MOLECULAR-ORBITAL MODELS OF SILICA RINGS AND THEIR VIBRATIONAL-SPECTRA

Amorphous silica (a-silca) has a distribution of various ring sizes. It is generally known that these rings vary in size from 2-membered rings through 6-membered (or possibly higher) rings of silica tetrahedra. These silica rings are modeled by a semiempirical quantum molecular orbital method named AM1. The calculations predict a range of Si-O bond lengths to be from 1.69 to 1.74 Angstrom and a range of Si-O-Si bond angles to be from 102 degrees to 174 degrees. These values compare well with previous ab initio and PM3 results. The predicted infrared spectra also agree well with previous models and experimental results.