Benchmark ab initio characterization of the complex potential energy surfaces of the HOO− + CH3Y [Y = F, Cl, Br, I] reactions

Abstract

The α-effect is a well-known phenomenon in organic chemistry, and is related to the enhanced reactivity of nucleophiles involving one or more lone-pair electrons adjacent to the nucleophilic center. The gas-phase bimolecular nucleophilic substitution (S_N2) reactions of α-nucleophile HOO⁻ with methyl halides have been thoroughly investigated experimentally and theoretically; however, these investigations have mainly focused on identifying and characterizing the α-effect of HOO⁻. Here, we perform the first comprehensive high-level ab initio mapping for the HOO⁻ + CH₃Y [Y = F, Cl, Br and I] reactions utilizing the modern explicitly-correlated CCSD(T)-F12b method with the aug-cc-pVnZ [n = 2–4] basis sets. The present ab initio characterization considers five distinct product channels of S_N2: (CH₃OOH + Y⁻), proton abstraction (CH₂Y⁻ + H₂O₂), peroxide ion substitution (CH₃OO⁻ + HY), S_N2-induced elimination (CH₂O + HY + HO⁻) and S_N2-induced rearrangement (CH₂(OH)O⁻ + HY). Moreover, besides the traditional back-side attack Walden inversion, the pathways of front-side attack, double inversion and halogen-bond complex formation have also been explored for S_N2. With regard to the Walden inversion of HOO⁻ + CH₃Cl, the previously unaddressed discrepancies concerning the geometry of the corresponding transition state are clarified. For the HOO⁻ + CH₃F reaction, the recently identified S_N2-induced elimination is found to be more exothermic than the S_N2 channel, submerged by ∼36 kcal mol⁻¹. The accuracy of our high-level ab initio calculations performed in the present study is validated by the fact that our new benchmark 0 K reaction enthalpies show excellent agreement with the experimental data in nearly all cases.