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

Abstract

Recently we reported experimental validation of the α-effect in the gas phase. However, an earlier study by our group showed a lack of enhanced reactivity in a series of SN2 reactions of α-nucleophiles with methyl chloride conflicting with computational predictions. In an attempt to resolve these discrepancies, we investigate the SN2 reactions for methyl chloride of low exothermicity where the smaller thermodynamic component of the activation barrier may expose α-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 Brønsted plot of normal and α-nucleophile reactions with methyl chloride. While the presence of an α-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 α-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 15kJmol−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 α-effect. Significantly lower electron affinities (∼0.6eV) are associated with the formation of the α-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 α-nucleophiles.