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

The high-resolution Fourier transform spectrum of CH3OH has been investigated in the 1400-1650 cm-1 CH3-bending region, and perpendicular D K = 1 subbands forming a consistent pattern have been identified with origins from 1490 to 1570 cm-1. The location of the subbands as the only significant spectral features towards the upper edge of the CH3-bending absorption favours their assignment to the n4 in-plane Aʹ asymmetric CH3-bending mode. The upper state term values have been fitted to J(J + 1) power-series expansions to obtain substate origins and effective B-values. The origins exhibit a linear K-dependence as well as the normal variation with K2. 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-vibration energies is anomalous. It appears to be inverted relative to the customary picture for n = 0 torsional levels, in agreement with a recent prediction, but has unusual periodicity significantly different from the ground state. A simple Fourier cosine series model for the energy curves gives a vibrational band origin of 1477.6 cm-1 for this CH3-bending mode, close to the best current calculated value for n4.