Raman spectroscopy of dimethyl sulphoxide and deuterated dimethyl sulphoxide at 298 and 77 K

Raman spectroscopy was used to determine the molecular behaviour of DMSO and DMSO-d6, and to compare it with that of DMSO in DMSO-intercalated kaolinites. For DMSO at 298 K two bands are observed at 2994 and 2913 cm-1 and are assigned to the antisymmetric and symmetric CH stretching modes. At 77 K the degeneracy of these bands is lost. Bands are now observed as antisymmetric bands at 3001, 2995 and 2988 cm-1 and symmetric bands at 2923, 2909 and 2885 cm-1, respectively. For the DMSO-intercalated low-defect kaolinite, the 2913 cm-1 band resolves into five component bands at 2882, 2907, 2917, 2920 and 2937 cm-1. The CD antisymmetric and symmetric stretching modes in the 298 K spectrum are found at 2250 and 2125 cm-1, respectively. Both bands show some asymmetry and further bands may be resolved at 2256 and 2244 cm-1 in the antisymmetric stretching region and at 2118 cm-1 in the symmetric stretching region. The spectra of the SO stretching region of DMSO and DMSO-d6 are complex with a band profile centred at 1050 cm-1. Three bands are curve resolved at 1058, 1042 and 1026 cm-1 attributed to the unassociated monomer and the out-of-phase and the in-phase vibrations of the dimer, respectively. Upon cooling to liquid nitrogen temperature, these three bands are observed at 1057, 1038 and 1019 cm-1. The spectra of the SO stretching region of DMSO-d6 are more complex because of the overlap of the DCD deformation modes with the SO stretching modes. The antisymmetric and symmetric stretching CS modes of DMSO are observed at 698 and 667 cm-1, shifting at 77 K to 705 and 672 cm-1. It is concluded that the structure of DMSO in DMSO-intercalated kaolinite is different from those of both liquid DMSO at 298 K and solid DMSO measured at 77 K.