Study of the Vibrational Spectra and Absorption Cross Sections of 1-Chloro-1-fluoroethene by a Joint Experimental and Ab Initio Approach

The gas-phase infrared spectra of 1-chloro-1-fluoroethene (geminal chloro-fluoroethene, ClFC=CH2, 1,1-C2H2ClF) were recorded at medium resolution in the range of 400-6400 cm(-1), and the vibrational analysis led to revised assignments for the nu(11) (A '' symmetry), nu(2) (A' symmetry), and nu(1) (A' symmetry) bands. Besides the fundamentals, all the most important spectral features were interpreted in terms of overtone and combination bands, thus obtaining an accurate description of the vibrational structure of ClFC=CH2. Accurate measurements of absorption cross-sectional spectra were carried out, and integrated band intensity data were determined. High-level ab initio calculations of harmonic and anharmonic force fields thoroughly supported and guided the analysis and the disentangling of the several strongly coupled polyads involving many vibrational levels. Diagonalization of the effective Hamiltonian with the off-diagonal elements involving several Fermi and Darling-Dennison resonance coefficients computed by the theoretical cubic and quartic force constants provided the predicted energy levels in good agreement with the vibrational assignments. The calculated infrared intensities, obtained by taking into account anharmonic corrections, were compared to the accurate experimental absorption cross-sectional data determined here.