Benchmark Ab Initio\nMapping of the F<sup>–</sup> + CH<sub>2</sub>ClI S<sub>N</sub>2 and Proton-Abstraction Reactions

The experimental\nand theoretical studies of gas-phase\nS<sub>N</sub>2 reactions have significantly broadened our understanding\nof the\nmechanisms governing even the simplest chemical processes. These investigations\nhave not only advanced our knowledge of reaction pathways but also\nprovided critical insights into the fundamental dynamics of chemical\nsystems. Nevertheless, in the case of the prototypical X<sup>–</sup> + CH<sub>3</sub>Y → Y<sup>–</sup> + CH<sub>3</sub>X [X, Y = F, Cl, Br, and I] S<sub>N</sub>2 reactions, the effect\nof the additional halogenation of CH<sub>3</sub>Y has not been thoroughly\nexplored. Thus, here, we perform the first high-level ab initio characterization\nof the F<sup>–</sup> + CH<sub>2</sub>ClI S<sub>N</sub>2 and\nproton-abstraction reactions utilizing the explicitly-correlated CCSD(T)-F12b\nmethod. Two possible S<sub>N</sub>2 channels leading to the Cl<sup>–</sup> + CH<sub>2</sub>FI and I<sup>–</sup> + CH<sub>2</sub>FCl products are distinguished, in which we investigate four\ndifferent pathways of back-side attack Walden inversion, front-side\nattack, double inversion, and halogen-bonded complex formation. In\norder to obtain the benchmark energies of the geometries of the stationary\npoints, determined at the CCSD(T)-F12b/aug-cc-pVTZ level of theory,\nadditional computations are carried out considering the basis set\neffects, post-CCSD(T) correlations, and core corrections. Using the\nbenchmark data, we assess the accuracy of the MP2, DF-MP2, MP2-F12,\nand DF-MP2-F12 methods as well. By comparing the present F<sup>–</sup> + CH<sub>2</sub>ClI system with the corresponding F<sup>–</sup> + CH<sub>3</sub>Y [Y = Cl and I] reactions, this study demonstrates\nthat further halogenation of CH<sub>3</sub>Y significantly promotes\nthe corresponding proton-abstraction and S<sub>N</sub>2 retention\nchannels as well as the halogen-bonded complex formation, and as a\nconsequence, the traditional back-side attack Walden-inversion mechanism\nbecomes less pronounced.