Benchmark ab initio characterization of the complex potential energy surfaces of the HOO - + CH 3 Y [Y = F, Cl, Br, I] reactions.

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 N 2) 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 - + CH 3 Y [Y = F, Cl, Br and I] reactions utilizing the modern explicitly-correlated CCSD(T)-F12b method with the aug-cc-pV n Z [ n = 2-4] basis sets. The present ab initio characterization considers five distinct product channels of S N 2: (CH 3 OOH + Y - ), proton abstraction (CH 2 Y - + H 2 O 2 ), peroxide ion substitution (CH 3 OO - + HY), S N 2-induced elimination (CH 2 O + HY + HO - ) and S N 2-induced rearrangement (CH 2 (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 N 2. With regard to the Walden inversion of HOO - + CH 3 Cl, the previously unaddressed discrepancies concerning the geometry of the corresponding transition state are clarified. For the HOO - + CH 3 F reaction, the recently identified S N 2-induced elimination is found to be more exothermic than the S N 2 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.