Initial Steps of the Sol−Gel Process: Modeling Silicate Condensation in Basic Medium

The initial step of the sol−gel process, that is, the condensation of two molecules of silicic acid has been studied by means of density functional theory. The chosen system represents the reagents under basic reaction conditions. Calculations were performed in the gas phase as well as by employing a solvent model for aqueous solution. For both systems, a reaction intermediate with one pentacoordinated silicon center was found as the most stable structure. The influence of intramolecular hydrogen bonds on the stability of the intermediate structure is discussed, and different pathways for the subsequent condensation step are investigated. Furthermore, the effect of fluoride substitution on the reaction path is investigated.