Rotational and torsional analysis of the OH-stretch third overtone in (CH3OH)-C-13

We record double resonance spectra of the 4nu(1) band of jet-cooled C-13-methanol using single rotational state selection in the nu(1) fundamental and subsequent promotion of the selected molecules to the fourth vibrational level. We then detect transitions to the final excited states by infrared laser assisted photofragment spectroscopy (IRLAPS). The assigned A symmetry transitions reach upper states with K = 0 and 1, and J from 0 to 5. For E symmetry, the transitions reach levels with Kin the range -3 to 2 and J from 1 to 7. The rotation-torsional analysis determines a value for the torsional tunneling splitting of 2.8 +/- 0.4 cm(-1) at nu(1) = 4. In a previous paper (J. Chem. Phys. 110, 11359-11367 (1999)), we reported a trend of monotonically decreasing tunneling splittings in (CH3OH)-C-12 for nu(1) = 0, 3, and 6 that we explained by a model that incorporates a linear increase in the torsional barrier height with OH stretch excitation. The (CH3OH)-C-13 tunneling splitting for the 4nu(1) band is in quantitative agreement with the trend found for (CH3OH)-C-12. (C) 2002 Elsevier Science (USA).