Theoretical examination of solvent reorganization and nonequilibrium solvation effects in microhydrated reactions

The general features of solution reactions were clarified by theoretically investigating the following four reactions in the presence of two water molecules as solvent: (1) NH3 + CH3Cl --> H3NCH3+ + Cl-, (2) F- + CH3Cl --> FCH3 + Cl-, (3) HO- + (CH3)(3)S+ --> HOCH3 + (CH3)(2)S, and (4) internal rotation in formamide. Ab initio calculations for these microhydrated reactions were used to determine the intrinsic reaction path on a solution-reaction surface that corresponds to the two-dimensional configuration space spanned by solute and solvent reactive coordinates. It was found for each reaction that the major component of the reaction-path motion in the reactant and product regions is the solvent reorganization, whereas the major reaction-path component in the transition-state region is the solute reactive motion. The solvent reorganization was shown to play a role in reducing the height of an activation barrier along the displacement of the solute reactive coordinate. Despite the moderate effect that nonequilibrium solvation was previously found to have on the contact-ion-pair formation of t-BuCl in the presence of four water molecules, the nonequilibrium solvation effect was found to be negligible for all the microhydrated reactions examined in the present study. This finding indicates that the solvent characterized in the transition-state region as a frozen spectator does not exert force on the solute reactive motion and that there is no simple relation between the strength of the solute-solvent interaction and the strength of the nonequilibrium solvation effect. It was also shown that the NES effect can be moderate if the motion along the intrinsic reaction path in the transition-state region comprises some solvent reactive motion. The present findings are expected to be the general features of solution reactions.