ELECTRONIC-STRUCTURE CALCULATIONS ON OCTANUCLEAR SILSESQUIOXANES AND ALUMINOSILSESQUIOXANES

Semiempirical electronic structure calculations have been performed on a series of octanuclear molecular silicates and aluminosilicates to determine the effects of substitution of tetravalent silicon by trivalent aluminum. By inclusion of molecular species that have already been isolated and characterized, these calculated results have been compared directly with experimental values to determine the reliability of the results obtained. Although these calculations consistently predict Si-O bonds to be slightly longer than observed experimentally, they are shown to be quite successful at reproducing geometric trends within a related series of molecules. The calculated relative stabilities for compounds containing two or more aluminum atoms predict that isomers containing Al-O-Al linkages should be thermodynamically less stable, consistent with Loewenstein's rule for solid-state aluminosilicates. Selected studies of monoprotonated species have also been performed to determine the relative stability of different protonation sites, especially with regard to the proximity to aluminum. As expected, protonation of bridging oxygen atoms near trivalent aluminum sites is predicted to result in thermodynamically more stable, and thus less acidic, species than those produced by protonation of oxygen atoms bridging two tetravalent silicon atoms.