

About the G value for the solvated electron, two values have

been quoted, 2.8 and 2.3, of which the former value seems to be more(7)

authoritative , Since the ionisation process leading to solvated

electron formation cannot distinguish molecules as belonging to a clu-

ster or otherwise, the G value of solvated electrons produced in clu-

sters will be 2.8 x 0.48 = 1.34. This value is appreciably less than

the previously calculatedvalue for the ionisation events in clusters.

This ie logical since some of the electrons are bound to be attracted

and neutralised. In the present model, this neutralisation does not

produce free radicals, since the energy corresponding to the ionisation

potential has alitsau^ Wen dissipated as an exciton. The neutralisa-

tion, therefore, gives two water molecules rather than free radicals.

+ OH] + e~ y 2^0 (2)

This process may take place Ihrough the fast reaction

HjO* + OH" v 2^0 (3)

the returning electron neutralising OH, rather than BLO . This re-

action has in fact been suggested recently . The net result is that

the OH radical produced in the fast proton transfer reaction is destro-

yed whenever the electron returns.

Now a quantitative estimate of the OH radical yield can be

made. The number of returning electrons is 1.6 - 1.34 = 0.26. There-

fore, the total number of OH radicals left in the clusters is 1.34.

Since the H atoms formed in the bimolecular exciton reaction combine

only when the cluster dissolves, some of them are bound to escape and

destroy a portion of the OH radicals. This fraction can be calculated

from the lower yield of Gw as compared with Gw n . The total H atoms"2 *2 2

generated as per the present model is 1.6. G_ ie 0.45. Therefore,

uncombined H atoms are = 1.6-0.9 = 0.7. Subtracting this from 1.34,

the OH radical yield in clusters is 0.64. To this must be added -tb<*

- 3 2 -

OH radicals formed outside c l u s t e r s . This value wil l be equal to

1 # 54 x 2*S|. = 1,45.. 1.45 + 0.64 = 2.09, very close to the accepted0.48

value of 2.2 for GntI. I t i s believed t ha t the OH radica ls do notOH

combine with each other sufficiently to contribute to G~ Q , for the

following reasons: ( i ) The OH radicals are formed within clusters

and outside c lus ters , ( i i ) In c l u s t e r s , 1fae OH radicals remain

hydrogen bonded to the l a t t i c e unt i l the cluster as a whole dissolves,

( i i i ) In the non-hydrogen bonded water, the high degree of close

packing of the H?0 molecules prevents rapid diffusion. When one c lus ter

dissolves and a new one i s formed out of previously non-bonded water,

the resulting s t ructural disruption leaves the OH radicals too widely

separated to part icipate in "spur" react ions .

The same i s true of solvated e lect rons . These are formed

by those electrons which have escaped the e lec t ros ta t ic a t t rac t ion of

the parent and are a considerable distance away (Here ELatzmann's view

point i s probably more appropriate) . Whatever non-uniformity i s there

in e dis tr ibut ion would mostly be along the track of the ionising

part ic le rather than in "spurs". This s l igh t non-uniformity would

account for any small i n i t i a l decrease in e

aq

With a value of 2.8 for G - , there might appear to be a

lack of mass balance as has been pointed out by Allen . However,

th i s deficiency would a r i s e only if a separate yield of H atoms i s

assumed. According to the present model, there are two p o s s i b i l i t i e s .

There might be a genuine yield of H atoms because a l l of them might

not have reacted with OH radica ls . The second poss ib i l i ty i s that

there may not be any primary H atoms, the so cal led H atom reactions

being due t o , as has been suggested, to 'excited1 water. In the

present model, the 'exci ted ' water would correspond to excitons pro-

duced in the non-hydrogen bonded portion of the water.

At the moment the second a l te rna t ive i s preferred. However,

the true position might l ive somewhere between the two extremes. In

fac t , the G value of free H atoms according to t h i s model i s 0.7, only

s l igh t ly higher than the accepted value of 0.6. Actually, the model

-33 -

can allow for a s l ight increase of G .

OH

The implici t assumption in the above arguments is that no

molecular products and no hydrogen atoms are produced in the non-

hydrogen bonded water. With the excitan. mechanism outlined above and

with low LET radiat ion this i s bound to be the case . With a disordered

structure and weak interaction between the molecules, neither the

Prenknel approximation, nor the Wannier approximation can be applied.

Whatever excitons are producod are l ikely to be of very short range,

essent ia l ly located on one water molecule. Most of them probably result

i n bond rupture giving; r ise to a H.OH pair, which quickly undergoes

geminate recombination and phonon production. This picture, of course,

wi l l no longer be true for high LET radiation. There is a poss ib i l i ty

tha t even in the non-cluster water, when a dense concentration of

excitons is produced, occasional bimolecular combination can occur,

leading to the observed increase in molecular product yields.

So f a r , only the excitons produced as consequence of ion i -

sation events have been taken in to account. But according to the

mechanics of energy depositions, many simple excited molecules should

be produced. Why can' t these give r ise to excitons, especially since

the Prenknel approximation has been used? I t i s believed that while

these may give r i s e to excitons with a wide range of energy d i s t r i -

bution, they do not give r i se to radical or molecular products. The

reason might be ( i ) they do not transfer t he i r energy to the l a t t i c e ,

( i i ) the resul tant excitons do not have a vibrational components.

In the case of the proton t ransfer exciton, the energy which should

have been carr ied away by the products H_0 and OH i s transmitted to

the l a t t i c e because these species remain strongly hydrogen bonded to

the l a t t i c e . The resultant exci tors , probably have both an electronic

and vibrational component. They can, therefore, readily give r i s e to

molecular products. The purely electronic excitat ions may not be

capable of giving r ise to molecular products. As a matter of f ac t ,

the quantitat ive treatment of many aqueous radiat ion chemical r eac t -

ions often ignores the primary radiation produced excited s t a t e s .

-34-

The explanation for their success, in spite of this obvious omission,

lies in the arguments given above.

Another point which has been omitted so far is the question

whether the excltons are in the singlet or the triplet s tate . For the

purposes of this present model this does not matter, since both singlet-

singlet and triplet-triplet reactions are possible. Nor does the fact

that the first excited state might be dissociative matter in the case of

a propagating wave. On the grounds of energy considerations alone t r i -

plets might be favoured, as the energy level of the lowest excited t r i -

plet might be lower than the first excited singlet. However, the t r i -

plets would s t i l l have to be formed through the proton transfer step.

Solutes reduce molecular products by interfering with exciton

+ exciton reactions. The presence of a solute molecule inside a cluster

will provide a centre for the localisation of exoitons, resulting in the

bimolecular reaction taking place in the close proximity of a solute

molecule. The net result will depend upon the nature of the solute.

The presence of species like Cl or Br which can react with OH will

interfere with H-0 production by removing a OH radical from the loca-

lised excitation through converting i t to OH . H? production, however,

will not be prevented.. The opposite is the case with oxidising species.

In this case selective H atom removal can take place. Interference with

molecular product production can also occur through charge separation

in the excitons after the trapping of the excitons, and the scavenging

of one charge. Both the mechanisms are possible since the exciton has

both electronic and vibrational components. The detailed kinetics of

solute effects on molecular products will be dealt with in a later

publication. For the present i t is obvious that the solutes interfere

with a physical process and not with diffusion controlled reactions.

This would qualitatively explain why even high concentrations of

solutes are unable to eliminate molecular products completely. The

present model also predicts the existence of solutes which will lower

the yields of both the molecular products.

-35 -

TESTS FOR THE MODEL

The following experiments are suggested for verification of

the premises on which the above model i s founded.

( i ) Oxygen Effects

Op has an ionisation potential of 12.3 ev and a f i r s t exci-

t a t ion potential of 4.9 ev. The s imi lar i ty with HpO i s obvious, nn-

fortunately, however, 0_ reacts readi ly with H atoms and solvated e l e -

c t rons , and 2H0 or 20 gives HO + 0 . Accurate measurements of

G(H_O ) at high ( >/ .1M) concentrations of 0 might give useful

information.

( i i ) Lowering of Molecular Product Yields Through the Use of Selective,Excitable Organic Compounds

There are a number of organic compounds, especially aromatics,

which have a number of excited s t a t e s . These can, therefore, be expected

to interfere with molecular product production through exciton quenching.

The diff icul ty , as in the above case i s , that these compounds react

readi ly with H and OH a lso . In sp i te of this i t might be possible to

use compounds l ike phenol which react only very slowly with solvated

elec t rons , under neutral or high pH conditions to t e s t whether the

reduction in molecular product yields agrees with the present model or

the diffusion model „

( i i i ) Trapping the Radiation Produced Exciton t o Produce FluorescentRadiation

There i s no evidence for fluorescence of water under gamma

i r rad ia t ion . The reason for t h i s i s that the excitons combine bimole-

cular ly in c lus te rs and if excitons are produced a t a l l in free water,

they eventually get converted in to phonons. I t should, however, be

possible to in ter fere with both these processes with ions or molecules

having suitable excitat ion levels and which can give fluorescence.

Oxygen would be an ideal case. This experiment would in fact be com-

plimentary to ( i ) i f a t a concentration of ^ 0.1M02, fluorescence

can be observed under i r rad ia t ion , then i t can be inferred that the

- 3 6 -

exci tons are being d i r e c t l y quenched by t h e 0 molecules in s t ead of

through r a d i c a l a b s t r a c t i o n . A number of paramagnetic inorganic ions

may a l s o be s u i t a b l e .

REFERENCES

1. A.A. Schwarz; Proceedings of the F i f t h Informal Conference on theRadia t ion Chemistry of Water, Notre Dame, p.51 (1966)

2 . Chakrapany Gopinathan; Proceedings of the DAE Chemistry Symposium,Chandigarh (1969)

3 . D . I . Dexter and R.S . Khox; ' E x c i t o n s 1 , In te rsc ience Publ i shers(1965)

4 . G. Nemethy and H.A. Scheraga; J.Chem.Phys. 36_, 3382 (1962)

5 . F.W. laiaple, F.H. F i e ld and J . I . F r a n k l i n ; J.Am.Chem.Soc.79, 6132 (1957)

6. J . Frenknel ; Phys.Rev. 37, 1276 (1931 )

7. A.0. Allen, 'Proceedings of the F i f t h Informal Conference on theRadiat ion Chemistry of Water, Notre Dame, p.6 (1966)

8-. J . C . Russel and G.R. Freeman; J . Chem.Phys. 43 , 90 (1968)

-37-

EVIDBNCE FOR. TRIPLET BXCITONS IN THE RADIOLYSIS OPFROZEN AQUEOUS SYSTEMS

P.N. Moorthy

I would l i k e to mention about some evidence fo r the t r i p l e t

exciton in the radiolysis of frozen systems. In Fig . i (a) i s shown the

ESR spectrum of gamma-irradiated O.5M H SO. where the two outermost

l ines consti tute the H-atom doublet and the central single l i ne , i s due

to the SO. radical ion, these two species being formed by reaction of

the primary species viz e and holes (h ) with the solute according t o :

e + HSO, —»• H + SO,4 4

h+ + HSO" —> H+ + SO"4 4

The rest of the spectrum i s due to the OH radical also present in

gamma-irradiated pure ice i t s e l f . When 0o5M HpSO. containing O»2M

CoSO or 0.2M NiSO was i r radiated at 77°K, the ESR spectra shown in

Figure i(b) and (c) were obtained. In both cases both the H-atcm

doublet and the SO. singlet are seen to be very much reduced in inten-+2 +2

sity. The effect of Co and Ni on the H-atom signal is not surpri-

sing since these ions are known to be good electron scavengers in

aqueous solution at room temperature. What is puzzling is the effect

on the SO." signal. Being electron scavengers, these ions cannot be

expected to be good hole scavengers. Also, if they acted as either

electron or hole scavenger, one should have been able to see the

corresponding ESR spectra, whereas the observed spectra show no

evidence of this. From the known quenching effect of paramagnetic

ions on triplet excited states, our interpretation of the observed

result is that the primary species are triplet excitons which, in the

presence of the para magnetic transition metal ions, get quenched,

whereas, in their absence, H atoms and SO ~ radical ions would have

been formed by reaction of the HSO ~ respectively with the electron

-38-

and hole ends of the triplet ezciton:

HSO." + e~ ... h4 H' + SO.

Co+2 or Ni+2

Quenched

- 3 9 -

0-5 M . H2SO4

(b)

0-5 M HaSO4, 02 M Ni S04

(O

H

05 M , 02 M CoSO4

FIG.1.

ON THE PRIMARY SPECIES INVOLVED IN THE ENERGY TBANSFER FROM GAMMA IRRADIA-TED ALKALI HALIDES ABD OTHER SOLIDS TO WATER 'T POSSIBLE

IHV0L7EMENT OP TRIPLET STATE OF WATER

J a i P . Mit ta lChemistry Division

Bhabha Atomic Research Centre, Trombay, Bombay-85

When gamma i r radia ted a l k a l i hal ides and some other organic sol ids

are dissolved in water, s tored energy i s t ransfer red to the solvent . Ea r l i e r

resul ts^ ' of l i gh t emission from ths above process have been ve r i f i ed and

extendedo I t i s argued tha t a t r i p l e t s t a t e of water i s populated v ia low

energy e l ec t rons , l iberated on dissolut ion of gamma-irradiated a l k a l i halides

in water. The emitted l igh t has been iden t i f i ed as coming from T ^ S1 o

transition in water. Emission from various acceptors also point towards the

involvement of triplet state. Ini t i ta l results from some kDown triplet induced

reactions such as cis-trans isomerisation, e t c . , also indicate ths presence of

triplet state of water.

These observations seem to be consistent with the recent observations oj

a triplet state of water by Hamill^ ' and Compton et aL* • A stable tr iplet

stsite of water will be of great interest in radiation chemical studies.

1. G. Ahn&trom et al . j Acta. Chem, Scand. 21, 855 (1959)3 ibid, ^ 300 (1965)

T. Westerrark et al.$ Arkiv Fern, 12, 139 (I96i)i Nature 188, 395 (i960)

2. D. Lewis and W.H. Harnill; J . Chem. Phys. _5J_, 456 (1969)

3« R.N. Compton et a l . ; J. Chem. Phys. 48, 901 (1968)

- 4 1 -

SECQHDAEY PTiOCESSES IN RADIATION CHEMISTRY

Jai P. MittalBhabha Atomic Research Centre

Chemistry DivisionTrombay, Bombay 85

The preceeding papers have dealt with the primary processes

involved in radiation chemical studies. Prom Table 1 i t will be seen

that on the time scale of events involved we have so far covered only

the very early periods. In the present paper we would like to learn

about the later processes which eventually lead to net chemical effects.

By virtue of their late coming on the time scale, these processes are

usually grouped together under the t i t l e of secondary processes. The

primary ionic and excited species produced in the first step take part

in several processes immediately following their formation. In

Table 2 are listed some of the more important of these processes. We

would review some of these in some details giving a few examples from

our own recent work.

I . POSITIVE IONS

Eyring, Hirschfelder and Taylor have summarized the pro-

cesses in which positive ions take part immediately after ionization

as follows : -

(i) Clustering - The ion may act as a clustering center

for one or more neutral molecules attached by polarization.

( i i ) Ionic fragmentation - The ini t ia l ly produced

odd electron radical ion species may dissociate to form an even

electron species such as carboniuzn ions.

( i i i ) Ion molecule reaction - The positive ion may react

with another molecule to form a new ion and a neutral radical.

(iv) Charge transfer - The initial ion may react with

another molecule and exchange electrons in a collision process.

This eventually leads to an energy transfer phenomenon.

- 4 2 -

(i) The clustering hypothesis was popular 20-30 years ago but went

down in importance in ninteen fifties-., HecentlyV once, again some interest in

this hypothesis has s t a r t ed . We will skip over detai ls of t h i s pro-

cess as t h i s is covered by Dr Vohra in h is paper (cf. pp.83-.-99)

( i i ) Ion fragmentation to form carbonium ions - Although the pre-

sence of carbonium ions have been well documented in the mass spectro-(2 3)metric studiesv ' and reactions such as

are well known, the extent of the parent positive ion fragmentation

in condensed phases is not known, but i t is presumably small because

the process RH R + H is in most cases, endothermic for ground

state radical ions. Table 3 shows the endothermicity of this process

for certain straight chain alkanes. But the above reaction can occur

spontaneously if there is ini t ia l ionic excitation. Libby^ has

postulated that carbonium ions are f oimed by ionizing radiation very(5)readily. Williams has given theoretical reasons for expecting the

formation of carbonium ions. As can be seen from Table 4, in case of

branched alkanes, the process even seems to become exothermic in

certain cases. Surprisingly enough, this point has only very recently

attracted attention of radiation chemists. The only earlier measure-

ment of radiation yields of carbonium ions is of Ward and Hamill .

Recently, we have made an attempt to count these even electron posi-

tive species (carbonium ions) by derivatising them with suitable

negative ionic species X~ produced in situ

RH

RH+

e +

R+ +

—-*

RH+

R+

R»X —> R1

• x " --•RX

+ e~

+ H

+ X~

The ra t ional i ty of th is approach i s that electron capture

by certain solute (R'x) wi l l give negative species x" by dissociat ive

electron capture. The X" wi l l then diffuse in the f ield of the

-43-

positive hole. If this positive species happens to be an even ele-

ctron species, earbonium ion formation of a salt like complex will

take place. The carbonium ion and negative ion will neutralize each

other having the two radicals very close neighbours. The products

can then be formed as H + X~ * RX

On thermoc hemical grounds recombination of a positive ion

with an electron should lead to decomposition. However, the primary

reaction of dissociative electron capture will dissipate as much as

3-4 eV of this recombination energy, thereby reducing the possibility

of a dissociative charge recombination. Furthsr, even in a low diele-

ctric medium, an additional 2 eV per ion will be dissipated by polari-

zation of the medium. The coulombic energy of the separated ion pair

partially converts to kinetic energy during recombination and is

expanded in over coming viscous drag, dissipating an additional

2 eV.

For the sake of comparison, two saturated alicyclic com-

pounds, cyclohexane and cyclopentane, along with on** heavily branched

hydrocarbon neohexane were tested for the production of csrbonium ions.

Three different sources for giving the appropriate negative species

were used, viz. , benzyl acetate, carbon tetrachloride, and sec. butyl

bromide. While making the choice, the important thing to keep in

mind is to see that the X radical has as high an electron affinity

as possible.

To avoid complications arising from free radicals, all

experiments were done in the presence of iodine.EX produced in the

presence of radical scavenger was determined by vapour phase chro-

matography. The results are given in Table 5. These results indicate

that earbonium ions are not very important in the condensed phase

radiolysis of saturated straight chain hydrocarbons. The results agree

well with the earlier work of Wsxd and Hamill .

In the case of neohexane, we find as expected from the ener-

getics point of view, a surprisingly higher G vslue obtained with the

-44-

help of all the three solutes. It must be remembered that here we are

counting a tertiary carbonium ion. The higher G value (0.8) is under-

standable in terms of the general theorem that hyperconjugation

effects due to attached methyl groups operate much more strongly in the

even electron carbonium ions than in the odd electron parent ion. In

the production of these from the parent radical ion, dissociation is

accompanied by a gain in resonance energy.

One very interesting observation is that the trend of the

production of these carbonium ions runs parallel to the values recently

obtained for "free ions" by conductivity experiments of Schmidt and(a)

Allonv '. I t strongly indicates that the explanation of different

values (structure dependent) of "free ion" yield may be correlated

with the i r carbonium ion y ie lds . We are now involved in considering

various interest ing theore t ica l implications of th i s s imi la r i ty .

I I . ION MOIECUIE REACTIONS IN THE COHDE1TSBD PHASE

The reaction of an ion with a molecule to form a new ion

has been commonly referred to as an ion-molecule reaction. This i s a

form of charge transfer A + B • C + D, Until quite recent ly ,

reactions of t h i s type have been considered re la t ive ly unimportant

in condensed phases, those of th'e familiar uncharged free radica ls(9)being stressed. Stevenson has pointed out that an ion may un-

dergo as many as 10 collisions before neutralization in atmospheric

gas if the dose rate is 300 R/sec. In other words, if ion molecule

reactions can occur, and if the results of these reactions are chemi-

cally significant to the overall results , these reactions should not

be neglected.

Ifeisels, Hamill and Williams^10'' studied radiolysis of

methane in rare gas atmosphere. They concluded th&t the results can

be explained by ion molecule reactions.

CH4+ + CH4 > CH^ + CH

-45-

The rates and cross sections of several ion molecule reactions known

to occur in methane and ethane have been investigated'11 '12 '1 .

It was found that such reactions have very high cross sect-

ions, particularly at low translational energy. Stevenson and

Schissler indicated that reaction could occur at every collision,

and that the cross sections were about three times the value expected

by the ordinary kinetic theory.

These reactions and others have been proposed for radiolytic

processes in the gaseous state ' . It is not as evident that ion()(15)molecule reactions occur in condensed phases , although they are

not unlikely ' and have been postulated to account for several

experimental observations in liquid phase radiolysis ' '

Even after so many proposals, the occurrence of ion mole-

cule reactions (the best known of these are proton transfer reactions)

in the radiolysis of saturated liquid and solid aliphatic or alicyolic

hydrocarbons remained an attractive postulate only. Energetically

such a reaction as

C6H12+ + C6H12 > C6H13+ +C6H11

tends to be endothermic by about 80 Kcal/mole

C6H12+ + e " —* C16H12 ^ H = ~ 2 2 8 Kcal/mole

n.H<o —> C,HH, + H. AH = + 94 Kcal/moleO ,d D 11

H —>-H+ + e~ AH = +315 Kcal/mole

H+ + C,H1O > C - H * AH = -104 Kcal/moleb \d b \j

C 6 H1 2

+ + C6H12 — * G 6 H1 5

+ + °6H11 * * + 7 ?

