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

**Ved Prakash Gupta* and Abhishek Kumar Mishra **

*Department of Physics, University of Lucknow, Lucknow 226007, India*

* Corresponding author: E-mail:vpgpt1@yahoo.co.in

**Abstract**

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 Å 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.

**Keywords:** Chlorocarbonyl ketene; Conformers; Anharmonic frequencies;
Spectroscopic constants; DFT; Perturbation
theory