Electron density distributions, rho(r), have been calculated for the partially and fully geometry optimized molecules (SiO)-O-I, (SiO2)-O-II, (H2SiO3)-O-III (H4SiO4)-O-IV, (H6S2O7)-O-IV, (H8SiO6)-O-VI, (H10Si2O10)-O-VI, and (H12SiO8)-O-VIII, using SCF Hartree-Fock wavefunctions calculated with a 6-311++G** basis set (the Roman numeral superscripts denote the coordination number of Si). The value of the electron density distribution, rho(r(c)), evaluated at (3,-1) critical (stationary) points, r(c), along the SiO bonds of these molecules increases linearly with decreasing SiO bond length, R(SiO), from rho(r(c))=0.50e/Angstrom(3) for R((SiO)-O-VIII)=1.96 Angstrom to rho(r(c))=1.36e/Angstrom(3) for R((SiO)-O-II)=1.48 Angstrom. A mapping of the Laplacian of the distribution, del(2) rho(r), together with a determination of the bonded radii, r(b)(O), calculated for the oxide ions indicate that the fractional ionic character of the bond increases with increasing coordination number from similar to 0.40 for an (SiO)-O-II bond to similar to 0.50 for an (SiO)-O-IV bond to similar to 0.60 for an (SiO)-O-VI bond to similar to 0.75 for a (SiO)-O-VIII bond. Unlike the ionic and crystal radii of the oxide ion, r(b)(O) is not constant for a given coordination number but increases linearly with R(SiO) from r(b)(O)=0.86 Angstrom for R(SiO)=1.48 Angstrom to r(b)(O)=1.20 Angstrom for R(SiO)=1.96 Angstrom. Concomitant with this increase in the size of the oxide ion, the atomic charge on the O atom increases as the volume of its basin expands and the fractional ionic character, f(i)(SiO), of the SiO bond increases. As f(i)(SiO) increases, not only is there a decrease in the value of rho(r) at r(c), but also the sharpness (curvature) of the maximum in rho(r) radiating perpendicular to the bond and the sharpness of the minimum in rho(r) along the bond are both diminished. Electron density distribution calculations for five H6Si2O7 molecules with geometries fixed to match those observed for the nonequivalent 'Si2O7 dimers' in coesite show that the value of rho(r(c)) for the bridging SiO bond increases as the SiOSi angle widens and R(SIO) shortens. The properties of rho(r) for the SiO bonds of the molecules are similar to those measured for coesite demonstrating that rho(r) is similar in both systems. The ellipticity of the electron density of the. SiO bond decreases slightly as the angle widens with the cross-section of the bond becoming circular at 180 degrees. Such an effect is consistent with the formation of weak pi-bonds where the pi-system perpendicular to the SiOSi plane becomes more dominant as the SiOSi angle narrows. Relief and level line contour -del(2) rho(r) maps calculated through the OSiO planes in H4SiO4 and H8SiO6 show that the electron density of the valence shell concentration of the oxide ion is enhanced in the directions of the Si atoms with the enhancement being greater for the (SiO)-O-IV than for the (SiO)-O-VI bond in conformity with the greater ionic character of the latter. Maps calculated for H12SiO8 show little dissipation of the valence shell of the oxide ion and little or no enhancement in the direction of the Si atom in agreement with the larger fractional ionic character of the (SiO)-O-VIII bond and a nearly spherical distribution of the valence shell enhancement of rho(r) for the oxide ion.
