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J. Phys. Chem. 1991, 95 , 2665-2668Gas-Phase Inorganic Chemistry: Laser Spectroscopy of Calcium and StrontiumMonothiolates and Monohydrosulfides

2665

W. . M . L. Fernando, R.S. Ram, L. C. O'Brien,? and P. F. Bemath*,$Department of Chem istry, University of Arizona, Tucson, Arizona 85721 (Received: July 17, 1990)

The gas-phase reaction of Ca and Sr vapors with various thiols and hydrogen sulfide was studied. These reactions producedthe meial monothjolates_CaSR and SrSR, where R = H, CH3, CH2CH3,etc. Three electronic transitions, CZAf-g2A',B2A"-X2A', and A2Af-X2A', were detected by laser-induced fluorescence. The spectra a re consistent with a bent Ca-S-Rstructur e. Some Ca-S and Sr-S stretching frequencies were determined from the spectra.

IntroductionThe interaction of metals with sulfur-containing molecules isan important area in inorganic chemistry.' Metal thiolates areoften studied because they provide models for naturally occurringsulfurcontaining compounds found, for example, in the active sitesof proteins. Zn and C d thiolates have been known for a long time.2These compounds are studied partly because of the biologicalimportance of the elements and partly because zinc dithio-carbamate and related compounds act as accelerators in thevulcanization of rubber by sulfur. Ag(1) and Au(1) thiolates havebeen extensively studied in relation to chemotherapy for arthritis.jAlthough the action of H2Son metals commonly gives insolublesulfides, some complexes of H2 S are known, although they arereadily oxidized to sulfur or deprotonated to SH - c o m p l e x e ~ . ~The re has been no previous experimental work on gas-phasealkaline earth metal monothiolates. The smallest members of thealkaline eart h monothiolate family (M-SR, M = Ca, Sr ) are thetriatomic hydrosulfides Ca SH an d SrSH . W e have previouslystudied many monovalent derivatives of Ca and Sr in our labo-rato ry, includ ing those with metal-oxygen," metal-nitrogen?JOm e t a l - ~ a r bon , " - ~ ~nd metal-boront4 bonds. Th e observation ofcalcium and strontium monothiolates extends our work to the nextrow of the periodic table.High-resolution analysis proves that alkaline earth mono-hydroxides are linear.s.6*15-17Recently, the gas-phase calciumand strontium hydrosulfide molecules were studied a t high res-olution in our laboratory.'* These high-resolution analyses provedthat CaSH and SrSH are bent. Electron propagator calculationsby OrtizI9 confirm that the ground and excited states of CaS Hare bent, having C, symmetry. T he optimum calculated eometryfor the ground state has a Ca-S bond length of 2.6141, S-Hbond length of 1.346 A , and a Ca-S-H bond angle of 100.Oo.In-the experiment: reported here, thr ee electronic transitions(A-X, 8-X, nd e-X) of the gas-phase calcium and strontiumthiolates were studied at low resolution. Th e method of synthesisof these compounds and the analysis of their spectra ar e discussed.

Experimental SectionThe calcium and strontiu m thiolates were made in a Broida-typeoven1*by the reaction of the electronically excited metal vaporwith the ap propriate thiol or H2S,similar to previous work in thisarea.s-18 The Ca or Sr metal was vaporized from a resistivelyheated crucible, carried to the reaction region by argon carriergas, and reacted with the oxidant. The total pressure inside theoven was maintained a t -1.5 Torr, with a few milliTorr of th eoxidant. All the thiols used in this study were purchase d fromthe Aldrich Chemical Co. (97% purity). Th e vapor pressures atroom temperature were sufficiently high for the production ofthiolates. For H$ the gas from a lecture bottle (Matheson) wasadmitted to the oven.'Curr ent address: Department of Chemistry, Southe rn Illinois University,Alfred P. loan Fellow; Camille and Henry Dreyfus Teacher-Scholar.To whom correspondence should be addressed.

Edwardsville, IL 62026.

0022-365419112095-2665$02.50/0

CaSH 15 366 15810 16082CaSCH , 15 509 15807CaSCH2CH3 1 5 463 15760CaSCH2CH2CH3 15455 15711bC a S C H ( C H 3 ) * 15 423" 15 694bCaS C(C H3), 15 434" 15 664bCaS CH (CH 3)C2 HS 15 432" 15 694b'Broad peak. bOverlapp ed with th e 8-2 transition of CaSH.

