Microsolvated Ion-Molecule S_N2 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 CH₃Cl. To investigate the generality of this effect, the competition between the solvent-induced HO⁻−S_N2 pathway and the normal HOO⁻−S_N2 pathway is studied for the microsolvated HOO⁻(H₂O)_n=1,2,3+CH₃X (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_N2 pathway. Interestingly, the barrier difference is insensitive to the leaving group. Calculations show that the water induced HO⁻−S_N2 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)(H₂O)_n-1 nucleophile more than that of the HOO⁻(H₂O)_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_N2 pathway than that of the HOO⁻−S_N2 pathway.