A cluster model study of contact ion pair formation of t-BuCl in aqueous solution:: Calculational evidence for nucleophilic solvent assistance in SN1 reaction

To achieve an understanding of the solvent effects on the contact ion pair formation of t-BuCl in aqueous solution from a microscopic point of view, we carried out ab initio molecular-orbital calculations for a cluster system consisting of t-BuCl and four water molecules. Each of the most stable contact ion pair and the relevant transition state was found to have a ten-membered ring structure in which one edge of a hydrogen-bonded chain consisting of four water molecules solvates nucleophilically on the back side of the central carbon of the t-Bu group and the other edge of the water chain hangs electrophilically on the chlorine. This finding suggested that the contact ion pair formation for the cluster system is assisted not only by electrophilic solvation but also by nucleophilic solvation of the water on t-BuCl. The calculated energy barrier for the contact ion pair formation was comparable with the experimental data for the corresponding solution reaction when the influence of bulk water is taken into account by means of a continuum solvation model. The calculated solvent kinetic isotope effects for the cluster system were in good accord with the experimental data for the solution reaction. The contact ion pair and the relevant transition state structures fur the cluster system were therefore suggested to have some relevance in the solution reaction. We concluded that the contact ion pair formation of t-BuCl in aqueous solution proceeds via the nucleophilically (and electrophilically) solvated transition state, followed by the formation of the nucleophilically (and electrophilically) solvated contact ion pair.