Quantitative dynamics of paradigmatic S_N2 reaction OH^- + CH₃F on accurate full-dimensional potential energy surface

The bimolecular reaction between OH^- and CH₃F is not just a prototypical S_N2 process, but it has three other product channels. Here, we develop an accurate full-dimensional potential energy surface (PES) based on 191 193 points calculated at the level CCSD(T)-F12a/aug-cc-pVTZ. A detailed dynamics and mechanism analysis was carried out on this potential energy surface using the quasi-classical trajectory approach. It is verified that the trajectories do not follow the minimum energy path (MEP), but directly dissociate to F^- and CH₃OH. In addition, a new transition state for proton exchange and a new product complex CH₂F^-⋯H₂O for proton abstraction were discovered. The trajectories avoid the transition state or this complex, instead dissociate to H₂O and CH₂F^- directly through the ridge regions of the minimum energy path before the transition state. These non-MEP dynamics become more pronounced at high collision energies. Detailed dynamic simulations provide new insights into the atomic-level mechanisms of the title reaction, thanks to the new chemically accurate PES, with the aid of machine learning.