The process would be exothermic i f

I.(donor) + A.(acceptor) ^ D.(H - CgH^) + I .(H')

As the bond dissociation energy of the C-H bond for arious donors

-46-

and the ionization potential of H* do not change much, the important

variables are the ionization potential of donor and proton a f f in i ty of

the acceptor. Since the ionization potent ials for most l iquid sa tura-

ted hydrocarbons l i e in the region of 9-11 eV, the only s ignif icant

variable i s the proton a f f in i ty of the acceptor. If the proton af f in i ty

of the acceptor i s 7-9 eV, such proton t ransfer reactions would be exo-

thermic even in a condensed phase. Table 6 l i s t s value for the proton

af f in i t ies of some representative polar and non-polar molecules.

Table 7 l i s t s the ionization potentials of some of the hydrocarbons.

Molecular radical cations derived from the parent molecules

(RH) can aHso be represented as protonated free rad ica ls , v i z . ,

RH = R" £H J • The acidic character of these species can be v e r i -

fied by an ion molecule reaction involving proton transfer to Brona 3d

baees such as water and ammonia. In the condensed phase, where f rag-

mentation of the parent ion i s shown (as above) to occur to a l e s se r

extent than in gas phase, these reactions may be favourable and

important.

(21)Williams , taking advantage of the high proton a f f i n i t y

of MH_, was the f i r s t to offer the only convincing evidence for proton

transfer from the parent posit ive ion of a saturated hydrocarbon. He(22)

l a t e r extended this work t o another acceptor, CpH_QD« Solutions

of deuterated ethanol and l i gh t cyclohexane were irradiated and HD was

found to be a significant product, explained by the reactions

RH+ + CJLOD > R° + C^H.QDH+

DH + e~ y CLHCQD + H

3 + D

and D" + RH » R° + HD

Proton transfer competes with the neutralization of cyclohexane ions.

He also studied the kinetics of proton transfer from C , H * ions toO 12

Ck.H5QD i n comPe*i*ion with geminate .recombination.

•47-

Guarino and Hamillv J ' have been able to interfere with

simple charge t ransfer to solutes by adding a second solute which i s

a good proton acceptor. Toraa and Hamill^ ' have recently presented

evidence that addit ion of a molecule with high proton affinity i n t e r -

fered with the neutral izat ion process C-H.,* + e~ ——> C H + 2Ho 12 6 10

by reducing the yield of cyclohexene.

We have, recently, re-examined th i s problem and gathered

further evidence for the above mentioned proton transfer reactions.

We util ized the approach of examining the neutral product formed in

an ion molecule reaction. A proton transfer reaction can be written

as

RH+ + B • r R* + BH> (a)

Every time the base, B, abstracts a proton from the p-~rent ion,

RH , one radical i s produced,, In theory, if one can now count the

neutral radicals produced, one should be able to study the extent of

the proton transfer reaction. But, i t i s well known that in the

radiolysis of xiydrocarbons, radicals and predominantly parent radi-

cals are produced even in the absence of any added base. This in-

terference can be overcome by adding a known efficient radical sea-

venger (such as I ) to the sample and irradiating with and without

added bases. If one studies only the increase of radicals , R°

produced in the presence of added bases, this increase in radical

will indicate the minimum yield of radicals produced via proton trans

fe r , reaction (a) . The G -alue of R* will be a measure of proton

transfer reaction.

The bases chosen for such studies must satisfy certain

conditions:

( i ) They must have high proton affinity values so that

reaction (a) will be exothermic in the absence of any solvation

effect.

(ii) Charge transfer and electron capture processes should

not interfere. The bases should have higher ionization potentials

-48-

than that of the hydrocarbon under study and low electron affinities.

In the results given in Tables 8-10, G values for the in-

crease of radicals by various proton acceptors are tabulated. Five

different alkanes having a range of ionization potentials have been

studied. Five bases having different thermochemical parameters such ai

proton affinity, electron affinity, etc. were tried. ;

In the case of cyclohexane (IP = 9.88 eV) and cyclopentane

(IP = 10.53 eV), i t is clear from the data of Tables 8 and 9 that

adding a small amount (2%) of any of the five proton acceptors

(e.g., methyl alcohol, isopropyl alcohol, acetonitrile, acetone, and

pyridine ) results in a definite increase of the parent radical

(cyclohexyl and cyclopentyl). Furthermore, the extent of proton

transfer seems to be directly proportional to the affinity values of

various proton acceptors. The various acceptors can be arranged in

order of decreasing proton accepting ability as : methyl alcohol "^

isopropyl alcohol ^ acetone ^ acetonitrile ^ methyl tetrahydro-

furan.

Table 10 shows the results when benzene (IP = 9»26 eV) was

used as the solvent. It is clear from the re PI'.Its that in this case

proton transfer does not take place even with the strongest base, pyri-

dine. This is in accordance with the hypothesis that for proton tra-

nsfer to take place, the ionization potential of the donor should he

high. Benzene is a borderline case.

To further test the validity of energy dependence for

reaction (a), a donor of high ioniaation potential (cyclohexane)was

coupled with various olefins having lower proton affinity, as the

proton acceptors. Results are given Table 11. It is obvious that

instead of increasing the yield of cyclohexyl radicals, the olefir.s

have in fact reduced the yield. This effect is attributed to a charge

transfer mechanism. All the olefins have a lower, ionization potential

than c yclohexane.

-49-

Figure 1 gives the results of the experiments when

3-methyl pentane (IP = 10.06) was used as a donor. In the case of

3 methyl pentane irradiated in the presence of iodine, i t is known

that in addition to fragment radicals methyl, ethyl and sec. butyl,

at least two isoaers of the parent radicals are also formed.

Figure 1 shows "this result. Though the individual peaks on the ehro-

matogram were not characterized, i t is deduced from retention items

that peak 1 is due to C. iodide, and peaks 3 and 4 to isomeric C4 6

products. Peak number 3 is characterized as due to the tert, product

fK,C-CH -C-dL-C^ and peak number 4 to the secondary product

H C-CH-CH-CHg-CH .

I

From the results of addition of various proton acceptors

(Fig.i), one very interesting fact emerges, viz. , that while the weak

bases (methyl tetrahydrofuran, acetone, acetonitrile, etc.) on proton

transfer increase the yield of peak Ho.3, representing the formation

of a more stable tert. radical, the stronger bases, pyridine and meth-

anol, are also found to increase the yields of secondary and even

primary radicals „ This indicates a very striking specificity in

ionic reactions. If the base is weak, proton transfer takes place

only from one (that giving rise to most stable radical) or the many

possible sites, but if base ^s strong enough, this specificity is

lost and proton transfer takes place mostly on the basis of statis-

tical availability of the protons. Similar results were obtained

from methyl cyelohesane (Fig. 2).

To study contributions of ionic processes in more detail,

various electron scavengers were added to the system. Results with

eyclohexane as the donor and five different electron scavengers in

Table 12 show that all electron scavengers tried have reduced the

yield of cyclohexyl radicals.

-50-

ELectron scavengers convert the electrons to another

negative species X~ (v iz . , reaction c ) , which on neutral izat ion

(reaction d) does not give the same products

e" + RH+ *• TH* * R" + H* (b)

e~ + R X T R1 + X~ (c)

X~ + RH*+ > RH + X or other products (d)

When both electron scavenger (cyclopentyl bromide) and proton acceptor

(methanol) were added to the solvent the resul ts in Table 13 bring out

a very interesting point : the increase in radicals produced by addition

of a proton acceptor to a sample i s the increase produced when the same

base i s added to a sample without electron scavenger. This indicates

that when a proton acceptor i s added to the system some positive ions

which were i n i t i a l l y not giving rise to radicals (on neutral izat ion)

are now forming radicals due to transfer of a proton to the added

ba se .

RH+ + B *> R* + BH+

This can be understood e i ther on the basis of two different s tates

of positive ions or due to some branching reaction involving positive

ions (not giving rise to radicals) . At present i t i s not possible to

distinguish between the two poss ib i l i t i e s .

I I I . CHARGE IIEUTRALIZATION

The third possible reaction in which ions take part i s

neutralization. This includes electron capture processes and reactions(25) (261

with a positive ion. NSagee and Magee and Burton have consi-

dered the theoretical aspects of the neutralization of charge in an

irradiated system by the reaction of positive and negative ions, i.e.A + B~ » AB

Experimentally, these processes open up new possibilities

o2 studying the mechanism of ionic neutralization. Here again we

would mention about a recently published work of ours where 2 7 ' an

- 5 1 -

attempt was made to study the dynamics of the chazge neutralization

process. The system studied was a dilute solution of methyl iodide

in 3-methyl pentane. In this work advantage was taken of the fact

that in a matrix of hydrocarbons such as 3-methyl pentane both the

positive hole and electrons migrate and can be trapped so both the

processes can be studied. In the experiments described in this work,

the mobility of the ion was controlled by varying the viscosity of

the matrix.

The nature of the ionic species produced was characterized

by adding various ionic scavengers. Both the irradiation and speetros-

copic examination were done at 77°K and the spectra w-i.-p scanned ino o

the region of 3000A to 8000A. The optical absorption bands produced

f a l l in three groups : 360 and 440 nm, X; 390 and 480 nm, Y ; 540 and

760 nm, Z. Mit tal and Bamill assigned the X group to RI and Z

group bands to (Rl) . But the most interest ing assignment turned out

to be of the Y se r i e s of bands. The authors argued and gave evidence

tha t the Y ser ies of bands should be assigned to a charge transfer

complex formed by the recombination of a positive ion C and a

negative ion A

3 MP

. . . i j charge transfer complex

3 MP

e + CH I —

CH,I+ + I" —

y

-*CH

—*>

CH_I3

.+r[CH,!

3

The identification of charge transfer complex such as above, as the

transient product of ionic recombination provides a useful new tool

in radiation chemistry. To the best of our knowledge,the only

identified products of charge recombination in radiation chemistry

are those reported by Mittal and Hamtn and the excited species(28)

identified through recombination luminescence

- 5 2 -

REFERENCES

1 . H. E y r i n g , J . O . H i r s c h f e l d e r and H.S. T a y l o r ; J . Chem. P h y s .£ , 479 (1936)

2 . F .E . F i e l d , J . L . F r a n k l i n snd F.W. Lampe; J.Air .Chem.Soe.79., 2419 (1957)

3 . S . Wexler and N. J e s se ; J.Am . Chem.Soc. 8 4 , 3425 (1962)

4 . W.F. Libby; J .Chem.Phys. 35_, 1714 (1961)

5 . T .F . Wi l l i ams ; Trans . F a r a d a y . Soc. 57., 755 (1961)

6 . J.W. Ward and W.H. Harail l ; J,AnioChemoSoc.87., 1853 (1965)

7 . longue t Higg ins ; Chem.Soc. Sp .Pub l . 9_, 5 (1957)

8 . W.F. Schmidt and A.O.Al len; S c i e n c e . 160, 302 (1968)

9 . D.P. Stevenson; J .Phys.Chem. 6J_, 1453 (1957)

10. G.G. M e i s e l s , W.H. Hamill and R.R. W i l l i a m s ; J .Chem.Hiys.25_, 790 (1956)

1 1 . D.P . Stevenson and D . O . S c h i s s l e r ; "The Chemical and B i o l o g i c a lActions of R a d i a t i o n s " , Academic Press London, 5_, 257 (1961)

12. F.H. F i e l d , J . C F r a n k l i n and F.W, Iampe; J.Am.Chem.Soc.78, 5697 (1958)

1 3 . C D . Wagner, R.A. Wadsworth and D.P . Stevenson:! J .Chem.Phys .2 8 , 517 (1958)

14. I .M. Dorfman; J . Phys.Chem. 62_,29 (1958)

15 . T.O. J o n e s , R.H. Luebbe, J . H . Wilson and J . E . Wi l l a rd ;J.Phys.Chem. 62_, 9 (1958)

16. J . S . Bur r ; J.Phys.Chem. 61_, 1483 (1957)

17. R.H. S e h u l e r ; J .Chem.Phys. 26_, 425 (1957)

18. H.A. Dawhurst ; J .Phys.Chem. 62_, 15 (1958)

19. C.H. Gevantman and R.R. Wi l l i ams; J.Phys.Chem 56., 569 (1952)

2 0 . W.F. Libby; J.Chem.Phys. 35_, 1714 (1961)

21. W.R. Busier, D.H. Martin and Ef. Williams; J.Chem.Phys.44, 4377 (1966)

22. J.W. Buchanan and Ff. Williams; J.Chem.Phys. 44, 4377 (1966)

23. J.P. Guarino, M.R.Ronayne, and W.H. Hamill; J.Chem. RadiationResearch V7, 379 (1962)

-53-

24. S.Z. Toma and W.H. Hamill; J.Aa.Chea. Soc.86_, 1478 (1964)

25. J.L. Magee; Disc .Faraday.Soc. 1£, 33 (1952)

26. J.I. Magee and M. Burton; J. Am.Chem.Soc. 72_, (1965)

27. J.P. Mittal and W.H. Hamill; J.Am., Chem. Soo. 89_, 5749(1967)

28. J.A. Leone and W.H. Hamill; J. Chem. Riys. 49, 5249 (1968)

- 5 4 -

TABLE 1

Approximate Time Scale of Events i n Radiat ion Chemistry

- log t(secs)

18

17

16

14

12

11

10

9

6

4

3

0

-2

Events

Past electron traverses molecule

MeV proton traverses molecule; timefor energy loss to fast secondaryelectrons

Time for energy loss to electronicstates.(vertical excitation)

Past ion-molecule reactions involvingH-atom transfer. Molecular vibrationPast dissociation

Electron thermalizes. Self-diffusiontime scales for liquids of lowmolecular weight

Dielectric relaxes in water. Basicneutralization time for polar media

Spur formed

Spur reactions

Latra-track reactions completed

Neutralization times in media of lowviscosity and dielectric constant

Escape time for electrons in mediaof low viscosity and dielectric constant

Radiative life-time of triplets

Neutralization times for media of highviscosity and low dielectric constant

Stage

•ac>

• HCO

• 1 -

•a

oow•H

•a11

-4

-55-

TABIE 2

RH -"« > RH+ + is

RH -"*—» HH* , . . . . . . ( 2 )

RH+ + e~ — t RH* (3)

Reactions of excited species (excluding non-chemical proceb0o~;.-such as fluorescence e t c .

HH* * It" + H' thermal . . . . . . ' ( 4 )

#

(5)def ine + H- (6)

#EH ^R'H + R"H Btable molecular products ..-.(7)

RH* + Q —*RH + Q* . . . . . . . . ( 8 )products

Reactions of e lectrons;- (in the presence of scavengers)

e + A —f [Aj ~ non-dissociative electroncaptuxe. (9)

e~ + RX —>R° + X~ diss.electron capture (10)X" + RH+ *RH* (11)

Reactions of pos i t ive i o n s : -

RH+ + R'H —•* RH + R'H+ charge transfer (12)

RH —>R + H carbonium ion formation (13)

RH + RH ——*RIL + R" ion molecule react ion . . . . ( 1 4 )

-56-

TABIE 3

Energetics of n-alkane Ion Dissociation

Hydrocarbon

n-pehtane

n-hexane

Secondaryi OYl "P T flff—

ment

S-°5H11

S"C4H9

S"C3H7

C2H*

S"C4H9+

Kcal/mole

170

185(+5)

190

224

262

185(+5)

190

224

Radical. AHf.(Rg ) AH f (R1 CH^R2 )

Kcal/mole

H 52

CH3' 31

CLHf." 24

n-C.Hq° 19

C-H,. * 24

n~C4H9* 1 9

Kcal/mole

208

208

208

208

208

202

202

202

A H p

Kcal/mole

14

8

6

38

73

7

10

41

TABLE 4

Energetics of Ion Dissociation in Branched Hydrocarbons

Hydrocarbon AHKcal/mole

tfeopentane -39.7

Ioniza-tion pot-ential ,

eV

10.29

Neohexace -44.4 10.19

f(parention)Kcal/mole

Secondaryion

198

191

Kcal/ ranamole cal mole

166

262

152

166

152

CH 31

:• €

H" 52

24

31

mole

-1

+70

+6

-1

-8

-58-

TABLE 5

Yields of Ion Recombination Products

F orce of X" G(RX)a'C

,c Solvents used (HH)Solutes used • _ , „ , -.

, N Cyclohexane Cyclopentane Heo-hexane

Benzyl acetate 0.21 0.14 0.81

Carbontetrachloride 0.25 0.17 0.84

Sec.butylbromide 0.19 0.12 0.78

20a. Dose: 1.52 x 10 eV/ml; a l l runs were at room temperatureb . All runs are in the presence of 5 z 10~ N iodinec . All solutes used were 2% in concentration (v/v)

* The values are for t -bu ty l aceta te , t -butyl chloride andt-butyl bromide

-59 -

TASXE 6

Proton Aff ini t ies of Some Eepresentative Polar and Non-polar

Compounds

Compound Proton/Affinity(KJeal/iaole )

Methane 129

n-hexane 103

benzene 144

iso-butene 196

acetaldehyde 166

acetone 173

dimethyl ether 168

aeetonitrile 191

water 171

methanol 177, 183

ethanol 185, 202

amnonia 200, 238

OH" 372

* All values are taken from J.A. Ward's Ph.D. thesis (University

of Notre Dame, 1964)

-60-

TABIE 7

Ionization Potentials of Some Compounds

Compound Ionization potent ia l(eV)

Cyclohexane 9.88 + 0.02

Cyclopentane 10.53 +0 .05

Methyl cyclohexane 9.85 + 0.03

3-methylpentane 10.08 + ?

Iso-pentane 10.32 + ?

Benzene 9.24 + 0.01

Toluene 8,82 + 0.01

Mesitylene 8.40 + 0.01

Methyl iodide 9.54 + 0.01

Methyl alcohol 10.85 + 0.02

Dimethyl ether 10.00 + 0.02

Hexene-1 9,46 + 0.02

* All the values are taken from K. Watanabe, J.Quant. Spect.

Eadiat. Transfer, 2_, 369 (1962)

- 6 1 -

TAELE 8

Badiolys is of Cyclohexane - I - Proton Acceptors a ' c

Proton acceptors G(cyclohexyl iodide)

Noiue 3.92

Ifethanol 5.67

Iso-propyl alcohol 5.23

Acetone 4.94

Acetonitrile 4.87

2-methyl tetra-nydrof uran 4.68

20a. Dose = 1.52 x 10 eV/ml; a l l experiments were done at

room temperature

All runs with 5 3

c . All proton acceptors were 2$ in concentration (v/v)

b. All runs with 5 x 10~3M iodine

-62-

TABIE 9

Badiolysis of cyclo'pentane - !„ - proton accep to r s a ' C

Proton acceptors G(cyclopentyl iodide)

None 2.98

Methanol

Acetone

Acetonitr i le

2-methyl tetrahydrofuran

Pyridine

3.72

3.66

3.48

3.06

3.96

20a. Dose = 1.52 x 10 eY/ralj a l l experiments were done a t

room temperatureb . All runs with 1 x 10~?M iodine

c . All proton acceptors were 2j6 i n concentration (v/v)

-63 -

TASLE 10

HadiolyBis of Benzene - I~ - Proton Acceptors6fC

Proton acceptor G(lodo benzene)

Hone 0.27

Mfethanoi 0.24

Pyridine 0.23

20 20a. Dose = 1.5? x 10 x 10 eV/ml; all runs are at room

temperature

b. All runs with 1.0 x 10~2M iodine

c. All proton acceptors were 2% in concentration (v/v)

-64-

TABIE 11

Badiolysis of Cyclohexane - Ig - olifinB '

Solute G(cyelohexyl iodide)

None

Octene-1

Pentene-2

2-Methyl pentene-1

Trans-octene-2

Benzene

3.92

2.91

2.92

3.18

3»36

3.21

20

a. Dose = 1.52 i 10 eV/mlj a l l runs are a t room temperature

b . All runs with 1 x 10"2M iodine

c . All solutes were 2% concentration (v/v)

-65-

TABLE 12

Radiolysie of Cyclohexane + Iodine + Electron Scavengersa>"

Solute G(cyclohexyl iodide)

None 3.92

Cyclopentyl bromide 1.83

Sec.butyl bromide 2,01

Nitrobenzene 2.96

Benzyl acetate 2.13

20a. Dose = 1.52 x 10 eV/ml; all runs are at room temperature

b. All runs with. 1 x 10~2M iodine

c. All solutes were 5% in concentration (v/v)

-66-

TABIE 13

Badiolysis of Cyclohexane }• Iodine + Various Offcher Solutee a 'C

Solutes G(cyclohexyl iodide) ,4&(cyclohexyliodide)

None 3.92 g 1.750

Methanol 5.67 5

Cyclopentyl g 1 e 5 g

bromide

Cyclopentylbromide + methanol

20a. Dose = 1.52 x 10 eV/mlb. All runs with 1 x 10~2M iodine

c . Both cyclopentyl bromide and metnanol were 2% in concentration

(v/v)

- 6 7 -

PIGURE 1

Chromatogram of products from radiolysis of 3-me*-hyl

pentane + iodine + various proton acceptors •

3-methyl pentane + iodine only

increase i n peak number 3 by adding any

of the proton acceptors (methanol,

ace tone , methyl t e t rahydrofuran ,

acetonitr i le , pyridine)

increase in peak number 2 obtained only

in case of laethanol

increase in peak number 4 obtained only

in case of pyridine

# All proton acceptors were used in same concentration {2% v/v)

FIGTJHE 2

Chromatogram of products from radiolysis of methyl cyclo-

hexane + iodine + various proton acceptors*,

______,»—______«_ methylcydohexane + iodine only

--—————— increase in peak number 2 Toy adding

any of the proton acceptors (methanol,

acetone, methyl tetrahydrofuran

acetoni tr i le , pyridine)

- . - . - . - . - . - . - T - . inorease in peak number 4 obtained

only in case of pyridine.

