FTIR spectroscopy of the 2 nu(3) overtone band for different BrCN isotopomers: an improved evaluation of the anharmonic force field of cyanogen bromide

The 2 nu(3) overtone band of six different isotopomers of cyanogen bromide has been recorded in the range from 4200 to 4400 cm(-1) with a resolution o f 0.02 cm(-1) using a Fourier transform infrared (FTIR) spectrometer. This allowed us to achieve complete knowledge of the energies for all levels cor responding to double vibrational excitation. An improved evaluation of the quartic force field of cyanogen bromide has been performed using the new da ta obtained together with those already known from previous works. Theoreti cal values derived from a recent ab initio calculation have been used to co nstrain the potential terms which cannot be determined with sufficient prec ision by the least-squares analysis of the available experimental data.