The ro-vibrational analysis of the v_4 fundamental band of CF_3Br from jet-cooled diode laser and FTIR spectra in the 8.3-μm region

摘要

The v_4 fundamental band of CF_3 ~(79)Br and CF_3 ~(81)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~(-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 transforminfrared spectrumcovering the entire spectral region of the v_4 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 v_4. In total, 4651 transitions were assigned to CF_3 ~(79)Br, 4047 to CF_3 ~(81)Br, with J "_(max) = K"_(max)=80; of these, 3171 for CF_3 ~(79)Br and 2755 for CF_3 ~(81)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 C3v symmetry. The v_4 band of both the isotopologues resulted essentially unperturbed, but the △l = △k = ±2 l-resonance was found to be active within the v_4 = 1 state. Precise values of the vibrational energy and of the ro-vibrational parameters of v_4 = 1 for CF_3 ~(79)Br and CF_3 ~(81)Br 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.