* All proton aceptora were used in same concentration (2# • / • )

CHART UNITS

a m

3

5

- 6 9 -

RADIATI02T POLYMERISATION

by

K.N. Rao, M.H. Rao and Vijay KumarChemistry Div i s ion

Bbabha Atomic Research Cen t re , Trombay? Bombay-85

Unsaturatec! compounds l i k e e thy lene , s t y r e n e , a c ry l a t e s and

othe r v iny l conpounds can be polymerised by c a t a l y s t s or by r a d i a -

t i o n , i . e . , UV, X- or gamma r a y s o Polymerisation of t t e s e can be

simply depic ted i n the following ways

X (Reactive species) Initiation

R R'I I

X + HC = CH

R

-XCH - CH

R R1

XCH - CH + CH - CH

r\

R R'1'

» iCH - CH

R

-CH - CH

Q PQ (Polymer)

Propogat-ion

Termination

In order to start iiie polymerisation a catalyst or ra&iaiien

is required, represented by I , which produces a reactive species X.

X then reacts with monomer molecule and conveys i t s reactivity from

molecule to molecule through the double bond until i t is annihilated!

Thus, in a l l polymerisations there is an initiation step, a propaga-

tion step and a termination stepo

By making certain simple assumptions and applying steady

state treatment t« the above, one can derive an. expression for the

rate of pelymerisation

-70=

where the k's are the rate constants for the three steps mentioned,

I, the initiator concentration and M is the monomer concentration«

Other useful relationships for radiation induced polymerisation

are:

G(~m) = 100 R B/I

G± = 1 0 0

DP = G(-m

These equations show that for biiaolecular termination the rate «f

polymerisation is prop»rtional to the square root of ihe radiation

intensity and the average degree of polymerisation is equal to the

monomer molecules consumed ts the number of active initiators pro-

duced.

The subject of radiation polymerisation can now be discussed

in terms of the ' init iators ' and the kinetics and mechanism of poly-

merisation o

Primary Initiating Species; One of the important aspects of radia-

tion polymerisation is the identification of the initiating species

and the measurement of i ts concentration,, In the conventional poly-

merisation, a catalyst like benzoyl peroxide, AIBN is added and is .

decomposed thermally or photolytically anione has a fair idea of the

radical that is produced. In the radiation process ? i t is mere

difficult to establish i ts identity. The result of the passage of

radiation through any substance is the formation of cations, elec-

trons, excited molecules and free radicals. Most of the early work-

ers assumed that neutralisation of ionic species is too rapid for

them to contribute to polymerisation and only free radicals lead to

polymerisation. It will be seen later that this concept had to be

modified as radiation promotes polymerisation both by cations and

anions.

- 7 1 -

There are a number of techniques available for 1fae detection

and estimation of free radicals in monomers. The most common method

has been the use of scavengers. Certain stable free radicals like

DEPH, galvinoxyl or compounds l ike 02 , Fed. , react very rapidly with

the radiation produced free radicals and thus make them unavailable

for polymerisation. A decrease in the rates of polymerisation by

any one of these radical scavengers i s taken as proof of the radical

init iat ion of the polymerisation phenomenon. This method also yields

a value for G(R) production.

In the case of a number of solid monomers, electron spin

resonance enables one to detect the presence of free radicals and

even for the l iquids i t is possible to estimate the radical yield

at low temperatures when they are frozen. Although much work has

not been done on the monomers, ESE measurement can be made while the

irradiation i s in progress. Similar studies have been made by

Fassenden on hydrocarbons^ .

•The degree of polymerisation i s equal to G(-m)/G^ and i f one

measures the average molecular weight of the polymer and the G(mono-

mer consumed), the G(initiator) can be calculated.

In the scavenger technique there Is no guarantee that the

compounds like DFPH are scavenging only the radicals and not the

electrons a lso . This throws .some doubt on the validity of the

method. The detection of free radicals by ESR in the solid state

i s no proof that they are the propogating species. This informa-

t ion has to be carefully correlated with the kinetics before coming

to a conclusion.

The detection of an ionic species in polymerisation presents

even more d i f f i c u l t i e s . Early conclusions that ions are involved

- 7 2 -

were arrived at by the rapid polymerisation rates observed at very

low temperatures for some monomers and alBo the retardation of

polymerisation observed for syrene^ , alpha-methyl-styrene and

several others by traces of water, ammonia and amines.

In the early studies of carefully purified monomers, re-

producibility of R was rather dif f icult afd even R was found to

be proportional not only to intensity ( i ) but also t o I where x

varied from 0,5 - 1» This broke down the criteria that in ion

molecule reactions R i s always proportional to L It i s possible

that termination may take place between a growing chain ion and a

counter ion rather than the chain terminating by simple charge trans-

fer to an inactive species unimolecularly.

In the case of styrene, i t was identified that the most

important impurity which inhibits ionic polymerisation i s water

which was established by adding known amounts of i t to carefully

dried styrene and polymerising i t , when i t reverted back to radical(2)

process ° The role of water is assumed to be the removal of proton

from a cation,

RH+ + ^ 0 > R + ^

and thus inactivating the propogating species o One might assume

that when a l l the water has been converted to H-0 , ionic polymeri-

sat ion would be resumed but actually E.0 gets neutralised by

negative species and H O i s restored. One can use other proton

scavengers like NH, and amines which do not catalyse a radical

reaction and observe a similar ef fect o Since then a number of

monomers have been studied which show a similar behaviour, though

not a l l of them.

In "dry" systems there must be both ionic aid radical poly-

merisations taking place simultaneously. However, recent evidence

-73-

suggests that some of the ions generate radicals when wet and hence

in ttie dry state the radical contribution may be insignificant.

Another important aspect of ttiese polymerisations is chain

transfer. If the G(-m) i s about 1Cr and the degree of polymerisation

10 , 1hen Gi would be about 100. This is definitely high and in

order to accommodate i t one must assume that a polymer chain trans-

fers i t s charge to another monomer which starts off another polymer

molecule p + M ——> P + M

In tho l ight of the above, a representative of each type,

vinyl benaoate for radical process and triozane for ionic type, i s

discussed.

Vinyl benzoate i s a liquid which has been polymerised by

radiation (gamma rays) and Fig. 1 shows tbe percentage monomer con-

version as a function of dose for several dose rates . To avoid

complication in kinetics due to increase in v iscos i ty , the conver-

sion of monomer i s kept below 20$. Fig.2 shows a similar study at

different temperatures. From the usual arrhenius plot (Fig.5) an

energy of activation of ~3&2 KCal for "the polymerisation reaction i s

obtained. This i s in the range of values obtained for radical poly-

merisation systems. In onler to measure the radical yield, DPPH scav-

enging was employed and from the threshold dose required (Fig.3) to

start the polymerisation G(R) was calculated to be 0.94. However,

wten G(-DPPH) was measured 1/ the optical density method (Fig.6),

G(E) was found to be 5.0. This needs to be explained as to why only

about 18$ of the radicals formed are effective. Perhaps the solution

l i e s in the fact 1iiat only the radicals at the vinyl site are effective

in init iating polyne risatian and those an the benzene ring are inac-

t i v e . Even tte radicals at the vinyl s ite might transfer the radical

to the benzene rii^ and end up as oligomers which are not shown as

-74 -

polymer. Hence the low value is due to radical transfer. Further

confirmation that chain transfer of this kind takes place can be

had from a study of log R vs log I . (Fig .4). The slope should

be 0.5 for a perfect radical polymerisation system,. If chain trans-

fer is involved, the value l ies between 0.5 - 1 «0 and the experi-2

mental value is 0o66o We have also determined k_/k. for this system-A * -4

whose values are given at 25°> 1.59x10 * and at 60°, 5.8 x 10 . The

degree of polymerisation is about 100 which also supports the chain

transfer explanation„ We are further investigating whether ultra-

drying will change the mechanism to ionic polymerisation,,

Another system of commercial importance is trioxane which

has also been investigated in our laboratory. Trioxane is a cyclic

ether which was found by Japanese workers to undergo radiation(5)

polymerisation . I t i s a crystalline solid with a m.po 63°. When

irradiated with gamma rays in the solid state, i t was found to under-

go polymerisation but the rate of polymerisation as well as the per-

cent conversion depended upon the temperature, the highest yield

being observed close to the melting point„ The limiting yield

was between 40 - 50$ and the results were not reproducible„ It was

also found that radical scavengers like 02 had no effect on the limit-

ing yield or the rate of polymerisation„ Prom these results i t was

concluded that the reaction proceeds by an ionic intermediate. Evid-

ence for the ion was found when trioxane was subjected to high pre-

ssure mass spectrometric study „

-75-

+ TOX > (C^O) H+

+ TOX > *

(7)BSE has shown that the reactions are:

y O £ CH

H + 0 = CHOCH 0 = CIL

Similar observations were nade for other cyclic compounds like

n -propiolactone, diketene, 3,3-bis (chloromethyl) oxetan, e t c . ,

which are crystalline monomers.

It was also observed that when trloxane was irradiated in

a i r , a further polymerisation took place on annealing after the irra-

diation was over. This post polymerisation complicated the situation

further. In liquid state there was either negligible or no poly-

merisation.

Trioxane being a crystalline sol id , i t was only natural to

look for an explanation of the temperature effect and irreprodueibi-

l i t y of the data in the crystal nature.

large single crystals of trioxane were irradiated and i t

was observed that the polymer growth was parallel to one of the cry-

s ta l axes and the temperature effect was to fac i l i ta te molecular(8)motion. A similar conclusion was arrived at by Adler by studying

post polymerisation of trioxane single crystal. However, more accurate(a)

studies were needed. H.H. Rao end Ballantine prepared samples

under vacuum and carefylly controlled, conditions and. concluded that

in. the liquid, state there, is no polymerisation, the late of polymeri-

sation and limiting conversion are not reproducible, the kin&ties de-

pends also on the size of the crystal - large crystals give higher

-77-

rate to traces of impurities once again emphasises the ionic nature of the

reaction!

To sum up, these studies emphasise the fact that radiation poly-

merisation is still a not well understood field. Cur knowledge of the

chemical identity of the ionic species is still very limited. In this,

pico second pulse radiolysis may enlighten us in the future.

Lastly, since it has been shown that dryness and purity play such

an important role, some of the older systems and data may be worth

reinvestigEiting.

REFERENCES

1. R.C. Pessenden, J . Phy. Chem, 68, 1508 (1964)

2 . C.L. Johnson and B . J . Ketzj Polymer P r e p r i n t s 4_» 440 (1963)

3 . F . Wil l iams; Discussions Faraday Soc. j5jS, 254 0 9 6 3 )

4 . S. Wexler and R. Marsha l l , J . Am. Chem. Soc. 66, 761 (1564)

5. S. Okamura and K. Hayashi; Macromol. Chem. 47_» ?3C (1961)

6. K. Hayashi; Paper presented a t I n t e r n a t l . Symposium on J'acromolecularChemistry, Budapest (1969)

7. V.I . Tupikov et a l ; Russian J . Phy. Cher.. jjE,, 243G (1

8. G. Adler; J. Folycer Sci. ill , j , , ZL-^ (i>'''6)

9. M.H. Rau ar.d L.S. Bal lan . in t ; Trans. Am. Nucl. Soc. 1_, 312 (1964)5J . Poly. Sci- A3, 2579 (1965)

10. K. Hayashi, A. I to and S. Okamura; Paper presented at In terna t l .Symposium on the u t i l i s a t ion of large radiation sources and Acceleratorsin Industrial Processing, I: nich, lAEA/STr-123/i6

-76-

• - 218 rads^«. wftoul prior Irmd.win prior wtoo.

X-410GD-TA-103 radvsac ' * 'A-6018 rads/Mc «fttwut prior kind.

-60»rads/uc i f * Prior brod

25TOOF IRRADIATION (mn)

F i j i . Plai of tprfymerisation vs. time of iradiation at different dose rates for vinyl benzoate.

250 500 7 0TIME OF IRRADIATION (mm)

Fig.2. Plot of * polymerisation vs. time of irradi ation at different temp.Dose rate. 114 4 rads/s e c .

-79-

- Without DPPH

D

O-2-187 xio

1000TIME OF IRRADIATION (min)

Fig. 3. Plot of contraction of Vinyl benzoate containing different concn. of DPPH

in ditaiometer vs. time of irradiation at 2 5 t -

Dose rate 114-4 rads/sec.

4 6 8 10 • 2 4 6 8 10*rads/sec

Fig.4.Plot of log Rp vs. log I

-80=

T

Fig. 5. Plot of log Rp vs. V T .

TIME OF IRRADIATION (miniFig.6. Plot of optical density (after 20 times dilution)

vs time of irradiation, at differentconcns. of DPPH in Vinyl benzoate.

Dose rate -114-4 rods ./sec.

-81.

1 2 3 4 5Time of Irradiation (hrs.)

Fblymerisation of tnoxane below M. P

PIS.7

ION

to 50a:LJ

o

Pre-irradiation

1 1

5 10

-82-

dose

, 1!

- 1 x106Rads at 0°C

> 20 24

TIME (brs.)

Post polymerisation of tnoxane under vacuum at b6C

FIG.8

1 2 3 4 5Time of Irradiation (hrs.)

Liquid state polymerisation at 70 C

PIG .9

-83-

EOLE OF ffAS PHASE IONIC BECOMBINATION REACTIOIB IN THEFORMATION OF EMBRYOS OF CONDENSATION NOTCIEI

by

K.G. Vohra, P..V.N. Nair and K.5 . VasudevanBhabha Atomic Research Cen t re , Bombay-85

1 . INTRODUCTION

In t h e formation of Aitken nuc l e i in the atmosphere by gas

phase r e a c t i o n s , i t i s important t o understand the mechanism of f o r -

mation of embryos. Although ex tens ive research has been carr ied out

to understand the o r i g i n of Aitken n u c l e i , the bas i c mechanisms are no t

f u l l y explainedo The embryos and n u c l e i may be formed by chemical

reactions in the atmosphere, including the possible photolytic and radio-

lytic reactions aided by solar radiations and ionizing radiations. Photo3y-

tic and radiolytie processes giving rise to ionic reactions are known to

be important in the D-region, ioeo, around an altitude of 80 km where

the concentration of ions formed by solar DV etc. , is far greater than

the concentration of ions in the lower atmosphere. Ion chemistry in the

lower atmosphere is likely to be important when we consider the ionic

reactions aided by photolysis in the formation of embryos of condensation

nuclei o This paper deals with experiments on radiolytic reactions con-

sisting of specific ionic recombination reactions to study a possible

mechanism of formation of condensation embryo So I t i s proposed that

similar reactions can take place in the atmosphere with certain trace

gases in the presence of water vapour, oxygen, solar radiation and natural

ionizing radiations*

The reactions studied deal with the conversion of certain

inorganic trace gases to the corresponding acids and the condensation

embryos and nuclei consist of one or more molecules of the acid with

several molecules of water. The trace gases used in this study are SOg,

-84-

C12» and oxides of nitrogen, which are normally present in the

atmosphere* The corresponding acids formed by pbotolytic and radio-

ly t i c reactions are HjSO., HC1 and HBO,, which form nuclei by hy drat ion.

The importance of this study i s due to the possibi l i ty that in the

atmosphere these gases are continuously converted to acid nuclei. At

saall concentration of gases normal chemical reaction would be slew

and therefore conversion process must be explained on the basis of photo-

ly t i c and radiolytic reactions.

The primary photolytic processes in the lower atmosphere are

limited to the formation of excited species and free radicals, and the

breaking of weak bonds since the energy of solar photons does not

exceed 3 to 4 eV. The ionizing radiation dose in the atmosphere i s also

extremely small ranging from 10-100 ar per hour near the ground and

200-1000 ur per hour at altitudes of 15-18 km (the region of maximum

dose for cosmic rays). These doses are too small for any laboratory

study of possible radiolytic reactions. However, i t i s found that the

radiolysis of gas mixtures during or after photolysis can lead to a

variety of interesting ionic reactions at very small dose rates of ion-

izing radiations. Such studies are particularly important in explaining

the possible reactions in the atmosphere where small doses of ionizing

radiations are absorbed by the air during or after irradiation by sun-

l ight . The experiments reported here are the preliminary studies on the

possible ionic reactions in the lower atmosphere.

2. NATURE OF IONS AND TEE BEACTI0K3 STUDIED

In the experiments to be described, the nuclei ere formed in the

gas phase, in a reaction vessel , by the ionic reactions aided by photo-

l y s i s . Therefore, i t i s necessary to give a brief description of the

nature of ions studied, in relation to ions in the lower atmosphere*

- 8 5 -

Hecent studies have shown that the predominant positive ions

in the lower atmosphere are likely to be H,0+(Ho0) , same as in the(•\ 2) 5 <i n

D-region of the atmospherev ' \ Although the evidence for the

existence of these ions in the D-region is based on mass spectrometric

measurements^5' , their importance in the lower atmosphere is based

on theoretical evidence*- ' K H,O+ ions formed in humid air cluster

very fast even at rexj low concentrations of water vapour, probably

by hydrogen bonding. The most stable cluster formed by suck hydro-

gen bonding is H,0 (Ho0), . There i s evidence that at higher humidity

the cluster can hold more than three molecules of water* , and the

total number of water molecules in the cluster may be as large as 12^ .

In the controlled experiments, the positive ions are funned

in a reaction vessel containing humid air or a suitable gas mixture,

by beta or gamma irradiation., Table I shows the reaction scheme

proposed for the formation of H,0 (H O) ions.

It i s alsj» important to know the nature of negative ions in the

lower atmosphere and the nature of ions formed in the reaction vesselo

Hohnesr'' has theoretically shown in a recent paper that after the

formation of o" ions in the atmosphere by three body electron attachment

reactions, subsequent clustering depends on the relative concentrations

of water vapour and C0_ in the airo He has proposed that O2(H2O)n i s

the likely negative ion in the lower atmosphere but i f the water con-

centration goes below that oi CO-, CO"(H2O)n ions are formed. In the

presence of trace gases S02, Cl2 and oxideB of nitrogen, however, the

nature of negative ions would be different, as also suggested by

Mohnen(7). 0~ or O ^ H ^ n e g a t i v e i o n s c a n t r a n s f e r the charge to

more electronegative free radicals and molecules formed by photolysis

of the air containing trace gases (Table 2 gives the electron affi-

nit ies of some of the speci«s relevant to this study)«, This i s an

-86-

important basi s of the present study. Photolysi s of S02 and 0 2

mixtures can form SO.!' diradical by excited oxygen mechanism (8) Or,(9)probably, through a tr ip let excited state of S O ^ ' . Photolysis

of Cl_ can form Cl atomic radicals and that of oxides of nitrogen

can give N0,« Tables 3 and 4 give the possible recctions.

In the present experiments trace amounts of S0_, Cl,, and

oxides of nitrogen are added to nitrogen with about 100 ppm of oxygen.

Although the oxygen content of the system i s far lower thai that in the

atmosphere, this was found to be sufficient for the formation of nega-

tive ions of interest for the study of recombination reactions. Photo-

lysis was carried out before or during radiolysis for the study of the2- - - -

reactions. Thus, radical ions SO, , HSO. , Cl and NO, are the likelynegative ions formed.

The products of recombination reactions of the above species

should be acids, because the negative and positive ions are similar to

those formed by solutions of acids in water. The following general type

of recombination reaction i s proposedt

where the product of recombination i s a complex cluster, consisting of an

associate of a positive ion and a negative ion, stabilized by water and

electrical ly neutral on the whole. Table 5 gives the proposed ionic re-

combination reactions leading to the formation of embryos of acid nuclei.

As will be explained in the last section of this paper, th is type of re-

combination reaction i s different from the known ionic recombination

reactions in the gas phase. Such reactions are l ikely to be of tremend-

ous importance in the formation of condensation embryos and nuclei in the

atmosphere., -

-87-

The formulation of reaction mechanism in the present study

is somewhat different from the conventional approach in ion chemistry -

The reaction products are particles or nuclei consisting of a mixture

of two components. Therefore, at the present stage, rate constants

cannot be determined, and the results are qualitative to a large

extent. The quantitative evaluation is complex since a large number

of variables are involved.

3. EXPERIMENTS

(a) Figure 1 gives a schema.vAo of the system used for the study of

recombination reactions for the formation of condensation embryos and

nuclei o The reaction vessel is a 12 1 Pyrex flask with about 100 ml of

distilled water placed at the bottom to ensure high humidity in the flasko

Light source used for the irradiation of the gas mixture is a mercury

vapour UV lamp of 200 W. After cut-off by Pyrex, the effective thresholdo

wavelength of light irradiating the gas mixture was found to be 2950 A«

The source distance is adjusted to get a total intensity of" 0<,1 mW/cm o

For gamma irradiation of the reaction vessel, a radium source is placed

at the bottom of the reaction vessel and the radiation dose at the centre

is 10 mr/hr. For beta irradiation a tritium source is suspended at the

centre of the reaction vessel and the estimated ionization rate of the

source is 10 ions/cc/sec

The carrier gas used was tank nitrogen or argon which contained

nearly 100 ppm of oxygeno Different trace gases were chemically produced

in the trace gas generator* Trace gases were generated by slow reactions

in very dilute solutions at room temperature. The reaction mixtures

were prepared aid aged to give a fairly steady concentration of the trace

gas when the flow rate over the trace generator was maintained constant.

For the measurement of the product nuclei, a Rich type of nuclei counter

was used, operated at an under pressure expansion of 20 en of mercury,

-88-

giving expansion ratio of 1.29. The minimum size of particles measured

by this counter is 8 A« The instrumeit was calibrated to cover a con-

centration range of 10 to 10 nuclei/cm3.

