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

The infrared Fourier transform spectrum of (CH3OH)-O-18 has been recor ded 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-b ending, and CH3-deformation fundamental bands. More than 10 000 lines have now been assigned in this region. The present paper focuses on th e n = 0 ground torsional subbands of the in-plane CH3-rocking fundamen tal. This band is principally of parallel a-type character, due to mix ing of the predominantly parallel CO-stretching and OH-bending coordin ates with the perpendicular rocking coordinate. Some b-type character was also observed, with partial assignments of a number of weak perpen dicular subbands. The parallel subbands have been fitted to J(J + 1) p ower-series expansions to obtain the subband origins, as well as compa ct representations of the data in terms of phenomenological state-spec ific expansion coefficients. The n = 0 excited state energies calculat ed from the subband origins follow a similar oscillatory pattern with K to that of the ground vibrational state but with significantly reduc ed amplitude. Analysis with our basic torsion-rotation Hamiltonian yie lds 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 vib rational energy is found to be 1058.46 +/- 0.62 cm(-1). An interesting J-localized level-crossing resonance between the CH3-rocking and CO-s tretching modes has also been observed through perturbations in the sp ectrum. (C) 1995 Academic Press, Inc.