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 B3LYP 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.

中文翻译：

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基于实验和量子化学数据相结合的Si（OH）4（g）的热力学性质

硅基陶瓷在燃烧环境中的挥发性主要由气态Si（OH）₄的形成来控制。利用B3LYP密度泛函理论对6-311 + G（d，p）基组进行了近似计算，研究了该物种在298.15-2100 K和P  =  0.1 MPa时的热容和熵。非谐振荡器，并针对受阻转子进行了校正。基于实验的424–1661 K的Si（OH）₄（g）的吉布斯能量已用于评估298.15 K和P 下Si（OH）₄（g）的形成吉布斯能量以及熵。= 0.1兆帕。我们发现，对于谐波和非谐振荡器近似，QC和“实验”值非常接近，而对于“受阻转子”近似，则不是。Si（OH）₄分子的OH旋转能的计算也支持该结论，其中发现势垒超过12 kJ / mol。最后，我们建议在298-2100 K的温度范围内，理想气体状态下P  = 0.1 MPa的Si（OH）₄的热力学性质。