(b) Experiments with

A continuous flow rate of 1 litre per minute was maintained

through the reaction vessel and the estimated concentration of S0_ in

the carrier gas was 0.1 ppm. In the S0 2 experiments, light irradia-

tion was carried out prior to irradiation by the gamma source. Light

irradiation was carried out for 30 sec. and after a delay of 1 min.

the gamma irradiation was started. The delay was given for the com-

pletion of reaction of the excited species to form SO." by the reac-

tion shown in Table 3. However, it is likely that SO.'* reacts with

water giving HSO.*. During the irradiation by the gamma source the nega-2— —tive ions SO. and HSO. are the likely ions formed. Gamma irradiation

is carried out for 3 min. during which continuous formation of the ions

and recombination may be expected. Figure 2 gives the number of nuclei

counted in the reaction vessel at different times, for three cases,

(i) with the gamma irradiation alone, (ii) with 30 sec, light irradiation

alone, and (iii) with gamma irradiation following 30 sec. light irradia-

tion. There is a marked increase in the number of nuclei formed in the

reaction vessel in case (iii), i.e., gamma irradiation following light

irradiation. This provides striking evidence that radiolytic reactions

of photolised gas mixture are significant in the production of nuclei.

(o) Experiments with Chlorine and Oxides of Nitrogen

In the case of chlorine since the likely mechanism of free

radical formation is the breaking of Cl - Cl bond, light irradiation was

carried out simultaneously with irradiation by the tritium source*

Figure 3 gives the number of nuclei formed in the reaction vessel at

-89-

different times with an without tritium irradiation, with Od ppm

of chlorine in the carrier gas. For studies with oxides of nitrogen,

fractional ppm levels of HO were introduced in the carrier gas. The

flow rate was maintained at 1 l i t re per minute and ifae reaction

vessel was irradiated with light for 5 minutesa A comparison of

nuclei formation with and without the gamma dose of 10 mr/hr showed

a striking increase in the formation of nuclei "by ionization0 Fur-

ther quantitative studies with oxides of nitrogen are proposed to be

carried out.

4. DISCUSSION

Tke present experiments deal primarily with a process of gas-

to-particle conversion aided by ionizing radiations.. In the basic

mechanism proposed which i s essentially a process of chemical changec

, the embryos of the particles are formed by recombination of hydrated

H,0 ions and certain negative ions.. The important points to be dis-

cussed ares i) mechanism of recombination of hydrated ions9 i i ) nature

of the embryos and, i i i ) growth of embryos to nuclei.

The process of recombination of hydrated ions in the gas phase

has not been investigated in detail. In any process of recombination,

i t i s important to consider the release of energy in the process,

because the stability of the product depends on i t - In the recombination

of atomic ions, the energy released is the difference between the first

ionization potential of the atom forming positive ion and the electron

affinity of the atom forming negative ion. In the case of molecular

ions, the only important reactions at atmospheric pressure are the three

body recombination reactions in which the excess energy released is lost

by an impact with the third body, generally a gas molecule. However,

in the case of recombination of hydrated ions in the gas phase, we have

-90-

also to consider the dielectric properties of water as the oppositelycharged bydrated ions make a close approach. In the gas phase re-combination process in the case of dry ions, when the two ions ofopposite charge are brought closer together by electrostatic attrac-tion, on reaching the c r i t i ca l distance, the ions lose their excesskinetic energy by an impact with a gas molecule and start executingclosed orbits around each other prior to recombination. The recom-bination process is complete after the electron transfer has taken

place from the negative ion to the positive ion, with the release ofenergy. After recombination the two ions lose their identity,giving r ise to one or more neutral atoms, molecules or free radicals*In the case of hydrated ions, the mechanism may be understood asfollowsi As these ions approach the cr i t ical distance by electrostaticattraction they do not neutralize by electron transfer, due to the pre-sence of water molecules around the hydrated ions. Sue to the possiblerole of water as a dielectric medium between the ions, the ions tend toform associates of the type formed in the ease of electrolytes in solu-tion. The cri t ical separation between the hydrated ions forming ionassociates would be smaller than the cr i t ica l separation between theions which are neutralised after recombination. The difference incritical separation would depend on the dielectric constant of waterand the number of water molecules separating the two ions. The maximumcritical separation Rc (known as Bjerrum distance) for such ions, eachhaving a unit charge, may be the same as in an electrolyte solution,given by<10>

where e is the electronic charge, k isthe Boltzman's constant, T is

absolute temperature of the system and £ i s the dielectric constant

of water. The Bjerrum distance is different from the cr i t ica l dis-

tance between the dry ions, as given by the Thomson theory of ion-ion

-91-

recombination^ ', due to the effect of the dielectric molecules

separating the ions. The actual distance between the hydrated ions

would depend on the number of water molecules attached to the ions.

The stabi l i ty of such an ion associate formed in the gas phase may

be due to lesser number of molecules of water than would be availa-

ble in liquid solution.

It is proposed that the complex clusters formed in the above

type of recombination processes constitute the embryos of nuclei

formed by the effect of ionizing radiations., The formation of such

embryos does not lead to any significant release of energy as in

the case of normal recombination processes because part of the energy

is stored in the system, as in an electrical condenser,. Formation

of similar embryos by the neutral species can also be postulated on

the basis of free radical reactions„

The growth of embryos to nuclei can proceed by rapid coagula-

tion when the number formed is very large, and by further hydratione

However, a detailed discussion of this mechanism is beyond the scope

of this paper and reference may be made to the e arlier work of the

The role of small doses of natural ionization in the forma-(U)tion of nuclei in the atmospheric air has been reported earlier .

The present experiments have shown that nuclei formation by ionic

recombination reactions is mpre rapid in photolysed a i r , and the

mechanism is likely to be of great interest in the formation of con-

densation nuclei in the earth's atmosphere.

-92-

REEERENCES

1. V. Mohnen; On the nature of tropospheric i ons , Planetary Elec-trodynamics, Gordon and Breach Science Publ i shers , New York,Vo l .1 , p.197 (1969)

2 . R. Siksna; Eole of the water substance in the s t ruc tu re and by-production of ions in the ambient atmospheric a i r , PlanetaryElectrodynamics, Gordon and Breach Science Publ i shers , NewYork, Vol .1 , p.207 (1969)

3 . R.S. Narc i s i ; On water c l u s t e r ions in the ionosphere DjreglonjPlanetary Electrodynamics, Gordon and Breach Science Publ ishers ,New York, Vol .11, p.447 (1969)

4 . P. Kebarle and E.W. Godbole; Mass spectrometric study of ions xfrom the alpha p a r t i c l e i r r a d i a t i o n of gases a t near atmosphericpressures , J . Chem. Phys. 39., 1131 (1963)

5* M.M. Shahin; Mass spectrometric s tud ies of corona discharges ina i r a t atmospheric pressures , J . Chem. Phys. 45., 2600 (1966)

6. K.G. Vohra, M.C. Subba Ramu and K.N. Vasudevan, Studies on thenucleat ion of water c l u s t e r i o n s , 7th I n t . Conf. on Condensa-t i o n and Ice Nuclei , Prague-Vienna (1969)

7 . V.A. Mohnen? Preliminary r e s u l t s on the formation of negativesmall ions in the troposphere; CACR symposium on AtmosphericTrace cons t i tuen ts and Atmospheric Circula t ion , Heidelberg (1969)

8 . E.R. Gerhard and H.F. Johnstone} Photochemical Oxidation of s u l -phur dioxide in a i r , 2nd. Engr. Chem. 47., 972 (1955)

9 . Linnel l H. Robert; Photochemical nucleat ion i n a i r po l lu t ion ,I n t e r n t . Symposium on Nucleation phenomenon, Case I n s t i t u t e ofTechnology, Cleaveland, Ohio, Abs t r ac t s , 13-14 (1965)

10. J , Hose; Dynamic Physical Chemistry, S i r Isaac Pitman and SonsL t d . , London, 520 (1961)

11 . Ear l W. Mcdaniel; Coll ision Phenomenon in Ionized Gases, JohnWiley and Son? I n c . , New York. London. Sydney, 571 (1964)

12. K.G. Vohra and P.V.N. Nair; Recent thinking on the chemicalformation of ae roso l s in the a i r by gas phase r e a c t i o n s ,J . of Aerosol Science, 2_ (1970)

-93-

13< K.G. Vohra, K.N. Vasudevan and F.VONO Nair; Mechanisms ofnuclei fonniiqg reactions in the atmosphere, CACB Symposiumon Atmospheric Trace Constituents and Atmospheric Circulation,Heidelberg (1969)

14* E.G. Vohra, H,C. Subba Ramu and K.N., Vasuderan; Role of Naturalionization in the fermatian cf condensation nuclei in theatmospheric a i r , Planetary Electrodynamics, Gordon and BreachScience Publishers, New York, Vol.1, p.127 (1969)

- 9 4 -

TABLE I

Proposed Reactions f o r the Formation of H,O__yL0)Ions i n Humid Gases

1 .

2 .

( i )

(ii)

(iii)

(IT)

(v)

Np + HpO

+

A+ + HgO+

AH + HgO

N2+ + O2

0 2+

+ H.0 +

$2 + fiUO +

O^.HgO + H.

N2+

+ H.0

F O + H 2 (

O + H20 +

+ OH

AH+ + OH

H 0+ + A

H 0+ + O2 + OH

H

OH

+ OH

0+ (H20) + M

n = 1 to 7 (Mass. Spec.)

* Hate constants of most of these reactions are known frommass spectrometry.

- 9 5 -

TABLE 2

Electron A f f i n i t i e s for the Formation of KegatlveIons of Interest

°2

Negative Ion Electron Affinity (eV)

so,

HO,'

01"

so2"

0H~

4.

3.

3.

2.

2,

1,

0

0

88

85

,80

.17

.7

.58

These axe the values selected from a number cfreferences *

-96-

TABIE 3

Photolytie Formation of Free Radicals by the Irradiationof Humid Gas Mixtures Containing Trace levels of SO ,01^

and Oxides of Nitrogen by Light of h j> 2950 &

1.

2 .

so2* +

°2

so,,

SO

SO,

3 . SO4 + HgO HSO,° + OH"4

4 . Cl ,

5 . NO

Cl° + 0 1 *

NO,, + 0

NO, NO + 0

NO .

1 .

2.

3.

-97 -

TABLE 4

Formation of Negative lone of the Free BadicalSpecies of Table 3

X + e•

•f

HSO/4X

m

XC]

X

x"

+ e

+

.* +

+ e

+

-

HSO4"

+ e

Cl#

e"

—•

NOg + NO2

NO " + a , 0 + M

—>

— ^

HSO ~ + X4

HSO ~4X~

Cl~ + X

X"

NO"" + X

NO NO

+ M

TABLE 5

Proposed Gas-phase Recombination React!one of HyflratedIonic Species for the Formation of Embryos

Add Nuclei

+ d

zcoL

EI

X)

o2HTO>

nmo

m

•

oo

n"030OocoTIO

N

O•n

cinmoTIOao

XoHor

TIC

>2O

oc

RE

-I

(It

r>X

1o-nHXfS

YS

m2 ^

INLE

T

1 •

>u.v

ys/. '/AI-- ^—v.r [ .

V S--—--

L*-TO VACUUM CHAMBERAND SUCTION PUMP.

L t -

T fi GPNERATOf?

3 FlLTER-2

P> STOPCOCK

f?EACTIONVESSEL

1 STOPCOCK

EXPANSION

NUCLEI COUNTER

NUCLEI PER CC

LIGHT STARTED O>29SQ)

I•GAMMA SOURCE

n IP •>I i ip r o

1 1 1I -< 2** _ >

ill5 5 s

l

/UNDER NATURAL lONIZATION

50TIME (.mts)

FIGURE-3. FORMATION OF CONDENSATIONNUCLEI BY lONIZATION OF NITROGENWATER SYSTEM CONTAINING O.I ppmOF CHLORINE WITH SIMULTANEOUSIRRADIATION WITH LIGHT ABOVE 295OA*

-100-

HADIATION CHEMISTRY OF SYSTEMS OP BIOLOGICAL INTEBEST

B.2 . SinghBiology Div i s ion

Bhabha Atomic Research Centre, Trombay, Bombay-85

IHTRODUCTIOH

In a symposium on "Radiation Chemistry" when seme one gets

up to speak on biological systems, one cannot miss a remark onee

made by Erof • Z«M. Baeqr about biologists and radiation chemists.

He wrote "Biologists are exceedingly embarrassed in public discussion

when some importent sc ient ist rises and says that you are talking

nonsense, your experiments are a l l wrong and your ideas are properly

unthinkablec Radiation chemists are never embarrassed, they have

answers ready for the dirt iest questions„ Their way i s that of rope-

daneers smiling in their permanently unstable equilibrium which those,

who do not belong to the guild are forced to admire". This remark

makes at least one point very clear that radiation chemistry has given

something admirable to biology. In the present communication attempts

wil l be made to discuss a few selected systems which are not of biolo-

gical origin but the radiation chemical studies on them have made a

considerable impact on radiation biology.

Amongst these systems radiosensitisers have attracted maximum

attention by radiation chemists. Iodoacetamide, iodoacetic acid and

N-ethylmaleimide are the most potent and thoroughly investigated

radiosensitisera (see ref e 2) o These compounds are a l so known to

bind eff iciently with -SH groups which protect organisms against

radiation damage. The sensitisation was, therefore, earlier thought

to be due to intraeellula* lowering of -SH levels• However, evidence

has now been obtained that some short-lived transients are involved

-101 -

in t h i s phenomenon al though -SB poisoning may also have a s l i g h t role

to p l ay .

IGD0ACETA1LIDE AND RELATED OOMFOUinS

With a view to t e s t whether shor t - l i ved toxic products of

s e n s i t i s e r s were involved, Dewey and Kichael^ ' i r r a d i a t e d iodoacetamide

and exposed i t to c e l l s a t various per iods a f t e r i r r ad ia t iono When the

period between the end of i r r a d i a t i o n and exposure to c e l l s was pro-

gressively increased, lethality of cells decreased,, The cells irradia-

ted in a similar manner and exposed to the unirradiated sensitiser did

not show any effecto I t can, therefore, be inferred taat some short-

lived transient of iodoacetamide reacted with cells to cause the enhanced

killingo Singh et al^ ^ subsequently studied the mode of formation of

various transients of iodoacetamide using pulse radiolysis technique,,

They postulated I , I " and "CHpCONHp as the intermediate species pro-

duced during radiolysis of iodoacetamide;

I CH2CONH2 + eaq. — > i " + 'CHgOOHHg . . . . (1)

I^+OH ^ I ° + 0H" . . . . (2)

I* + I" ^ I 2 " • . . . . (3)

oooo ( 4 )

Amongst these "CHgCONHg was thought to toe relatively unimportant for

radiosensitisation since i t could be produced by reaction of hydroxyl

radical's with acetamide^^ which is ineffective in modification of

radiation response of cel ls . I" and 1~ can also be produced during

radiolysis of other iodine compounds such as EE and methyl iodide in

addition to iodoacetic acid. On investigation, they also proved to be

-102=

excellent radiosensitisers'*'» Purtbexaore tha sensitisation dueto these chemicals could be enhanced by scavenging eaq with MO

o r SO, and reduced on scavenging OH radicals with thiocynate ion.The mechanism of formation of sensitising species, therefore, invol-ved a reaction between the seasitiser and the hydroxyl radicals«

O O D O O D O \S "f" OH ' —" m^ S

S° + C e l l ? C e l l d e a t h o o t , , o o o ( 6 )

where S is the sens i t i ses

Further evidence for involvement of halogen transients inradiosensitisation was obtained by Shenoy et al^ '„ Using 5 Ilabelled iodoacetic acid and potassium iodide, they showed thatEo coli B/r cells became iodinated on irradiation in presence ofthese sensitiserso The radioactivity associated with cells was in-significant without irradiation or on irradiation of cells andsensitiser separately and then mixing togethero They also observedthat the amount of radioactivity in cells increased with dose ofradiation and i t was higher under oxic than anoxic conditions ofirradiationv o The presence .of KDKS during irradiation was foundto reduce the amount of J I associated with cel ls and fractionation

of cells into various morphological parts and chemical constituentsrevealed that iodine was incorporateproteins at all amino acid residuesrevealed that iodine was incorporated mainly in the bacterial membrane

(8)

The indiscriminate iodination of prote ins a t a l l amino acidresidues would lead tb the inactivatlon of enzymes and disturb variousmetabolic functions vital, to cell survival. The syntheses of proteinsand DNA were found to be grossly Inhibited in cells which becameiodinated due to irradiation or due to exposure to iodine atoms pro-

-105-

duced during the reaction of sensitiser with OH radicals in

Penton's reaction^ * '« In addition, two respiratory enzymes

namely, ATPase and succinoxydase were also found to be inactivated

as a result of iodination. In fact , in vitro irradiation of

alcohol dehydrogenase resulted in enhanced inactivation of the

enzyme in presence of iodine compounds ' although results contra-

ry to this have been reported in trypsin^ '•

It can, therefore, be inferred that though they react with

-SH groups, the'sensitising property of iodoaeetamide or iodoac-

e t i c acid i s mainly due to I-atoms released during the radiolysis.

Being highly reactive, iodine atoms combine with proteins/enzymes

causing inhibition of vital metabolic processes in cells* The

enhanced ki l l ing of ce l l s in presence of sensitiser, therefore, i s

a result of "total protein—poisoning" due to iodination of proteins

and enzyme So

N-ETHYLMALEIMIIJE (HEM) AND BELATED COMPOUNDS

The r a d i o s e n s i t i s i ng property of HEM was a l s o ascribed to i t s

high r e a c t i v i t y with -SH compounds. However, when Adams and c o -

workers^12^ i rrad ia ted HEM and bac ter ia l c e l l s with e lectrons using

the rapid mixing technique they observed t h a t ,

(a) before i rrad ia t ion the contact of ce l ] , s with HEM

for 4 - 40 msecs was sufficient to exhibit full

sensitisation. The half time of the second order

reaction between equimolar solution of HEM and

cysteine groups under optimum conditions was earlier

estimated to bs about 10 sees. The 4 - 40 mseos of

-104=

contact of HEM with ce 11B would not, therefore,

1« sufficient for SH-bindmg to occur and to cause

sensitisationfl

(b) cells exposed to irradiated HEM after 4

msecs did not show any effect ,

(c) cells irradiated and exposed to unirradiated NEM

after 4 msecs exhibited slight killing (WF » 1.5)°

On the basis of these results -SH binding cannot account fc

sensitisationo Two further inferences can be drawn involving .shor

lived transientso

(i) NEM reacts with some transients of cells,

( i i ) -fee sensitising transients of NEM are shorter-lived

than 4 msecso

The results obtained by Ward et al ^ ' supported the first

alternatives They demonstrated that MEM reacts with eaq, as well a1 0 — 1 — 1 Q —1

OH, the ra te constants being 3o2 x 10 M sec end 5 = 9 x 107~M

respectively-, In addition, they also investigated the destruction

NEM in presence of various consti tuents of cells» G(-NEM) was fourk

decrease with increasing concentrations of thymine in the solution !

at 10 m9 whese a l l the species produced in water would have been

scavenged by thymine9 G(-NEM) was s t i l l about :>o3<> In presence of ]

during i r rad ia t ion , th i s residual G(-NEM) almost doubledo I t was,

thereforep suggested that NEM reacted with the rad ica l s formed by

OH attack on thymine

Thymine + OH '• ^ TOH „ „„ <, oo „ „„ (7)

TOH + NEM > destruction of HEM « . . . (8)

-105-

0n irradiation in presence of oxygen, however, they reporteda pure competition of thyiaine and HEM for ttie radiolytically producedhydro y l radicals. HEM would have less chances of reaction withTOH due to the presence of O? and the reaction,

TOfi + Og —* TDBOg . . . . . . . . . (9)

Using 4C-HEM Johansen et al' ^' also presented evidencefor reaction of HEM with transients of DHA during in vitro irradia-tion. I t , therefore, appears that during radiosensitisation by HEM,a reaction between the sensitiser and some transients of biomolecule si s involved.

Adams , however, favours the other alternative suggestinginvolvement of HEM transients during radiosensitisation. He reportedthat HEM is capable of stabilising eaq. by way of i t s resonating

structure,

I

If reaction of eaq. with certain target molecules in cells was causingthe damage, the presence of many other molecules would reduce its

chance of reacting with such sites. The electron stabilising coo-pounds could transport eaq" to these sites provided of course theelectron affinity of sites is higher than that of the sensitiser.Various other organic chemicals which possessed such electron stab-i l is ing groups were, in general, found to cause radiosensitisation.

-106-

This hypothesis has been well appreciated by biologists and i t

satisfactorily explains how indirect effects of radiations can

be enhanced by the presence of such sensitisers^

The recent work in Adams' ^ laboratory has, however,

demonstrated electron transfer from nucleic acid constituents to

sens-tisers such as MEM. This i8 a traffic in the reverse direc-

tion. Instead of transporting electrons to the vital molecules

in cells, HEM has picked up electron from the radiation induced

transients of nucleic acid constituents. This phenomenon would

be extremely importait for direct effects of radiation. As Adams

and Cookes J have pointed o at, 3n electron knocked out from one

part of a macromolecule would get thermalised at some other part

causing almost a polarisation effect. Charge neutralisation would

restore the molecule but if the electron is transferred to the sen-

sit iser, the macromolecule carrying a positve charge would be un-

stable and would break down giving rise to free radicals. The

sensitiser would thus enhance the direct effects of radiation on

cells. Whether a reaction between the transients of sensitisers

and biomolecules or vice versa, is responsible for sensitisation, is

s t i l l an open question* In addition, the reaction of some of the

sensitisers with intracellular -SH groups and their inhibitory effect

on post-irradiation repair processes may also be involved in this(18)processv •

MISCELLANEOUS COMKHJHDS AND A FDTUBE APPROACH

Some short-lived transients of sensitisers are believed to

be involved in sensitisation also by vitamin K_. and i t s various(7) ~

anologuesv ' . Although the identity of these transients could not

be established, these were shown to be formed due to reaction of the

sensitiser with radiolytioally produced hydroxyl radicals. Using

-107-

scavengers for eaq, H and OH and the rapid mixing technique,

detailed studies on various radiosensitisers are urgently warran-

ted. Whenever found responsible, the radiosensitising transients

which are usually free radicals or radical ions can he easily

investigated with the help of pulse radiolysis, electron spin

resonance and simple methods of product analysis. Such in-

vestigations are now in progress in our laboratory using the

e.s.r. technique. In addition, the differential response of

various living systems to radiosensitisers is also being studied.