TABLE II: Band Centers of the Strontium Thiolates (in em-')A2Al-22Af BzA'C22A' eZA"22AfmoleculeSrSH 14 293 14815 15 026SrSCH, 14407 14780 15468SrSC H2CH 3 14 355 14745 15 499SrSC H2CH 2CH3 14351 14734 15 466"SrSCH(CH ,), 1432 9 14 732SrSC(CH,) , 14318 14732SrSCH(C H3)C2H S I4 323 14728' road peak.

Two CW broad-band (N 1 cm-I) dye lasers pumped by th e alllines output of Coherent Innova 90 and Coh erent Innova 70 argon

(1) Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry, 5th ed.;(2 ) Blower, P. J. Coord. Chem. Rev. 1987, 76 , 121.(3) Sadler, P. J. Strucr. Bonding (Berlin) 1976, 29 , 171.(4) Casewit, C. J.; DuBois, M. R. J . Am . Chem. Soc. 1986, 108, 5482.(5 ) Bernath, P. F.; Kinsey-Nielsen, S.M. Chem. Phys. Lett. 1984, 105,(6) B razier, C. R.; Bernath, P. F. J . Mol. Specrrosc. 1985, 114, 163.(7) Brazier, C. R.; Ellingboe, L. C.; Kinsey-Nielsen,S.M.; Bernath, P. F.(8) OBrien, L. C.; Brazier, C. R.; ernath, P. F. J. Mol. Specrrosc. 1988,(9) Brazier, C. R.; Bernath, P. F. J . Chem. Phys. 1988,88, 2112.(10) Ellingboe, L. C.; Bopegedera, A. M. R. .; Brazier, C. R.; ernath,P. F. Chem. Phys. Leu. 1986, 126, 285. O'Brien, L. C.; Bernath, P. F. J .Chem. Phys. 1988,88, 2117.(1 1) Bopegedera, A. M. R. P.; Brazier, C. R.; Bernath, P. F. Chem. Phys.Letr. 1987, 136, 97; J. Mol. Spectrosc. 1988, 129, 268.(12) Brazier, C. R.; Bernath, P. F. J . Chem. Phys. 1987,86 ,5918; 1989,97 , 4548.(13) OBrien, L. C.; Bernath, P. F. J . Am . Chem.Soc. 1986, 108, 5017.(14) Pianalto, F. S.; opegedera, A. M. R. P.; Fernando, W. T. M. L.;Hailey, R. N.; OBrien, L. C.; Brazier, C. R.;Keller, P.C.; Bernath, P. F.J . Am . Chem. Soc. 1990, 112, 7900.(15) Hilborn, R. C.; Zhu, Q.; arris, D. 0. J. Mol . Specrrosc. 1983, 97,13 .(16) Nakagawa, J.; Wormsbecher, R. .;Harris, D. 0. J . Mol. Specfrosc.1983, 97, 31 .(17) Kinsey-Nielsen,S.M.; Brazier, C. R.; Bernath, P. F. J . Chem. Phys.1986, 84 , 698.(18) Fernando, W. T. M. L.; Hailey, R. N.; O'Brien, L.C.; Ram, R.S.;Bernath, P.F. Unpublished results and work in progress.

Wiley-Interscience: New York, 1988.

663. Bernath, P. F.; Brazier, C. R. Asfrophys. J . 1985, 228, 373.J . Am. Chem. Sot. 1986, 108, 2126.130, 33 .

0 1991 American Chemical Society

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2666 The Journal of Physical Chemistry, Vol. 95, No. 7, 1991A

Fernando et al.

620 640 nmFigure 1. Laser excitation spectra of calcium thiolates. The corre-sponding transitions for the different thiolates are connected by a line.CaSH is also observed in the spectra of the larger thiolates in the upperfour panels. The transitions of these four larger thiolates are badlyobscured by the transitions of CaSH.ion lasers were used in this experiment. The dye lasers wereoperated with Pyridine 2, DCM , and Rhodamine 6G dyes. Onedye laser beam was focused into the oven to probe the moleculartransition. The second dye laser was used to excite the alkalineearth 3P,-1S0tomic transition (6892 A for Sr, 6573 A for Ca)and to promote the chemical reaction.Two types of low-resolution spectra were recorded. Laserexcitation spectra were obtained by scanning the probe laserthrough a spectral region where MSR (M = Ca and Sr) absorband detecting the to tal fluorescence through red-pass ilters (SchottRG 695, RG 9, and RG 780)with a photomu ltiplier tube. Re-solved fluorescence spectra were ob tained by fixing the dye laserat the frequency of a CaSR or SrS R molecular transition andresolving the fluorescence with a small monochromator.Results

