MILLIMETER-WAVE AND INFRARED-SPECTROSCOPY OF (BRCN)-C-13 - ANHARMONIC-FORCE FIELD OF CYANOGEN-BROMIDE FROM SPECTROSCOPIC DATA AND AB-INITIOCALCULATIONS

The rotational spectra of (BrCN)-Br-79-C-13-N-14 and (BCN)-B-81-C-13-N -14 in the low-lying 01(1)0, 10(0)0, 02(0)0 and 02(2)0 vibrational sta tes were observed in the millimetre-wave region, and the nu(3) fundame ntal band of the C-13 containing species was recorded from 2110 to 217 0 cm(-1) with a diode laser spectrometer. The anharmonic force field o f cyanogen bromide was calculated using the data obtained together wit h those already known for the normal and N-15 isotopomers. The experim ental determination of the force field was supported by an ab initio c alculation of the intramolecular potential energy surface, which was o btained by the CASSCF method. A further refinement of the equilibrium structure was made using the equilibrium rotational constants of six d ifferent isotopomers.