MOLECULAR-ORBITAL MODELS OF AM1 AND PM3 MULTIRING SILICA CLUSTERS ANDTHEIR INFRARED-SPECTRA

Amorphous silica (a-silica) has a distribution of various ring sizes. Large clusters of silica tetrahedra containing various ring sizes from 8- to 3-members were modelled using both AM1 and PM3 molecular orbita l methods. Bond lengths, bond angles, heats of formation and infrared spectra were compared to previous work and experimental values. The be st fit to the experimental infrared spectra for AM1 was a silica clust er with one 8 membered ring, a 4 membered ring and a 3 membered ring. The best fit for PM3 appeared to be a silica cluster with one 8 member ed ring, a 4 membered ring, a 3 membered ring and a 5 membered ring.