The millimeter wave spectrum of aGg′ ethylene glycol: The quest for higher precision

The rotation-tunneling spectrum of the aGg′ lowest energy conformer of ethylene glycol (1,2-ethanediol) has been studied between 54 and 370 GHz employing conventional absorption spectroscopy. Almost 450 different transitions between and within the v = 0 and v = 1 tunneling substates have been recorded with an accuracy of mostly 10 to 20 kHz and with J and Ka quantum numbers up to 51 and 25, respectively. The current investigation was guided by fits and predictions based on a reduced axis system (RAS) Hamiltonian. Despite considerable Coriolis interaction between the two tunneling states, the new lines together with more than 100 transitions from a previous study could be fit within experimental uncertainties using a Hamiltonian with only 34 spectroscopic parameters. These are the rotational, quartic, and sextic centrifugal distortion constants, the energy difference with quadratic and quartic distortion corrections along with eleven terms describing the Coriolis interaction between the two states.