Investigating the α‑Effect in Gas-Phase S_N2 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 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 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 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 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

Substituent Effects on the Nonradical Reactivity of 4-Dehydropyridinium Cation

Recent studies have shown that the reactivity of the 4-dehydropyridinium cation significantly differs from the reactivities of its isomers toward tetrahydrofuran. While only hydrogen atom abstraction was observed for the 2- and 3-dehydropyridinium cations, nonradical reactions were observed for the 4-isomer. In order to learn more about these reactions, the gas-phase reactivities of the 4-dehydropyridinium cation and several of its derivatives toward tetrahydrofuran were investigated in a Fourier transform ion electron resonance mass spectrometer. Both radical and nonradical reactions were observed for most of these positively charged radicals. The major parameter determining whether nonradical reactions occur was found to be the electron affinity of the radicalsonly those with relatively high electron affinities underwent nonradical reactions. The reactivities of the monoradicals are also affected by hydrogen bonding and steric effects