Determination of reduced Hamiltonian parameters for the CD3OH isotopomers of methanol

Abstract

Abstract Torsion–rotation parameters have been calculated for 12C- and 13C-methanol isotopomers of CD3OH with 16O, 17O, and 18O from potential energy surfaces for methanol using an approach developed by Duan and Takagi [Phys. Lett. A 207 (1995) 203]. With the help of these calculated parameters and on the basis of the theory of correlation problems for a molecule with an internal rotor, an appropriate reduction scheme and the corresponding reduced Hamiltonian for the CD3OH molecule have been obtained. To test the validity of the reduction scheme, a global analysis for CD3OH has been carried out to spectral data containing 491 microwave/mm-wave transitions and 5780 far-infrared lines in the first three torsion states with J⩽29 and K⩽12. A good fit was obtained using a torsion–rotation Hamiltonian which has been reduced by the contact transformation formalism so that it can be characterized by 53 adjustable parameters. The root mean square (rms) deviations were 0.099 and 5.8 MHz for the microwave/mm-wave and far-infrared data blocks, respectively. These rms residuals are of the order of the corresponding experimental uncertainties. The calculated parameters agree well with the fitted parameters. The rapid convergence in the global fits demonstrates that the reduction of the Hamiltonian with the help of calculated parameters provides a powerful tool for analysis of the spectrum data for the torsional–rotational transitions for a slightly asymmetric molecule with a threefold internal rotor.