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 sol vent 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 st ructure of the solutions is discussed in terms of radial distribution funct ions, orientation of the DMSO molecules, and their geometrical arrangement in the first solvation shells. Dynamics of the first and second solvation s hells are studied in terms of mean square displacement of the centre-of-mas s of DMSO molecules, and the reorientational autocorrelation functions of d ipole moment and S-O bond vectors. It is found that the first solvation she ll of Naf consists of 6 DMSO molecules located at the vertices of a distort ed 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, Translatio nal and reorientational mobilities of the DMSO molecule within the first so lvation shell of the cation are significantly fewer than in bulk solvent. T he first solvation shell of Cl- does not indicate regular polygonal arrange ment. It consists of 10 DMSO molecules oriented by their methyl groups and sulphur atoms towards the anion with a preferable angle about 90 degrees be tween bisector of angle CSC and vector pointing from Cl- to the median poin t between two methylgroups. Dynamic behaviour of the solvent molecules in t he first solvation shell of the anion and in bulk solvent are found to be s imilar. (C) 2001 Elsevier Science B.V. All rights reserved.