Beyond the Harmonic Approximation: Impact of Anharmonic Molecular Vibrations on the Thermochemistry of Silicon Hydrides

Abstract

Thermodynamic state function (enthalpy, entropy, and heat capacity) were calculated for several types of silicon hydrides taking into account the strongly anharmonic character of some of the molecular vibrations (internal rotation, inversion, and pseudorotation). The anharmonic motions were treated as one-dimensional motions taking place along the harmonic normal coordinates, neglecting anharmonic coupling terms. Partition functions were calculated from the idealized numerical eigenvalue spectrum in the case of pseudorotation; for the other types of large amplitude motions, we used quantum-corrected classical partition functions. Following the work of Knyazev and Tsang, we derived a novel partition function for an asymmetric double well potential. We then used the data to calculate enthalpies, entropies and free energies of reaction for several types of chemical reactions among silicon hydrides, at both the harmonic and the anharmonic level. Differences arising from the inclusion of anharmonicity are discussed.