Millimeter-wave and Far-infrared Spectroscopic Characterization of Torsionally Excited Ethylene Glycol

Abstract Ethylene glycol has been detected in various astronomical sources, with rotational excitation temperatures ranging from ∼8 to 300 K. The higher end of this range approaches the energies of this molecule’s lowest torsionally excited states, suggesting significant population of them in warmer interstellar molecular clouds. This study examines these torsionally excited states through high-resolution millimeter-wave (75–110 GHz and 140–220 GHz) and synchrotron-based far-infrared (50–560 cm −1 (1.5–16.8 THz)) spectroscopy. We observed a dense array of well-resolved rovibrational bands in the far-infrared, involving the two lowest energy conformers, labelled G1 and G2. Ten of the bands were assigned to C–C torsional ( ν 24 ) and C–O torsional ( ν 23 and ν 21 ) transitions that involve a change in the tunneling state (A 1 ↔A 2 ), and two were assigned to the CCO bending ( ν 20 ) fundamental (one each for G1 and G2), which do not involve tunneling-state changes. Particular attention is given to the ν 23 fundamental, for which we performed a detailed analysis of its lower tunneling subband, ν gs (G1,A 2 ) → ν 23 (G2,A 1 ), which is conveniently well isolated from other features. A combined fit incorporating 1866 far-infrared lines from this subband in addition to 227 submillimeter-wave lines from torsionally excited ethylene glycol was performed, providing an extensive set of spectroscopic constants for the ν 23 (G2,A 1 ) state. These constants allow for reliable predictions of pure rotational transitions within this excited torsional state to be made, many of which will likely appear in astronomical surveys given the high sensitivity of modern radio telescopes.