This is by no means unimportant since the extent of radiosensiti-

sation has been found to depend on the nature of organisms; i t being

greater in radioresistant than radiosensitive -trains (see ref.2).

This would most probably need an explanation not in terms of radia-

tion chemistry but in biological language.

AC HNOWLEDGE&ENTS

The author wishes to than*: Dr. A«R. Gopal-Ayengar for advice

and encouragement and Shri M.Ac Shenoy and D.S. Joshi, who collaborated

in this project.

REFERENCES

1o Z.M. Bacq; Chemical Protect ion against ionising radia t ion,P,218 Published by Charles S. Thomas (1965)

2« B.A. Bridges? Adv. in Rad. Bio l . £ (136$) ( in Press)

3 . D.L. Dewey and B.D. Michael; Bioch. Biophy. Res. Commns. 21.392 (1965)

4 . B.B. Singh, A Charlesby, J . P . Keene and A.J. Swallow; 3rd Int l . .Congrc Radiat . Research, Cortina D1 Ampezzo, (1966) (Abstract) .

5. L. Mullenger, B.B, Singh, M.G. Ormerod and C.J. Dean; Nature216. 372 (1967)

-108-

6. M.A. Shenoy, B.B. Singh and A.R« Gopal-Ayengarj Science 160.999 0968)

7. B.Bc Singh, M.A. Shenoy and A.R. Gopal-Ayengar; Studia Biophysica15/16, 263 (1969)

8. A.R. Gopal-Ayengar, D.S. Joshi, M.A. Shenoy and B.B. Singh;I I I Intli . Biophysics Congress, Mass (1969) To be publishedin Adv. in Biol. Med. Phys.

9. B.B. Singh, V.T» Srinivasan, B.Y. Bhatt, II.A. Shenoy, D.S. Joshiand V.W. Bedekar; B.A.R.C. - 451 (1969)

1,0. M. Quintilliani and L. Bernardini; Experime"fitia 2£ 630 (1967)

11. B.B. Singh and A. Kabi; Proc. Natl. Ins t t . Sciences (India)2£, 291 (1969)

12. G.E. Adams, M.S. Cooke, B.D. Michael; Nature 212, 1368 (1968)

13. J .F . Ward, I . Johansen and J . Aaeen; I n t l . . J . Rad. Biol. 1^163 (1969)

14. I . Johansen, J«P. Ward, K. Siegel and A. S le t ten ; Biocho Biophy.Ree. Conmms. ^5_, 949 (1968)

15* G.E. Adams; Current topics in Sad. Res. ][, 35 (1967)

16 . C.L. Greenstock, RSL. Wilson and G.E. Adams; I I I n t l . . Symp. onRadiosensitising and Radioprotective drugs, Rome (1969)

17. G.E. Adams and M.S. Cooke; Intl .-; . J.Rad. Biol . 1^, 457 (1969)

18. L. Mullenger and M.G. Ormerodj I n t i * . . J . Rad. Biol . 1£ (1969)

-109=

EXPERIMENTAL TECHNIQUES

R» Mahadeva I y e rChemistry Div is ion

Bhabha Atomic Research C e n t r e , Trombay, Bombay~85

Whenever h i g h energy r a d i a t i o n i s absorbed by mat ter , i t

results in extensive, sometimes short-lived, perturbation of the

system which may later manifest i tself in the form of excitation, ion-

isation and decomposition of the substance under investigation. Thus,

the aim of a radiation chemist is to find out the proper chronological

sequence of events which ultimately end up in the final stable products

of radiation damage to the system,,

The time required for the energy deposition is so short compared

to the subsequent physico-chemical changes that with our present day"~12

techniques we can at best look back 10 seconds after the energy de-

position took placeo The modern techniques available to the radiation

chemist are sufficiently sophisticated that a wealth of information on

specific reaction mechanisms has been well elucidatedo

I shall not refer here in a comprehensive way on the various

methods adopted, but instead deal with some of the major ones and

their specific advantages,,

(A) Let us f i rs t look at the conventional methodB used. If we were

to irradiate a substance with "/=rays or electrons and after the

irradiation analyse the products by gas chromatography or other highly

sensitive techniques,, we will get an idea about the yield and nature

of the products produced., If from this information one has to work

back on the sequence of events which ]ed to the formation of the stable

products, the available data becomes obviously inadequate and several

=110-

alternate routes can be envisaged for the production of the same

product. Therefore, what one does is to intercept a possible reac-

tion and see the effect of such an interception on the final products.

The classical example of this type is the use of scavengers

which can be free radical scavengers like halogens, C labelled CgH.

or ionic scavengers like NgO, SFg, etc. , or even scavengers for

specific excited states. Extensive data of this type is available

in the li terature- However, i t is worth-while to mention that if

halogen scavengers are used, use of an electron capture detector with

a gas chromatograph enhances the analytical capability of the study

because in many instances the electron capture detector is at least

1CT times more sensitive than the well-known flame ionisation detector;

consequently one can work with much lower concentrations of halogen

scavengers, thus minimising Hie rolevof nalpgennin intercepting ionic

reactions. The; study-can therefore, be extended to much lower radiation doses,

We have found^ ' using such a detector that Br? radiolysed in

hexane gives many more alkyl bromides than so far reported. We have

not yet been able to identify all the products; photochemical reactions

of the same system also produce similar products.

Much more surprising in the fact that I- in hexane or cyclo-

hexane photolysed in Pyrex tubes with room light produced alkyl(2)

iodides* ' which can be detected only by an electron capture detector.

Dr. Gaumann and his collaborators^' have been able to label

hexane with deuterium instead of hydrogen at various specific positions

of the molacule and, by analysing the products by gas chromatography,

have been able to show the G values for radical fragmentation of

different bonds,.

(B) I have mentioned that, in order to, work back on the sequence

of events which lead up to the final products, i t i s necessary to know

-111-

the major routes through which the chemical changes take place,

e.g., ionisation vs free radicals and excitation. In order tj

estimate the ionic yield on irradiation, either one could use

specific ion scavengers which are not always easy to come ty or

apply an electric field to the system and from the measured current,

characteristics of the cell and an idea of the mobilities •-•? the

species, ionic yields and life-times can be estimated. Elegant

work of this nature has been reported by Freeman'^' and Allen' .

For example, Schmidt and Allen^ ' used a clearing field method

wherein, after a pulse of X-rays lasting 5 x 10 seconds, a high

voltage of 3 - 5 kv was applied for as long as 1 - 2 seconds so that

drift of the ions to the cell electrode was complete- Using a con-

denser circuit, the collected charge was measured in an electro-

meter amplifier. They showed that G,_ . \ yield ranges from

0.05 for benzene to 0,85 for neopentane*

If, instead of estimating the yield of ions, one is interested

in the role of ionic reactions in the formation of end products, the produ-

ct distribution could be analysed with and without an applied field.

However, i t i s worth-while to bear in mind that the field strength

must be kept sufficiently low so that no electronic excitation takes

place by collision with the accelerated electrons. But, if one works

in the saturation current region in gases up to the point where electron

multiplication occurs, the results may be able to distinguish between

fast ion-molecule reactions which are unaffected franincreased molecular

excitation and their consequent effects* =

There is some scattered information on the effect of magnetic

fields on enhancement of photo-conductivity of molecular crystals

like anthracene, tetracene, etc. These are suggested to be due to the(7)role played by excitons which are sensitive to magnetic fields

-112-

gIt has been reported*1 J that application of a constant magnetic

field of 6000 Oersted enhanced the rate and yield of polymerisation of

HCHO at -196° by a factor of 2, perhaps suggesting the role of charge

transfer excitons in polymerisation.

(C) It is obvious that the primary and secondary electrons pro-

duced in radiolysis enter into chemical reaction only when their

energies have been brought down to 10 to 20 ev above which no stable

reaction product i s possible. Therefore, controlled electron aniflsion in a

mass spectrometer with the analytical advantages lends itself to the

study of the radiation chemistry of gaseous systems. Unlike con-

ventional mass spectrometric studies at high pressures where the

interest is i s ion molecule type reactions, Rudolphw/ has used what

he calls a wide range radiolysis source with a mass spectrometer to

study the radiation chemistry of methane. The advantage of this

method is that by the use of multiple ionisation cells neutral and

transient species and their subsequent reactions can also be studied.

Thus, i t was shown that methane produces on radiolysis about 50/0+ve

ions and ^Ofo neutral primary species.

There are two basic ways by which one can look at the dynamic

changes produced in a system under irradiation.

(i) 5y keeping the observation time and sensitivity comparable

to the dynamic changes being produced, i .eo, pulse radiolysis and

(i i) by momentarily freezing out the charges or speoies produced

in such a way that in many cases the observation can be made some time

after irradiation. This i s , of course, the matrix isolation method.

The comparison of data between such a frozen or solid system with that

of a gas or liquid is not always straight forward aid hence the method

has i ts limitations.

-113-

Extensive work has been done on absorption spectroscopy of

t rans ient species produced and trapped in glassy matrices^ ' a r !

pa ra l l e l ESR studies have also been carr ied out' ' . The matrices

used general ly are* 3-methyl pentane, methyl tetrahydrofuran, ethyl

alcohol , and alkal ine i c e , because of t h e i r optical transmission

advantage. However, s igni f icant r e s u l t s have been reported for two

types of so called opaque samples. Shida and Hamill^ ' , by freezing

CCl. in 1 mm optical c e l l s , were able to get translucent samples good

enough for absorption spectroscopy and thus eliminated the l imitat ion

imposed by glas3 forming compounds. Iyer and 7,riliard' ^ , using

p e l l e t s of C-Br/- of about 0.2 mm thickness have been able to get

meaningful da ta on radlolysis -f th is compound by absorption spectro-

scopy.

Perhaps the most widely used and easy- to- interpret i s pulse

rad io lys i s of which Hart has given us a great deal of information.

Excellent reviews and two books on pulse radio lys is are now availa-

ble^ . Time resolut ions of the order of 10 seconds have been

achieved by Bronskill and Hunt- ' using the fine s t ructure of a

l inac beam and detection by Cerenkpv l i g h t .

In those cases where the t rans ien t species produced by pulse

r a a i o l y s i s do not exhib i t optical absorption in a convenient region of

the spectrum, an ESR spectrometer has been coupled to a pulsed beam of(16^

electrons giving time resolut ions as small as 1 U.second ' .

REFERENCES

1. K. Annaji Rao and R.M. Iyer (Unpublished data)

2. H.M. Supta and R.M. Iyer (Unpublished data)

3 . T. Gaumann and B. Reipso; Radiation Chemistry - I I . Advances in Chemistryse r i e s 82, p.441 (5968)

4. G.R. Freeman and J.M. Payadh; J . Chem. Phys. 4JL» 8 6 (1965)

-114-

5. W.F. Schmidt and A.O. Allen; J. Phy. Chem. 22» 3730 (1968)

6. P. Ausloos and S.G. Lias; Chapter 1. in Actions Chimiques etBiologiques des Hadiations (19^7)

7. V.L. Talrose; Chapter II in Action Chimiques et Biologiques desRadiations (1967)

8. I. Tabataj Chem. Technol. Polym. £, 3 09^5)

9. P.S. Rudolph; Radiation Chemistry II. Advances in Chemistry Series,82, p.101 (1968)

10. W.H. Hamill; in "Radical Ions", Interscience (1967)

11. J.E. Willard in "Fundamental Processes in Radiation Cheinistry",John Wiley (1968)

12. T. Shida and W.H. Hamill; J. Chem. Phys. 4ii 2369 (1966)

13. R.M. Iyer and J.E. Willard; J= Chem. Phys. 46, 3501 (1967}

14> See for examples

L.K!. Dorfman and M.S. Matheson; Progress in Reaction Kinetics,Vol.3 (1965)

E.J. Hart; Ann. Rev. Nuclear Sci. 1J>, 125

M. Ebert, J.P. Keene, A.J. Swallow and J.H. Baxendale (eds);"Pulse Radiolysis", Academic Press (1965)

M.S. Matheson and L.M. Dorfman; "Pulse Radiolysis", MIT Press (1969)

15. M.J. Bronskill and J.W. Hunt; J. Phy. Chen. J2, 3762 (1968)

16. B. Smaller, J.R. Remko end E.C. Avery; J. Chem. Phys. 48, 5174 (1968)

C0MME5TS

C. Gopinathan 1 It is interesting that the effect of a magnetic

field en the yields in certain radiation chemical

systems should be attributed to excitons. He

had suggested in 1966 (F.S. Bainton and

C. Gopinathan, Transo Faraday Soc, 6£, 143)

that mobile energy, as an exciton, might be in-

volved in the radiation chemistry of low temper-

ature sulphuric acid glasses. It is interesting

that more evidence should accumulate about the

possible role of excitons in radiation chemistry.

-115-

RADIOLYSIS OF FERROUS-XYLENOL ORANGE SYSTEMIN ACIDIC SOLUTIONS

B.L. GuptaDi rec to ra te of Radiation Protect ion

Bhabha Atomic Research CentreTrombay, Bombay - 85

INTRODUCTION

Hakoto OtonKr ^ has reported t he formation of di f ferent com-

plexes of f e r r i c ions wi th xylenol orange. His r e s u l t s show that in1 1 J

acidic medium, Fe reacts with excess of xylenol oraige (XO) to form

1 1 1 and 1 : 2 complexes with absorption maximal at 545 nm, respectively,

while excess of Fe forms a 1 :1 complex with absorption maximal at

586 nm. The optimal formation of the complex having the maximal absorp-

tion at 545 Dm has been shown to be in 0.034 M perchloric acid. Molot(2)

et alv ' also used this complex formation for the estimation of Fe

They buffered their solutions to a pH of 2.6 using chloride-acetate

buffer.

In the present work the formation of the complex between Fe1

and xylenol orange has been studied in aqueous sulphuric acid solutions.

The complex has been used for the estimation of the ferric ions produced

by radiolysis of a solution containing both ferrous ions and xylenol

orange.

EXPERIMENTAL

A tetra-sodium s a l t of xylenol orange, reagent grade, was d i s -

solved i n t r i p l e d i s t i l l e d water t o give about 10"5M concentrat ion.

The ex t inc t ion coe f f i c i en t value of 3*12 x 10V" cm" a t 580 nm a t

pH 10.0 was used fo r t h e estimation of xylenol orange concentration^ ' .

-116-

Sulphuric acid solution was prepared by diluting BDH

AnalaR grade material in triple distilled water. Stock solutions of

ferric ammonium sulphate and ferrous ammonium sulphate were pre-

pared by dissolving AnalaR grade materials in 0»1 ft sulphuric acid.

Sodium chloride solution was prepared "by dissolving AnalaR grade salt

in triple distilled water.

Stoppered tubes of I.D. 12.0 mm and wall thickness 1.0 mm

were used for irradiations. The irradiations were performed in

cobalt-60 gamma cells. The calibration of the cells was done using

Fricke dosimeter and dose absrobed by the solutions was calculated

after correcting for the difference in electron density of the solu-

tions. The irradiations were done in aerated solutions.

All measurements were made on a Beckman SB spectrophotometer.

In the study of Fe ( i l l ) and xylenol orange complex formation, the

solution without Fe ( i l l ) was used as blank. In radiolysis studies,

the unirradiated solution was used as "blank.

RESULTS

Fig.1 shov/s that the connlex between fe r r i c ions and xylenol

orange has a maximal absorption at 540 run. The complex for re t ion

increases with increase in sulphuric acid concentration and i t i s

optimal in 0.05 S sulphuric acid (Fig .2) . Under these condit ions,

ferrous ions do not form any complex with xylenol organce (F ig .1) .

There i s no complex formation of f e r r i c ions with xylenol orange in

0.8 N sulphuric acid.

Fig*3 shows the increase in the absorbance at 540 nm with

increase in dose at different i n i t i a l concentrations of ferrous ions

in the solut ion. In a l l experiments, the i n i t i a l concentration of

xylenol orange was kept constant at l O " ^ .

-117-

The increase in the absorb an ce at 540 rm with increase in

dose at different i n i t i a l concentrations of xylenol orange ii« the

solution i s shown in Fig.4« In a l l these experiments, the in i t i a l

concentration of ferrous ions in the solution was kept constant at

10 **!!. "/hen the i r rad ia t ions were continued, the abscr'oance at

540 nm decreased (Fig.5)- The atsorbance increased again i f more of

xylenol organce was added to the solution after i r radia t ion .

The addition of sodium chloride in the solutions before

irradiation did not change the radiolytic yield (Table I ) .

Ferric xylenol orange complex does not show very significant

decomposition on radiolysis (Table I I ) •

DISCUSSION

The following mechanism can be proposed to explain the results 1-

H 0 —A.AA/VV ? e , H , O H , ^ o ^ P ' ^ 2 • • • • • • • •

i + H+ H (2)aq

H + O2 = H02 (3)

g H 2O 2 (4)

Fe+"* +H2O2 = F e ^ ^ + O H + OH (5)

Fe4^ + OH - Fe4** + OH . ' ( 6 )

XO + OH - XO.OH (7)

Fs"1^" + XO.OH = F e + + + + (XO.OH)" . . . . . . . . (8)

In the radiolysis of the aqueous solutions of xylenol orange in

aci<?ic solutions, i t has been observed that hydroxyl radicals oxidize

-118-

zylenol orange very fast whereas H02 radicals react very slowly. In

dilute solutions, H-0 has no appreciable reaction with xylenol orange(3)but i t has been found to oxidize the ferrous ions quantitatively even

in solutions containing xylenol orange.

When ferrous ions are added to the xylenol orange solution,

HOL and H_0_ oxidize ferrous ions whereas OH radicals react with xylenol

orange and ferrous ions both. The rate of the reaction of OH radicals

with xylenol orange i s about four times higher than with ferrous ions .

Since the in i t ia l G-value for the oxidation of ferrous ions has been

found to be 15.6, i t seems that the in i t i a l product from the radiolytic

oxidation of xylenol orange also oxidizes ferrous ions. The ferric ions

formed by the radiolytic oxidation of the ferrous ions form a complex

with xylenol orange.

At low concentrations of ferrous ions probably a l l the oxidized

xylenol orange does not react to oxidize ferrous ions and thus a lower

absorbance i s obtained at 540 nm (Fig.3). With increase in ferrous ion

concentration the absorbance increases t i l l a l l the oxidized xylenol

orange has reacted with them. At high concentration of ferrous ions, the

absorbance i s lower because there i s not enough xylenol orange present

in the solution to complex a l l the ferric ions which are i n i t i a l l y present

in the solution and produced on radiolytic oxidation.

The increase in absorbance with increase in concentration of

xylenol orange (Fig .4) i s expected since more of the ferric ions will

complex with i t . Fig.5 shows that, as the dose i s increased, i n i t i a l l y

the absorbance at 540 ma increases and then i t gradually decreases. The

decrease i s due to the non-complexing behaviour of the oxidized xylenol

orange since the absorbance again increases i f more of xylenol orange

i s added after irradiation i s over. The oxidation of xylenol orange i s

very significant, since OH radicals produced in reactions (1) and (5),

react with xylenol orange in reaction (7) .

The non-interference by the chloride ions can also be explai-

ned since a l l the OH radicals react to oxidize ferrous ions . The traces

-119-

of Impurities can not compete with the highly reactive xylenel orange.

Figure 6 shows the radialytic transformations broaght about

in xylenol orange in the ferrous xylenol orange system. The decolora-

tion of phenolphthalein by hydroxyl ion addition in a similar way is

well known . Mckeown and Waters1- ' have also shown that in alkaline

solutions phthalein dyes add HOL at the same position. The-observed

value of 15.6 for ferric ion yield also supports this mechanism; other-

wise the ferric ion yield would have been higher due to peroxide for-

mation. The non-complexing properties of irradiated xylenol orange

also show that quinoid oxygen which is known to take part in complex

formation has undergone transformation. The structure of metal com-

plexes of xylenol orange has been given by Kovalenko et al .

-4To sum up, solutions containing 10 M each of ferrous ions

and xylenol orange in 0.05 N sulphuric acid can be used to measure

the radiation dose in the range 5 - 3000 rads using a calibration

curve. Higher doses can also be estimated when the concentration of

xylenol orange in the solution is increased either before or after

irradiation. The same system can also be used to estimate the HpO

produced in irradiated solutions since equivalent oxidation'of ferrous

ions by HpO has been observed.

ACKNOWLEDGEMENT S

The author i s grateful to P.N.Krishnamoorthy and

G. Subrhamanian fo r t h e i r keet i n t e re s t in and encouragement of t h i s

work. Thanks are a l so due to Dr. A.K. Ganguly for h i s valuable gu i -

dance and t o P.H.Kamath for useful suggestions.

