Benchmark ab initio stationary-point characterization of the complex potential energy surface of the multi-channel Cl + CH3NH2 reaction

Abstract

We characterize the exothermic low/submerged-barrier hydrogen-abstraction (HCl + CH₂NH₂/CH₃NH) as well as, for the first time, the endothermic high-barrier amino-substitution (CH₃Cl + NH₂), methyl-substitution (NH₂Cl + CH₃), and hydrogen-substitution (CH₂ClNH₂/CH₃NHCl + H) pathways of the Cl + CH₃NH₂ reaction using an accurate composite ab initio approach. The computations reveal a CH₃NH₂⋯Cl complex in the entrance channel, nine transition states corresponding to different abstractions, Walden-inversion substitution, and configuration-retaining front-side attack substitution pathways, as well as nine post-reaction complexes. The global minima of the electronic and vibrationally adiabatic potential energy surfaces correspond to the pre-reaction CH₃NH₂⋯Cl and post-reaction CH₂NH₂⋯HCl complexes, respectively. The benchmark composite energies of the stationary points are obtained by considering basis-set effects up to the correlation-consistent polarized valence quadruple-zeta basis augmented with diffuse functions (aug-cc-pVQZ) using the explicitly-correlated coupled-cluster singles, doubles, and perturbative triples CCSD(T)-F12b method, post-(T) correlation up to CCSDT(Q) including full triples and perturbative quadruples, core correlation, and scalar relativistic and spin–orbit effects, as well as harmonic zero-point energy corrections.