FOURIER-TRANSFORM SPECTROSCOPY OF (CH3OH)-O-18 - THE INPLANE CH3-ROCKING BAND

The infrared Fourier transform spectrum of (CH3OH)-O-18 has been recorded in the region from 1020 to 1620 cm(-1) at high resolution in order to study the low-lying bending modes, including the CH3-rocking, OH-bending, and CH3-deformation fundamental bands. More than 10 000 lines have now been assigned in this region. The present paper focuses on the n = 0 ground torsional subbands of the in-plane CH3-rocking fundamental. This band is principally of parallel a-type character, due to mixing of the predominantly parallel CO-stretching and OH-bending coordinates with the perpendicular rocking coordinate. Some b-type character was also observed, with partial assignments of a number of weak perpendicular subbands. The parallel subbands have been fitted to J(J + 1) power-series expansions to obtain the subband origins, as well as compact representations of the data in terms of phenomenological state-specific expansion coefficients. The n = 0 excited state energies calculated from the subband origins follow a similar oscillatory pattern with K to that of the ground vibrational state but with significantly reduced amplitude. Analysis with our basic torsion-rotation Hamiltonian yields an effective torsional barrier height for the rocking state of 474.5 +/- 2.8 cm(-1), a 27% increase over the ground state value. The vibrational energy is found to be 1058.46 +/- 0.62 cm(-1). An interesting J-localized level-crossing resonance between the CH3-rocking and CO-stretching modes has also been observed through perturbations in the spectrum. (C) 1995 Academic Press, Inc.