Microsolvated Ion-Molecule S_N 2 Reactions with Dual Nucleophiles Induced by Solvent Molecules

Singly-hydrated HOO^- anion was found to induce alternative nucleophile HO^- via proton transfer from water molecule when reacting with CH₃ Cl. To investigate the generality of this effect, the competition between the solvent-induced HO^- -S_N 2 pathway and the normal HOO^- -S_N 2 pathway is studied for the microsolvated HOO^- (H₂ O)_n=1,2,3 +CH₃ X (X=F, Cl, Br, I) reaction by quantum chemistry calculations. Incremental hydration increases the barrier heights of both pathways and enlarges the barrier difference between them, which favors the HOO^- -S_N 2 pathway. Interestingly, the barrier difference is insensitive to the leaving group. Calculations show that the water induced HO^- -S_N 2 pathway is highly suppressed when the degree of hydration increases beyond two. The differential barrier under incremental hydration can be explained by solvent molecules stabilizing the HOMO level of HO^- (HOOH)(H₂ O)_n-1 nucleophile more than that of the HOO^- (H₂ O)_n nucleophile. Comparison between the HO^- - and HOO^- -nucleophiles suggests that α-effect exists. Activation strain analysis attributes the barrier differences to stronger TS distortion of the HO^- -S_N 2 pathway than that of the HOO^- -S_N 2 pathway.