MILLIMETER-WAVE AND DIODE-LASER SPECTROSCOPY OF (ICN)-N-15 - ANHARMONIC-FORCE FIELD OF CYANOGEN IODIDE FROM SPECTROSCOPIC DATA AND AB-INITIO CALCULATIONS

The nu(3) fundamental band (C-N stretch) and four associated hot bands of (ICN)-N-15 have been recorded in the 2115-2170 cm(-1) range using a diode laser spectrometer. The analysis of the infrared bands has bee n supported by measurements of new millimetre wave transitions in the ground and low lying 01(1)0, 10(0)0, 02(0)0, and 02(2)0 vibrational st ates. The anharmonic force held of cyanogen iodide has been calculated using the data obtained together with those already known for the nor mal and C-13 containing species. An ab initio calculation of the intra molecular potential energy surface, obtained by the CASSCF method, has provided useful constraints for those force constants which are not d eterminable directly from the experimental data.