Investigation of thermodynamic properties of gaseous SiC(X and a ) with accurate model chemistry calculations

Density functional theory, high-level model chemistry at G3(MP2), CBS-Q, G3//B3LYP, G3(QCI) and QCISD(T)/aug-cc-pv5z levels of theory combined with statistical thermodynamics have been employed to explore the thermodynamic properties of gaseous SiC(X ) and SiC(a ). The heat capacities and entropies are obtained via statistical thermodynamics with the structure, vibrational frequency and the electronic excitations calculated at B3PW91/6-31G(d) and B3PW91/6-311G(d) levels. The importance of the anharmonic corrections and the electronic excitations is examined. The reliability of the electronic excitation energies calculated with the time dependent density functional method is tested. The heat capacities and entropies of the ground state calculated in this work are consistent with other theoretical work but different from those in the JANAF Tables. The enthalpies of formation and the Gibbs free energies of formation are in excellent agreement with the calculated results from one experiment but are higher than another observation