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

Molecular dynamic simulations of pure dimethyl sulfoxide, DMSO (216 molecul es) and its "infinitely diluted solutions" of Na+ and Cl- (1 ion and 215 so lvent molecules) have been performed at 298.15 K in NVT ensemble by using a force field model introduced by Liu Muller-Plathe and van Gunsteren (J. Am er. Chem. Sec., 117 (1995) 4363) and a potential of shifted force. The stru cture of the solutions is discussed in terms of radial distribution functio ns, orientation of the DMSO molecules, and their geometrical arrangement in the first solvation shells. Dynamics of the first and second solvation she lls are studied in terms of mean square displacement of the centre-of-mass of DMSO molecules, and the reorientational autocorrelation functions of dip ole moment and S-O bond vectors. It is found that the first solvation shell of Na+ 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. Translationa l and reorientational mobilities of the DMSO molecule within the first solv ation shell of the cation are significantly lower than in the bulk solvent. The first solvation shell of CT does not indicate a regular polygonal arra ngement. It consists of 10 DMSO molecules oriented by their methyl groups a nd sulphur atoms towards the anion with a preferable angle of about 90 degr ees between the bisector of angle CSC and the vector pointing from Cl- to t he median point between two methyl groups. The dynamic behaviour of the sol vent molecules in the first solvation shell of the anion and in the bulk so lvent are found to be similar. (C) 2000 Elsevier Science B.V. All rights re served.