15min:

J. R. COOPER, Department of Physics and Astronomy, University of Calgary, Calgary, Alberta T2N 1N4, Canada; L.-H. XU, Department of Physical Sciences, University of New Brunswick, Saint John, New Brunswick E2L 4L5, Canada; N. MOAZZEN-AHMADI, Department of Physics and Astronomy, University of Calgary, Calgary, Alberta T2N 1N4, Canada.

A new method has been developed for the ab initio determination of several vibration-torsion-rotational spectroscopic parameters for a symmetric top molecule with an internal rotor. In contrast to existing methodologies ^, which employ vibrational contact transformations either numerically or algebraically, the present model treats the molecule as vibrationally static but with a density distribution characteristic of the vibrational wavefunction. The second-order rotational constants A and B and torsional constant F, distortion parameters D_J, D_K, and D_JK, and torsional distortion parameters D_m, D_Jm, and D_Km have been determined for a series of ethane-like molecules from the results of ab initio calculations done at the CCSD(T) level. The technique has been applied to the molecules CH₃CH₃, CH₃CD₃, CD₃CD₃, and CH₃SiH₃ with very promising results. Preliminary results for the potential constants F_3J and F_3K are also in excellent agreement with global fit values.