Experimental and theoretical studies of the vibrational spectra of cis-1-bromo-2-fluoroethene

The gas-phase infrared spectrum of cis-1-bromo-2-fluoroethene has been studied at low resolution in the range 200–6500 cm⁻¹, leading to a complete assignment of the fundamentals, except the lowest vibrational mode ν₉ predicted at 167 cm⁻¹. The remaining vibrational structure has been mainly interpreted in terms of first overtone or two quanta combination bands. Isotopic ^79/81Br shift has been observed only in the ν₈ fundamental. The equilibrium structure and the quadratic force field have been investigated theoretically at CCSD(T) level of theory employing Dunning’s correlation consistent triple-zeta basis set. Cubic and semidiagonal quartic force field have been calculated using second-order Møller–Plesset perturbation theory and Ahlrich’ split valence (SV) contracted basis set. After a minor scaled quantum mechanical (SQM) adjustment of the quadratic force constants, the vibrational analysis, based on the second-order perturbation theory, has been carried out with the calculated force constants.