Similar to microhydrated hydroperoxide anion HOO-(H2O)n, the HOO-(NH3)n=1-3 anion can induce alternative nucleophiles by proton transfer (PT) from the solvent molecule NH3. The PT-induced species NH2-(H2O2)(NH3)n-1 is higher in energy than HOO-(NH3)n, obeying the proton affinity (PA) prediction that HOO- has a higher PA than NH2-. The potential energy profile of HOO-(NH3)n reacting with CH3Cl shows that the transition states of the traditional HOO--SN2 pathway are ∼10 kcal mol-1 lower in energy than those of the PT-induced NH2--SN2 pathway, indicating the latter path is unlikely to compete. The differential solvation energy for reactants and transition states with incremental solvation increases the barrier height of both HOO--/NH2--SN2 pathways and makes the transition structures more product-like. For HOO-(sol)n + CH3Cl → CH3OOH + Cl-(sol)n reactions, the barrier heights for sol = H2O are higher than those for sol = NH3, because H2O is more polar than NH3, and the electrostatic interaction is strengthened, hence H2O molecules stabilize the microsolvated nucleophiles more. In addition, because the H2O molecule is a better proton donor than the NH3 molecule, the PT-induced HO-SN2 pathway is more likely to compete with the HOO-SN2 pathway. The HOMO level of nucleophiles, which negatively correlates with the SN2 barrier heights, is found to be a good descriptor to predict the SN2 barrier height of a microsolvated system with the same attacking nucleophile. This work adds to our understanding of the differential solvent effect on the prototype ion-molecule SN2 reactions.