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 CH 3 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 - (H 2 O) n=1,2,3 +CH 3 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)(H 2 O) n-1 nucleophile more than that of the HOO - (H 2 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.