Tunable diode laser spectroscopy of cis-1,2-difluoroethylene:: Rovibrational analysis of the ν9 and ν5+ν10 bands and anharmonic force field

cis-1,2-Difluoroethylene was synthesized, and the gas-phase infrared spectrum was investigated in the upsilon(9) and nu(5)+nu(10) band region (1350-1391 cm(-1)), at a resolution of about 0.002 cm(-1), employing a tunable diode laser spectrometer. These vibrations of symmetry species B-1 yield strong a-type bands. Most of the J and K structure was recorded in different subbranches and the rovibrational analysis led to (t)he assignment of 2313 (J less than or equal to 64, K-a less than or equal to 26) and 1172 (J less than or equal to 57, K-a less than or equal to 20) transitions of the nu(9) and nu(5)+nu(10) bands, respectively. Using Watson's A-reduction Hamiltonian in the I-r representation a set of accurate spectroscopic parameters, up to four sextic centrifugal distortion terms, was obtained for both the excited states nu(9) = 1 and nu(5) = nu(10) = 1. A Fermi resonance coupling constant W = 4.2(5) cm(-1) was estimated for the perturbation between the nu(9) and nu(5)+nu(10) levels. Correlated harmonic force constants were obtained from coupled cluster CCSD(T) calculations with the cc-pVTZ basis set while the anharmonic force constants were computed at MP2 level using the same basis set. There is a good agreement with the available experimental data, in particular the equilibrium geometry compared with the experimental r(0) structure and the fundamental modes. In addition, the ab initio anharmonic force field of cis-1,2-difluoroethylene provided a critical assessment of the experimental spectroscopic parameters, especially in the treatments of strong Fermi interactions.