Concentration-Dependent Hydrogen-Bonding Effects on the Dimethyl Sulfoxide Vibrational Structure in the Presence of Water, Methanol, and Ethanol

The effects of hydrogen bonding between dimethyl sulfoxide (DMSO) and the co-solvents water, methanol, and ethanol on the symmetric and antisymmetric CSC stretching vibrations of DMSO are investigated by means of Raman spectroscopy. The Raman spectra are recorded as a function of co-solvent concentration and reflect changes in structure and polarizability as well as hydrogen-bond donor and acceptor ability. In all cases studied a nonideal mixing behavior is observed. The spectra of the DMSO/water system show blue-shifted CSC stretching modes. The antisymmetric frequencies are always further blue-shifted than the symmetric stretching ones. The DMSO/methanol system also features blue-shifted CSC stretching frequencies but at high mole fractions a pronounced red shifting is observed. In the binary DMSO/ethanol system, the co-solvent also gives rise to blue shifts of the CSC stretching frequencies but restricted to mole fractions between x=0.38 and 0.45. The different magnitudes and occurrences of both blue- and red-shifted spectral lines are comprehensively and critically discussed with respect to the existing literature concerning wave-numbers and Raman intensities in both absolute and normalized values, In particular, the normalized Raman intensities show a higher sensitivity for the nonideal mixing behavior because they are independent of the mole fraction.