Thermodynamic properties of Si( <scp>OH</scp> ) ₄ (g) based on combined experimental and quantum chemistry data

Abstract

Abstract The volatility of silicon‐based ceramics in combustion environments is primarily controlled by the formation of gaseous Si( OH ) 4 . The heat capacities and entropies of this species at 298.15‐2100 K and P = 0.1 MPa have been studied with the B3 LYP density functional theory for the 6‐311+G(d,p) basis set in different approximations: a harmonic oscillator, an anharmonic oscillator, and with corrections for hindered rotors. Experimentally based Gibbs energies of Si( OH ) 4 (g) at 424‐1661 K have been employed to evaluate the Gibbs energy of formation, , and the entropy, , of Si( OH ) 4 (g) at = 298.15 K and P = 0.1 MPa. We found that the QC and “experimental” values are very close for the harmonic and anharmonic oscillator approximations, but not for the “hindered rotor” approximation. This conclusion is also supported by calculations of the OH rotational energy for Si( OH ) 4 molecule, where the potential barrier was found to exceed 12 kJ/mol. Finally, we recommend the thermodynamic properties of Si( OH ) 4 in the ideal gas state at P = 0.1 MPa over the temperature range of 298‐2100 K.