Torsion-Rotation Energy Levels and the Hindering Potential Barrier for the Excited Vibrational State of the OH-Stretch Fundamental Band nu1 of Methanol.

The absorption spectrum of methanol vapor has been recorded at room temperature from 3200 to 4200 cm-1 with a 1.24-M path-difference Fourier transform spectrometer. Over 10 000 transitions to the torsion-rotation states n, tau J, K of the OH stretching fundamental band nu1 have been assigned, thereby greatly expanding the results of previous investigations. Approximately 4000 of the cleaner lines have been used to determine 665 excited state torsion-rotation levels with principal torsional quantum number n = 0 as well as 494 levels with n = 1, 286 levels with n = 2, and 14 levels with n = 3. These levels correspond to 86 values of the quantum numbers n, tau, K. Most of the levels of a given n, tau, K are perturbed resulting in irregular P-, Q-, and R-branch series. The J = 0 origins for 64 values of n, tau, K have been fitted and a value of 410 cm-1 obtained for the hindering potential barrier. This is in agreement with a previous study of grating spectra but is lower than that obtained in more recent analyses of n = 0 <-- 0 transitions observed for K </= 3 in very low temperature spectra. The criteria used here to select states to be fitted and the difficulties in estimating the J = 0 origins because of the many perturbing level crossings are discussed. Copyright 1998 Academic Press.