『Abstract
Energetics for the condensation dimerization reaction of monosilicic
acid:
2Si(OH)4 ⇒ 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, Eg, 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, ΔGVRT,
sum to +2.5 kcal/mol and the hydration free energy contribution
calculated with a polarizable continuum model, ΔΔGCOSMO,
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 〜10-1.5, in reasonable agreement with experiment.
As T increases, ΔGVRT increases slowly. As
the dielectric constant decreases (for example, under high T and
P conditions in the super critical region), ΔΔGCOSMO
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 PΔV contribution to the enthalpy is also considered,
but cannot be accurately determined.
We have also calculated 29Si-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 29Si-NMR spectra 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.』
1. Introduction
2. Computational methods
3. Results and discussion
3.1. Energetics
3.2. Spectra
4. Conclusions
Acknowledgments
References