Ion Pair Formation and Solvation Energies in Solutions of Alkali Halides in Dimethyl Sulfoxide. II. Model Calculations

Abstract

Energy calculations were carried out on models of molecular-level structures likely to be present in solutions of alkali halides in dimethyl sulfoxide (DMSO). Classical electrostatic interactions were assumed, and polarization of a DMSO molecule was assumed due to the fields of the ions only. The validity of this assumption was tested. DMSO molecules were represented by increasingly detailed models, with most calculations carried out with each molecule represented by 10 point charges and 9 polarizable bonds. A program including up to 14 such molecules and two ions was used for energy and distance calculations, and is made available. Polarization effects are as important as interactions between permanent charges for energy calculations. The configurations of minimum energy determined by classical electrostatics often do not involve overlap of the "hard-sphere radii" of neighboring species, so that the neglect of quantum mechanical repulsive forces seems justified. Energy cycles using the calculated energies for ion–solvent complexes predicted experimental cation enthalpies with some success. The form of the potential for vibration of a cation in a solvent shell was investigated and found in cases not to have an energy minimum at the shell center. Calculations including next-nearest solvating DMSO's indicate a rather loose structure. An energy profile for an anion moving from a solvent-separated ion pair position to a contact-ion pair position is presented.