Global Fit of Rotational and Torsional-Rotational Transitions in the Ground and First Excited Torsional States of Methanol

Abstract

Methanol data for $\nu_{t} = 0$ (below the barrier) and $\nu_{t} = 1$ (straddling the barrier) have been treated, using a program based on the formalism of Herbst et.al. This program has been rather successful in fitting torsion-rotation levels of acetaldehyde below or at the barrier to internal rotation, and we wished to test it also for methanol. A careful test of this one-large-amplitude-motion formalism for methanol is of some interest, since the OH bending motion, if its amplitude is large enough, could have a very large effect on the internal rotation barrier. Indeed, this barrier, and even the internal rotation degree of freedom itself, must disappear at the $C_{3}\nu$ configuration. If such a bend-torsion interaction is important, accurate energy level calculations for methanol will be possible only if a two-dimensional large-amplitude-motion formalism is used. So far, no obvious evidence has been seen for the necessity of the two-dimensional treatment for methanol since we have achieved very satisfactory global fits for the $\nu_{t} = 0$ and 1 torsional states, as shown below $(J_{\max} = 20): \nu_{t} = 0$ (J.M.S.169. 396-409. 1995): 732 MW (4 kHz or 50 kHz), 96 TuFIR (50 kHz) and 1320 FTFIR $(0.0002 cm^{-1})$ lines are fit to RMS residuals 6.3 kHz, 50.1 kHz and $0.00021 cm^{-1}$, respectively, with 43 adjusted and 3 fixed parameters. $v_{t} = 0$ and 1 (preliminary results): some 1020 MW (50 kHz or 100 kHz), 240 TuFIR (100 kHz) and 6000 FTFIR $(0.0002 cm^{-1})$ lines are fit to approximately 100 kHz for MW, 200 for TuFIR lines and $0.0002 cm^{-1}$ for FTFIR data using 54 parameters.