The effect of the primary solvate shell on the mechanism of the Stöber silica synthesis. A density functional investigation.
The target of the present computational study was the acid catalyzed bond cleavage of the Si-O and C-O bonds in siloxane, alkoxysilane and ether in aqueous media. In the present study the effect of water as a solvent has been modeled using a full primary solvate shell built up from water molecules connected via hydrogen bonds around the reacting molecules. The interaction energy between the embedding water cluster and the "solvated" molecule gives an estimate for solution effects. The cleavage of the Si-O bonds in these molecular clusters proceeds with low barriers; furthermore the reaction energies corrected with the solvent interaction energies gives a reaction thermodynamics, which is in accordance with the experimental results. Molecules with a Si-O bond form stable pentavalent silicon with the solvent water molecules if protonated, while in the case of the neutral molecules tetracoordinate silicon is obtainable. The summary of the calculated reaction paths gives a possible route of siloxane formation from methoxysilane in aqueous media. The same computational methodology predicts that the hydrolysis of dimethyl ether is hindered by a substantial barrier.
Terleczky
Péter
P
Department of Inorganic and Analytical Chemistry, Budapest University of Technology and Economics, Hungary.
Nyulászi
László
L