title: Role of structures with penta- and hexacoordinate silicon in the nucleophile-catalyzed hydrolysis of tetramethoxysilane.
abstract: The mechanism of the base catalyzed hydrolysis of tetramethoxysilane (TMOS), proposed earlier on the basis of experimental data, is assessed by theoretical methods, i.e. MP2 and B3LYP with 6-31G(d) and Dunning correlation-consistent basis sets. Models considered involve one and two hydrolyzing water molecules attacking (MeO)(4)Si in the frontside position with a nucleophile (NH(3) and OH(-)) in the backside position. This approach allowed us to simulate uniformly the catalytic action of weak and strong bases. It was shown that the presence of a base in the backside position considerably lowers the activation barrier for hydrolysis. The inclusion of the additional water molecule which results in a substantial lowering of the barrier for uncatalyzed hydrolysis does not change noticeably the catalytic effect. In both one- and two-water molecules models the structure of the transition state in the presence of a nucleophile becomes nearly octahedral in which hexacoordinate silicon and four oxygens of the TMOS moiety have a planar arrangement.
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