Exit Channel Dynamics in a Micro-Hydrated S N 2 Reaction of the Hydroxyl Anion

We report on the reaction dynamics of the monosolvated SN2 reaction of cold OH–(H2O) with CH3I that have been studied using crossed beam ion imaging. Two SN2 reaction channels are possible for this reaction: Formation of unsolvated I– and of solvated I–(H2O) products. We find a strong preference for the formation of unsolvated I– reaction products with respect to the energetically favored reaction toward solvated I–(H2O). Angle differential cross section measurements reveal similar velocity and angular distributions for all solvated and parts of the unsolvated reaction products. We furthermore find that the contribution of these two products to the total product flux can be described by the same collision energy dependence. We interpret our findings in terms of a joint reaction mechanism in which a CH3OH(H2O)···I– complex is formed that decays into either solvated or unsolvated products. Quantum chemical calculation are used to support this assumption