Far-infrared spectra and ab initio calculations of nitrosyl chloride

The far-infrared spectra of ON 35 Cl and ON 37 Cl were recorded from 20 to 100 cm −1 at a resolution of 0.004 cm −1 . Thirteen pure rotational Q branches were assigned for each molecule in the ground vibrational state from which the rotational constants A and the distortion constants φ K and φ K were obtained utilizing previously reported values of the constants B and C from microwave studies. Rotational Q branches were also observed for both molecules in two vibrationally excited states. Ab initio calculations up to those using fourth-order perturbation (MP4) with configuration interaction utilizing the 6-31G* basis set were carried out. Optimized geometries obtained at this level are compared with those obtained with smaller basis sets without configuration interaction and with the experimentally determined values. Force constants were calculated which are compared with the values previously reported. The potential energy distributions and quartic distortion constants were obtained at all levels of ab initio calculations up to MP2/6-31G*.