High-resolution infrared spectroscopy of CH281BrF near 8 μm: rovibrational analysis of the ν3 and ν8 fundamentals and resonances with the dark states 2ν5 and ν6 + ν9

The infrared spectrum of isotopically enriched CH281BrF was investigated in the ν3 and ν8 region between 1150 and 1370 cm−1 at a resolution of 0.003 cm−1. The ν3 vibration of symmetry species A′ gives rise to an a-/b-hybrid band with a-type predominance, while the ν8 mode of A″ symmetry produces c-type absorption. Due to the proximity of the band origins to those of closely lying overtones and combination bands, the v3 = 1 and v8 = 1 levels were found perturbed through Coriolis resonance by the v5 = 2 (A′) and v6 = v9 = 1 (A″) states, respectively. The spectral analysis resulted in the identification of 3132 transitions (J″ ≤ 98 and Ka″ ≤ 14) for the ν3 and 2958 transitions (J″ ≤ 68 and Ka″ ≤ 19) for the ν8 bands. The assigned data were fitted using the Watson's A-reduction Hamiltonian in the Ir representation and the perturbation operators. Although no transitions belonging to the perturbers were observed, the band origins and excited state parameters for fundamentals and ‘dark states’ together with coupling terms for the ν3/2ν5 and ν8/ν6 + ν9 dyads were determined