Fourier transform spectra and inverted torsional structure for a CH3-bending fundamental of CH3OH

The high-resolution Fourier transform spectrum of CH3OH has been investigat ed in the 1400-1650 cm(-1) CH3-bending region, and perpendicular DeltaK = 1 subbands forming a consistent pattern have been identified with origins fr om 1490 to 1570 cm(-1). The location of the subbands as the only significan t spectral features towards the upper edge of the CH3-bending absorption fa vours their assignment to the v(4) in-plane A' asymmetric CH3-bending mode. The upper state term values have been fitted to J(J + 1) power-series expa nsions to obtain substate origins and effective B-values. The origins exhib it a linear K-dependence as well as the normal variation with K-2. The mean effective B-value of 0.82 cm(-1) is higher than that of the ground state, consistent with a bending vibration. The pattern of K-reduced torsion-vibra tion energies is anomalous. It appears to be inverted relative to the custo mary picture for n = 0 torsional levels, in agreement with a recent predict ion, but has unusual periodicity significantly different from the ground st ate. A simple Fourier cosine series model for the energy curves gives a vib rational band origin of 1477.6 cm(-1) for this CH3-bending mode, close to t he best current calculated value for v(4).