Polarizability anisotropy relaxation in pure and aqueous dimethylsulfoxide

A molecular dynamics simulation study is presented for the relaxation of the polarizability anisotropy of liquid dimethylsulfoxide (DMSO) and DMSO-water mixtures of DMSO mole fractions x(D)=0.05, 0.10, 0.25, 0.50, and 0.75. The system's collective polarizability is computed through a dipolar induction mechanism involving the intrinsic polarizability and first hyperpolarizability tensors for water and DMSO, obtained from ab initio quantum chemical calculations at the MP2/6-311++G(d,p) level. The rotational-diffusion components of the anisotropy relaxation of the pure liquids increase upon mixing to a maximum near 25% DMSO, showing consistency with other dynamical properties of these mixtures. Features of the optical Kerr effect (OKE) nuclear response of liquid water, previously ascribed to hydrogen bonding distortions, show significant enhancement upon addition of DMSO due to the formation of strong DMSO-water H-bonds. The OKE spectrum for DMSO is in close agreement with experimental measurements, but there are discrepancies for pure water in the vicinity of 60 cm(-1), pointing to the existence of inaccuracies in our description of OKE sensitive polarizability fluctuations of water. The mixtures OKE spectra feature an enhancement in the high frequency water librational band. (C) 2003 American Institute of Physics.11942181218