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

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• 作者：Ditte L. Thomsen ; Jennifer N. Reece ; Charles M. Nichols ; Steen Hammerum ; Veronica M. Bierbaum
• 刊名：Journal of the American Chemical Society
• 出版年：2013
• 出版时间：October 16, 2013
• 年：2013
• 卷：135
• 期：41
• 页码：15508-15514
• 全文大小：308K
• 年卷期：v.135,no.41(October 16, 2013)
• ISSN：1520-5126

文摘

The 伪-effect鈥攅nhanced reactivity of nucleophiles with a lone-pair adjacent to the attacking center鈥攚as recently demonstrated for gas-phase S_N2 reactions of HOO^{鈥?/sup>, supporting an intrinsic component of the 伪-effect. In the present work we explore the gas-phase reactivity of microsolvated nucleophiles in order to investigate in detail how the 伪-effect is influenced by solvent. We compare the gas-phase reactivity of the microsolvated 伪-nucleophile HOO鈥?/sup>(H₂O) to that of microsolvated normal alkoxy nucleophiles, RO鈥?/sup>(H₂O), in reaction with CH₃Cl using a flowing afterglow-selected ion flow tube instrument. The results reveal enhanced reactivity of HOO鈥?/sup>(H₂O) and clearly demonstrate the presence of an 伪-effect for the microsolvated 伪-nucleophile. The association of the nucleophile with a single water molecule results in a larger Br酶nsted 尾_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鈥?/sup> relative to the normal nucleophiles at the transition states, indicating that differential solvation may well contribute to the 伪-effect. The reactions of the microsolvated anions with CH₃Cl can lead to formation of either the bare Cl鈥?/sup> anion or the Cl鈥?/sup>(H₂O) cluster. The product distributions show preferential formation of the Cl鈥?/sup> anion even though the formation of Cl鈥?/sup>(H₂O) would be favored thermodynamically. Although the structure of the HOO鈥?/sup>(H₂O) cluster resembles HO鈥?/sup>(HOOH), we demonstrate that HOO鈥?/sup> is the active nucleophile when the cluster reacts.}