SiO and GeO bonded interactions as inferred from the bond critical point properties of electron density distributions

Abstract

The topological properties of the electron den- sity distributions for more than 20 hydroxyacid, geometry optimized molecules with SiO and GeO bonds with 3-, 4-, 6- and 8-coordinate Si and Ge cations were calculated. Electronegativities calculated with the bond critical point (bcp) properties of the distributions indicate, for a given coordination number, that the electronegativity of Ge (1.85) is slightly larger than that of Si (1.80) with the electronegativities of both atoms increasing with de- creasing bond length. With an increase in the electron density, the curvatures and the Laplacian of the electron density at the critical point of each bond increase with de- creasing bond length. The covalent character of the bonds are assessed, using bond critical point properties and elec- tronegativity values calculated from the electron density distributions. A mapping of the (3, u3) critical points of the valence shell concentrations of the oxide anions for bridging SiOSi and GeOGe dimers reveals a location and disposition of localized nonbonding electron pairs that is consistent with the bridging angles observed for sil- icates and germanates. The bcp properties of electron den- sity distributions of the SiO bonds calculated for represen- tative molecular models of the coesite structure agree with average values obtained in X-ray diffraction studies of coesite and danburite to within5%.