Attenuated total reflection Fourier-transform IR spectroscopic study of dimethyl sulfoxide self-association in acetonitrile solutions

Dimethyl sulfoxide (DMSO) solutions in acetonitrile have been studied using the method of attenuated total reflection Fourier-transform IR spectroscopy (ATR FTIR) to elucidate the nature and the quantities of DMSO self-associates in solution. Results obtained are discussed together with earlier evidence of DMSO self-association in the literature. The results of the present study were used to resolve existing controversies in the assignment of bands in the DMSO vibrational spectrum. The structure of the bands due to the skeletal vibrations was found to be complex. It was shown that DMSO molecules in solution associate in cyclic dimers. Two independent calculations were performed to estimate the dimerization constant as well as the respective Gibbs energy using the data for the SO-stretch and CSC-asymmetric stretch of DMSO.