New insights into hydrogen bonding: hyper-Raman spectroscopy of DMSO-water solution

Solutions of dimethyl sulfoxide-water (DMSO-H₂O) have served for decades as a model system to study the direct and indirect effects of hydrogen bonding between a polar functional group and water molecules. Despite decades of study, two major points of contention remain (1) the proposed geometrical arrangement of the hydration shell formed around DMSO and (2) the degree of intermolecular bonding between H₂O molecules and the DMSO methyl groups caused by partial polarization/charge transfer. We use hyper-Raman spectroscopy to probe these two questions. Hyper-Raman is a nonlinear optical scattering process that complements traditional Raman via the difference in their selection rules which allows hyper-Raman to probe Raman inactive vibrational modes. We exploit the high sensitivity of hyper-Raman scattering to the low frequency librations of H₂O to observe a change in librational mode population providing insight into the coordination of H₂O around DMSO molecules and the formation of the hydration shell around DMSO molecules. Furthermore, we find evidence of intermolecular bonding between H₂O and DMSO methyl groups by detecting differences in the frequency shifts of C-S and C-H modes for DMSO concentrations below 33 mol %.