The A 1A″ state of isocyanogen (CNCN)

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.