REFERENCES

1. Makoto Otomo; Composition of the xylenol orange complexes ofPe4*"*" and t h e i r appl ica t ion t o the determination of iron or xylenolorange; Bunseki Kagaku jU (&), 677 (1965)

2 . L.A. Molot, I . S . Mustafin, and R.P. Zagrebina; Determination ofcoexistent aluminium and iron with xylenol orange; Izv. Vyssh.Ucheb. Zaved., Khim. Technol. 9. ( 6 ) , 873-5 (1966)

-120-

3. B.L. Gupta (Unpublished work)

4. I.I. Finar; Organic Chemistry, Volume I, Ifege 791. Longmans,Green and Co. Ltd., (1964)

5. E. Mckeown and W.A. Waters; Use of ESR spectroscopy in tracingthe mechanism of oxidation of phtnalein dyes, J.Chem.Soc.679(1966)

6. P.N.Kovalenko, K.R. Bagdasarov, O.E. Shelepin and M.A. Shemyakina;Spactrophotometric study of xylenol orange and its complexes withbismuth, Zh. Obshch. Khim. 38 (9), 2015

-121-

TABLB I

Effect of Sodium Chloride on thePe rrous-Xylenol Orange

Cone, of

Cone, of

Cone. of

BoBe

Cone

(M

0.0

0.8

2 . 0

4 .0

10.0

ferrous ions

xylenol orange

sulphuric acid

Badiolytic Yield ofSystem

=s

=

-410 *M

10~4M

0.05 N

1960 rads

. of NaCl Absorbance at 540 run

x 103) (Cm"1)

0.31

0.31

0.31

0.31

0.31

-122-

TAHiE I I

Badiolytic Decomposition of Ferric - XylenolOrange Complex

Cone, of f e r r i c ions = 5 x 10" M

-4Cone, of xylenol orange = 1 0 M

Cone, of sulphuric acid = 0.05 N

Dose Absorbance a t 540 na

(rads x 1O~3) (Cm"1)

0 0.40

1.31 0.40

2.25 0.39

3.19 0.38

4.12 0.37

- 123 -

|0 • 20

360 520 480Wove length, nm

FIG. 1 Compiling behaviour of Iron(II) and Iran I l l l l

with lylenol orange

A F* Hil l , 2XU^ ,XO.10 4 M and* Hill, 2XU^,XO.10M and HjSO^.OOISN.

B t C ; F* 1111, 5X IOSM,XO, W4M a>it H2SO4.0 05H.

on

0*0

030

Ft( I I I ) : 3.IIXIOSM

XO 10'*M

0-20C01

FIO 2

002 0C7003 004 &05Cong of H2 SOt ,N

Effect of lulphur-r acid eonc on Fe (III)-XO cample* formation

-124-

0.50-

1.0 BO2 0 3 0 4 0OOM ( rate X Iff3

FIO.S RadlaCytO ot F»(H)-X0 syctwn atdlft«wnl concentration* Of F»m)

FIG. 4

3-0DoM.radixiO3

Rodlelyslt of F«<II)-XO •yattmat dlttwcnt eoneanlratlona of XO

- 1 2 5 -

2.3X10 X0 added af•« IIrradMHpo - f f

1 .

10 20 29 30

Dos», rads< 10

FIO. S EMtet of lyltnal orang* addition after Irradiation

OH ;<I '

* ^

OH

J*\(CH2COOHJ2

OH

sCH2-N(CHjCOOH)2

K (CH2COOH32H

H

(XO)

-XO complM

CMj

rt €=>-'

(Non-comp»»xlng XO?

FIO. 0 RadJolytlc «ronsfonnattor» erf xylenol orangs In Fe - XO »yst«m

-1 26-

RADIOLYSIS OP HiUTOHIUM( I I I ) IN AQUEOUS MEDIUMe «

M.S. Nagar and P.R. NatarajanRadiochemistry Division

Bhabha Atomic Research CentreBombay-85

This paper presents the r e s u l t s of our invest igat ions on the

effect of cobalt-60 gamma radiation on millimolar solut ions of pluto-

nium(l l l ) in 0.8N hydrochloric acid in argon atmosphere. The relevant

work t h a t has been carr ied out e a r l i e r in our Division on the radio-

lys i s of plutonium(ill) in hydrochloric and sulphuric acids i s f i r s t

reviewed.

Ghosh-Mazumdar, Singh and Srinivasan ' have reported t he i r

resu l t s on the oxidation of plutonium(il l) in 1.0 - 8.0M hydrochloric

acid solutions by gamma rays from the 'Apsara1 reac tor . They found

that in argon equil ibrated solutions a t 1M HC1, G(Pu(lv)) was 0.35 and

that a t higher hydrochloric acid concentrations the yield was d i rec t ly

proportional to the molarity of the hydrochloric acid used. Hydrogen

peroxide was not found in detectable quan t i t i e s . The observed propor-

t iona l i ty of G(Pu(lV)) with hydrochloric acid concentration was i n t e r -

preted t o mean that the oxidation of P u ( l l l ) was proceeding mainly

through the species formed by the ' d i r e c t ' action of radiat ion on

hydrochloric acid . I t was decided to invest igate the reactions that

could occur between the radio ly t ic products of water and plutonium .

Since the nature and yields of the rad io ly t ic products of water in

0.8N sulphuric acid medium are well known* , Ghosh-Jfezumdar,

Natarajan and Srinivasan^ ' investigated the gamma radiolys is of

plutonium(ll l) in 0.8N sulphuric ac id . The proximity of the redox

potential values in sulphuric acid medium for the plutonium(ll l) -

XlV) aad the ferrous-ferr ic systems^ ' ' presented fur ther i n t e r e s t ,

since the radiat ion chemistry of the l a t t e r system has been well(7)studied . I r radia t ion of argon equil ibrated plutonium(il l) solut ion

in 0.8N sulphuric acid yielded plutonium(iv) and hydrogen peroxide

-127-

with i n i t i a l yields of G(Pu(lv)) = w^ 5 and S ( L O L ) = ~ 0.8, the(8)

yields fa l l ing with dosev . The measurement of hydrogen yield gave

a value of about 3.6 for the i n i t i a l G ^ ) while i t decreased to about

2.5 a t about 1.1 x 1019 ev / ia l ( 9 ) . The following reactions were proposed

to explain the observed y i e l d s ' 4 ' .

P u ( l l l ) + OH > Pu(iv) + OH" - (1)

H + H202 ^ 1^0 + OH (2)

PU(III) + H + H+ > Pu(lV) + H2 (3)

H + H —-* Hg (4)

PU(IV) + H » Pu ( l l l ) + H+ (5)

Reaction (1) would give a G(Pu(lv)) value of about 2.95. Reaction (2)

was proposed to explain the observed G(HpO ) which was l ess than 0.8

a t doses greater than 0c3 x 10 ev/ml. Reactions ( i ) and (2) would

not explain the observed G(Pu(lV)) of about 5. I t was, therefore,

suggested that reaction (3) was a lso taking place. I t appeared that

a l l the hydrogen atoms did not undergo reaction (3) only, since in

that case the G(Pu(lV)) would have been more 1ian 6.6 . The observed

G(Hp) also indicates t ha t a l l the hydrogen atoms did not undergo rea-

ct ion (3) only, since in that case the G(BL) would have been 4 . 1 . I t ,

therefore , appeared that reactions(4) and (5) might a lso be taking

place .

Prom the above review i t may be concluded that plutonium(lll)

i s oxidized to plutonium(iv) by gemma radiation through the interaction

of the radiolyt ic products of water in 0.8N sulphuric acid medium,

while there appeared to be no net ' indi rec t effect1 in hydrochloric acid

so lu t ion . In order t o get an answer t o this apparent anomaly, we

decided to re-examine the work on the radiolysis of plutonium(lll) in

hydrochloric acid medium. We have so far carried out a few i r radiat ions

of millimolar solut ions of plutonium(lll) on 0.8N hydrochloric acid18

medium in an iner t atmosphere of argon in the dose range 1.5 x 10

ev/ml to 30.6 x 101 8 ev/ml and measured the yields of plutonium(iv)

-128-

and hydrogen peroxide produced. The results are given in Table 1.

I t nay be seen that the G(Pu(lv)) is between 4 and 5 upto a

dose of about 6 x 10 ev/ml and falls at higher doses. At a dose of

was22.9 x 10 ev/ml, i t has fallen to a value of about 0.7. fiCH^)

not more than 0.83 at a l l the doses studied. Most of the measurements

were carried out after about 2 hours from the time of irradiation. It

was o-hecked in a few cases for 'after effect1, if any, and i t was

observed that there was practically no after effect for about 4 to

5 hours.

The oxidation of plutonium(lll) to plutonium(iv) in 0.8U

hydrochloric acid medium may be explained by the following reactions.

OH + Cl" + H+ y Cl + HO (6)

Pu(lll) + Cl * Pu(lV) + Cl" (7)

The ini t ia l yields appear to be somewhat higher than 2.95,

the primary yield of OH radicals. It is possible that reaction (2)

may be contributing partly• T^e low yield of BLO may also be explained

by reaction (2). It i s possible that reaction (3) may be talcing place

in 0.8H hydrochloric acid medium to significant extent only at lower

doses. The low yield of Pu(lV} at higher doses can be explained by

invoking reaction (5). The fact that there appears to be a distinct

possibility of reactions (2) and (5) taking place is of interest , since

the Pu(lll) solutions used may be containing not more than 5% of the

total plutanium concentration as Pu(iv) and similar amounts of HoO,.

The analytical method used cannot definitely exclude the possibility

of small quantities of Pu(iv) and HgOg present ini t ia l ly in the

Pu(lll) solutions.

-129 -

TABLE 1

Hadio lys i s of p l u t o n i u m ( l l l ) i n 0.8N hydrochloricac id - y i e l d s of plutonium(iv) and hydrogen peroxide

3 .No.

1

2

3

4

5

6

7

8

9

10

Coseev/ml

x 1O"1 S

1.53

1.53

3.44

5.35

11.46

11.46

15.28

19.1

22.92

30.56

G(Pu(IV))

3.66

3.79 *

4.91 *

5.14

3.01

2.94 *

1.73

1.65 *

0.730.68

0.46

0.83

0.12

0.73

0.58

0.08

0.19

nil

0.06

0.11

* In these cases the 'after effect' wa3 found to be negligible

In order to check the effect of the presence of HO prior

to irradiation on the G(Pu(lv)) measured, two irradiations of solutions

containing millimolar solutions of Pu(lll) and H_0 in 0.8N hydrochloricIfl 18

acid were carried out upto 1.53 x 10 ev/ml and 11.46 x 10 ev/sni.

There was a definite decrease of &(Pu(lV)) in both the cases. In the

case of 1.53 x 10 ev/ml irradiation, G(Pu(r/)) fell from 3.8 to

2.8, while in the case of 11.46 x 1018 ev/ml irradiation, i t fell

from 2.9 to 1.2. This may probably indicate the scavenging of OH

radicals by HgO .

It is proposed to carry out further work to understand more

about the reactions induced by gamma radiation in hydrochloric acid

solutions of plutonium.

-130-

EXPERIMENTAL

Triple d i s t i l l e d water and quartz d i s t i l l e d Analar HC1 were

used in i^iring the s o l u t i o n s . Plutonium was purif ied by ion-exchange

and peroxide p r e c i p i t a t i o n methods. P l u t o n i u m ( l l l ) was prepared by

electrolytic reduction. Irradiation was carried out with cobalt-60

gamma radiation at a dose rate of 0.38 x 10 ev/ml per minute. Dosi-

metry was carried out with a Frieke dosimeter. The measurements of

plutonium(ill) oxidized and hydrogen peroxide produced were carried out

ACKN OWLEDGEMENT

The authors wish t o express t h e i r s incere thanks t o

Dr M.V. Eamaniah, Head, Radiochemistry D i v i s i o n , f o r h i s keen i n t e r e s t

i n the work.

REFERENCES

1. A.S. Ghosh Mazumdar, R.N. Singh and 5 . Srinivasan; Indian J .Chem. 2_, 226 (1954)

2 . A.S. Ghosh Mazum&ar, F.R. Natarajan and B. Srinivasan; Proc .Hucl. Rad. Chem. Symp. Bombay, p . 79 (1964)

3» A.O. A l l e n ; The Radiation Chemistry of Water and Aqueous S o l u t i o n s ,(S . Van Nostrand Company, I n c . Princeton, New Jersey ) , 41 (1961)

4 . A.S. Ghosh Mazumdar, P.R. Natarajan and B. Srinivasan; Proc . Nucl .Rad. Chem. Symp., Waltair , p . 119 (1966)

5 . J . J . Rowland J r , J .C. Hindman and K.A. Kraus; The TransuraniumElements, ed i t ed by G.T. Seaborg, J . J . Katz and W.M. Manning(ifcGraw H i l l Book Co. , I n c . , New York) 133 (1949)

6 . G. Chariot , Se lected Constants Oxidation Reduction P o t e n t i a l s(Pergaman Press , New York), 12 (1958)

7 . W.G. Rothschild and A.O. Al len; Rad.Res. 8 , 101(1958) and thereferences cited "therein

8. Bo Srinivasan; M.Sc thes is , Bombay University (1966)

9. A.S. Ghosh Mazumdar, P.R. Natarajan and B. Srinivasan(Unpublished work)

10. P.V. Balakrishnan, A.S. Ghosh Mazumdar and R.N. Singh;Talanta r\_, 977 (1964)

- 1 3 1 -

RADIATION CHEMISTRY OP NITRIC ACID SOLUTION

P.K. Bhattacharyya and R. SainiRadiocheraistry Division

Bhabha Atomic Research Centre

INTRODUCTION

I t was our i n t e r e s t t o inves t iga te the rad ia t ion chemistry

of n i t r i c acid i n connection with the nuclear fuel reprocessing r i t h

long chain t e r t i a r y amine . The r ad i a t i on chemistry of aqueous

n i t r a t e has been s tudied by several workers in neu t ra l and basic

solu t ion^ * . Recently Mahlman ' reported the rad io lys i s of n i t r a t e

in su lphur ic acid medium and subsequently Miner e t . a l described

some of the r e s u l t s on rad io lys i s of aqueous n i t r i c acid solut ion. I t

i s now c e r t a i n t ha t the important r a d i o l y t i c products in n i t r a t e and

n i t r i c acid so lu t ions are mainly NOu and H^O- besides 0 . I t i s also

known t h a t the HNO? r e a c t s with H_Op i n acid so lu t ion , thus making i t

extremely d i f f i c u l t t o estimate the y ie ld of the products . However,

i n t h i s paper we w i l l show tha t the n e t G(HNO?) decreases with dose

when aqueous n i t r i c acid (ill) i s radio lysed . The G-values were con-

s ide rab ly lower when r ad io ly s i s was car r i ed out in presence of a i r

in comparison with t h a t in argon swept so lu t i on . The G(HffOg) obtained

here i s ca lcula ted on the basis of the est imation of HNOg remaining

just after the irradiation.

EXPERIMENTAL

Materials

Ordjnary d i s t i l l e d water was further purified by d i s t i l l i n g

from alkal ine potassium permanganate solut ion and f i n a l l y in an a l l -

quart z d i s t i l l a t i o n system. The water was stored in a quartz vessel

for use.

The analytical reagent grade HNO_ was further purified by

distilling in an all quartz distillation unit.

-132-

Sample Preparation for Badiolysis

The purified MO was diluted with triple distilled water

to give 1M HMO, solution. For irradiation in air, 5 ml portion of

111 HNO, solution was transferred to a glass irradiation cell and sealed.

In case of radiolysis in presence of argon gas, the 1M HNO_ solution

was transferred to a fritted glass apparatus with a provision for

passing argon gas from the bottom. The gas was bubbled for 20 minutes

and then the solution ( r v- / 5 ml) was transferred to the irradiation

cell through a standard joint. The irradiation cell was then sealed

in presence of argon. The volume of the solution was determined from

the weight of the solution and the density.

Irradiation

Co y-radiation source was used with a dose rate of18 ™1 —1

0.364 x 10 ev ml min . Irradiation was carried out for a parti-

cular time continuously.Analytical Technique

Just after irradiation the solution was frozen at 77°K and

the calculated amount of NaOH was added to make the solution alkaline.

The sample was then brought to room temperature and the N0~ was esti-(6)

mated on the basis of diazo colour formationv . The method involved

adding 0.5$ sulphamilamide solution in acetic acid to the N0~ solution,.

then n - 1 naphthyl ethylenediamine dihydrochloride solution in acetic

acid was added after about 5 minutes. A pink coloured solution was

obtained. The absorbance was recorded with Cary-14 spectrophotometer.

The concentration of HNO,, thus produced in HtfO, after irradiation was- 1 -1

calculated on the basis of B = 50,000 M cm a t 542 nm.max

RBSUDPS AND DISCUSSION

The resultB of our experiments with 1M HRO_ so lu t ion showed

that the rad lo ly t i c product, HUO_ increases with the i r r a d i a t i o n time

and hence with dose. The concentration of HNO versus the i r r ad i a t i o i

time when i r r ad i a t i on was car r ied out in presence of a i r i s shown in

-133-

Fig .1 . The G(HN02) a i r calculated from the smooth curve was found to

decrease with dose (Pig.1). The decrease of G values is also shown

in Fig.2, for the case of argon saturated solution. The G values were

based on the estimation of HJTO2 just after irradiation and hence could

be considered as net values.

It has been reported that the G(tfO~) in dilute nitrate

solution ( ^ 0.1M) in basic medium is £& 0.5 whereas in 4M nitrate- (7)

solution, G(N02) was 4.0v ' . This increase of G values with N0~ con-

centration was explained as due to combination of "indirect" and

"direct" effect of radiolysis. Indirect effect is through the water

radiolysis ( i . e . HgO-"***! , HgOg, OH) and subsequent reaction with the

solute, i . e . ,

H + NOj > M02 + 0H~ (1)

2H02 + 1^0 * 2H+ + N0~ + N0~ (2)

OH + NO" —> N02 + OH" (3)

whereas in direct effect reaction (4) is important

2N0~ * 2N0~ + 02 (4)

Mahlman^4' found that G(HN0o) in 0.8N H,,SO, for 1M nitrate

solution was nearly 1.7. Becently, Miner e t .a l v obtained G(HN02)=2.6

on radiolysia of 1M HNO, in a i r , whereas in vacuum they obtained a

value = 2 . 0 . Thus, in acid solution ambiguity s t i l l remained. It

was known that HLO reacts w.th HNO_ in acid solution hencs Miner et a l

attempted to protect the HN0o thus formed from reacting with ILO by(3)

adding p-nitro anil ine. I t has been observed earlier that the HNOg

decomposition in 0.5M HNO, with time was quite significant (See Pig.3).

The DPPH scavenger study in HNO -HNO, system also showed the existence

of free radicals (Fig.4). These were explained on the basis of the

following reactions

* 2N0 + HO + NOj (5)

HN02 + H O — * HgNO2 + HgO (6)

-134-

HgNoJ - > NO+ +H2O (7)

NO+ + NO* - ^ N2O4 (8)

N O . —> 2 NO. (9)2 4 ^

It is not certain how these reactions are significant in affecting the

G(HNO2).

Since we have not protected e i ther HNO or ILOg during i r r a -

diation time, we cannot in f ac t compare our values with others reported.

But our resu l t showed a s ignif icatnly lower value of G(MO2 ) & i r when

0o was present in the system, in comparison with G(HNO_) . This was£L & I& Oil

contrary to the fact Miner e t a l reported, although they could not

explain the effect of 0 for increasing G(HNO ) . However, we explain

the present r esu l t s on the basis of the reactions ( i ) and (1O).

H + NO" > N02 + 0H~ K1 = 1.7 x 107 ( 0

10 / \H + 02 > H02 K1Q = 1.2 X 10 (10)

Prom the ra te constants K and K i t i s c lea r that the react ion (1O)

would be s ignif icant in presence of 0? and thus would hamper the for-

mation of HN02 by indirect e f f ec t . Prom P igs . 1 and 2 i t i s seen that

the G(HN02) decreases in both the cases with dose or the i r r ad i a t i on

time. This i s a t present assumed as due to the reaction of H202 and

HN02 formed on radiolys is . Further work in t h i s direction i s under

progress.

REFERENCES

1. P.K. Bhattacharyya and R. Veeraraghavan; Report BARC-384 (1968)

2 . A.K. Pikaev; Russ. Chem.Rev. 29_, 235 (i960)

3 . M. Daniel and E.E. Wigg; J . Phy. Chem. 71 , 1027 (1962)

4. T.J. Sworeki, R.ff. Ifathews and H.A.Mahlman; Adv. in Chem. Series8J_, 169 (1968)

5. F.J . Miner e t a l ; Report RFP-1299 (1969)

6. T.G. Ward, G.E. Boyd and R.C. Axtman; Rad.Res. 33, 447 (1968)

7. M. Daniels; Adv. in Chem. Series 81_, 153 (i°68)

8. P.K. Bhattacharyya and R. Veeraraghavan (Unpublished d a t a ) .

-135-.

0 4

S 10 19 20 29 30 39 40 49 50 55 60 Kb 0Irradiation Time (minutaa)

Flgurt-1. Variation of G(HN02 )a | r with Irradiation time for IM nitric acM. Tha 6(HNO2 )o n . I* colculotad

en th» boils of tha amooth eurva drown for eoncantrotlon of HN02 Vi Irradiation fImt •

3 0

0 10 20Irradiation tlma (mlnutm)

FHurt-2.Vorlotlonof GCHNOa)^,,,,, for Argon Saturofad IM nllrleadd aohjflon with Irradiation tlma

-156-

2 0

0 30 60 90Storage Tlm« (mlnuttt)

Figure - 3. Dtcrtoit of absorbanc* of 002M HNO2 IN 0-9M HNO3solution In Air.

2 0

15 -

1-0 -

0 51-5OS 10

Cone- Of Nitric Acid

Flgure-4. Ratio of dacraoe* of [CPPH] to that of [HNO2] V« coneanfraflonof nitric odd.

