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 (pO→σ*SiX)π. In silanolate anions, the (pO→dSi)π delocalization becomes more important. Thermodynamic parameters were calculated for the transformation of some ionized silanols, X(HO)nH2−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.