Ab initio determination of spectroscopic parameters for ethane-like molecules in the ground vibrational state

The quadratic rotational constants A and B and torsional barrier V3, distortion parameters DJ, DK, and DJK, torsional distortion parameters Dm, DJm, and DKm, and barrier-dependence parameters F3J, F3K, and F3m have been determined for CH3CH3, CH3CD3, CD3CD3, and CH3SiH3 from the results of ab initio calculations done at the CCSD(T) level. Calculated values for the first six parameters are consistently within about 1% of experimental values, while the relative errors for Dm, DKm, F3J, and F3K are generally less than 20%. Calculation of the parameters DJm and DsJ is found to be more problematic, even with the application of vibrational averaging in the harmonic oscillator approximation. There is evidence that this is due to the influence of vibrational contact transformations in the experimental values.