Competition of F/OH-Induced S _N 2 and Proton-Transfer Reactions with Increased Solvation

The potential energy profiles of F/OH-induced nucleophilic substitution (S _N 2) and proton-transfer (PT) channels evolving with solvation for reactions of F ^- (H ₂ O) _n=1-2 + CH ₃ I were characterized using B3LYP/ECP/d method. The hydrogen-bonded F ^- (H ₂ O) _n - -HCH ₂ I prereaction complex at the entrance of potential energy surface (PES) has a significant role on the reaction dynamics for each channel. Among the above three channels, the F-S _N 2 channel is the most preferred and OH-S _N 2 could be competitive. In contrast, the PT channel will occur at much higher collision energy. Importantly, for each channel, the central barrier is gradually increased with the addition of water molecules. This phenomenon indicates that the reactivity will decrease with degrees of solvation and this has been confirmed by experiment and direct dynamics simulations. Moreover, compared with the previous trajectory simulations, a non-IRC behavior has been uncovered. The water delivering process from fluorine to iodine side as illustrated on PES is barely observed, and instead, the reaction tends to dehydrate before passing through the S _N 2 barrier and proceeds with the less hydrated pathway in order to weaken the steric effect. The work presented here shows the comprehensive potential energy surfaces and structures information on the F-S _N 2, PT, and OH-S _N 2 channels, and predict their competitive relationship, which would be helpful for better understanding the dynamics behavior of the title and analogous reactions.