Influence of the Leaving Group on the Dynamics of a Gas Phase SN2 Reaction

TL;DR

This study investigates how the leaving group affects the reaction dynamics of gas phase SN2 reactions, revealing that orientation and impact parameters significantly influence the reaction mechanism beyond simple mass effects.

Contribution

It uncovers the influence of leaving groups on SN2 reaction dynamics through combined experimental and simulation approaches, highlighting the importance of reactant orientation.

Findings

Different leaving groups lead to distinct reaction dynamics.

Impact parameters and orientation are crucial in reaction mechanisms.

Mass scaling alone does not explain dynamic differences.

Abstract

In addition to nucleophile and solvent, the leaving group has a significant influence on nucleophilic substitution (SN2) reactions. Its role is frequently discussed with respect to reactivity, but its influence on the reaction dynamics remains obscured. Here, we uncover the influence of the leaving group on the gas phase dynamics of SN2 reactions in a combined approach of crossed-beam imaging and dynamics simulations. We have studied the reaction F- + CH3Cl and compared it to F- + CH3I. For the two leaving groups Cl and I we find very similar structures and energetics, but the dynamics show qualitatively different features. Simple scaling of the leaving group mass does not explain these differences. Instead, the relevant impact parameters for the reaction mechanisms are found to be crucial, which is attributed to the relative orientation of the approaching reactants. This effect occurs on short time scales and may also prevail under solution phase conditions.