Structure and dynamics of Na+ and Cl- solvation shells in liquid DMSO:: molecular dynamics simulations

Molecular dynamic simulations of pure dimethyl sulfoxide, DMSO (216 molecules) and its "infinitely diluted solutions" of Na+ and Cl-(1 ion and 215 solvent molecules) have been performed at 298.15 K in a NVT ensemble by using a force field model introduced by Liu, Muller-Plathe and van Gunsteren (J. Amer. Chem. Sec., 117 (1995) 4363) and a potential of shifted force. The structure of the solutions is discussed in terms of radial distribution functions, orientation of the DMSO molecules, and their geometrical arrangement in the first solvation shells. Dynamics of the first and second solvation shells are studied in terms of mean square displacement of the centre-of-mass of DMSO molecules, and the reorientational autocorrelation functions of dipole moment and S-O bond vectors. It is found that the first solvation shell of Naf consists of 6 DMSO molecules located at the vertices of a distorted octahedron and oriented by their polar S-O bonds towards the cation with a preferable angle of 156 degrees between S-O and Na-S vectors, Translational and reorientational mobilities of the DMSO molecule within the first solvation shell of the cation are significantly fewer than in bulk solvent. The first solvation shell of Cl- does not indicate regular polygonal arrangement. It consists of 10 DMSO molecules oriented by their methyl groups and sulphur atoms towards the anion with a preferable angle about 90 degrees between bisector of angle CSC and vector pointing from Cl- to the median point between two methylgroups. Dynamic behaviour of the solvent molecules in the first solvation shell of the anion and in bulk solvent are found to be similar. (C) 2001 Elsevier Science B.V. All rights reserved.