Thermochemistry of Si n O x (OH) y Vapor Species from Quantum Chemical Calculations.

The thermochemistry of the Si-O-H system has been extensively studied both experimentally and theoretically due to its importance in chemical processes, degradation of silica-protected materials in combustion, and geological processes. In this paper, we review past studies and use quantum mechanical methods to generate a new data set. Molecular geometries were generated with DFT using the B3LYP functional. Energetics were calculated with RCCSD(T) methods extrapolated to the complete basis set (CBS/45) limit. Particular attention was given to the treatment of the vibrational modes. A rigid rotor model was used, corrections for anharmonicity were applied, and the Pitzer-Gwinn treatment of the hindered rotation of the M-OH groups was applied. The generated enthalpies of formation at 298 K are compared to those of experiments and other calculations. Generally, the agreement is good. A set of thermodynamic data (enthalpy of formation at 298 K, entropy at 298 K, and heat capacity polynomial to 3000 K) is presented for SiOH, SiO(OH), Si(OH) ₂ , SiO(OH) ₂ , Si(OH) ₃ , Si(OH) ₄ , Si ₂ O(OH) ₆ , and Si ₃ O ₂ (OH) ₈ . These can be added to any of the common computational thermodynamics packages. The application of these data to high-temperature corrosion and geological problems is discussed.