The electron density delocalization of hydrogen bond systems

Abstract

Recent experimental and theoretical researches have gradually proved that hydrogen bond (H-bond) interactions are not simple traditionally considered electrostatic interaction. Instead, they involve electron density delocalization. In this work, we outline the studies of electronic structures of the H-bond systems in water systems and biological organic molecules systems. Theoretical researches based on the first-principles method have found important evidences for electron density delocalization in H-bond region. Topological analysis based on natural bond orbital (NBO) analysis proves that during the formation of the H-bonds, electrons transfer from B orbitals to A–H anti-bond orbitals. Energy decomposition analyses revealed that the delocalized electronic structures show strong relations with orbital interactions. Moreover, penetrating molecular orbitals (MOs) are proved to contribute to the electron density delocalization of the H-bonds, and the quantitative contributions for such MOs could be obtained with the electronic density projected integral (EDPI) method. The electronic delocalization and the corresponding penetrating MOs could be visualized in water clusters, based on the first-principles method. These researches open a new sight for understanding the electronic structures of H-bonds from atomic level, and even contribute to further controlling proton tunneling as well as charge and energy transfer processes.