How the generalized anomeric effect influences the conformational preference.

The generalized anomeric effect refers to the conformational preference of a gauche structure over an anti structure for molecules with a R-X-C-Y moiety. Whereas there are conflicting reports regarding the origin of this ubiquitous effect, a general consensus is that both the steric (more specifically electrostatic) and hyperconjugative interactions contribute. Here we employed the block-localized wavefunction (BLW) method, which is the simplest variant of ab initio valence bond (VB) theory and can define reference electron-localized states self-consistently, to evaluate the magnitude of the hyperconjugation effect in a number of acyclic molecules exhibiting the generalized anomeric effect. The BLW-based energy decomposition analysis revealed that both the steric and hyperconjugation effects contribute to the conformational preferences of methoxymethyl fluoride and methoxymethyl chlorides. But for the other systems under investigation, including methanediol, methanediamine, aminomethanol and dimethoxymethane, the hyperconjugative interactions play a negative role in the conformational preferences and the steric effect is solely responsible for the generalized anomeric effect.