Deciphering the Complexity in the Rotational Spectrum of Deuterated Ethylene Glycol.

Ethylene glycol (CH₂OH-CH₂OH) is an abundant "complex organic molecule" (COM) detected in different astronomical objects, but the steps of its interstellar synthesis are not yet fully understood. In this respect, the observation of deuterated isotopologues could offer insights into its formation mechanism as well as into its chemical evolution in space. Such observations, however, require detailed spectroscopic knowledge of their rotational features. Here, we present an extensive analysis of the rotational spectrum of oxygen-deuterated ethylene glycol, including the singly and doubly deuterated forms. The new measurements, carried out between 75 and 450 GHz, significantly expand the spectroscopic knowledge of the aGg' conformers of the CH₂OH-CH₂OD, CH₂OD-CH₂OH, and CH₂OD-CH₂OD species. We also report, for the first time, the laboratory identification of the gGg' conformers of the two mono-deuterated species. Our results reveal previously unobserved perturbations arising from the interaction between CH₂OH-CH₂OD and CH₂OD-CH₂OH, which has been modeled by including Coriolis coupling and Fermi constants in the Hamiltonian and allowed the accurate determination of the energy difference among them. Additionally, we observed significant anomalies in the spectrum of the doubly deuterated species, which seem to be caused by accidental degeneracies between the levels of the two tunneling substates. Despite the complexity and difficulties, the improved spectroscopic parameters derived from our analyses provide a solid base for future interstellar searches of deuterated ethylene glycol, enhancing our understanding of the evolution of COMs in the interstellar medium.