title: Mechanism of the catalytic activity of nucleophiles in the stepwise hydrolysis and condensation reactions of tetramethoxysilane.
abstract: Active ingredients: A model for the simplest hydrolysis reaction is applied to all stages of stepwise hydrolysis and condensation taking place during a sol-gel process. The picture shows the molecular structures of the transition states of the ammonia- (left) and OH(-)-promoted (right) condensation reactions of two Si(OH)(4) molecules, including an additional water molecule.The previously proposed model of the catalytic activity of nucleophiles in the hydrolysis reaction of tetramethoxysilane is expanded to the subsequent stages of hydrolysis, that is, the stepwise hydrolysis of (MeO)(4-x)(OH)(x)Si (x=1-3) molecules, and to the condensation reaction of completely hydrolysed species. The estimate of the reaction barrier heights by using the B3LYP and MP2 methods with correlation-consistent double-zeta basis sets allows us to predict the catalytic activity of bases for the different steps of hydrolysis and for condensation reactions. In general, the catalytic activity of bases decreases with the number of hydrolysed groups, vanishing in the case of ammonia at x=2 and remaining in the case of the hydroxyl anion up to x=3. In addition, the value of the catalytic activity of NH(3) and OH(-) is of the same order of magnitude for hydrolysis reactions, while the effect of the OH(-) anion substantially exceeds that of NH(3) for condensation reactions.
authors.affiliation: Department of Chemistry, Radiochemistry Laboratory, St. Petersburg State University, 199034 St. Petersburg, Russia.
authors.forename: Igor S
authors.initials: IS
authors.lastname: Ignatyev
authors.forename: Manuel
authors.initials: M
authors.lastname: Montejo
authors.forename: Juan Jesús
authors.initials: JJ
authors.lastname: López González