A PM3 MOLECULAR-ORBITAL MODEL OF SILICA RINGS AND THEIR VIBRATIONAL-SPECTRA

The PM3 molecular orbital model was applied to various silica rings from twofold to sixfold on size. From these optimized structures, the bridging oxygen (BO) and non-bridging oxygen (NBO) bond lengths, the Si-O-Si (theta) angles, the O-Si-O (psi) angles and final heats of formation were tabulated. These data were compared with earlier MNDO and ab initio molecular orbital calculations. The theoretical vibrational spectra were then calculated and compared with earlier work as well as the experimental IR response. The PM3 method was found to be a viable calculational tool for modeling silica structures.