Revealing a Heavy-Atom Assisted Rotation Mechanism in the H + NHCl Multi-Channel Reaction.

Identifying atomic-level mechanisms in elemental chemical reactions is crucial for understanding complex reaction processes. This study focuses on the typical multichannel H + NHCl reaction, which plays a significant role in environmental science. High-level ab initio calculations determined seven distinct reaction pathways, leading to three product channels: H + NHCl, HCl + NH, and Cl + NH. A full-dimensional, globally accurate potential energy surface was constructed by fitting 143,333 ab initio energy points, calculated at the UCCSD(T)-F12a/aug-cc-pVTZ level. The atomic-level mechanisms of the reaction along these seven pathways were identified and visualized using quasi-classical trajectory calculations. Interestingly, a novel reaction mechanism, termed "heavy-atom assisted rotation", was discovered. In this light-heavy-heavy reaction, the attacked heavy atom (either Cl or N) acts as a "gangplank", propelling the light H atom in front of the other heavy atom through rotational motion. This mechanism results in forward and sideward scattering of products at small impact parameters, which contrasts with any known direct mechanisms.