RAMAlq SPECTRUM OF DIMETHYL SULFOXIDE (DMSO) AND THE INFLUENCE OF SOLVENTS

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1 RAMAlq SPECTRUM OF DIMETHYL SULFOXIDE (DMSO) AND THE INFLUENCE OF SOLVENTS BY A. SELVARA~AN (From the Department of Physics, Indian Institute of Science, Bangalore-12) Received September 6, 1965 (Cornmtmicated by Professor R. S. Krishnan, F.A.SC.) ABSTRACT The Raman spectrum of DMSO is recorded with a Hilger two-prism spectrograph and ~4358 A excitation. In addition to all the Raman lines reported earlier, six new lines at 898, 925, 1223, 1309, 2811 and 2871 cm. -x are observed and tentative assignments are given. The influence of solvents (CC14, CHC13, CH3COOH) on the S = O bond is also studied. A shift from the liquid phase value, i.e., 1043 cm. -1 to 1054, 1052 and 1009cm. -1 in the respective solvents is observed. The possibilities of association effects and hydrogen bonding are discussed. 1. INTRODUCTION THE Raman spectrum of DMSO was first investigated by Vogel-Hogler (1948) and subsequently the infra-red and Raman spectra of DMSO and DMSO-de were investigated by Horrocks, W. D. and Cotton, F. A. (1961) anda normal co-ordinate analysis of the vibration spectrum of DMSO was also carried out by them. The solvent effect on the S = O frequency in inflared has been investigated by Barnard, Fabian and Koch (1949), Bellamy, L. J. et al. (1959), Milton Tamres and Scott Seades (1959) and Russel S. Drago and Devon Meek (1961). References to the study of DMSO complexes and the theoretical studies on the S = O bond have been given by Horrocks and Cotton (loc. cit.). In view of several unusual properties shown by DMSO and also because its bonding properties have not yet been studied by Raman effect the author took upa detailed investigation of its Raman spectrum and the influence of different solvents on ir. 2. EXPERIMENTAL DETAILS The DMSO used in the present study was a generous gift from Messrs. Crown Zellerbach, U.S.A., and it was further purified by repeated distiuation 44

2 Raman Spectrum of Dimethyl Sulfoxide and Influence of Solvents 45 under reduced pressure. The final sample was practically free from water also. A Toronto type zig-zag mercury arc run at a current of 5 amps., was used for exciting its Raman spectrum. A consisting of a saturated solution of sodium nitrite was used to cut of A 4047 and the shorter wavelengths. On account of the proximity of the aro, the temperature of liquid at the time of recording the Raman spectrum was roughly 40 ~ C. A Hilger two-prism spectrograph having a dispersion of 150 cm.-x/mm, in the ~ 4358 region was employed for recording the spectrum. Using a slit width of 0.05 mm. and Agfa Raman orthochromatic plates an exposure of about 15 hours had to be given to get a satisfactory spectrum. 3. RESULTS AND DISCUSSION The observed Raman shifts along with the values reported by the earlier workers are given in Table I. The observed spectra of pure DMSO and also mixtures of DMSO with carbon tetrachloride, chloroform and acetic acid can be seen in Plate III, Fig. 1. The microphotometer records are reproduced in Text. Figs. 2 and 3. TABLE I Horrocks Author and cotton Vogel-Hogler Assignment 306 (5) (6) (4) (10) (9) (1) (2) (3) (5) (2) (1) (2) (7) (2) (2) (8) (8) C-S-C bend, va~ C-S-O antisymmetric bend, v28 C-S-O symmetrir bend, vxl C-S symmetric stretch, i,10 C-S antisymmet stretch, vas... CHs rocking (va) CH3 roeking (va1) S:O stretch, v 7 v, + "~a (.9) C-H symmetric deformation, v 6 C-H symmetrie deformation, v~, CHa degenerate deformation, yo v vx~, 1,1 s 2v4 (.9) C-H symmet stretch, vx, va, v a C-H degenerate stretch, vx,, v~5, vxe The nomenr of (~'s) is acr to Horror aad Cotton (1961).

3 46 A. SELVARAJAIq i 0 i i mm II_ "8" ]1.,li ~ / Ii i i I FIo. 2. Mierophotometer Record of (a) DMSO (b) DMSO in CCI 4 In addition to all the Raman fines reported by the earlier workers 6 new Raman lines have been observed in the present study. The numbers within brackets gire the visually estimated intensities of the Raman lines relative to the intensity of the most intense Raman line of 668 (10)cm_ 1 Before we discuss the most probable assignment it may be pointed out that most of the values reported by Horrocks and Cotton (1961) and used for normal co-ordinate analysis are for the vapour phase and therefore numerieal agreement of the frequencies with the corresponding Raman data may not exist. It is known from the heat of mi~ng data that in DMSO, the molecules may

4 Raman Spectrum of Dimethyl Sulfoxide and Influence of Solvents 47 be associated. In fact this view is supported by the observation that while in the infra-red spectrum of the vapour the S = O frequency has a value of ll02cm. -~, the corresponding Raman line in the liquid is at 1043 cm. -1 Further if one interprets this as arising from a lowering of the bond order due to association through the oxygen atom, then the lowering of the values of ve and v~9 from the vapour phase infra-red valuesindicate that the hydrogens of the methyl group also take part in the association. For the same reason and the consequent steric effects, the CH3 rocking modes may be expected to have higher frequencies in the liquid state and therefore the observed depolarized Raman line at 951 cm. -~ has been attributed to va~ and the Raman line 925 cm. -1 to v 9, The origin of the weak Raman line at 898 cm. -x which is observed in infra-red also is not clear. The Rayleigh line is found to be accompanied by a wing which extends up to 150 cm. -~ and there is no Raman in! (Q) - 1 (b) 3. Microphotometer Record of DMSO in CHClm. (a) 50% Concentration (b) 25% Concentration

