Determination of the Temperature-Dependent OH^- (H₂O) + CH₃I Rate Constant by Experiment and Simulation

Experimental and simulation studies of the OH^-(H₂O) + CH₃I reaction give temperature dependent rate constants which are in excellent agreement. Though there are statistical uncertainties, there is an apparent small decrease in the rate constant as the temperature is increased from - 60 to 125 °C, and for this temperature range the rate constant is ∼ 1.6 times smaller than that for the unsolvated reactants OH^- + CH₃I. Previous work [J. Phys. Chem. A 117 (2013) 14019] for the unsolvated reaction found that the S_N2 and proton transfer pathways, forming CH₃OH + I^- and CH₂I^- + H₂O, have nearly equal probabilities. However, for the microsolvated OH^-(H₂O) + CH₃I reaction the S_N2 pathways dominate. An important contributor to this effect is the stronger binding of H₂O to the OH^- reactant than to the proton transfer product CH₂I^-, increasing the barrier for the proton transfer pathway. The effect of microsolvation on the rate constant for the OH^-(H₂O)_0,1 + CH₃I reactions agrees with previous experimental studies for X^-(H₂O)_0,1 + CH₃Y reactions. The simulations show that there are important non-statistical attributes to the entrance- and exit-channel dynamics for the OH^-(H₂O) + CH₃I reaction.