Silanones and metasilicates from negatively charged ≡SiO(-) and =SiO2(2-) precursors.: Theoretical study

Ab initio calculations were performed to compare the electronic effects in unionized, partly and fully ionized silanols, XH2SiOH, and silanediols, XHSi(OH)(2), X = H, OH, OSiH3, Cl. The delocalization energy strongly increases on ionization. The silicon bond to negatively charged oxygen is significantly shortened gaining a partial double bond character, while the adjacent bonds to geminal oxygen, chlorine and hydrogen are elongated and effectively weakened. The weakening of the Si-X bond is particularly large in the dianionic structures, XHSiO2(2 - ). The delocalization energy in neutral silanols is mainly due to the negative hyperconjugation (p(O) --> (sigma(Si) (X)*)(x). In silanolate anions, the (p(O) --> d(Si))(pi) delocalization becomes more important. Thermodynamic parameters were calculated for the transformation of some ionized silanols, X(HO)(n)H2-nSiO(-), and silanediols, XHSiO2(2 - ) into silanones and metasilicates, respectively, by elimination of X(-). In the gas phase, the formation of silanones by this route is energetically very costly. However, thermodynamical calculations suggest that the process may occur in a polar solvent, In contrast, the formation of metasilicates is thermodynamically favoured even in the gas phase, (C) 2002 Elsevier Science B.V. All rights reserved.