A FREE-ENERGY PERTURBATION STUDY OF SOLVATION IN METHANOL AND DIMETHYL-SULFOXIDE

Relative differences in free energies of solvation, ΔG, between two different solutes in water, methanol, and dimethyl sulfoxide have been determined by a coordinate coupled free energy perturbation method. The solutes examined in this study are some closed shell ions, tetraalkylammonium ions, normal alkanes, and tetraalkylmethane molecules. The ΔG varies differently in different solvents for the mutation of a solute into another solute. These differences are shown to be resulting from the differences in the solvation processes in different solvents. For closed shell ions, the variation of ΔG shows greater sensitivity to change in the size of an anion in protic solvents than in dipolar aprotic solvent, while it is almost equally affected by the change in the size of a cation in all the three solvents. The structural results of pure liquid and ionic solutions of dimethyl sulfoxide obtained by the molecular dynamics simulations are discussed to rationalize the shape of the ΔG curve for mutations of the closed shell ions in dimethyl sulfoxide. In the case of large tetraalkylammonium ions and alkanes, the variation of ΔG in water is dictated by both the structure of the tight solvent shell around these solutes and the solute-solvent interaction energies. On the other hand, the variation of ΔG for these solutes in methanol and dimethyl sulfoxide are determined mainly by the solute-solvent interaction energies. © 1990, American Chemical Society. All rights reserved.