Tunable Diode Laser Spectroscopy of cis-1,2-Difluoroethylene: Rovibrational Analysis of the ν9 and ν5+ν10 Bands and Anharmonic Force Field

Abstract

cis-1,2-Difluoroethylene was synthesized, and the gas-phase infrared spectrum was investigated in the ν9 and ν5+ν10 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 B1 yield strong a-type bands. Most of the J and K structure was recorded in different subbranches and the rovibrational analysis led to the assignment of 2313 (J ≤ 64, Ka ≤ 26) and 1172 (J ≤ 57, Ka ≤ 20) transitions of the ν9 and ν5+ν10 bands, respectively. Using Watson's A-reduction Hamiltonian in the Ir representation a set of accurate spectroscopic parameters, up to four sextic centrifugal distortion terms, was obtained for both the excited states v9 = 1 and v5 = v10 = 1. A Fermi resonance coupling constant W = 4.2(5) cm-1 was estimated for the perturbation between the ν9 and ν5+ν10 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 r0 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.