Chemical dynamics simulations of the monohydrated OH^-(H₂O) + CH₃I reaction. Atomic-level mechanisms and comparison with experiment

Direct dynamics simulations, with B97-1/ECP/d theory, were performed to study the role of microsolvation for the OH^-(H₂O) + CH₃I reaction. The S_N2 reaction dominates at all reactant collision energies, but at higher collision energies proton transfer to form CH₂I^-, and to a lesser extent CH₂I^- (H₂O), becomes important. The S_N2 reaction occurs by direct rebound and stripping mechanisms, and 28 different indirect atomistic mechanisms, with the latter dominating. Important components of the indirect mechanisms are the roundabout and formation of S_N2 and proton transfer pre-reaction complexes and intermediates, including [CH₃-I-OH]^-. In contrast, for the unsolvated OH^- + CH₃I S_N2 reaction, there are only seven indirect atomistic mechanisms and the direct mechanisms dominate. Overall, the simulation results for the OH^-(H₂O) + CH₃I S_N2 reaction are in good agreement with experiment with respect to reaction rate constant, product branching ratio, etc. Differences between simulation and experiment are present for the S_N2 velocity scattering angle at high collision energies and the proton transfer probability at low collision energies. Equilibrium solvation by the H₂O molecule is unimportant. The S_N2 reaction is dominated by events in which H₂O leaves the reactive system as CH₃OH is formed or before CH₃OH formation. Formation of solvated products is unimportant and participation of the (H₂O)CH₃OH-I^- post-reaction complex for the S_N2 reaction is negligible.