Benchmark Ab Initio Characterization of the Abstraction and Substitution Pathways of the Cl + CH 3 CN Reaction.

We investigate the reaction pathways of the Cl + CH 3 CN system: hydrogen abstraction, methyl substitution, hydrogen substitution, and cyanide substitution, leading to HCl + CH 2 CN, ClCN/CNCl + CH 3 , ClCH 2 CN + H, and CH 3 Cl + CN, respectively. Hydrogen abstraction is exothermic and has a low barrier, whereas the other channels are endothermic with high barriers. The latter two can proceed via a Walden inversion or front-side attack mechanism, and the front-side attack barriers are always higher. The C-side methyl substitution has a lower barrier and also a lower endothermicity than the N-side reaction. The computations utilize an accurate composite ab initio approach and the explicitly correlated CCSD(T)-F12b method. The benchmark classical and vibrationally adiabatic energies of the stationary points are determined with the most accurate CCSD(T)-F12b/aug-cc-pVQZ energies adding further contributions of the post-(T) and core correlation, scalar relativistic effects, spin-orbit coupling, and zero-point energy corrections. These contributions are found to be non-negligible to reach subchemical accuracy.