Anharmonic Vibrational Properties of Chlorocarbonyl Ketene Conformers Using Second-order Perturbation Theory

The present paper reports main results of a comprehensive study of the structure and vibrational spectra of the stable conformers of chlorocarbonyl ketene computed using second-order perturbation theory treatment based on quadratic, cubic and semidiagonal quartic force constants. It is found that the s-trans conformer is more stable than the s-cis conformer by ∼0.69 kcal/mol. The vibrationally averaged bond lengths and bond angles, both for the cis and trans conformers, are within 0.005 إ and 0.14°, respectively, of the equilibrium geometry. The ab initio and DFT based anharmonic vibrational analysis using second-order perturbation theory provides reliable frequencies (r.m.s. deviation ±20 cm–1) and assignments to the vibrational bands. DFT calculations using the same method and basis function for the harmonic frequencies and anharmonic corrections give frequencies in better agreement with the experimental values than those in which the harmonic frequencies from a high level quantum mechanical method (B97-1/aug-cc-pVTZ) are coupled with anharmonic corrections from a cheap model (B3LYP/6-31+G**); the r.m.s. deviation in the latter case is ±47 cm–1. The errors in the calculation of the fundamental modes are reflected in the overtones and combination bands. Some spectroscopic constants namely, the anharmonic constants, rotational constants and rotation-vibration coupling constants of the two conformers have been calculated by density functional theory and compared with literature, where available.