Self-Association and Picosecond Dynamics in Liquid Dimethyl Sulfoxide

Dimethyl sulfoxide (DMSO) has been known for almost 150 years, but its properties as a solvent and reaction medium are far from being understood. In particular, association equilibriums in liquid DMSO have been characterized in just four papers, and the enthalpy of its self-association is unknown. The aims of this paper are to study self-association equilibriums in neat liquid DMSO at various temperatures by means of Raman spectroscopy, to find the enthalpy of self-association, and to solve the problem of hydrogen bonding in this liquid. Time correlation functions of vibrational dephasing and reorientation of coexisting monomers and dimers studied by the C-S-C and C-H symmetric vibrations indicate that external perturbations and vibrational and reorientational dynamics of these particles occur on different time scales. No signatures of H-bonding in DMSO are found. The association constants vary from 0.20 L mol(-1) (23 degrees C) to 0.081 L mol(-1) (100 degrees C). Their temperature dependence gives the enthalpy of association of DMSO as -11.7 +/- 0.9 kJ mol(-1).