Origin of the gauche preference of n-propyl halides and related molecules investigated by ab initio MO calculations: Importance of the CH/n hydrogen bond

Ab initio MO calculations were carried out to investigate the conformational preference of n-propyl halides, isobutyl halides, sec-butyl halides, and n-butyl halides. It has been found in most cases that the conformer in which a methyl group is close to the halogen atom is favored. The distance between the halogen atom and one of the hydrogens in the interacting CH3 group has been shown, in every case, to be shorter than the van der Waals distance. Natural bond orbital (NBO) charges have given results consistent with this finding. We suggest that the CH/n hydrogen bond contributes in determining the conformation of these molecules.