Unveiling a flip-over retention mechanism in the gas-phase Cl− + (CH3)3CI SN2 reaction

Abstract Bimolecular nucleophilic substitution (SN2) and base-induced elimination (E2) reactions play crucial roles in chemistry. Here, we report a combined study of ion-molecule crossed-beam 3D velocity map imaging experiments and dynamics simulations on an accurate 39-dimensional potential energy surface for the Cl− + (CH3)3CI reaction, allowing the most rigorous investigation of the atomistic dynamics. Good agreement between experimental and theoretical product angular and energy distributions is achieved, revealing that predominantly direct E2 reactions produce the majority of highly excited neutral products and slow ion product distributions. Moreover, we uncover a direct “flip-over” retention mechanism for the SN2 reaction, where substitution occurs via the direct flipping of the tert-butyl group, leading to retention of the tetrahedral carbon center. This mechanism manifests as a predominant forward-scattering feature in the product angular distribution and provides an important perspective on substitution dynamics in organic reactions.