Nucleophilic substitution reactions of microsolvated hydroperoxide anion HOO⁻(NH₃)_n with methyl chloride and comparison between ammonia and water as the solvent

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⁻-S_N2 pathway are ∼10 kcal mol⁻¹ lower in energy than those of the PT-induced NH₂⁻-S_N2 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⁻-S_N2 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⁻S_N2 pathway is more likely to compete with the HOO⁻S_N2 pathway. The HOMO level of nucleophiles, which negatively correlates with the S_N2 barrier heights, is found to be a good descriptor to predict the S_N2 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 S_N2 reactions.