Proton transfer-induced competing product channels of microsolvated Y - (H 2 O) n + CH 3 I (Y = F, Cl, Br, I) reactions.

The potential energy profiles of three proton transfer-involved product channels for the reactions of Y - (H 2 O) 1,2 + CH 3 I (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 N 2) product channel, and (3) oxide ion substitution (OIS) product channel that gives CH 3 O - and HY products. The reaction enthalpies and barrier heights follow the order OIS > PT CH3 > HO - -S N 2 > Y - -S N 2, and thus HO - -S N 2 can compete with the most favored Y - -S N 2 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 - (H 2 O) n -CH 3 I, in the entrance of the potential energy profile, signifying the importance of the pre-reaction complex. For HO - /Y - -S N 2 channels, we considered front-side attack, back-side attack, and halogen-bonded complex mechanisms. Incremental hydration increases the barriers of both HO - /Y - -S N 2 channels as well as their barrier difference, implying that the HO - -S N 2 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 N 2 barrier change trend with incremental hydration and varying Y - . In brief, this work computes the comprehensive potential energy surfaces of the HO - -S N 2 and PT CH3 channels and shows how proton transfer affects the microsolvated Y - (H 2 O) 1,2 + CH 3 I reaction by competing with the traditional Y - -S N 2 channel.