A b i n i t i o calculations of dissociative electronic states of ClCN: Implications to the photodissociation dynamics of the cyanogen halides

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Abstract
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 rCCl. We find that the low‐lying excited states derive from three electronic configurations: π3σ*, π3π*, and σπ4σ*. The lowest excited triplet and singlet states derive from the π3σ* configuration and give rise to bent 1,3A’ and 1,3A‘ states. States arising from the π3π* configuration are linear (Σ and Δ states). There is evidence of surface crossings along the reaction coordinate between triplet states arising from the π3π* configuration and those arising from the π3σ* and σπ4σ* 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.