5 48 A. SELVARAffAN line in the low frequency region of 150 cm. -I which can be attributed to the torsional oscillations of the methyl groups. The asymmetric intensity distribution of the wing accompanying the exciting radiation, shows clearly that it is not due to any parasitic iuumination and photographic hmation but must be largely due to hindered oscillations. The weak Raman lineat 1141 cm. -I has therefore been attributed to a combination of the S = O stretching with the torsional mode of the same species. The Raman ]ines at 2811 and 2871 cm. -x ate not easily accounted for on the basis of the normal co-ordinate analysis of Horrocks and Cotton and one has to account for them only as an overtone of the one of the CH8 deformation modes v v vs, v,~ or vas split due to interaction. The solvent effect on the S = O band has been studied in the infra-rcd by several workers referred to earlier. But no work on the Raman spectra of mixtures of DMSO and other liquids of different hydrogen bonding ability has been done. Therefore the Raman spectra of mixtures of DMSO and carbon tetrachloride, chloroform and acetic acid ate studicd and the observed values of the S = O frequencies ate given in Table II. TABLE II Solvent Solvent by volume DMSO S=O band cm. -1 CC14.. CHCI3 -. ~ 0 ~ CH3COOH In infra-red, the S = O band of liquid DMSO occurs at 1071 cm. -1 while its value in CC14 and in CHCIs is 1071 cm. -1 and 1055 cm. -1 respectively (Bellamy et al., 1959). The change in the S = O frequency in CHCls solvent according to these authors is due to the formation of hydrogen bond. This view is confirmed by the other workers also. But in the Raman spr of mixtures of DMSO and CC14 and CHCls, one finds the S = O band occur- in nearly the same position, viz., 1054 and 1052 cm. -x respectively and

6 Raman Spectrum of Dimethyl Sulfoxide and Influence of Solvents 49 the shifts from the value of the pure liquid are nearly the same. In the IR the conccntration of DMSO in CCI~ and CHCIa employed was roughly 15% while in the present work a concentration of 50% in CCI~ anda concentration of 10 to 50% in CHCIa arc uscd. One interesting point to be noted is that though the DMSO molecule has only the symmetry Cs (m) and so au the modes ate IR as well as Raman active, there exists a difference in value for the S = O bond (IR ~ 1070 cm.-1; R ~ 1045 cm.-1). In this connection it may be recalled that there is an association of the molecules in organic acids in sucia a way that the associated molecule acquires a centre of symmetry and each mode splits into two, one active in IR and the other in Raman effect. Ir a similar explanation holds good in the case of DMSO also then one can justify the difference between the IR and R. E. values for the S : O band, but it is not easy to give the exact configuration of the molecular complex. The larger shift obtained in the case of CHaCOOH shows the formation of a strong hydrogen bond. The present observation shows also that the S = O banal is broader in CH~COOH and in CHC13 than in CC14. The C-H stretching frequency seems to show a weak splitting in 10% concentration of the DMSO in CHCI3. 4. ACKNO~~.s The author expresses his deep sensr of gratitude to Professor R. S. Krishnan and Dr. P. S. Narayanan for suggesting the problem and for the many useful discussiom he had with them. 5. Rs 1. Barnard, Fabian and Koch, J. Chem. Soc., 1949, H.P. 2. Bellamy, L. J., Conduit, Trans. Farad. Soe., 1959, 55, C. P., Pace, R. J. and WiUiams, R. L. 3. Cotton, F. A., Francis, R. J. Phys. Chem., 1960, 64, and Horrocks, W. D. 4. Cotton, F. A. and Francis, J. Amer. Chem., 1960, 82, R. 5. Horror W.D. and Spectrochim. Acta, 1961, 17, 134, Cotton, F. A,

7 5O 6. Milton Tamres and Scott Searles 7. Paolo Biscarini and Sergio Ghersetti ~ 8. Russell, S. Drago and Devon Meek 9. Ruth Vosel-H{~gler Aotto, K. Salonen.. A. SELVARA~AN J. Amer. Chem. Soc., 1959, 81, Gazy. Chito. Ital., 1962, 92, 61. J. Phys. Chem., 1961, 65, Acta Phys. Austriaca, 1948, 1, 311. Ann. Acad. Sci. Fennicae Ser, 1961, 6A, (67).

8 A. Selvara#an Proc. Ind. Acad. Sci., A, VoZ, LXIV, PI, III O~~ +,li 0 - ~ 0,~-7~: t.+ " :, , (a) (bj ~.'i!i:+, r i +i!i +_,~ J :+.., , (dj (I) l+ i ii+fil i~ "'.,.+ + ii i,,, i: 91 i~ ~+ ~ s.. ' + ~ 9 ' Flo. 1. Raman Spectrum of (a) Pure DMSO (b) DMSO in CCI l (c) DMSO in CHa COOH (d, e,f) DMSO in CHC]3,