The à 1A″ state of isocyanogen (CNCN)

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Abstract
Ab initio electronic structure theory has been used to characterize the lowest 1A‘ excited state of isocyanogen. The geometries, vibrational frequencies, and infrared intensities of the X̃ 1Σ+ and à 1A‘ states have been determined using the single and double excitation configuration‐interaction (CISD) method in conjunction with a triple‐ζ plus double polarization (TZ2P) basis set. Adding the Davidson correction and including a set of f polarization functions in the basis set (CISD+Q TZ2Pf), we estimate the energy difference between the ground and first excited singlet state of isocyanogen to be Te = 41 700 cm−1. With a C–N–C bond angle of 117°, this bent 1A‘ excited state may be useful in stimulated emission pumping (SEP) experiments on the NCCN■CNCN isomerization.