Analysis of torsion-rotational transitions in the first three torsional states of CH3OD Original Research Article

In the framework of systematic analysis and fitting of transition frequencies of methanol isotopomers with a reduced torsion-rotational Hamiltonian that is obtained from the one-large-amplitude internal rotation model, 1126 Fourier transform far-infrared (FIR) transitions involving the second excited torsional level (vt=2) have been added to the data set of Duan and McCoy [J. Mol. Spectros. 199 (2000) 302] to achieve a global fit of the observed high resolution microwave (MW), millimeter wave (MMW) and Fourier transform FIR spectra for CH3OD. The CH3OD data set contains 460 MW and MMW transitions and 3474 Fourier transform FIR transitions with vt⩽2 and J⩽21. The MW and MMW transitions have been fit with a root-mean-square (rms) deviation of 0.12 MHz, whereas FIR transitions have a rms deviation of 0.00026 cm−1. These deviations are approximately equal to the experimental uncertainties, indicating that the MW, MMW and FIR spectral transitions have been fit to an accuracy approaching the experimental uncertainties and the reduced torsion-rotational Hamiltonian model is capable of accurately describing CH3OD energy levels up through the second excited torsional level. The success of the fit demonstrates that the increased general asymmetry in CH3OD can be taken care adequately by the reduced Hamiltonian model.