Abstract
Molecular orbital results, obtained using an approximate self-consistent-field method, are presented for two systems, one consisting of two silicate tetrahedra sharing a common edge and saturated with hydrogens at their periphery (Si2O6H4), and the other, consisting of a silicate tetrahedron sharing an edge with a Mg-containing octahedron and saturated with hydrogens (SiMgO8H10). For both systems the Obr–Si–Obr angle opposite the shared edge has been varied while all distances and angles not involved in the shared edge have been held fixed. For Si2O6H4, an energy minimum is found at an Obr–Si–Obr angle of about 85°; this is significantly less than the undistorted value of 109.5° but in fairly good agreement with the observed Obr–Si–Obr angle (93°) in silica–W and Si2O2. The energy stabilization at this minimum is about 100 kcal/Si2O6H4 unit. For SiMgO8H10, the minimum energy obtains when (Obr–Si–Obr) ∼ 103°, in good agreement with the observed data for forsterite. For the Si-Mg cluster, the energy stabilization at the minimum is only 4 kcal/SiMgO8H10 unit. An important determinant of total energy is found to be the covalent overlap repulsion between the metal atoms, manifested in large negative bond overlap populations. Decreases in M–O bond overlap populations and charge redistribution from M to the exterior of the bridging oxygen atoms also contribute significantly to total energy changes.
