The rotational spectrum of methanol in the third excited torsional stateThis article is part of a Special Issue on Spectroscopy at the University of New Brunswick in honour of Colan Linton and Ron Lees.We dedicate this paper to Dr. R.M. Lees, one of the early pioneers of methanol spectroscopy and internal rotation theory. Indeed, inspired by his 1968 methanol paper (Lees and Baker. J. Chem. Phys. 48, 5299 (1968)), numerous methanol studies have been matured and brought to the next level of ever challenging internal rotational problems. Among them, in the last 40 years, Ron himself has made large contributions to the understanding and advancement of the complicated methanol spectrum from pure rotation to vibration-torsion-rotation interactions. His energy maps constructed from observed transitions are continuously evolving and represent one of the most comprehensive and valuable high resolution methanol databases for a variety of applications.

The rotational spectrum of methanol in its third excited torsional state, vt = 3, as well as the K = 0 A-state of vt = 4, has been recorded and analyzed in the 8–1654 GHz frequency range. The vt = 3 state lies well above the top of the torsional barrier and the majority of the energy origins for the various K sub-bands lie below the C-O stretching fundamental vibration. Only the a-type R-branch spectrum was observed in this frequency range, suggesting that the near torsional degeneracy required for a b-type spectrum in the microwave accidentally reduces the torsional overlap, making these transitions very weak. The A-state K = 4 of vt = 3 interaction with the C-O stretch torsional ground state K = 0+ and K = 1+ has been accounted for. The E-state K = –5 level could be fit directly with a power series in spite of its resonance with the C-O stretch torsional ground state K = –5 level. The aR-branch of the K = 0 A-state vt = 4 sub-band is also provided and the origin of this band resonance with the C-O stretch is discussed. The data have been modeled with a power series to near experimental accuracy. The measured and calculated spectrum should facilitate radio astronomical assignments of vt = 3 transitions in hot sources.