2 0 M

-137 -

HEACTION OP HYDRATED ELECTRONS WITH PERTECHNETATE

P.K. Mathur and K.S, VenkateswarluBhabha Atomic Research CentreChemistry Div i s ion , Trombay, -

Bombay 85

There have been several reports in the literature on the

inhibition of corrosion by ions of the type X0.n~. Among these,( )(1) -

Cartledge has found that TcO affords excellent corrosion pro-

tection in aqueous solutions. In view of this attempts are under way

in the United States to try out perteohnetate ion as a corrosion

inhibitor in an operating boiling water reactor.

As such i t was thought worthwhile to determine the para-

meters involved in the radiation chemistry of this ion. Preliminary

work by Dr Manohar Lai and Shri J.V. Jashnani^ ' showed that in

steady state V-radiolysis there is virtually no observable

G /„ n - \ . It is obvious that the radiation induced oxidation -reduc t ion r eac t ions a re evenly balanced i n t h i s system. However, i n

order t o determine the r a t e constant of any individual react ion i t i s

e s sen t i a l t o suppress other reac t ions . The ava i l ab i l i t y of the

phot ogene r a t ion unit for hydrated electrons set-up by E . J . Hart in

our labora tory has now afforded t h i s opportunity.

Using the now famil iar technique, the rate constant of

the TcO ~ with hydrated e lec t ron has been found to be 1.15+0.1x

10 I M Sec" . Since there i s no net decomposition during steady

s t a t e radiolyBis , i t i s obvious t h a t the ra te constant for the oxi-

dat ion of TcO, ~ by OH radicals should also be of the same order of

magnitude.

REFERENCES

1. G.H. Cartledge; J.Am. Chem.Soc. 77, 2658 (1955)

2 . Science News Let ters 8 3 , 264 (1963)

3 . J .V. Jashnani; M.Sc. t h e s i s e n t i t l e d , "Radiation Chemistryof Inorganic Substances" submitted to Bombay University (1968)

*~tel&tiw^o~ihe~tt^co^twt~tf~u5^+~0~2Vx 101° L ll"1 Sec"1 forM 0 ~ determined with the same experimental set up.n. 4.

- 1 3 8 -

ELBCTHDN TRANSFER REACTION IN THE RADIATION CHliMISRRY OF SOME BIOLO-GICALLY BHPOHTANP HE SULPHIDE COMPOUNDS

B. L a l i t h a and Ja i P . M i t t a lChemistry Division

Bhabha Atomic Research Centre, Trombay, Bombay-85

INTRODUCTION

Much i n t e r e s t has been focussed i n t h e l a s t decade o r so on

the r a d i a t i o n chemistry of sulphur containing organic compoundsr This

in t e r e s t arose a f t e r the knowledge that many of theBe compounds were

rad iopro tec t ive substances- Considerable experimental evidence has

accumulated from various rad iobio logica l l abo ra to r i e s throughout the

world tha t many of these compounds, such as -SH containing compounds

like, t h i o l s , mercaptans and organic d i su lph ides , exert a d e f i n i t e

rad iopro tec t ive effect on t he b io log ica l ly important biomolecules ,

microorganisms, c e l l s and whole organisms, The organic d isu lphides

which are frequently used a re cys t ine , cystamine, th ioglycol and

glu ta th ione . Even though considerable experimental data i s ava i l ab le

about their relative protective abilities, the radio-protective action

ie s t i l l predominated by ' t r i a l and error1 methods only*

The present study was undertaken to understand more about the

nature of the protective ability of these disulphide compounds at

molecular level by learning their behaviour towards primary species

produced in the radiolysis of water, which is the most abundant consti-

tuent of all biological systems. The familiar radiolytic oxidation of

ferrous ions in the Fricke dosimeter solution was. taken as the indicator

solution.

EXPERIMENTAL

Material s

1-cystine of the purest grade and the oxidised form of gluta-

thione (acid-free, grade I I I , ethanol-free) from Sigma Chemical Company

-139-

and cystamine from Fluka A.G., BUCHS S.Go (Puram) were used.

Sulphuric acid and ferrous ammonium sulphate used for the doai-

metry solution were of B.D.H. " Analar " grade. The dosimetry

solution contained 1 x 10~5M ferrous ions and 0.8 N(0,41l) of sul-

phuric acid• Solutions were prepared in pre-irradiated triply dis-

t i l led water.

Irradiations

Solutions of cystine, cystamine and glutathione of concentrations-4. -2.

ranging from 10 IS to 5 x 10 Tl were irradiated in Pyrex glass-tubes.

The distilled water was purified by irradiating i t over-night and then

redistill ing from alkaline KMnO. and acidic K_Cr,,07. 5 ml aliquots60

were used for each irradiation. A'gamma cell1 type Co source having1 ft

dose rate of 0.371 x 10 ev/ml/mt was used and the air saturated solu-

tions were irradiated for 2.5i 5» 10 and 15 minutes.

Analysis

Fe formed was measured by spectrophotometric analysis at

305 nm using molar extinction coefficient of 2320 at 25°C The optical

density readings were taken soon after irradiation so that the adventi-

tious oxidation of the solution was minimised. The value of &Fe5+ used

for calculating dose rate was 15»5»

Observations

From the yield (ferric) vs total dose curves, al l of nMch were

linear through the origin, G 3+ values were calculated and plotted as a

function of the concentration of various added disulphides. The <»Fe3+

values for the various disulphidess of different concentrations are

given in the following table for 5 minutes irradiationt

-140-

Ko. Coacentration

Cystine Cystaroine Glutathione(Oxidised) 5 ^ ^ ^

1. 1 x 10-4M 14.79 13.52 12.06 11.91

2. 5 x 1 0 " ^ 12.21 9.98 8.43 11.12

3. 1 x 10"5M 10.47 8.21 6,54 10.92

4. 5 x 1O"3M 9.74 6.69 5.89

5. 1 x l O " ^ 9.59 6.32 4-2

6. 5 x 10-2M 7.99 5.75

Among these compounds i t i s noticed that the reduction of G-_ 5+re

was maximum in the case of glutathione.

DISCUSSION

2+The Fe in the Fricke dosimeter solution i s oxidised by the

active free radicals , H" and *0H and by H2O_ which are produced as

primary species in the radiolysis of water. The. well-known mechanism

i s as followsi

'OH + Fe2+ > Fe5+ + OH"

H2°2 * Fe2+ "~~-> Pe?+ + 0H~ + 0H

H + 02 — - ^ HOg

HO" + H+ + Fe 2 + -^ Fe3+ + HjOg

C(Fe3+) a i r " 2.OH

As early as 1934 » i t was found that the presence of small amounts

of certain organic compounds in solution affect to a considerable extent

the rate of radiolyt ic oxidation of Fe2 + . DewhursV2 '^ studied the

- 1 4 1 -

effect of the systematic introduction of aliphatic alcohols on the

value of G 3+ in the dosimetry solution.

Certain other chemicals like formic acid, cyclic and aromatic

hydrocarbons, unsaturated compounds like allyl alcohol, styrene,

thiourea, ailyl thioureaj acetylene, phenol and benzene increased

the &Fe3+ value and the effect varied with the structure of the organic

compound. Dewhurst explained the action of the impurities by the

following sequence of reactions.

°0H

R"

BD2

RH =

+ HH

+ °2

RCCH + Fe

RO* + H+

impurity

f

+ F e 2 + _ ^

2+ ^

+ Fe2+ _

present

R' +

E02

Fe3* H

Fe5+ +

. F e 5 ++

H20

y RCOH

RO" + OH

ROH

Thus, we see that each «OH radical "brings about the oxidation

of three Fe instead of only one, which results in spuriously high

value of &„ 3+ when organic compounds are added to the system*

In the case of the disulphides, as seen from the table, the

& 3+ value instead of increasing, decreased markedly by the intro-c e

duction, of RSSR. I t clearly indicates that these compounds intercept

the oxidising species and in the process do not produce any chain

propagating species.

Several schemes have been proposed to explain the radiolysis

of cystine and related disulphideso The intermediate products of

oxidative degradation of cystine were studied by Grant et al and

Rotheram et a l ^ c ' o Aqueous solutions of cyBtine were studied by i any

other workers a l so ' ^"^ . Cystamine was subjected to detailed Btudy by

an* Oa.Ujcu.M . The mechanism recently offered by Purdiev

-142-

has the initial reaction as follows in the aerated systems.

RSSR + *0H > BSOH + RS"

I t should be noted that the sulphonic acid, RSOH, was supposed to be

unstable and other reactions were explained on this basis. If this

mechanism i s correct9 then there is no explanation to the question why

the BS8 radical formed does not react with the oxygen present in the

aerated system to give r ise to an organic peroxide, which should

again react with the ferrous ions giving r ise to an increase in the

&_ 3+ value just like any other organic peroxide. We therefore,re

postulate that a radical of the type RS* is not being formed in the

ini t ia l step, but suggest instead an electron transfer mechanism

"OH + RSSR => RSSR+ + OH"

An electron i s t ransferred from the disulphide to the hydroxyl rad ica l(12)to give r i s e to a disulphenium cation^ y . The radical ca t ion formed

can undergo fur ther rac t ions and can d i s soc ia t e into non-chain p ro -

pagating specieso

OH + RSSR > RSSR+ + 0H~

RSSR+ H y RSH + RS+

To tes t the validity of the above hypothesis, the limiting

value of S, 3+x for higher concentrations of the disulphide (when

compared to Pe+2) w e r e computed as followsj

The rate constants of "OH radical reacting with ferrous and

disulphides are known as

kc (°0H + Pe+2) . 3.2 x 108 1 M™1 sec"1 at pH=1 (10)

k (OH+RSSR) = 8.8 x 109 1 if1 sec"1 at pH = 1 (11)

( if RSSR = glutathione)

-143-

Therefore, at higher concentrations, the hydroxyl radicals are

preferably taken by the disulphide . Then the remaining oxidising

species axe H>2 and HgOgo Recently Jayson showed'12' that at higher

concentrations of the disulphide, the ED- will be preferably taken

by the disulphide than by the ferrous ions and under the conditions

of irradiation the disulphides do not react with the hydrogen peroxide.

H +

RSSR

H2°2

°2 —*+ H0 2 + I

+ Fe2+ -

- HO,

I+—*- RSSR+

-^ Fe 3 + +

+ H2(

0H~

°2+ °0H

The hydroxyl radical thus produced will then be taken by

the disulphide againo Hence,

3»65 + 0,8

4»45

In the case of glutathione, experimentally i t was found that at a

concentration of 10°"TI the ^ 3 + was reduced to 4o2.^ 3

G 3+ values were also calculated for equal concentration of

the disulphide and Fe2+o In this case,virtually all the °OH radicals

will be scavenged by the disulphide, while^ ' the HOg" radicals will

react with ferrous ionso So,

Fe2+

H2°2

H+

-144-

All ("OH) radicals will be taken up by RSSR. Hence, in this case

theoretically,

Gr_ 3+ = &/TI r\ \ + 2 dfrr\Fe {UnOr,) \B.)

m 0.8 + 2 x 3.65 « 8.1

The experimental value was found to be 802 at 1 mM concentration of

cystine and ferrous ions<>

It would be very interesting to study the effect of these com-

pounds in deaerated system because theoretically at high concentration

of RSSR, the limiting G-, 3+ should again come out to be 4<>45« Presently^re

work is in progress to test this point*

I t is also planned to study the absorption spectrum of RSSR at

(420 nm) which is supposed to be formed as intermediate.) Nicolau et alv '

have recently studied in a flow-system, the reactions of "OH radicals with

the sulphydryl compounds and the disulphides. They were able to identify

a triplet pattern of the sulphur radicals (RS°) in the case of sulphydryls9

but no EPR signal corresponding to the formation of RS* in the case of

disulphides was obtained» They have also suggested an electron transfer

mechanism to explain;their observations. The postulation of the RSSR

species also agrees well with some of the results reported recently by

Jayson(i4>.

I t is suggested that if many more compounds of known biological

activity are studied, i t may be possible to construct some sort of com-

parative scale whereby knowing the protective ability of a compound towards

the Pricke dosimetry solution, one would be able to predict i t s biological

protective activity, without doing much of biological testingo

-145-

ACKNOWLEDGEMENT

The a u t h o r s w i s h t o thank Dr . J . Shankar , Head, Chemistry

Division, BARC, for hie continued i n t e r e s t and encouragement through-

out t h i e work.

REFERENCES

1. G. Harker; Nature 1J&, 378 (1934)

2. H.A. Dewhurst; J. CLam. Phys. 9_, 1329 (1951)

3. H.Ae Dewhurst; Trans. Faraday Soc. 4J3, 905 (1952)

4. D.W. Grant, S.N. Mason and II.A. Link; Nature t9_3_, 352 (1961)

5. (a) W.M. Dale and J.V. Davies; Biochem. J. 48, 129 (1951)

(b) A.J. Swallow? J. Chen. Soc. 1334 (1952)

(c) S.Lo Witcher, M. Rotheram and N. Todd; Nucleonics 1J_, 30 (1953)

(d) P. Markakis and A»L. Tappel; J. Am. Chem. Soc. 82, 1613 (i960)

(e) J.C. Fletcher and A. Robson; Nature 1<£, 1308 (1962)

6. J.W. Purdie; J, Am. Chem. Soc. 89_, 226 (1967)

7. Bo Shapiro and L. Eldjarn; Rad. Res. £, 393 (1955)

8. (a) W.F. Forbes and W.E. Savige; Photochem. Photobiology I, 77 (1962)

(b) W.F. Forbes, D.E. Rivett and W.E. Savige; ibid, 1, 97 (19^2)

9. R. Bridicka, Z. Spurny and A. Fojtik; Coll. Czech. Chen. Commun.28, 1491 (1963)

10. (a) F.S» Dainton and T.J. Hardwickj Trans. Faraday. Soc. 52., 333 (1957)

(b) B. Bunn, F.S. Dainton, G.A. Salmon and T.J. Hardwick; Trans.Faraday Soc., , 1760 (1959)

11. G.E. Adams, J.W« Boag, J. Currant and B.D. Michael; In PulseRadiolysis (j.H. Bexendale, M. Ebert., J.P. Keene and A.J. Swallow,Eds), p.131, Academic Press (1965)

12. G-.G. Jayson, T.C. Owen and A.C. Wilbrabm; J. Chem. Soc. (B)944 (1967)

13. Claude Nicolau and Hermann Dertinger - Submitted to Rad. Res. (1970)

14. G.G. Jayson and A.C. Wilbrahm; (Unpublished work).

1x102MConcentration of RSSR

-147-

EFFECTS OP CO ° ~tf -RAY IRRADIATION OH L-THREONINE

Sav i t a V. Marathe and K.S. KorgaonkarBiophysics Div i s ion , Cancer Research I n s t i t u t e , Bombay - 1 2

INTRODUCTION

E a r l i e r i n v e s t i g a t i o n s (Maxwell e t a l 1954, 1955; Kopoldova'

e t a l 1961 j, 1965a and Korgaonkar et a l 1968) reveal t h a t i r r a d i a -

t i on of oC-amino ac id s i n aqueous s o l u t i o n s produces a l a r g e var ie ty

of i r r a d i a t i o n p roduc t s . Screening by se rve ra l independent proced-

ures seems, t he re fo re , necessary to i d e n t i f y and i s o l a t e most of these

compounds. I r r a d i a t i o n s t u d i e s on aromatic amino acids using u l t r a -

v i o l e t and inf ra - red spectrophotometry have been repor ted from th i s

l a b o r a t o r y (Donde and Korgaonkar 1962a, 1962b). S imi lar observations

on L- threonine are presen ted here.,

MATERIALS AND METHODS

L-threonine was obtained from B.DeHo The amino acid was

t e s t e d f o r i t s p u r i t y by paper chromatography, which showed only a

s ing le spot when s ta ined with ninhydrino The pur i ty of t h e compound

was f u r t h e r checked from i t s infrared spectrum.

For irradiation, solutions were made in triple distilled water

to 0.01 M concentration.

Irradiation% The solutions were irradiated with % -rays in 20 ml

aliquots using cylindrical polythene vials which were introduced into

the central space of the, Co cage type source supplied by the Atomic

Energy of Canada Ltd. (Orammacell 100). Dose rate, as measured by

ferrous-ferric dosimetry, was 0.16 M rad/hr» ultraviolet absorption

studies of the non-irradiated as v/ell as irradiated solutions were

carried out on Hilger and Watt OTspek spectrophotometer. Quartz

-148-

cuvettes of 1 cm path length were used, 'he solutions were diluted

(1:8) in order to bring the optical density values in ihe sensitive

region of the instrument» Studies were carried out in the spectral

region from 195 fcrnto 300 7ir»i. The instrument was flushed with N?

gas for about half an hour at a pressure of about 10 psi before and

during the optical density measurements in the 195 nm to 220 nm

region. Infra-red spectra in the region 2p to 15 of nan-irradiated

as well as irradiated samples of the amino acid were recorded on Ernst

Leitz self-recording double beam spectrophotometer with a rocksalt

prism using KBr disk technique,, The procedure for preparation of KBr

disks was as described earlier (Donde and Korgaonkar, 1962a) with the

following modifications

The dried sample collected from the evaporating dish con-

tained only 30C ing , in place of 500 mg of pure and finely ground KBr

powder. This mixture, after regrindingp was spread over the central

portion of the ring placed on the anvil of the dish making assembly.

The protion near the rim of the ring was fil led with 225 mg of the dry

and finely ground KBr powder which did not contain any sample „ This

was then pressed in Paul and Weber hydraulic press uAder vacuum for2

JS at about .-300 Kg/cm .

The disks prepared in this manner were used to record the

2three minutes at about .-300 Kg/cm .

spectra.»

To determine the radio-sensitivity and G value for destru-

ction of "the amino acid, several infra-red spectra were recorded with

different known amounts of non-irradiated Ii-threonine. Then the

optical density values for a sharp band at 10.8ju, which had least

interference by the adjacent bands, were plotted against the corres-

ponding amounts of the amino acid used. From ihe calibration curve

thus obtained, the residual amounts of threonine in the irradiated

samples were estimated and G values calculated.

RESULTS

The irradiated solution of L-threonine showed a steady increase

in pH from 6.00 (for non-irradiated) to 7.4 (at 5.2 M rad ) .

The u l t r a -v io le t spectra of non-irradiated as well as i r radia-

ted samples of L-threonine are shown in Pig do

The infra-red spectra of the solid residue from the non-irradi-

ated as well as i r radiated samples of L-threonine are shown in Fig»2.

The ca l ibra t ion curve for O.D. of 10.8 u band in relation to

quantity of the L-threonine is shown in Pig .3 .

G values for different doses are given in Table I .

DISCUSSION

The G values obtained in the present work are comparable with

those obtained e a r l i e r by Bageau and Mehran (1966). The fa l l in G value

a t higher doses could be due to loss of the material rather a decrease

in intrinsic radio-sensitivity.

U.VO absorption data presented in Pig. 1 shows increase in

the absorption in the region 195 nm to 300 nm which develops into a

sharp peak at 205 ran at a dose of 5.2 M rad. A new broad and rela-

tively weak absorption band which increases with dose is also seen at

270 ran. Two C = 0 groups separated by two carbon atoms are reported

by Henri (19 ,9 ) xo ^-J* a relatively weak band at 270 nm. Formation

of a new 0 = 0 group at r carbon position in some of the radiation

products of L-threonine cannot be ruled outo

Infra-red absorption data presented in Pig«2 shows bands at

4.95 Ji, 8.1 / i , 9.0^1, 9.7 ji, 10.8^, 11.5 jx, 13.0 ji, and 14.28^ to

decrease in their absorption intensities whereas bands at 3.4 ji and

6.2 fx are relatively unaffected. A new band at 12.05 )x is clearly

-150-

seen for doses above 2.6 M rad. A strong band at liiis site (833 cm )

is reported to be the characteristic of cis epoxy compounds by

Shreve et al (1951) and Guntnard et al (1953). A chemical test speci-

fic for expoxides as described by Puchs et al (1952) was therefore

carried out and came positive for the irradiated sample.

#P- Por comparison, the infra-red spectra of 1 ,2 epoxy-propane

- CH - CH was recorded, which showed i ts strongest band in the

2 u. to 15 y. region also at 12.05 u. Thus, formation of some epoxy

compound as a major irradiation product of L-threonine, particularly at

higher doses, i s indicated in the present work.

According to Leibster and Kopoldova (1964) carbon in 1he Y

position is the most reactive s i te for the radiation damage in the

<X--amino acids. Hydroxylation a t this point followed by elimination

of a water molecule could lead to the formation of an epoxy group„ The

following mechanism is therefore suggestedt

POSSIBLE MECHANISM OF REACTION

COOH COOH COOH

WHg— C""~H n i l NH^~™C—H _» j /* NHj>— C~~H

H-C-OH *" H-C-OH ** H-C.

CHt CHgOH H *™ C

K

The above suspected formation of epoxy compound in the irradiated

l-threonine seems to have much biological significance due to the known

carcinogenic property of epoxy compounds. Haddow (1958) has reported

that 1:2:3:4 diepoxy butane which is the simplest of the diepoxides was

tumour inducing. Hendry et al (1951) tested vinyl cyclohexene dioxide

- 1 5 1 -

wbieh produced carcinomas and sarcomas when applied to skin of mice.

I t would be of much i n t e r e s t to see i f the i r radia t ion product from

L-threonine also showed carcinogenic property. Animal experiments to

t e s t such biological implications of the present work are in progress.

