A QUANTUM-MECHANICAL INVESTIGATION OF SILSESQUIOXANE CAGES

Electronic structure calculations have been performed on a series of molecular silsesquioxane cages [HSiO1.5](n) (n = 4, 6, 8, 10, and 12) using ab initio quantum mechanical methods to examine factors that determine the relatively stability of different sizes of silicon oxide cages. While previous ab initio studies have been reported on the relative stability of molecular silicate rings, silsesquioxane cages are expected to be better models for solid-state silicates due to their rigidity and more similar molecular environment. To determine the relative stability of silicate cages, calculated total energies at optimized geometries (6-31G(d)//6-31 G(d)) for a series of silsesquioxane cages were compared. Consistent with both experimental observations and prior theoretical investigations, molecules containing (Si-O-)(3) rings were calculated to be significantly less stable than molecules containing only larger rings. Much smaller differences in relative stability were calculated to occur between larger cages that do not contain this type of ring.