ABINITIO CALCULATIONS OF DISSOCIATIVE ELECTRONIC STATES OF CICN - IMPLICATIONS TO THE PHOTODISSOCIATION DYNAMICS OF THE CYANOGEN HALIDES

Ab initio configuration interaction calculations with full geometric optimization along the Cl-CN dissociation coordinate have been carried out for several electronic states of ClCN. The calculations treat all low-lying singlet and triplet states and yield the vertical transition probabilities and oscillator strengths, as well as angular dependences near the Franck-Condon (FC) region and least energy paths for several electronic states as a function of r(CCl). We find that the low-lying excited states derive from three electronic configurations: pi-3-sigma-*, pi-3-pi-*, and sigma-pi-4-sigma-*. The lowest excited triplet and singlet states derive from the pi-3-sigma-* configuration and give rise to bent 1,3 A' and 1,3 A" states. States arising from the pi-3-pi-* ocnfiguration are linear (SIGMA and DELTA states). There is evidence of surface crossings along the reaction coordinate between triplet states arising from the pi-3-pi-* configuration and those arising from the pi-3-sigma-* and sigma-pi-4-sigma-* configurations. These crossings can be induced by slight bending of ClCN causing lowering of the symmetry. The calculated vertical excitation energies are in good agreement with features of the absorption spectrum, and suggest that the A continuum of ClCN involves transitions to the 2 1A' and 1 1A" states. The implications to the photodissociation dynamics of the cyanogen halides are discussed.