The ro-vibrational analysis of the v4 fundamental band of CF3Br from jet-cooled diode laser and FTIR spectra in the 8.3-μm region

The v4 fundamental band of CF379Br and CF381Br, 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−1, 1208.0–1210.1 cm−1 and 1212.5–1214.5 cm−1. 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−1. A Fourier transform infrared spectrum covering the entire spectral region of the v4 band, between 1190 and 1220 cm−1, was recorded at 298 K with a resolution of 0.004 cm−1. The experimental wavenumbers from the different spectroscopic techniques were combined to accomplish the complete ro-vibrational analysis of v4. In total, 4651 transitions were assigned to CF379Br, 4047 to CF381Br, with Jmax  = Kmax =80; of these, 3171 for CF379Br and 2755 for CF381Br 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 C3v symmetry. The v4 band of both the isotopologues resulted essentially unperturbed, but the Δl = Δk = ±2 l-resonance was found to be active within the v4 = 1 state. Precise values of the vibrational energy and of the ro-vibrational parameters of v4 = 1 for CF379Br and CF381Br were obtained. The bromine isotopic splitting amounts to 6.9 × 10−3 cm−1. In addition, the equilibrium geometry and the harmonic force field were calculated ab initio using the large-size basis set def2-QZVP in conjunction to the PBE0 functional.