Effect of pressure on local order in liquid dimethyl sulfoxide

Raman spectroscopy was used to study intermolecular interactions, dynamics and the structure of neat liquid dimethyl sulfoxide (DMSO). Isotropic and anisotropic spectra were recorded in the SO stretching vibration region at temperatures between 30 and 70 degrees C and under pressures up to 2200 bar. To separate the temperature and density effects, the density of DMSO was also determined over the same region of temperature and pressure. By analyzing the isotropic and anisotropic spectra, the Raman noncoincidence effect was measured. This coupling parameter which reflects intermolecular interactions exhibits a nonlinear increase with density but is relatively insensitive to temperature. With increasing density, drastic changes in the first moment and the shape of the isotropic SO band were detected. The SO stretching vibration band shifts to lower wavenumbers. whereas its full width at half-height increases significantly with increasing pressure, The isotropic SO stretching band is resolved into three components which show markedly different intensity changes upon pressurization. The results are discussed in terms of pressure-induced formation of stronger molecular aggregates leading to an enhanced local order in liquid DMSO at elevated pressures. In a general sense, the results of this study provide yet another example of the dominance of volume effects over kinetic effects on the dynamic structure of liquids.