Microwave and high-resolution infrared studies of CHBrF2: the ground and v4=1 states

A detailed spectroscopic investigation on bromodifluoromethane (CHBrF2, Halon 1201) is of considerable interest in view of its suggested variety of applications. Due to its shorter atmospheric lifetime and good fire-suppression properties, it has been proposed as a suitable interim replacement to fully halogenated halons. Besides, it has been highlighted as one of the most efficient working molecules in the context of practical 13C enrichment by infrared multiphoton dissociation (IRMPD). In this work we report the results of the combined microwave (MW) and infrared (FTIR) studies of CHBrF2. The ground-state rotational spectra of both bromine isotopologues have been investigated in the millimeter-/submillimeter-wave frequency region (70-400 GHz, with an accuracy of 15 kHz). Accurate spectroscopic constants (up to sextic cetrifugal-distortion constants) and the full quadrupole-coupling tensor have been determined. Furthermore, the Lamb-dip technique has been exploited to resolve the hyperfine structure of the spectra (with an accuracy of 2 kHz on the retrieved frequencies), thus allowing the experimental determination of the spin-rotation constants. The spectra assignment and the fitting procedures have been supported by high-level quantum-chemical calculations of the parameters involved. Concerning the bromine quadrupole-coupling tensor, relativistic corrections turned out to be essential for reliable predictions and those are available with sufficient accuracy from a second-order direct perturbation theory (DPT2) treatment. The high-resolution infrared spectrum (0.0015 cm-1) of the isotopically enriched CH79BrF2 sample (purity > 95%) has been recorded in the range 695-735 cm-1 by means of FTIR spectroscopy employing the highly brilliant synchrotron radiation source provided by the MAX-I electron storage ring at MAX-Lab, Lund University (Sweden). The spectral region investigated is characterized by the absorption of the v4 fundamental, approximately corresponding to the HCBr deformation mode, which gives rise to an A/C-hybrid band. Despite the presence of a single bromine isotopologue, the IR spectrum is very dense due to the small values of the rotational constants and the hot band features. More than 4200 transitions with J<99 and Ka<23 have been assigned to provide accurate rovibrational constants of the v4 = 1 state using Watson's A-reduced Hamiltonian in the Ir-representation. The obtained parameters are very close to those of the ground state thus indicating that v4 is most likely unaffected by perturbations.