Ab initio study of the ground state and the first excited state of the rectangular (D2h) N4 molecule

The dissociation pathway of D2h N4 has been investigated at the CASSCF level of theory. A new C2v transition state to dissociation was found and characterized on the potential energy surface. The effective barrier of dissociation from D2h N4 to 2N2 is estimated to 6.5 kcal/mol from MR-AQCC/VTZ//CAS(12, 12)/VTZ calculations. Vertical excitation energies and oscillator strengths for the lowest 20 singlet states of D2h N4 have been calculated using the EOM-CCSD method. The geometry of the first excited state 1B3u, considered as the lowest optically accessible state, has been optimized and characterized at various computational levels. This state might be useful for detection of D2h N4 by means of laser-induced fluorescence (LIF) spectroscopy.