Theoretical Studies of Si and Al Distributions in Molecules and Minerals with Eight Tetrahedrally Coordinated Atoms (T₈) in Double-Four-Ring (D4R) Geometries:  Octasilasesquioxanes, Gismondite, and Zeolite A

The inorganic compound octasilasesquioxane, Si 8 O 12 H 8, the mineral gismondite, CaSi 2 Al 2 O 8 ·4H 2 O, and synthetic Linde zeolite A all contain structural units in which eight tetrahedrally coordinated atoms (T 8, T = Si and/or Al) and their bridging oxygens form a double-four-ring (D4R) geometry. D4R compounds are much more stable than the single four rings that occur in some crystalline silicates and silicate glasses, since hydrolysis by nucleophilic attack at the T-atom is hindered by the cage geometry. Recently techniques have been developed for systematically incorporating non-Si-atoms into such T 8 units. We have calculated equilibrium geometries by using Hartree−Fock self-consistent-field molecular orbital methods and 6-31G* basis sets for Si 8 O 12 H 8, Si 8 O 12 (OH) 8, and a number of related molecules. Vibrational spectra have been calculated for Si 8 O 12 H 8, Si 8 O 20 8-, and the stable “alternating” isomer of the Si 4 Al 4 O 12 H 8 4- anion, which contains only Si−O−Al bridges. The calculated vibrational spectra for Si 8 O 12 H 8 are in good agreement with those measured, and the assignment of the spectra to the various normal modes is consistent with earlier models based on empirical force fields. Several other “paired” isomeric forms of Si 4 Al 4 O 12 H 8 4-, which contain some Al−O−Al bridges, are found to be higher in energy than the alternating structure, in accordance with Loewenstein's rule. However, the energies of the Si 4 Al 4 O 12 H 8 4- isomers depend upon the spatial separation of the Al−O−Al bridges as well as upon their number. For the Si 6 Al 2 O 12 H 8 2- anion, we have evaluated the stability of three different isomers in which the two Al-atoms are separated by increasing numbers of bonds. We find the lowest energy for the structure with a maximum separation of the Al-atoms, in accordance with Dempsey's rule. The energy penalty for the presence of Al−O−Al bridges is very similar in magnitude to that previously calculated in single-four-ring compounds. However, the relative energies of the various isomers of the Si 4 Al 4 ... and Si 6 Al 2 ... structures can be dramatically altered by the addition of H + to different bridging oxygens. The most stable form of Si 6 Al 2 O 12 H 10 is in fact calculated to have protonated Al−O−Al and Si−O−Al bridges. If the H-atoms bonded to the bridging O's could later be removed without reconstruction of the T 8 cage, a free Al−O−Al linkage could be produced that would be a strongly basic site within the material. In addition to structural and energetic properties, we have calculated the photoemission spectra, nuclear quadrupole coupling constants at O, vertical and adiabatic electron attachment energies, and Si core ion term energies for Si 8 O 12 H 8 .