Nucleophilic substitution reactions of microsolvated hydroperoxide anion HOO - (NH 3 ) n with methyl chloride and comparison between ammonia and water as the solvent.

Similar to microhydrated hydroperoxide anion HOO - (H 2 O) n , the HOO - (NH 3 ) n =1-3 anion can induce alternative nucleophiles by proton transfer (PT) from the solvent molecule NH 3 . The PT-induced species NH 2 - (H 2 O 2 )(NH 3 ) n -1 is higher in energy than HOO - (NH 3 ) n , obeying the proton affinity (PA) prediction that HOO - has a higher PA than NH 2 - . The potential energy profile of HOO - (NH 3 ) n reacting with CH 3 Cl shows that the transition states of the traditional HOO - -S N 2 pathway are ∼10 kcal mol -1 lower in energy than those of the PT-induced NH 2 - -S N 2 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 - -/NH 2 - -S N 2 pathways and makes the transition structures more product-like. For HOO - (sol) n + CH 3 Cl → CH 3 OOH + Cl - (sol) n reactions, the barrier heights for sol = H 2 O are higher than those for sol = NH 3 , because H 2 O is more polar than NH 3 , and the electrostatic interaction is strengthened, hence H 2 O molecules stabilize the microsolvated nucleophiles more. In addition, because the H 2 O molecule is a better proton donor than the NH 3 molecule, the PT-induced HO - S N 2 pathway is more likely to compete with the HOO - S N 2 pathway. The HOMO level of nucleophiles, which negatively correlates with the S N 2 barrier heights, is found to be a good descriptor to predict the S N 2 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 N 2 reactions.