### Abstract

Language | English |
---|---|

Pages | 4435-4443 |

Number of pages | 8 |

Journal | Geochimica et Cosmochimica Acta |

Volume | 65 |

Issue number | 24 |

DOIs | |

Publication status | Published - 15 Dec 2001 |

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### Keywords

- thermochemistry
- silicic acid
- gas
- DFT calculations
- quantum chemical calculations
- thermodynamic cycles
- geochemical quantities

### Cite this

*Geochimica et Cosmochimica Acta*,

*65*(24), 4435-4443. https://doi.org/10.1016/S0016-7037(01)00739-6

}

*Geochimica et Cosmochimica Acta*, vol. 65, no. 24, pp. 4435-4443. https://doi.org/10.1016/S0016-7037(01)00739-6

**Thermochemistry of silicic acid deprotonation: Comparison of gas phase and solvated DFT calculations to experiment.** / Sefcik, J.; Goddard, W.A.

Research output: Contribution to journal › Article

TY - JOUR

T1 - Thermochemistry of silicic acid deprotonation: Comparison of gas phase and solvated DFT calculations to experiment

AU - Sefcik, J.

AU - Goddard, W.A.

PY - 2001/12/15

Y1 - 2001/12/15

N2 - Theoretical approaches to the thermochemistry of silicate anions have so far focused on gas-phase 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 five-coordinated anion H5SiO5−. However, experimental evidence from in situ nuclear magnetic resonance (NMR) experiments clearly shows that deprotonated silicic acid in alkaline aqueous solutions is four-coordinated, H3SiO4−. We compare gas-phase 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 four-coordinated one (by 18 and 5 kcal/mol in the self-consistent 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.

AB - Theoretical approaches to the thermochemistry of silicate anions have so far focused on gas-phase 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 five-coordinated anion H5SiO5−. However, experimental evidence from in situ nuclear magnetic resonance (NMR) experiments clearly shows that deprotonated silicic acid in alkaline aqueous solutions is four-coordinated, H3SiO4−. We compare gas-phase 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 four-coordinated one (by 18 and 5 kcal/mol in the self-consistent 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.

KW - thermochemistry

KW - silicic acid

KW - gas

KW - DFT calculations

KW - quantum chemical calculations

KW - thermodynamic cycles

KW - geochemical quantities

UR - http://dx.doi.org/10.1016/S0016-7037(01)00739-6

U2 - 10.1016/S0016-7037(01)00739-6

DO - 10.1016/S0016-7037(01)00739-6

M3 - Article

VL - 65

SP - 4435

EP - 4443

JO - Geochimica et Cosmochimica Acta

T2 - Geochimica et Cosmochimica Acta

JF - Geochimica et Cosmochimica Acta

SN - 0016-7037

IS - 24

ER -