Off-atomic site charges in some anions, metal cations, and complexes: Significance for electrostatic properties and binding

Electronic structures and properties of several anions, metal cations, and their complexes with neutral molecules were investigated at the HF/6-31G** and B3LYP/6-31G** levels of theory. Charges shifted from atomic sites due to atomic orbital hybridization called hybridization displacement charges (HDC) were investigated in detail. It has been found that many components of HDC are associated with each atom of ion that are shifted from the atomic sites, those associated with metal cations being shifted by large distances as found previously in electrically neutral systems. It is shown that atomic orbitals are appreciably rehybridized in going from neutral molecules to anions and cations. Molecular dipole moments and surface molecular electrostatic potentials (MEP) are obtained satisfactorily using HDC for the various types of species mentioned above. In the OH--H2O complex, reversal of direction of shift of an HDC component associated with the hydrogen atom of H2O involved in hydrogen bonding, indicates that the hydrogen bond between OH- and H2O would have some covalent character. Other atomic site-based point charge models cannot provide such information about the nature of bonding. (C) 2006 Wiley Periodicals, Inc.