The Coriolis-interacting ν₆ and ν₄ bands of ethylene-cis-1,2-d₂ (cis-C₂H₂D₂) by high-resolution synchrotron Fourier transform infrared (FTIR) spectroscopy

The infrared spectrum of the ν₆ band of ethylene-cis-1,2-d₂ (cis-C₂H₂D₂) was recorded at the Australian Synchrotron in the 980–1100 cm⁻¹ region at an unapodized resolution of 0.00096 cm⁻¹. Some of the transitions of the ν₆ band centered at 1039.768335(30) cm⁻¹ were perturbed by the upper energy levels of the infrared inactive ν₄ band at 980.364(24) cm⁻¹ by an a-type Coriolis interaction. Rovibrational analysis of a total of 941 unperturbed and perturbed infrared transitions of the ν₆ 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 2 sextic constants for the ν₆ = 1 state and 3 rotational constants (A, B, and C) for the ν₄ = 1 state with a rms deviation of 0.00028 cm⁻¹. From the perturbed analysis, the a-type Coriolis resonance parameter Z_6,4^a for the ν₆ and ν₄ interacting bands was determined to be 0.5249(14) cm⁻¹. The band center and the rotational constants of the ν₆ = 1 state were found to agree within 1% to the calculated values using B3LYP/cc-pVTZ and MP2/cc-pVTZ levels of theory. Furthermore, the a-type Coriolis coupling constant of these two bands derived from this work were compared to those experimentally determined previously and presently calculated.