MOLECULAR-ORBITAL MODELS OF SILICA

Amorphous silica (a-silica) has a distribution of various ring sizes. It is generally known that these rings vary in size from 2-member rings through 6-member (or possibly higher) rings of silica tetrahedra. These silica rings, as well as chains, are modeled by the following semi-empirical quantum molecular orbital (MO) methods: AM1, PM3, and INDO.(1) The calculations predict a range of Si-O bond lengths to be from 1.65 to 1.74 Angstrom and a range of Si-O-Si bond angles to be from 102 to 174 degrees. These values compare well with previous ab-initio results. The predicted electronic and infrared spectra also agree well with previous models and experimental results. These MO models yield significant insight into the understanding of structure-property relationships for bulk amorphous silica, as well as for porous type VI sol-gel silica. The same MO methods are applied to a variety of topics: quantum clusters of Si-SiO2, silica densification, silica-water interactions, silica pore models, Cr3+-doped silica, silica fracture, and silica-amino acid interactions.