Microsolvated Ion‐Molecule SN2 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- -SN 2 pathway and the normal HOO- -SN 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- -SN 2 pathway. Interestingly, the barrier difference is insensitive to the leaving group. Calculations show that the water induced HO- -SN 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- -SN 2 pathway than that of the HOO- -SN 2 pathway.