Volume 48, number 3 CIIEMICAL PHYSICS LETTFRS ISJune

THE EFFECT OF INTERMOLECULAR INTERACTION ON THE ASYMMETRIC STRETCHING VIBRATION OF CS,

Cabor JANCSi)* and W. AIexander VAN HOOK Chemistry Department. Uniwrsity of Tennessee. Knoxville. l’etmessee 3 7916, USA

Rcccrved I March 1977

A qUantltiitlVC intcrpretdtron of thC “C/‘*C VdpOr prCSsUrC lsotopc cffcct (Vi%.) in C’Sz irrdicntcs that the frcqucncy slnft on conderrsatlon 111 the asyrnmctrlc stretching mode. 9, to be ascrrbed to the effect of internrolecu[ar forces LS 135 1 cm-‘. When combined wrth the spectroscopically observed shift, 27 cm-’ , a soundiy ba~cd value for the “dtekctric correc- tion” for this absorption band is obtained. The result is of interest, and mdrcates tbat thermodymuuic data on isotope cf- fects un be uccd to test this agpcct of spectroscopic theory.

Akopyan et al. [l], Lefranc ct al. [2], and Chfford and Crawford 131, among others, have pointed out that m molecules with very intense IR absorption bands the observed shifts of these frequencies on 8 phase change (particularly condensation) must be in- terpreted as being the sum of an intrinsic effect due to the action of the mtermolecular forces and a “diclec- tric correction”. The latter effect takes account of the difference between the effective field of the light wave acting on the molecule m a condensed medium and the rtvcra~c fictd in this medium [I ]_ The calculatior~

of the “dielectric correction” is difficult and the re- sults to date, at least in the CS2 system, are only ap- proximate. In the materrai which follows we present an analysis of experimental data which enables a di- rect calculation of the intrinsic effect. This can be combined with the observed phase frequency shaft to deduce a value for the “dielectric correction”.

Betts and Buchannon [4] have recently presented data on the vapor pressure isotope effects, tQ/t3C and 3%132S, for CS2. Their measured carbon effects are in reasonable agreement with the earlier reports at the boning point [S,G]. A critical analysis by Jancsci and van Hook [7} leads to the conclusion that the t2C/ 13C vapour pressure isotope effect (VPIE), ex-

* Permanent address. Central Research institute for Phyacs, Hun~r~n Academy of Sciences, Budapest. Hungary,

pressed as the Iogarithrmc vapor pressure ratio, is rca- sonably well established across the liquid range, and is given satisfactorily by the empirical tit to their more complete peculations,

In (p12,,32S2/pt3C32s ) = - 0.529/1’(K) + 0.00035. .2

In the In-rear molecule, CS,, the carbon atom is situated at the center of mass so there is no isotopic shift in the symmetric stretching motion or in the mo- ments of inertia or mean square torques. Therefore none of these motions contribute to the isotope er- ftXt, Which IIlUSt bC dUc to a lincrrr combination of contributrons from the hindered cxtcrnal tmnsIationq,

the doubly degenerate bending motion and the asym- metric stretch. The transIatio~~~ and bending confri- buttons to the isotope effect are expected to be smah [7-91 and this is borne out in the detailed normat coordinate analysis [7 j. At 300 K they amount to I .O X 10e4 and -4.7 X 10 -4, respectively, contpJred to the total effect -14.1 X IO-4 Not only is the contri- bution of the combined transIariona1 and bending ef- fects small in comparison to the overatt one, it is cspc- cially well defined in terms of the spectroscopic data [IO,1 I]. It follows that the major portion of the ef- fect is due to the shift on condensation of the very in- tense asymmetric stretching absorption, and from the measured VPIE we find the shtft in this frcqucncy due

4gt

Volume 48, number 3 CHLMICAL PIiYSiCS Lf=-t-l-ERS 15 June 1977

to iRtermolecu~~r forces as 13+2 cm-? The result, ifl- dicating a “dichxtric correction” of 14 cm-l (the ob- served frcqucncy shift is 27 cm-l) is nicely in the mrddle of the X0-15 cm-1 estimated by Akopyan et al. [I ] and the 15 cm-l (Lorentz correction) and 8 cm--I (Onsagcr correction) calculated by Jacob and Benarch [12]. It indicates &hat thermodynamic data on isotope effects can be used to test this aspect of spectroscopic theory_ The method is applicable to other molecules (Ccl,, CBr4, WI,, etc.) and we are currctttiy engaged in a more complete analysis of the approach which will bc reported clsewhcrc.

Support of the Natronal Science Foundatton through its Office of International Programs and Chemicat Thermodynamics Program, and from the National Institutes of Health is gratefully acknowlcdg- ed.

References

S.Kh Akopyan. O.P. Gi~rnn and N.G. Bakhshicv, Opt. Spectry. 36 (1974) 185. R. Lefnnc, J. Jacob and J. VincentCeisse, Spectrochim. Acta 22 (1966) 1057. AA. Clifford and B. Crawford, J. Phys. Chcm. 70 (1966) 1536. R.H. Bctts and W.D. Buchannon, Can. J. Chcm. 54 (1976) 3007. P. Baertschi and W. Kuhn, in: Proceedingc of the fnter- ~,atron~I Symposium on isotope Scpdration, eds. J. Krstcmakcr. J. Bigcleiscn and A.O.C. Nicr (North- Holland. Amsterdam, 1958) p. 65. E. JCreII and W. Nattermdllcr, Abhandl. Dcut. Akad. Wirt. Berlin Kl. Chem. Ccoi. Blot. (1965) 131. G. Jan& and A.W. van Hook, Can. J. C&m., submitted for publication. C. Jam6 and A-W. van Hook, Chern. Rev. 74 (1974) G83. J. Btgelcisen. R.C. Ilom and T. ishida, J. Chcm. Phys. 64 (1976) 3303. 11.1’. Smrtb and J. Ovcrcnd, J. Chcm. Phys. 54 (197 II 3632. S.V. Ribnikar and O.S. PUZIC, Spectrochim. Acta 29A (1973) 307. J. Jacob and J.P. Renaich, J. Chim. Phys. 64 (1967) 1282.

482