The ro-vibrational analysis of the <i>v</i><sub>4</sub> fundamental band of CF<sub>3</sub>Br from jet-cooled diode laser and FTIR spectra in the 8.3-μm region

<div><p>The <i>v</i><sub>4</sub> fundamental band of CF<sub>3</sub><sup>79</sup>Br and CF<sub>3</sub><sup>81</sup>Br, present in natural isotopic abundance, was investigated in the 8.3-μm region by high-resolution infrared spectroscopic techniques. Tuneable diode laser spectra were recorded in the ranges 1202.5–1205.0 cm<sup>−1</sup>, 1208.0–1210.1 cm<sup>−1</sup> and 1212.5–1214.5 cm<sup>−1</sup>. The tuneable diode laser spectra were obtained at the reduced temperature of 200 K and in a free-jet expansion. The latter technique was used to reduce spectral congestion, achieving a rotational temperature of about 50 K, with a resolution up to 0.0008 cm<sup>−1</sup>. A Fourier transform infrared spectrum covering the entire spectral region of the <i>v</i><sub>4</sub> band, between 1190 and 1220 cm<sup>−1</sup>, was recorded at 298 K with a resolution of 0.004 cm<sup>−1</sup>. The experimental wavenumbers from the different spectroscopic techniques were combined to accomplish the complete ro-vibrational analysis of <i>v</i><sub>4</sub>. In total, 4651 transitions were assigned to CF<sub>3</sub><sup>79</sup>Br, 4047 to CF<sub>3</sub><sup>81</sup>Br, with <i>J</i>″<sub>max </sub> = <i>K</i>″<sub>max </sub>=80; of these, 3171 for CF<sub>3</sub><sup>79</sup>Br and 2755 for CF<sub>3</sub><sup>81</sup>Br are from diode laser measurements. The data of each isotopologue were analysed using the model Hamiltonian for a degenerate vibrational state of a molecule of <i>C</i><sub>3</sub><sub><i>v</i></sub> symmetry. The <i>v</i><sub>4</sub> band of both the isotopologues resulted essentially unperturbed, but the Δ<i>l</i> = Δ<i>k</i> = ±2 <i>l</i>-resonance was found to be active within the <i>v</i><sub>4</sub> = 1 state. Precise values of the vibrational energy and of the ro-vibrational parameters of <i>v</i><sub>4</sub> = 1 for CF<sub>3</sub><sup>79</sup>Br and CF<sub>3</sub><sup>81</sup>Br were obtained. The bromine isotopic splitting amounts to 6.9 × 10<sup>−3</sup> cm<sup>−1</sup>. In addition, the equilibrium geometry and the harmonic force field were calculated <i>ab initio</i> using the large-size basis set def2-QZVP in conjunction to the PBE0 functional.</p></div>