Symmetry effects in rotationally resolved spectra of bi-deuterated ethylene: Theoretical line intensities of cis, trans, and as-C₂H₂D₂ isotopomers

In this paper, we report accurate first-principles variational rovibrational spectra predictions for the three double deuterated ethylene isotopologs denoted as cis, trans, and as-¹²C₂H₂D₂. Calculations were performed in the framework of the normal-mode approach using our ab initio ¹²C₂H₄ (D_2h) Born-Oppenheimer potential energy and dipole moment surfaces. Symmetry breaking effects under bideuterated H → D substitutions (D_2h → C_2v/C_2h) and their impact on infrared spectra are studied from normal mode transformations. All theoretical spectra simulated at 296 K up to J = 38 are in good agreement, both for line positions and in absorption cross sections, with the experiment. Accurate theoretical line lists providing for the first time intensities of rovibrational transitions are computed for the three species in the range (0–4500) cm⁻¹ and will be available on our TheoReTS information system (http://theorets.univ-reims.fr and http://theorets.tsu.ru). These results can be used for remote sensing retrieval of isotopic species using predicted line intensities and experimentally optimized line positions.