Improved rovibrational constants for the v₇ = 1 state of ethylene-cis-1,2-d₂ (cis-C₂H₂D₂) by high-resolution synchrotron FTIR spectroscopy

Using the far-infrared beamline of the Australian Synchrotron, the spectrum of the ν₇ band of ethylene-cis-1,2-d₂ (cis-C₂H₂D₂) was recorded in the 640–990 cm⁻¹ region at an unapodized resolution of 0.00096 cm⁻¹. A rovibrational analysis of a total of 2823 infrared transitions of the ν₇ band was carried out using an asymmetric rotor fitting program based on the Watson's A-reduced Hamiltonian in the I^r representation to derive up to four sextic constants with a rms deviation of 0.00035 cm⁻¹. From the fitting of 2634 ground state combination differences (GSCDs) of cis-C₂H₂D₂ which were derived from the infrared transitions of the ν₇ band of this work, and ν₁₀ and ν₁₂ bands of previous studies, together with 22 microwave frequencies, accurate ground state constants of cis-C₂H₂D₂ up to four sextic terms were obtained. The rotational constants (A, B, and C) of the v₇ = 1 state of cis-C₂H₂D₂ were found to agree within 0.5% with the calculated values using B3LYP/cc-pVTZ and MP2/cc-pVTZ levels of theory. From this work, the band center of ν₇ at 842.209489(20) cm⁻¹ and the rovibrational constants of the v₇ = 1 state of cis-C₂H₂D₂ were determined with better accuracy than previously reported.