Thermochemistry of silicic acid deprotonation: Comparison of gas phase and solvated DFT calculations to experiment
Sefcik, J. and Goddard, W.A. (2001) Thermochemistry of silicic acid deprotonation: Comparison of gas phase and solvated DFT calculations to experiment. Geochimica et Cosmochimica Acta, 65 (24). pp. 44354443. ISSN 00167037 (http://dx.doi.org/10.1016/S00167037(01)007396)
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Theoretical approaches to the thermochemistry of silicate anions have so far focused on gasphase molecular orbital and density functional theory (DFT) calculations. These calculations predict that in the presence of hydroxide ligands the most stable singly charged anion of the silicic acid H4SiO4 is the fivecoordinated anion H5SiO5−. However, experimental evidence from in situ nuclear magnetic resonance (NMR) experiments clearly shows that deprotonated silicic acid in alkaline aqueous solutions is fourcoordinated, H3SiO4−. We compare gasphase and solvated DFT calculations of monomeric anions of silicic acid in order to assess solvent effects on the thermochemistry of silicic acid deprotonation. We show that appropriate inclusion of solvation in quantum chemical calculations is critical for correct prediction of coordination and thermochemistry of silicate anions in aqueous solutions. Multiply charged anions of silicic acid are found to be electronically unstable in the gas phase and thus it is not possible to use thermodynamic cycles involving these species in thermodynamic calculations. However, a high dielectric constant solvent is sufficient to stabilize these anions, and solvated calculations can be used to directly compute their thermodynamic quantities. When we include the zero point energy (ZPE) and statistical mechanics contributions to the Gibbs free energy, we obtain accurate free energies for successive deprotonations of silicic acid in aqueous solutions. Although the pentacoordinate hydroxoanion of silicon is more stable in the gas phase than the fourcoordinated one (by 18 and 5 kcal/mol in the selfconsistent field (SCF) energy and the Gibbs free energy, respectively), it is less stable by 5 kcal/mol in the Gibbs free energy when hydration effects are appropriately accounted for. Solvated DFT calculations, validated here by their accurate description of silicate anions in aqueous solutions, should lead to more reliable predictions of important geochemical quantities, such as surface acidities and detailed reaction coordinates for dissolution of minerals.


Item type: Article ID code: 3777 Dates: DateEvent15 December 2001PublishedSubjects: Technology > Chemical technology Department: Faculty of Engineering > Chemical and Process Engineering Depositing user: Strathprints Administrator Date deposited: 04 Jul 2007 Last modified: 08 Apr 2024 15:21 URI: https://strathprints.strath.ac.uk/id/eprint/3777