Electronic and infrared absorption spectra of NCCN+

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The D2Πu←X2Πg electronic transition of the NCCN+ cation has been observed in a 5K neon matrix with origin band at 33,522±23cm−1. The origin of the B2Σu+←X2Πg system is assumed to be the band at 11,486±3cm−1. In the infrared, the σu+ vibrational mode is detected at 1799.5±1cm−1. Theoretical calculations demonstrate that D2Πu is a nonlinear shake-up state and possesses cis- and trans-bent/bent Renner–Teller minima, in contrast to the X2Πg and C2Πu states which are linear. The complex vibronic progression in the absorption spectrum in a neon matrix to the D2Πu state does not provide direct evidence for the two possible isomers. The stretching modes are assigned while the active bending modes coupled by the Renner–Teller effect will require additional studies. In the energy region of the D2Πu state lie several quartet states. The D2Πu state forms a conical intersection in Cs symmetry with B2Σu+ close to its barrier to linearity. The latter state crosses in linear structures also the C2Πu state and along the cis-bending coordinates the A2Σg+ state. Because the B2Σu+ state intersects conically with three electronic states, the vibronic and Renner–Teller couplings complicate the assignments of the vibronic progressions observed.