Trimeric silicate species, cyclic formation

From the development of resonance lines in the different spectra, a qualitative reaction mechanism can be extracted for the oligomerization process. In this mechanism, formation of cyclic structures is a predominant phenomenon. In the presence of TMA cations, no linear trimeric silicate anions are found in the silicate solutions, as can be deduced from Figures Id through lh. The use of alkali metal hydroxides as bases shows, aside from the cyclic trimeric silicate anion, the linear structure the relative concentration of each depends on the alkalinity and the alkali metal used. The formation of cyclic trimeric silicate anions occurs before formation of the linear trimeric species (6). The formation of double-cyclic silicate anions can be observed in Figure le two cyclic trimeric silicate anions combine to form the prismatic hexameric silicate anion (Q36). This prismatic hexameric silicate anion forms, through addition of two monomers or one dimer, the well-known cubic octameric silicate anion. This cubic octameric silicate anion has been proposed to be predominantly... 

In Figure 18-lc, the formation of cyclic trimeric silicic acid (0 3) is observed. Probably due to the high ionization of silicic acid species in the aqueous silicate solution, oligomerization of the d eric silicic acid proceeds via the cyclic trimeric isomer of trimeric silicic acid. In a later stage of the dissolution process, the formation of the linear isomer of trimeric silicic acid is encountered in the silicon-29 nmr spectrum (Figure 18-Id). Preferential formation of cyclic trimeric sUicic acid is attributed to charge stabilization of the trimeric species. Ab initio quantum mechanic calculations of cyclic structures of silicate units (Van Beest, Verbeek, and Van Santen... 

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