Facilitated inversion complicates the stereodynamics of an S(N)2 reaction at nitrogen center

Bimolecular nucleophilic substitution (S(N)2) reactions at carbon center are well known to proceed with the stereospecific Walden-inversion mechanism. Reaction dynamics simulations on a newly developed high-level ab initio analytical potential energy surface for the F(−) + NH(2)Cl nitrogen-centered S(N)2 and proton-transfer reactions reveal a hydrogen-bond-formation-induced multiple-inversion mechanism undermining the stereospecificity of the N-centered S(N)2 channel. Unlike the analogous F(−) + CH(3)Cl S(N)2 reaction, F(−) + NH(2)Cl → Cl(−) + NH(2)F is indirect, producing a significant amount of NH(2)F with retention, as well as inverted NH(2)Cl during the timescale within the unperturbed NH(2)Cl molecule gets inverted with only low probability, showing the important role of facilitated inversions via an FH…NHCl(−)-like transition state. Proton transfer leading to HF + NHCl(−) is more direct and becomes the dominant product channel at higher collision energies.