Benchmark Ab Initio Mapping of the F^- + CH₂ClI S_N2 and Proton-Abstraction Reactions

The experimental and theoretical studies of gas-phase S_N2 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^- + CH₃Y → Y^- + CH₃X [X, Y = F, Cl, Br, and I] S_N2 reactions, the effect of the additional halogenation of CH₃Y has not been thoroughly explored. Thus, here, we perform the first high-level ab initio characterization of the F^- + CH₂ClI S_N2 and proton-abstraction reactions utilizing the explicitly-correlated CCSD(T)-F12b method. Two possible S_N2 channels leading to the Cl^- + CH₂FI and I^- + CH₂FCl 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^- + CH₂ClI system with the corresponding F^- + CH₃Y [Y = Cl and I] reactions, this study demonstrates that further halogenation of CH₃Y significantly promotes the corresponding proton-abstraction and S_N2 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.