Molecular orbital study of bond-valence sum rule for hydrogen bond systems using Lewis-electron pair theory

Theoretical background of the bond-valence sum rule for hydrogen bond systems O-H⋯O is examined using molecular orbital methods related to Lewis-electron pair theory proposed by Okada and Fueno [T. Okada, T. Fueno, Bull. Chem. Soc. Jpn, 49 (1976) 1524]. The hydrogen valence conservation is considered. The bond order analysis for (HCOOH)2, α-(COOH)2·2H2O and H2SO4·4H2O shows that the bond-valence sum rule for hydrogen-bond systems is realised by the fact that the hydrogen valence conservation is approximately achieved thanks to the polarization effect and the charge transfer effect. Based on the bond order analyses for the symmetric hydrogen bond systems and B2H6 molecule, a revised valence concept is proposed as a rewritten expression of the bond-valence sum rule.