Investigating the α-Effect in Gas-Phase S_N2 Reactions of Microsolvated Anions

摘要

The a-effect-enhanced reactivity of nucleo-philes with a lone-pair adjacent to the attacking center-was recently demonstrated for gas-phase S_N2 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 a-effect is influenced by solvent. We compare the gas-phase reactivity of the microsolvated a-nudeophile HOO~-(H_2O) to that of microsolvated normal alkoxy nucleophiles, RO~-(H_2O), in reaction with CH_3Cl using a flowing afterglow-selected ion flow tube instrument. The results reveal enhanced reactivity of HOO~- (H_2O) and clearly demonstrate the presence of an α-effect for the microsolvated α-nudeophile. The association of the nudeophile with a single water molecule results in a larger Brφnsted β_(nuc) 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 a-effect. The reactions of the microsolvated anions with CH_3Cl can lead to formation of either the bare Cl~-anion or the Cl~-(H_2O) duster. The product distributions show preferential formation of the Cl~- anion even though the formation of Cl~-(H_2O) would be favored thermodynamically. Although the structure of the HOO~-(H_2O) duster resembles HO~-(HOOH), we demonstrate that HOO~- is the active nudeophile when the duster reacts.