GLOBAL FIT OF TORSIONAL-ROTATIONAL TRANSITIONS IN THE GROUND AND FIRST EXCITED TORSIONAL STATES OF METHANOL

Methanol data for upsilon(t) = 0 (below the barrier) and upsilon(t) = 1 (straddling the barrier) have been treated, using a program based on the formalism of Herbst el al. (J. Mol. Spectrosc. 108, 42-57, 1984). Altogether, 909 microwave hues (mostly assigned a 50 kHz measurement uncertainty) and 197 tunable far-infrared lines (mostly assigned 100 o r 200 kHz uncertainties) in the range K less than or equal to 12, J le ss than or equal to 20 and 5549 Fourier-transform far-infrared lines ( assigned an uncertainty of 0.0002 cm(-1) = 6 MHz) in the range K less than or equal to 14, J less than or equal to 20 have been globally fit to give a unitless weighted standard deviation of 1.032, using 56 adj ustable and 8 fixed parameters. Some difficulties remain in fitting hi gh K and J b-type tunable far-infrared lines. Although these difficult ies may arise from deficiencies in the model, it also seems possible t hat some measurement problems still remain in the tunable Ear-infrared data set. Apart from the high K and J difficulties with this latter d ata set, a very satisfactory fit has been achieved, leading us to the conclusion that up through the first excited torsional level, a tradit ional one-dimensional internal rotation model is capable of accurately describing methanol energy levels, without consideration of other lar ge-amplitude-motion effects, such as those arising, for example, when the OH bending vibration carries the molecule to a C-3 upsilon configu ration and the internal rotation barrier vanishes. (C) 1995 Academic P ress, Inc.