A comparison of laser excitation spectra of CaS R and SrS R(R = H, CH,, C2H5, C3Hs, CH(CH3 )2, C(CH3),, and CH-(CH3)C 2H,] is provided in Figures 1 a_nd 2, zespectively.-Thethree-observed elec tron ic transi tions ar e C2A-XA, h2A-XZA,and A2A-g2A. Th e observed band cen ters for these transitionsobtained from laser excitation and resolved fluorescence spectraare presented in Tables I and 11.The intensities of the features in these spectra a re distorted.The red-pass filters used to eliminate the scattered laser radiationin recording the excitation spectra enhance some fe atures overothers. The relative intensities are also dependent on the outputpower of the dye laser.In genercl for calcium_ hiolates, as the alkyl group becomeslarger, the A-X and 8-X band centers shift to the red (Figu re1) . Th e C-R ransition was not observed for calcium thiolates.For the strontium thiolates (Figure 2), the spectral shifts aresimilar. Note that the A-g transition of both CaS H and SrSHshifts to the blue when the H is replaced by CH,. Th e e-%(19) Ortiz, J. V.Chem. Phys. L t t . 1990,169,116; J . Chem. Phys. 1990,(20) W e t , J . B.; Bradford, R. S.;Eversole. J. D.; Jones,C. R.Reu. Sci.92, 6728.Instrum. 1975, 46 , 164.

660 680 700 nmFigure 2. Laser excitation spectra of strontium thiolates. The corre-sponding transitions for the different thiolates are connected by a line.SrSH is also observed for the larger thiolates in the upper four panels.The sharp feature at 6892 A is the LP,rlSotomic line of Sr, whichexcites th e 1-0 vibronic band of the A-X transition of SrSH.transition is observed for SrSH, SrSCH,, and SrSCHzCH3.Excitation spectra of larger thiolates show an almost identicalspectral pattern. Since these electronic transitions involvedmetal-centered molecular orbitals, the natu re of th e alkyl groupis expected to have a small effect on the transition energies. Ca SHan d SrSH were also observed in the spectra of larger thiolatesand grett ly mask the spectral features of the heavier thiolates.The C-5 ttansition of large Sr thiolates ar e probably obscuredby :he C-X transition of SrSH. t o r calcium compound s, the8-X transition is obscured by the A- 2 transition of Ca SH .C a s a nd SrS (AZ+-XZ+ system) appeared as an impurityin many spectra.21-22Changing the conditions in the oven alsominimized the production of sulfides. The reac tion of metal vaporwith the thioethers (CH3SCH3 nd C2H 5SC2HS)nd the disulfide,CH3S SCH3, produced the strong features identical with thoseobserved with the corresponding thiols, CH 3SH and C H3CH $H.However, the larger thioethers and disulfides are not availablewith high purity and the vapor pressures are less than th e thiols.The observed electronic states for the alkaline ear th thiolatescan be explained with th e aid of the energy level diagram in Figure3. The bonding and electronic structu re in these molecules aredescribed by an metal ion perturbed by the ligand. Th e thiolatespectra resemble the corresponding monohydroxides. The valence4s, 3d, and 4p atomic orbitals of the Ca+ ion give rise to theelectronic states of CaOH. The OH- ligand also mixes the p a p tatomic orbitals of the stat es so hat, for CaOH, the B2Z nd A 2 nstates are pu-da and pd*mixtures. CaSH is a bent moleculeand belongs to the C, point group. This symmetry lowering fromC of CaOH lifts the degeneracy of the in-plane p/d orbitalsand out-of-plane p/d orbitals. Therefore, the 211state split jnto2A out-of-plane) and 2A (in-p1ane)jtates in CaSH. The A 2 nand h2Z+states of Ca OH become-the AZA,D2A, and C2Astatesof CaS H. The symmetry of the A, 8,and states is determined

(21) Blues, R.C.; Barrow, R.F. Trans. Faraday Soc. 1969, 65 , 646.(22) Pianalto, F. .; Brazier, C. R.; Brien,L.C.; Bemath, P. .J . Mol.Spectrosc. 1988, 132, 80.

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Ca and Sr Monothiolates and Monohydrosulfidescm-

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