Newly proposed proton-abstraction roundabout with backside attack mechanism for the SN2 reaction at the nitrogen center in F− + NH2Cl

Abstract

Recent studies have improved our understanding of the mechanism and dynamics of the bimolecular nucleophilic substitution (S_N2) reaction at the carbon center. Nonetheless, the S_N2 reaction at the nitrogen center has received scarce attention and is less understood. Herein, we propose a new reaction mechanism for the S_N2 reaction at the nitrogen center in the F⁻ + NH₂Cl reaction using ab initio molecular dynamics calculations. The newly proposed mechanism involves the rotation of NHCl with one proton of NH₂Cl abstracted by the nucleophile, followed by the classical backside-attack process. The double-inversion mechanism revealed recently for the S_N2 reaction at the carbon center is also observed for the title reaction at the nitrogen center. In contrast to the F⁻ + CH₃Cl reaction with a proton abstraction-induced first inversion transition state, the F⁻ + NH₂Cl reaction is a hydrogen bond-induced inversion. This newly proposed reaction mechanism opens a reaction channel to avoid the proton abstraction mechanism at low collision energy. The double-inversion mechanism of the title reaction with a negative first-inversion transition relative to the energy of the reactants is expected to have larger contribution to the reaction rate than the F⁻ + CH₃Cl reaction with a positive first-inversion transition state.