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

Abstract

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

Ti4+ and Zr4+ inside Aluminosilicate and Borosilicate Cages: A Computational Study

“…The reactant molecule Si 4 Al 4 O 12 H 8 Na 4 is of C 4 v symmetry with the Na + ions on the outsides of four faces of the T 8 cube. This molecule is shown in Figure of ref (identified as Si 4 Al 4 O 12 H 8 Na 4 , Al···Si···Al···Si), and its structure agrees well with experiment, as discussed in ref .…”

Interaction of Molecular Nitrogen and Oxygen with Extraframework Cations in Zeolites with Double Six-Membered Rings of Oxygen-Bridged Silicon and Aluminum Atoms: A DFT Study

“…Significant shifts of the vibrational frequencies may be observed. 23 The introduction of balancing point charges may prevent correct geometry optimization. Different Si,Al distributions in D6R's were examined in order to compare the gasphase ion-exchange energies: clusters with C 2h symmetry have with maximum separation of the Al atoms within the ring were used to examine the SII sitesin the case of monovalent cations, the clusters were negatively charged.…”

Vibrational Structure of Monosubstituted Octahydrosilasesquioxanes

“…They are discussed as oligomeric supramolecular materials, , as sources for new organosiliceous polymers, , and as precursors to organolithic macromolecular materials (OMM's) − or hybrid inorganic−organic materials. − Moreover, it has been shown that atomic hydrogen can be encapsulated stably in the Si 8 O 12 cage . Hydrosilasesquioxanes (R = H) have been fruitfully viewed as readily available model compounds for studying specific aspects of zeolites or silica surfaces. − Especially H 8 Si 8 O 12 has been thoroughly investigated by X-ray and neutron diffraction, , as well as NMR and vibrational spectroscopies. , Hydrosilation of H 8 Si 8 O 12 leads to monosubstituted and higher substituted octanuclear silasesquioxanes, − which can be separated by size exclusion liquid chromatography . The compounds RH 7 Si 8 O 12 with R = Co(CO) 4 , CHCHPh, and CH 2 CH 2 Ph illustrated in Scheme have been synthesized by this method. , PhH 7 Si 8 O 12 in contrast was obtained starting from a mixture of HSiCl 3 and PhSiCl 3 …”