A theoretical approach to substituent effects. Stabilities and rotational barriers of β-substituted ethyl anions

Abstract

Ab initio molecular orbital theory is used to study substituent effects in a series of substituted ethyl anions, XCH₂CH₂^- (X = Li, BeH, BH₂, CH₃, NH₂, OH and F). All substituents stabilize the anion relative to the corresponding neutral ethane. For electronegative substituents (NH₂, OH and F) as well as CH₃, this stabilization is achieved through enhanced hyperconjugative donation from the occupied 2p(C^-) orbital to a vacant π*(CH₂X) or π*(CH₂) orbital; electropositive groups (Li, BeH and BH₂) stabilize the anion primarily by 1,3-overlap between the occupied 2p(C^-) orbital and the vacant 2p(X) orbital where such an interaction is possible. The importance of 1,3-interactions contrasts with the situation for cations XCH₂CH₂⁺ in which hyperconjugative interactions appear to be dominant.