Ab initio description of the ground and excited states of cyanogen isomers

Ab initio calculations are used to characterize the ground and low lying excited electronic states of several linear and cyclic isomers of cyanogen. In particular, the calculations consider the excitation energies for selected singlet and triplet Σ states of cyanogen (NCCN), isocyanogen (CNCN), di-isocyanogen (CNNC), diaza-dicarbon (NNCC), and corresponding states of three cyclic cyanogen isomers, including many predictions that are presented here for the first time. Our calculations for the linear structures agree well with available experimental data, thereby providing a benchmark for assessing the predictions for other isomers, such as the cyclic isomers. The present calculations are also used to identify the possible emitting species for some unexplained bands in the emission observed from certain low temperature matrices. Since, the emitter could not be assigned to any of the cyanogen isomers, some other molecule must be responsible for the emission in low temperature matrices.