Quantitative Dynamics of the Paradigmatic SN2 reaction OH− + CH3F on a Chemically Accurate Full-Dimensional Potential Energy Surface

The bimolecular reaction between OH− and CH3F is not just a prototypical SN2 process, but has three other product channels, the proton exchange, the proton abstraction and the O2− substitution. In this work, we develop an accurate full-dimensional potential energy surface (PES) for this reaction based on 191 193 points calculated at the level CCSD(T)-F12a/AVTZ. A detailed dynamics and mechanism analysis were carried out on this PES by 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 the proton exchange and the CH2F−‧‧‧H2O complex for the proton abstraction were discovered on this accurate PES. The trajectories avoid the transition states or this complex, instead directly dissociating to H2O and CH2F− through the ridge regions. These non-MEP dynamics become more pronounced at high collision energies. Detailed dynamics simulations provide new insights into the atomic-level mechanisms of the title reaction thanks to the new chemically accurate PES.