Exit Channel Dynamics in a Micro-Hydrated S_N2 Reaction of the Hydroxyl Anion

Abstract

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.

Effect of higher order solvation and temperature on SN2 and E2 reactivity

“…The addition of a small number of solvent molecules to gas-phase ions provides insight into this transition. Recent investigations of microsolvated ions have addressed the impact of solvation on mechanisms [2], reactivity [3], reaction dynamics [4][5][6][7][8], vibrational energy transfer [9,10], and delocalization of charge [11].…”

Reaction cross sections and thermal rate constant for Cl− + CH3Br → ClCH3 + Br− from J-dependent quantum scattering calculations

“…It has to be pointed out that the molecular beam ion-imaging experiments of the Wester research group have provided quite detailed non-thermal dynamics for S N 2 reactions. [22][23][24][25][26][27][28][29] While in our previous 4D calculations carried out under the constraint of C 3v symmetry, 55,57,[59][60][61] i.e. with inclusion of the umbrella bending and C-H stretching vibrations, a lot of information on mode-selectivity and the influence of the excitation of different internal degrees of freedom on the reactivity was obtained, the overall rate constant could not be reproduced exactly.…”

Imaging the dynamics of ion–molecule reactions

“…187,188 In the crossed beam experiments, the unsolvated product ion is found to be the dominant product ion. Specifically, the 189 showing no dependence on collision energy for the low-energy complex mediated mechanism. The abundance of the thermodynamically favored solvated product was found to be less than 5%.…”