Low-Frequency Vibrations of Molecular Crystals. VII. DMSO and DMSO-d6

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Abstract
The far‐infrared spectra of polycrystalline dimethylsulfoxide and dimethylsulfoxide‐d6 have been measured from 600 to 33 cm−1 at various temperatures from −30 to −194°C. Raman spectra of both compounds in the liquid and solid states have been recorded with 6328‐Å helium–neon excitation. Assignments are made for internal and lattice vibrations in terms of the crystallographic unit cell P2 1 / c (C 2h 5 ) with four molecules per unit cell. A distinction between librational and translational lattice modes was made on the basis of the isotopic shift factors. The external fundamentals are readily explained in terms of a layer lattice unit cell with intralayer coupling between molecules being of greater significance than interlayer coupling. The interesting methyl torsion was observed in the infrared at 257 cm−1 in the “light” compound shifting to 189 cm−1 with deuterium substitution. The threefold barrier to internal rotation was calculated to have a value of 4.4 kcal/mole for the solid. This barrier is compared to that found in the vapor as well as those obtained for structurally similar phosphorous compounds. It appears that oxygen raises the barrier by about 1 kcal/mole. No evidence for short‐range ordering could be found in the Raman spectrum of the liquid.