Benchmark Ab Initio Mapping of the F + CHClI S2 and Proton-Abstraction Reactions.

The experimental and theoretical studies of gas-phase S2 reactions have significantly broadened our understanding of the mechanisms governing even the simplest chemical processes. These investigations have not only advanced our knowledge of reaction pathways but also provided critical insights into the fundamental dynamics of chemical systems. Nevertheless, in the case of the prototypical X + CHY → Y + CHX [X, Y = F, Cl, Br, and I] S2 reactions, the effect of the additional halogenation of CHY has not been thoroughly explored. Thus, here, we perform the first high-level ab initio characterization of the F + CHClI S2 and proton-abstraction reactions utilizing the explicitly-correlated CCSD(T)-F12b method. Two possible S2 channels leading to the Cl + CHFI and I + CHFCl products are distinguished, in which we investigate four different pathways of back-side attack Walden inversion, front-side attack, double inversion, and halogen-bonded complex formation. In order to obtain the benchmark energies of the geometries of the stationary points, determined at the CCSD(T)-F12b/aug-cc-pVTZ level of theory, additional computations are carried out considering the basis set effects, post-CCSD(T) correlations, and core corrections. Using the benchmark data, we assess the accuracy of the MP2, DF-MP2, MP2-F12, and DF-MP2-F12 methods as well. By comparing the present F + CHClI system with the corresponding F + CHY [Y = Cl and I] reactions, this study demonstrates that further halogenation of CHY significantly promotes the corresponding proton-abstraction and S2 retention channels as well as the halogen-bonded complex formation, and as a consequence, the traditional back-side attack Walden-inversion mechanism becomes less pronounced.