Rotation-torsion-vibration term-value mapping for CH3OH: Torsion-mediated doorways and corridors for intermode population transfer

Fourier transform infrared spectra of CH3OH from 930-1650 cm−1 have been analyzed to reveal details of the rotation-torsion-vibration energy manifold of the CO-stretching, CH3-rocking, OH-bending and CH3-deformation modes and their torsional combination states. Mapping of the upper-state term values as a function of the rotational quantum number J has shown the locations of numerous substate crossing resonances that give rise to J-localized spectral perturbations and substate mixing and thereby create “doorways” for collision-induced population transfer among the different modes. Other near-degenerate substates are more globally mixed over a wide range of J, corresponding to “corridors” of doorways. Where both partner substates in a doorway resonance have been identified, the perturbations have been analyzed to find estimates of the interaction matrix elements and the degree of mixing between the coupled states. Many of the resonances are between substates of differing torsional quantum number, highlighting the importance of torsion in generating the doorway channels and enhancing intermode vibrational population transfer.