Microwave, High-Resolution Infrared, and Quantum Chemical Investigations of CHBrF₂: Ground and ν₄ = 1 States

A combined microwave, infrared, and computational investigation of CHBrF2 is reported. For the vibrational ground state, measurements in the millimeter- and sub-millimeter-wave regions for CH79BrF2 and CH81BrF2 provided rotational and centrifugal-distortion constants up to the sextic terms as well as the hyperfine parameters (quadrupole-coupling and spin-rotation interaction constants) of the bromine nucleus. The determination of the latter was made possible by recording of spectra at sub-Doppler resolution, achieved by means of the Lamb-dip technique, and supporting the spectra analysis by high-level quantum chemical calculations at the coupled-cluster level. In this context, the importance of relativistic effects, which are of the order of 6.5% and included in the present work using second-order direct perturbation theory, needs to be emphasized for accurate predictions of the bromine quadrupole-coupling constants. The infrared measurements focused on the ν4 fundamental band of CH79BrF2. Fourier transform investigations using a synchrotron radiation source provided the necessary resolution for the observation and analysis of the rotational structure. The spectroscopic parameters of the v4 ) 1 state were found to be close to those of the vibrational ground state, indicating that the ν4 band is essentially unaffected by perturbations.