Ab initio theory of hydrogen bonding in vitamin B6

Ab initio MO computations have been carried out for the crystalline form of the B₆ vitamin, pyridoxinium chloride. In the crystal, the pyridoxine is protonized to pyridoxinium, with the formation of a hydrogen bond N? H⁺ ? Cl^?. For an isolated molecule, the calculations predict a single potential well, with H⁺ placed close to Cl^? at the minimum. When neighboring molecules are included in the calculations, a double well is formed, and the lower minimum occurs for the proton placed near the experimentally observed position, in the vicinity of the N atom. Mulliken populations, which depend on the proton position, are described.