Torsion-rotation energy levels and the hindering potential barrier for theexcited vibrational state of the OH-stretch fundamental band nu(1) of methanol

The absorption spectrum of methanol vapor has been recorded at room tempera ture from 3200 to 4200 cm(-1) with a 1.24-M path-difference Fourier transfo rm spectrometer. Over 10 000 transitions to the torsion-rotation states n, tau J, K of the OH stretching fundamental band nu(1) have been assigned, th ereby greatly expanding the results of previous investigations. Approximate ly 4000 of the cleaner lines have been used to determine 665 excited state torsion-rotation levels with principal torsional quantum number n = 0 as we ll 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. M ost 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 poten tial 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 t 0 trans itions observed for K less than or equal to 3 in very low temperature spect ra. The criteria used here to select states to be fitted and the difficulti es in estimating the J = 0 origins because of the many perturbing level cro ssings are discussed. (C) 1998 Academic Press.