Theoretical study on the dimerization of Si(OH)4 in aqueous solution and its dependence on temperature and dielectric constant

Energetics for the condensation dimerization reaction of monosilicic acid: 2Si(OH)(4) double right arrow Si2O7H6 + H2O have been calculated quantum mechanically, in gas-phase and aqueous solution, over a range of temperatures and dielectric constants. The calculated gas phase energy, E-g, for this reaction is -6.6 kcal/mol at the very accurate composite G2 level, but the vibrational, rotational and translational contributions to the free energy in the gas-phase, DeltaG(VRT), sum to + 2.5 kcal/mol and the hydration free energy contribution calculated with a polarizable continuum model, DeltaDeltaG(COSMO), for a dielectric constant of 78.5, is about + 6.2 kcal/mol. Thus, the free energy change for the reaction in aqueous solution at ambient conditions is about + 2.1 kcal/mol and the equilibrium constant is similar to10(-1.5), in reasonable agreement with experiment. As T increases, DeltaG(VRT) increases slowly. As the dielectric constant decreases (for example, under high T and P conditions in the supercritical region), DeltaDeltaG(COSMO) decreases substantially. Thus, at elevated T and P, if the effective dielectric constant of the aqueous fluid is 10 or less, the reaction becomes much more favorable, consistent with recent experimental observations. The PDeltaV contribution to the enthalpy is also considered, but cannot be accurately determined. We have also calculated Si-29-NMR shieldings and Raman frequencies for Si(OH)(4), Si2O7H6 and some other oligomeric silicates. We correctly reproduce the separation of monomer and dimer peaks observed in the Si-29-NMR spectrra at ambient T and P. The Raman spectral data are somewhat ambiguous, and the new peaks seen at high T and P could arise either from the dimer or from a 3-ring trimer, which is calculated to be highly stabilized entropically at high T. Copyright (C) 2005 Elsevier Ltd.