The origin of the generalized anomeric effect: possibility of CH/n and CH/pi hydrogen bonds.

Ab initio MO calculations were carried out at the MP4/6-311++G(3df,3pd)//MP2/6-311++G(3df,3pd) level to investigate the conformational Gibbs energy of a series of methyl ethers CH(3)O-CH(2)-X (X=OH, OCH(3), F, Cl, Br, CN, C triple bond CH, C(6)H(5), CHO). It was found that the Gibbs energy of the gauche conformers is lower in every case than that of the corresponding anti conformers. In the more stable gauche conformers, the interatomic distance between X and the hydrogen atom was shorter than the sum of the van der Waals radii. The natural bonding orbital (NBO) charges of group X were more negative in the gauche conformers than in the anti conformers. We suggest that the CH/n and CH/pi hydrogen bonds play an important role in stabilizing the gauche conformation of these compounds.