Millimetre-wave and infrared spectroscopy of Br¹³CN: anharmonic force field of cyanogen bromide from spectroscopic data and<i>ab initio</i>calculations

Abstract

The rotational spectra of 79Br13C14N and 81Br13C14N in the low-lying 0110, 1000, 0200 and 0220 vibrational states were observed in the millimetre-wave region, and the ν3 fundamental band of the 13C containing species was recorded from 2110 to 2170 cm−1 with a diode laser spectrometer. The anharmonic force field of cyanogen bromide was calculated using the data obtained together with those already known for the normal and 15N isotopomers. The experimental determination of the force field was supported by an ab initio calculation of the intramolecular potential energy surface, which was obtained by the CASSCF method. A further refinement of the equilibrium structure was made using the equilibrium rotational constants of six different isotopomers.