Proton transfer-induced competing product channels of microsolvated Y⁻(H₂O)_n + CH₃I (Y = F, Cl, Br, I) reactions

The potential energy profiles of three proton transfer-involved product channels for the reactions of Y⁻(H₂O)_1,2 + CH₃I (Y = F, Cl, Br, I) were characterized using the B97-1/ECP/d method. These three channels include the (1) PT_CH3 product channel that transfers a proton from methyl to nucleophile, (2) HO⁻-induced nucleophilic substitution (HO⁻-S_N2) product channel, and (3) oxide ion substitution (OIS) product channel that gives CH₃O⁻ and HY products. The reaction enthalpies and barrier heights follow the order OIS > PT_CH3 > HO⁻-S_N2 > Y⁻-S_N2, and thus HO⁻-S_N2 can compete with the most favored Y⁻-S_N2 product channel under singly-/doubly-hydrated conditions, while the PT_CH3 channel only occurs under high collision energy and the OIS channel is the least probable. All product channels share the same pre-reaction complex, Y⁻(H₂O)_n-CH₃I, in the entrance of the potential energy profile, signifying the importance of the pre-reaction complex. For HO⁻/Y⁻-S_N2 channels, we considered front-side attack, back-side attack, and halogen-bonded complex mechanisms. Incremental hydration increases the barriers of both HO⁻/Y⁻-S_N2 channels as well as their barrier difference, implying that the HO⁻-S_N2 channel becomes less important when further hydrated. Varying the nucleophile Y⁻ from F⁻ to I⁻ also increases the barrier heights and barrier difference, which correlates with the proton affinity of the nucleophiles. Energy decomposition analyses show that both the orbital interaction energy and structural deformation energy of the transition states determine the S_N2 barrier change trend with incremental hydration and varying Y⁻. In brief, this work computes the comprehensive potential energy surfaces of the HO⁻-S_N2 and PT_CH3 channels and shows how proton transfer affects the microsolvated Y⁻(H₂O)_1,2 + CH₃I reaction by competing with the traditional Y⁻-S_N2 channel.