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

The PM3 molecular orbital model was applied to various silica rings fr om twofold to sixfold on size. From these optimized structures, the br idging 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 format ion were tabulated. These data were compared with earlier MNDO and ab initio molecular orbital calculations. The theoretical vibrational spe ctra were then calculated and compared with earlier work as well as th e experimental IR response. The PM3 method was found to be a viable ca lculational tool for modeling silica structures.