Does α-Effect Exist in E2 Reactions? A G2(+) Investigation

The gas-phase base-induced bimolecular elimination (E2) reactions at saturated carbon with 13 bases, B- + CH3CH2Cl -> BH + CH2=CH2 + Cl- (B = HO, CH3O, CH3CH2O, FCH2CH2O, ClCH2CH2O, Cl, Br, FO, ClO, BrO, HOO, HSO, and H2NO), were investigated with the high-level G2(+) theory. It was found that all alpha-bases with adjacent lone pair electrons examined exhibited downward deviations from the correlation line between the overall barriers and proton affinities for the normal bases without adjacent lone pair electrons, indicating the existence of the alpha-effect in the gas phase E2 reactions. The sizes of the alpha-effect for the E2 reaction, Delta H-alpha(E2), span a smaller range if the alpha-atoms are on the same column in the periodic table, in contrast to the corresponding S(N)2 reactions, where the Delta H alpha(S(N)2) values significantly decrease from an upper to a lower column. The origin of the alpha-effects in E2 reactions can be interpreted by the favorable orbital interaction between the lone-pair electrons and positively charged anti-bonding orbital. It is worth noticing that the neighboring electron-rich pi lobe instead of lone pair electrons could also cause the alpha-effect in E2 reaction.