ELECTRONIC SPECTROSCOPY OF THE CYANOGEN HALIDES

The electronic absorption spectra of the cyanogen halides, XCN where X = Cl, Br, and I, have been investigated in the range 3100-1050 angstrom. The spectra are analyzed in terms of vibronic structure, oscillator strengths, and effective quantum numbers. The spectra of the cyanogen halides exhibit no regular Rydberg structure. The absence of regularity is shown to be a direct consequence of the presence of intravalence excitations and Rydberg/valence interactions. In confirmation of the above, the intravalence transitions arising from the 2-pi-->(5-delta, 3-pi, and 6-delta) configurational excitations are observed and assigned. In addition, we presume to extract "term values" for the antibonding 5-delta, 3-pi, and 6-delta MOs and then use these to predict the energies of the remaining nine low-energy intravalence excitations of the cyanogen halides, {4-delta; 1-pi; 3-delta}-->{5-delta; 3-pi; 6-delta}. There is a danger in this, in that we seem, too blithely perhaps, to make use of a simple one-electron MO model in situations where it is known that many-electron effects may dominate. We believe that the use of one-electron considerations is moderated by the extensive use of a vast amount of empirical, correlative experimental data. Finally, all excited states that arise from the 12 low-energy intravalence excitations are correlated with the states of the separated halogen atom and CN radical such that photoprocesses in the cyanogen halides may be rationalized. The A-band and alpha-band continua are assigned as 2-pi-->5-delta, 1,3II and 2-pi-->3-pi; 1,3-DELTA, 1,3-SIGMA+,1,3-SIGMA- configurational excitations, respectively. The 4-delta-->5-delta;1-SIGMA+ transition is associated with the discrete structure atop the alpha continuum; the intense, discrete band systems that lie to the blue of the B and C Rydberg band systems are associated with the 2-pi-->6-delta;1,3II intravalence transitions. The states that arise from the remaining eight configurational excitations are shown to be mostly dissociative in nature. The correlation scheme predicts (i) CN (X2-SIGMA+) to be the primary product of photolysis within the A continua; (ii) CN (A2II(i)) to be the primary product of photolysis from the onset of the alpha continua up to photon energies of 80 000 cm-1 for each of the cyanogen halides, and (iii) CN (B2-SIGMA+) to be a primary product for photon energies greater than approximately 105 000, 94 000, and 80 000 cm-1 in CICN, BrCN, and ICN, respectively.