15min:

YUN-BO DUAN, Department of Electronics and Computer Science, Toyama University,
Toyama 930, Japan; KOJIRO TAKAGI, Department of Physics, Toyama University, Toyama 930, Japan.

Based on the analysis of order of magnitude of the term H_mln in a Hamiltonian, a new scheme of the ordering by magnitude is proposed to analyze the vibration-torsion-rotation interactions for molecules with internal rotations, in which the order of the torsional angular momentum term with P_\gamma² is considered to be just between those of the vibrational angular momentum term with p_k² and the rotational angular momentum term with P² (or P_a²). Therefore, the importance of torsional problem is emphasized. The sequential contact transformation technique is applied to the vibration-torsion-rotation Hamiltonian for a molecule containing a threefold symmetric internal rotor, which makes it possible to analyze clearly various higher-order vibration-torsion-rotation interactions and their effects in the vibration-torsion-rotation spectra. A reduced torsion-rotation Hamiltonian for the molecule with C_3v(M) symmetry is given in the new ordering scheme and is applied to analyze the spectra of CH₃OH and CH₃¹⁸OH. Some new lines are observed for CH₃¹⁸OH.