REFERENCES

1. R.B. Done and K.S. Korgaonkarj I n t . J . Rad. Biol . 4_, 285 (1962)

2 . R. Fuchs, R.C. Waters and C.A. Vanderv/erf; Anal. Chea. 2£, 1514 (1952)

3. HB.H. Gunthard, H. Heusser and Ho Furst ; Helv. Chem. Acta. j>6_1900 (1953)

Cited from - 'The application of infra-red and Raman spectrometryto the elucidation of molecular s t ruc ture 1 by R.N. Jones andC. Sandorfy in Chemical Applications of Spectroscopy1 (Editor -W. West) p.440Interscienee Publ ishers , I n c . , New York (1956^

4. J.A. Haddow; B r i t . Med. Bull . 14, 79 (1958)

5« J.A. Eendry, R.F. Homer, F.L. Rose and A.C. Walpole: Br i t . J .Pharmacol. Chemot. 6, 235 (1951)

6. V. Henri; (1919) Etudes de Photochemie (6,9. 89); Cited fromAbsorption Spectrophotometry by G.F. Lothian. Second edition (1958)Po90« Hilger and Watts L td . , London,

7. J . Kopoldova, J . Liebster and A. Babicky; I n t . J . Appl. Rad. IsotopesU f 139 (1961)

6. J . Kopoldova, J . Liebster and A. Babicky, I n t . J . Appl. Rad. Isotopes,

H , 489 (1963a)9. J . Kopoldova and J . Liebster; I n t . J . Appl. Rad. Isotopes 1£, 493 (1963*0

10. -K.S. Korgaonkar and R.B. Lone; In t . J . Rad. B io l . , £ , 67 (1962b)

1 1 . K.S. Korgaonkar, S.V. Marathe and K,A. Chaubal; Science and Culture24 , 100 (1968)

12. J . Leibster and J . Kopoldova; 'Radiation Chemistry of Amino Acids ' ,In'Advances in Radiation Biology1. Editors-L.G. Augenstein, Mason andH. Quastlerj Vol. 1,, p.165 (1964), Academic Press , Hew York and London.

13. Ch.R. Maxwell, D.C Peterson and N.E. Sharpless; Rad. Res. 1, 530 (1954)

14. 'R/Pageau and A.R. Mehran; Nature 2i]2, 98 (1966)

-152-

15. H.E. Shaiplese, A.E. Blair and Cfc.H. Maxwell; Had. Res., 2, 155 0955)

16. O.D. Shreve, M.R. Heether, H.B. Knight end D* Swernj Anal. Chem. 2^, 277 (1951)J|

TABLE I

G Values

Dose in M rad. G

1-3 3.0

2.6 2,7

3.9 2,4

- 1 5 3 -

O.V. SPECTRA OF L-TNREONIHE

MRAMATEO WITH * RAYS FROM tt"

UN ICMfn ** ^r

INFRARED 8K0TRA Of L-THREONINE IRRADIATCDWITH /RAYS FROM CO"

• •

CAUMATIOH CURVE FOR L-THREONMCAT 10 8 /r

t l«MOUHT I

» t

0-1

o

A• /1/ i

iA A

A• //

•WWCLEHSTH W Ji

-154-

GAMMA-IRRADIATION STUDIES WITH SYNTHETIC POLY-L-LYSINE-HfDRO-BROMIDE USING MONOLAYER TECCHNIQUE

S.V. Joshi and K.S.-KorgaonkarBiophysics Division, Cancer Research Institute

Tata Memorial Centre, Bombay - 12.

INTRODUCTION

Our earlier studies'1*2 ' with dilute aqueous solutions of poly-

L-tyrosine, poly-L-serine showed that marked and significant changes in

the monolayer properties of these polyamino acids are produced on their

irradiation with Co gamma rays. These studies were, therefore, ex-

tended to other synthetic polyamino acids and the results on poly-L-

lysine-hydrobromide are presented here.

MATERIALS AND METHODS

Poly-L-lysine-hydrobromide (YEDA PREPARATION - LY 45 • Mole-

cular Weight es t imated by v i s c o s i t y measurements-40,000) was obta ined

from New England Nuclear Corpora t ion , Boston, U.S.A. The sample was

dissolved in triple distilled water (45 ug/ml). The solution was irra-

diated in 20 ml aliquots by Co gamma rays at a dose rate 1.2 x 10

rad/hr and the changes in pH, ultra-violet spectral absorption and

monolayer properties were studied. The pH measurements were carried

out two hours after irradiation using Elico pH meter - Model L1-10c

The procedure for irradiation of the samples, for their ultra-violet

spectral absorption measurements and for the study of their monolayer

pr§pertiea were as described in the previous papers^112 ' . The mono-

layers were formed on 0.2 M phosphate buffer + 4$ NaCl at pH 6.0 as

sub-phase. Poly-L-lysine was spread on the sub-phase after inixing

i ts solution with n-propyl alcohol in the ratio 111 (v/v). No mono-

layer of the polymer could be fouued above pH 10.0.

-155-

RESULTS

The plots of pressure, P (dynes/cm) vs area, A(m2/mg), of

product of pressure and area, PA vs P in t t e low pressure region for

non-irradiated as well as i rradiated samples of poly-1-lysine-

hydrobromide are given in Figs. 1 and 2 respectively. The variation

of area compressibility f = (- SA/A. £*P) with area A as obtained

from P-A curves was also studied, and the values of area at minimum

compressibility (A ^m) were noted. The various character is t ic area

values and number average molecular weights as determined from the

above p l o t s , along with the data on changes in pH of the polypeptide

solution af ter i r radia t ion are given in Table I . The ul t ra-violet

spectral absorption curves are given in Fig<>3»

DISCUSSION

Prom the data for non-irradiated sample of poly-L-lysine-hydro-

bromide presented above, i t i s seen that j

1. The P-A isotherm (Fig.1) shows a small region of sudden r i se

in compressibility. Isemura et a l ^ ' * ^ who observed a similar sharp

r ise in compressibility (so called 'plateau region') in t h e i r P-A curves

of analogues of synxhetic proteins,associated i t with the presence of

long side chains in these molecules.

2. The values for l imit ing, actual and packing areas per

residue (Table I ) appear ro be re la t ive ly high as compared to those noted

ea r l i e r fox poly-L-tyrosine, poly-DL-alanine (1) and poly-L-serine (2) .

These values indicate absence of compact structure for th i s molecule at

t h i s pH. A similar expanded character has also been reported for another

€ -amino polymer 'amilan' by Hotta^ ' * ' and Loebv ' .

Molecular weight obtained from the PA-P plot i s nearly double the

value as supplied by the manufacturers. ?his indicates that the associa-

tion in t h i s aolecule i s re la t ive ly weak as compared with tha t noted

-156-

earlier for poly-L-tyrosine, poly-DL-alanine and poly-L-serine. This

points towards some inter-relation between molecular association end

molecular weight of these molecules. Similar findings are also due to(a)

Schellman and Schellman^ ' who observed that the agregation or associ-

ation in polyamino acids is restricted to low molecular weight peptides.

The data on the irradiated samples show thatj

1. pH of the solution is gradually reduced with increase in

dose (Table I ) .

2. The P-A curve is shifted to lower area at the f i rs t dose9

i , e . , 1.6 x 10 rad. At the next two higher doses used, however,, i t i s

shifted back to higher areas. At the highest dose, i . e . , 6.8 x 10" rad,

the effect i s again reversed and a drastic reduction in area is noted.

At this dose the "plateau" region disappears. No measureable changes in

molecular weight are noted from the monolayer data at all the doses used.

The shift of P-A curves to lower area without change in molecular

weight at the lowest dose could axis.* out of increased solubility of the(12}of the moleculeo Liebster et alv ' i n their irradiation studies on

simple DL-lysine hydrochloride observed hydroxylation at C carbon atom.

Probably such an effect on the side chains of this molecule would render

i t more soluble and cause the observed shift.

The disappearance of the 'plateau* region at the highest dose used

indicates further action on the side chains resulting in narked reduction

in their length.

Explanation far the reversed trend of expansion of area without

much change in molecular weight for the two intermediate doses, requires

such changes in the side chains which would bring them back on the

surface. If the effect of irradiation was such as to knock off HH2

groups from some of the side chains in the same molecule, such chains

would become hydrophobic a&d will have a tendency to be away from aqueous

-157 -

phase. Even the intact groups will now be in greater hydrophobic

surroundings and may be lifted into non-aqueous phase, i . e . ai

This seems to be a possible explanation for the observed reverseA A

trend at doses 2.6 x 10 rad and 4.6 x \Q rad. Such a behaviour has

also been noted by Desai and Korgaonkaar ' , particularly in their

irradiation studies with basic proteins. Deamination in tii e 6

position has also been noted in radiation studies referred to above'12 '1 ' ' ' .

This emphasises the probable important role of BH9 group in ihe expansion

phenomenon.

3. The ultra- violet absorption data (Fig.3) show small but

gradual increase with dose in absorption of the peptide band^ * ' with

a peak at ?05 nm. It is difficult to associate the increase in absorption

with increase in peptide chain length since increase in ultra-violet

absorption in the neighbourhood of 220 nm seems to be even higher and a

possibility of a new weak band near 220 nm may not be ruled out. Lack

of major attack on the peptide bond i s , however, clearly evident^

The present report shows that, as in our earlier studies with(12)poly-L-tyrosin, poly-])L-alanine and poly-L-serine^ , the main target

of radiation injury in poly-L-lysine-hydrobromids, is not the peptide

bond but the side chains of the polypeptide molecule. These observations

appear significant in the context of the earlier results of Southern ani

Rhodes ' who formed cleavage of peptide bond as the major effect of

radiation in polyamino acids. Their conclusion i s , however, based on

viscosity study, a method which is highly sensitive to chain length

as compared to chemical damage in the side chains of the molecule..

Moreover, the radiation doses used by these workers were much higher -

in the megsrad range.

-158-

phase. Even the intact groups will now be in greater hydrophobie

airroundings and nay be lifted into non-aqueous phase, i .e . air^

This seems to be a possible explanation fi>r the observed reverse

trend at doses 2.6 x 10 rad and 4.6 x 10 rad. Such a behaviour has

also been noted by Desai and Korgaonkar*1 , particularly in their

irradiation studies with basic proteins. Deamination in the 6

position has also been noted in radiation studies to above ' ,

This emphasises the probable important role of NEL groups in the expansion

phenomenon.

5. The ultra-violet absorption data (Fig.3) show small but

gradual increase with dose in absorption of the peptide band^ ' ' with

a peak at 205 nm. It is difficult to associate the increase in absorp-

tion with increase in peptide chain length since increase in absorption

with increase in peptide chain length since increase in ultra-violet

absorption in the neighbourhood of 220 nm seems to be even higher and a

possibility of a new weak band near 220 nm may not be ruled out. Lack of

major attack on the peptide bond i s , however, clearly evident*

CONCLUSIONS

The present report shows that, as in our earlier studies withfi 2)

poly-L-tyrosine, poly-DL-alanine and poly-L-serinev ' , the main target

of radiation injury in poly-L-lysine-hydrobromide, is not the peptide

bond but the side chains of the polypeptide molecule. These observations

appear significant in the context of the earlier results of Southern and

Rhodes ' who formed cleavage of peptide bond as the major effect of

radiation in polyamino acids. Their conclusion i s , howeverrbased on

viscosity study, a method which is highly sensitive to chain length

as compared to chemical damage in the side chains of the molecule»

Moreover, the radiation doses used by these workers ware much higher - in

the megarad range*

-159-

REFERSNCES

1. K.S. Korgaonkar and Sindhu 7 . Joshi j Gamma i r rad ia t ion studieswith synthetic polyamino acids - Studies with poly-L-tyrosineand poly-DL-alanine using monolayer technique, Rad. Res. 35,213-226 (1968)

2. Sirflhu Vo Joshi and K.S. Korgaonkar; Monolayer studies with normaland gamoa-irradiated poly-L-serine Indian J . Biochem. (In press)(1969)

3 . T. Isemura, K. Hamaguchi, H. Rani, J . Noguchi and H. Yuki;Monolayers of synthetic protein analogues; Nature 168, 165-166 (1951)

4. K. Eda and Y. Masuda; On the mono-molecular layer of o£-amino laur icacid polymer, Bul l , Chem. Soc. Japan 24, 140 (1951)

5« T. Isemura and K. Hamaguchi;Surface chemistry of synthetic proteinanalogueso I . Surface P-A re la t ion of synthetic polypeptides as themodels of p ro te ins , Bull . Chem. Soc. Japan 25, 40 (1952)

6. H. Hotta5 Surface chemistry of high polymers I I Non-electrolyticf lexible l i nea r polymers at o/w in ter face , Bull Chem. Soc. Japan, g_6_386 (1953)

7 . H. Hotta; Surface chemistry of high polymers I I I Some relationshipsbetween the monolayer of non-electrolyt ic l inea r polymers, Bull. Chem.Soc. Japan 2$,586 (1955)

8. G.I . Loeb; Cited in 'Surface chemistry of proteins and polypetides'p . 20, N.R.L. Report 6318, I . S . Naval Res. Lab,, Washington, D.C. (1965)

9. J.A. Schellman and C. Schellman; In the proteins (H. Neurath-Ed) I Ip»43» Academic Press , New York, London (1964)

10. A.Re Goldfarb, L.Jo Saidel and E. Mosovich; The u.v, absorption spectraof pro-ceins, J . Biol , Chem. 122., 597 (1951)

11. J . S . Ham and J.R. P l a t t ; Far U.V. spectra of peptides, J . Chem. Phys.20, 355 (1952)

12. J . Liebster , J . Kopoldova, J. Kolousek and A. Babick'y; in 2nd Intern .Conf. Peaceful Uses Atomic Energy, Geneva, 22| P« 492, I .D.S. ,Columbia Univ. Press , N.Y. (1958)

13. J.To Daviesj Some factors influencing the orientat ion of (rAminogroups in monolayers of proteins and and.no acid polymers, Biochem. J .5§, 509 (1954) 6Q

14. A.M. Desai and K.S. Korgaonkar; Studies on the effects of Co r-rayson protamine sulphate, lysozyme and insulin by monolaBrsr technique. Rad.Res. 21, 61 (1964)

-160-

15. J . Kopoldova and J . Liebster ; The radiat ion chemistry of amino acids,p. 157-226, In 'Advances in radiation biology1 Vol.1.(L.G. Augenstein,R. Mason and H. Quastler Eds.) A.P. (1964)

16, E.M. Southern and D.N. Rhodes; Radiation Chemistry of polyamino acidsin aqueous solutions, In 'Advances in chemistry' s e r i e s , No,65,p.58-77, "Radiation preservation of Foods" (1967)

- 1 6 1 -

TABLE I

Monolayer Characterist ics of Poly-Ii-Lyaine-HydroftromLde

(Areas expressed as angstrom square per residue)

Low Pressure region High Pressure regionDose in

Rad. Limiting Actual area Packing Area at M o l „area occupied area minimum „.

compressibility

19.8Nil

1.6x1O4

2.6xiO4

4.6x1O4

6.8x1O4

> 2 6

>23

>24

>25

> 22

25.5

21.7

23-9

?3«9

15.8

17.1

11-24

17*19

16-21

17-22

~s 16

9,8x104

t i

••

it

ti

5.4

4-7

4.6

4.4

4.2

- 1 6 2 -

LEGENDS FOR THE FIQUBES

Pig . 1 . P-A curves of poly-L-lysine-byd:robroinide at pH 6.0

Pig . 2 . PA-P curves of poly-L-lysine-hydrobromide at pH 6.0

Pig. 3 . U.V. absorption spectra of poly-L-lysine-hydrobromide.

-163-

P-A CURVES OF POLY-L-LYSINE-HBrSTUDIED AT p H « 6 0

(O CM PHOSPHATE

1) • •» X X

«J4 •

5) • •

CONTROLumaoiaTED

• •

t-e

t •

< f

XIO*r4

X ;•) r

« io*r

s a x ia*r

PA-P CURVES Of POLY-L-i.YStNC -H8»STUDIED AT pH-BO

: nuiATi* i < i »>

mm

1111ig

I-S -

i-« -

M -

10 .

0-* -

OH •

••r •

o « •

0-B -

0-4 -

OS -

o-t -

0-5 -

11

I I

»

81

>

6-t e-a i-o M t-4 i *4 B 12 W 20 £4

PRESSURE (<lf*/em)

FlGd FIG.2

U.K aaaowrrtow apEcnm OF PdT-i.-i.YsmE wwwwwompti

WH.T-t-l.TtW

PIG.3

APPLICATION OF ESB FOR THE STUDY OF RADIOLYSIS OF FROZEN AQUO-ORGANICSYSTEMS

¥.1$, Moorthy, C. Gopinathan and K.N. Rao

We have employed the ESR technique to study the behaviour of

organic compounds such as the a lcohols , ethers and ketones in radiolysed

frozen aqueous systems. Thus, in gamma-irradiated frozen aqueous 1M H^SO.

system, the presence of these compounds in concentrations from 0.1 - 211

has , what one might a t f i r s t s igh t say, peculiar e f f ec t s : the H-atom and

SO." ESR signals normally observable in the absence of these addi t ives

are both absent9 the OH radical ESR signals are p rac t i ca l ly unaffected,

and there are new ESR s igna ls , d i f ferent for each compound, whose in ten-

s i t i e s are several times l a rge r than what they are in the corresponding

acid-free system of the same concentration in the organic compound. These

effec ts are shown for the case of ethanol in F i g . 1 . Also var ia t ions of

the i n t ens i t i e s of these new s ignals in the ac id-f ree systems as a fun-

c t ion of the concentration of the organic compounds indicated tha t the

species responsible for these s ignals are formed by the d i rec t ef fec t

of radiat ion on the organic eompopnd. This i s shown in Fig.2 fox* the

cese of e thanol . Analysis of the hyperfine s t ructure of the new signals

permitted the assignments as shown in Table 1. Our previous work on

ESR study of frozen aqueous systems had led us t o the conclusion tha t in

frozen aqueous HpSO the H-atoms and SO.™ rad ica l ions are formed by the

reaction of the solute ions (HSO ~) with the primarily formed e lec t ron

and hole, respect ive ly :

e~ + HSO ~ >. H + SO ~4 4

h+ + HSO " —-* H++ SO -4- 4

Also, we could show that the e lec t ron react ion i s responsible f o r the

in tens i f ica t ion of the R rad ica l spectrum, fo r in presence of e~ sca-

vengers t h i s in tens i f i ca t ion was not observed. That the above organic

compounds, espec ia l ly the a lcohols and e the r s $ cause the disappearance

of the SO. s ignals i s perhaps not surprising because these can be

expected to be hole scavengers, for as revealed in the study of the

-165=

radiolysis of alcohols in the pure state at 77°K, the following

reactions occurs

RH HH

so that in the frozen aqueous systems we can expect the reactions:

h+ + RH * RH+ *2° H ' + H^0+

More dif f icul t t o understand is the observation that thb.=a compounds

can act as electron scavengers, for a l l of. them, with the exception of

acetone, have very low react ivi ty with hydrated electrons in water a t

room temperature. Also, one has to explain why in presence of the

acids the R* radical ESR spectrum i s intensified as compared to the

acid-free samples. Both these effects have been explained on the

postulate that in presence of the acid "the compounds RH (al l of which

are oxy compounds) are protonated a t the oxygen atom (or the acid is

solvated partly by water and part ly by RH):

HHHIO-H

This species, unlike the molecule RH, reacts with the electron according

to the following scheme:

A-«- e H'

R8

H

HO-H

- 1 6 6 -

TAHEE I

Organic B a d i c a l s (H") formed i n Gamma-irradiated Frozen AqueousSystems C o n t a i n i n g ^ S O and Organic Compound (RH) a t 77 °K

EH

OL.OH

.CO

*l—IH*

CH CHOH

CH-CHOCELCH-

\:H,

Relative G ( ( y y ) H )

(arbitrary units)oo i=o aft roo

ON

I ••

-1 68-

ENERGY TRANSFER STUDIES IN BENZENE SOLUTIONS OPMETAL ACETYLACETONATESs FLUORESCENCE QUErCHING

O.P. K a l a n t r i and S0B. S r iva s t avaChemistry Divis ion

Bhabha Atomic Research Cent re , Trombay, Bombay-85

Quenching in benzene solutions containing s c i n t i l l a t o r PPo and

quenchers Al(acac),., Fe(acac), and Co(acac), was studied using both Co

2T -rays and 250 nmLTF ae sources of excitationso l inear Stern Volmer

relationships were obtained for Al(acac), which yielded > * ' - . = 854 M

and 536 M corresponding to specif ic ra te of quenching (kq) = 3»16 x

10 Nl~ see" and 2-56 x 10 M~ sec for high energy and photoexcited

cases respectively,, No effect of N_0 and 0.1M CHC1_ was observed ind i -1eating that the donor species i s predominantly 3^ s ta te of benzene.

Addition of Al(acac), showed decrease in phosphorescence in tens i ty of

benzene with concomitant increase in phosphorescence emission from the

quencher, suggesting the poss ib i l i ty of energy transfer to 4«32 ev level

of quencher from Bo« (4.71 ev) s ta te of beazene.

On the other hand. S tem Volmer plots for Fe(acac) end Co(acac)_

are concave in nature for botn Hie types of exci ta t ions . On extrapola-* ~1 4 - 1

tion of i n t i a l points, % '—' 9 x 10"M and ^—*3 x 10 M was obtained

for Co(acac)_ and Pe(acac) respectively fo r high enefgy exci ta t ion o

Relatively lower values, though of the same order, were obtained for

2 50 nm excitation„ These values correspond t o anomalously high kq values-

much in excess of the prescribed rate of diffusion controlled react ions .Data have been analysed in terms of Porster model of long range

cresone nee transfer and a c r i t i c a l distance (Ro) ~ 22 A evaluated forAl(acac),= Since the overlap integrals for Al(acac) Pe(acac) and

/ \ -15 ' -15 '

Co(acac)3 are of the same order, v iz . 8 x 10 „ 7.2 x 10 respectively,

one expected kq of the sane order for a l l the three quenchers« Further

quenching experimente carried out as a function of viscosi ty have revealed.

the poss ib i l i ty of diffusion controlled mechanism of quenching in the

present system,,