Resolving the α-effect in gas phase SN2 reactions: A Marcus theory approach

Recently we reported experimental validation of the alpha-effect in the gas phase. However, an earlier study by our group showed a lack of enhanced reactivity in a series of S(N)2 reactions of alpha-nucleophiles with methyl chloride conflicting with computational predictions. In an attempt to resolve these discrepancies, we investigate the S(N)2 reactions for methyl chloride of low exothermicity where the smaller thermodynamic component of the activation barrier may expose alpha-nucleophilicity. The efficiencies for the reactions of several normal nucleophiles [C6H5O-, HC(O)O-, CH3C(O)O-] and alpha-nucleophiles [HC(O)OO-, CH3C(O)OO-] with CH3Cl are added to our previous Bronsted plot of normal and alpha-nucleophile reactions with methyl chloride. While the presence of an alpha-effect is suggested in some of the reactions with methyl chloride at lower basicities, the homologous properties of the "normal" ions in this region deviate from straight-chain alkoxides making the definition of "normal" reactivity more difficult. Application of Marcus theory provides insight into the intrinsic nature of the alpha-effect and how easily intrinsic differences can be masked. Computational barriers were utilized to estimate an "average" Marcus intrinsic barrier for several reactions at two different levels of theory. The "average" intrinsic barrier for the identity reaction of HOO- lies roughly 15 kJ mol(-1) below those of the "normal" 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 "normal" reactivity will play a key role in the magnitude of the alpha-effect. Significantly lower electron affinities (similar to 0.6 eV) are associated with the formation of the alpha-oxyanions compared to the normal oxyanions (X + e(-) -> X-) suggesting that the ease of charge transfer between the nucleophile and transition state is responsible for the lower barriers of the alpha-nucleophiles. (C) 2012 Elsevier B.V. All rights reserved.