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

摘要

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(N)2 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(N)2 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(N)2 and proton transfer pre-reaction complexes and intermediates, including [CH3--I--OH](-). In contrast, for the unsolvated OH- + CH3I S(N)2 reaction, there are only seven indirect atomistic mechanisms and the direct mechanisms dominate. Overall, the simulation results for the OH-(H2O) + CH3I S(N)2 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(N)2 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(N)2 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(N)2 reaction is negligible. (C) 2015 AIP Publishing LLC.