Investigating the α‑Effect in Gas-Phase SN2 Reactions of Microsolvated Anions

The α-effectenhanced reactivity of nucleophiles with a lone-pair adjacent to the attacking centerwas recently demonstrated for gas-phase SN2 reactions of HOO–, supporting an intrinsic component of the α-effect. In the present work we explore the gas-phase reactivity of microsolvated nucleophiles in order to investigate in detail how the α-effect is influenced by solvent. We compare the gas-phase reactivity of the microsolvated α-nucleophile HOO–(H2O) to that of microsolvated normal alkoxy nucleophiles, RO–(H2O), in reaction with CH3Cl using a flowing afterglow-selected ion flow tube instrument. The results reveal enhanced reactivity of HOO–(H2O) and clearly demonstrate the presence of an α-effect for the microsolvated α-nucleophile. The association of the nucleophile with a single water molecule results in a larger Brønsted βnuc value than is the case for the unsolvated nucleophiles. Accordingly, the reactions of the microsolvated nucleophiles proceed through later transition states in which bond formation has progressed further. Calculations show a significant difference in solvent interaction for HOO– relative to the normal nucleophiles at the transition states, indicating that differential solvation may well contribute to the α-effect. The reactions of the microsolvated anions with CH3Cl can lead to formation of either the bare Cl– anion or the Cl–(H2O) cluster. The product distributions show preferential formation of the Cl– anion even though the formation of Cl–(H2O) would be favored thermodynamically. Although the structure of the HOO–(H2O) cluster resembles HO–(HOOH), we demonstrate that HOO– is the active nucleophile when the cluster reacts.