Octamer, cubic, silicon

Effect of the Structure of Silicon Sources. Hoebbel et al. used silicic acid sols, silicic acid gels, or Aerosil as a silica source of tetramethylammonium silicate aqueous solutions (9). In the solutions at the conditions that a N/Si ratio is 1.0 and Si02 concentration is ca. 1.4 mol dm-, the distributions of silicate anions are almost the same, and the cubic octamer is a dominant species, although the degradation rates of the silica sources are different. This suggests that the cubic octamer is formed in the tetramethylammonium silicate aqueous solution, regardless of the type of silica source with tetra-functionality. Tetraalkoxysilanes (Si(0R), R denotes an alkyl group) can be used as a silica source as well (4,12,14). 

As discussed for the hydrolysis reaction, the specific mechanism may be influenced by the local environment of the silicon undergoing condensation, the solvent, and nature of the catalyst. For example, Klemperer et al. [38] proved that two silicon species contained in separate cubic octamers undergo condensation under neutral conditions with essentially complete retention of configuration (see pathway A in Fig. 20), whereas condensation reactions between monomers may result in the inversion of one of the silicate tetrahedra. 

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