LAUSR

Abstract Using the far-infrared beamline of the Australian Synchrotron, the spectrum of the ν 7 band of ethylene- cis -1,2- d 2 ( cis -C 2 H 2 D 2 ) 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 ν 7 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 -C 2 H 2 D 2 which were derived from the infrared transitions of the ν 7 band of this work, and ν 10 and ν 12 bands of previous studies, together with 22 microwave frequencies, accurate ground state constants of cis -C 2 H 2 D 2 up to four sextic terms were obtained. The rotational constants ( A , B , and C ) of the v 7  = 1 state of cis -C 2 H 2 D 2 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 ν 7 at 842.209489(20) cm −1 and the rovibrational constants of the v 7  = 1 state of cis -C 2 H 2 D 2 were determined with better accuracy than previously reported.