Resolving the 伪-effect in gas phase Sb>Nb>2 reactions: A Marcus theory approach

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• 作者：John M. Garver^a ; ¹ ; Zhibo Yang^a ; Charles M. Nichols^a ; Benjamin B. Worker^a ; ¹ ; Scott Gronert^b ; Veronica M. Bierbaum^a ; ^{veronica.bierbaum@colorado.edu}
• 关键词：伪-Effect ; SN2 reaction ; Marcus theory
• 刊名：International Journal of Mass Spectrometry
• 出版年：2012
• 期刊代码：43_13873806
• 类别：ch
• 出版时间：15 April, 2012
• 卷：316-318
• 期：Complete
• 页码：244-250
• 文件大小：492 K

摘要

Recently we reported experimental validation of the 伪-effect in the gas phase. However, an earlier study by our group showed a lack of enhanced reactivity in a series of Sb>Nb>2 reactions of 伪-nucleophiles with methyl chloride conflicting with computational predictions. In an attempt to resolve these discrepancies, we investigate the Sb>Nb>2 reactions for methyl chloride of low exothermicity where the smaller thermodynamic component of the activation barrier may expose 伪-nucleophilicity. The efficiencies for the reactions of several normal nucleophiles [Cb>6b>Hb>5b>O^{鈭?/sup>, HC(O)O鈭?/sup>, CHb>3b>C(O)O鈭?/sup>] and alpha-nucleophiles [HC(O)OO鈭?/sup>, CHb>3b>C(O)OO鈭?/sup>] with CHb>3b>Cl are added to our previous Br酶nsted plot of normal and 伪-nucleophile reactions with methyl chloride. While the presence of an 伪-effect is suggested in some of the reactions with methyl chloride at lower basicities, the homologous properties of the 鈥渘ormal鈥?ions in this region deviate from straight-chain alkoxides making the definition of 鈥渘ormal鈥?reactivity more difficult. Application of Marcus theory provides insight into the intrinsic nature of the 伪-effect and how easily intrinsic differences can be masked. Computational barriers were utilized to estimate an 鈥渁verage鈥?Marcus intrinsic barrier for several reactions at two different levels of theory. The 鈥渁verage鈥?intrinsic barrier for the identity reaction of HOO鈭?/sup> lies roughly 15 kJ mol鈭? below those of the 鈥渘ormal鈥?nucleophiles, but this intrinsic difference is a maximum that can be significantly masked by leaving group barrier contributions to the overall Marcus activation barrier and thermodynamic driving forces. Variations in the intrinsic Marcus barriers of the anion(s) defining 鈥渘ormal鈥?reactivity will play a key role in the magnitude of the 伪-effect. Significantly lower electron affinities (鈭?.6 eV) are associated with the formation of the 伪-oxyanions compared to the normal oxyanions (X + e鈭?/sup> 鈫?#xA0;X鈭?/sup>) suggesting that the ease of charge transfer between the nucleophile and transition state is responsible for the lower barriers of the 伪-nucleophiles.}