Patterns in synchrotron near-free-rotor FIR spectra of CH3OH and CD3OH – The tau of methanol

Analysis of high-resolution synchrotron far-infrared spectra of CH3OH and CD3OH has been extended up to high vt = 3 to 5 torsional levels in the near-free rotor region. In this region, the free-rotor angular momentum quantum number m becomes the natural label for the torsional levels, and sub-bands with Δm = 0 have torsional overlap matrix elements close to unity giving rise to surprisingly strong features in the spectrum. A simple graphical picture of free rotor parabolas gives a convenient spectral predictor for the locations of the strong sub-bands, and shows that they display systematic patterns when classified by Dennison’s torsional index τ instead of the customary A and E symmetries. The initial lines of observed near-free-rotor sub-bands with the same τ connections fall in tight τ-groups encompassing a wide range in K with a constant separation between groups of different Δτ, confirming a pattern predicted from the earliest literature on internal rotation in methanol. Together with previously known data, results for newly assigned sub-bands illustrate the τ-grouping and extend the information database of ground-state methanol term values.