Anharmonic resonances in the CH chromophore overtone spectra of CHBrF2

The gas-phase Fourier transform infrared spectra of bromodifluoromethane, CHBrF2, have been recorded up to 10,100 cm(-1) with a resolution of 0.5 cm(-1). All the most relevant absorptions in the overtone region have been ascribed to the CH chromophore. Twenty-six bands involving either the CH-stretching (nu(1)) and bending (nu(2), nu(7)) modes have been assigned and their relative band strengths have been determined. Vibrational analysis of the corresponding polyads (up to N = 7/2) has been performed by means of an effective Hamiltonian that takes into account anharmonic resonances, thus leading to the determination of Fermi and Darling-Dennison coupling terms. The obtained effective spectroscopic parameters were able to quantitatively describe both the position and intensity of all the absorption bands, and also to rationalize the effects and the magnitudes of the various resonances in each polyad. The results obtained using this approach have been compared with those computed using high-quality ab initio electronic structure calculations employing second-order vibrational perturbation theory (VPT2). Overall, satisfactory agreement between the observed and the ab initio computed values was obtained by properly taking into account in the final calculations the relative magnitude of the Fermi and Darling-Dennison resonance effects in each polyad.