Based onabinitioelectronic structure calculations, the threehyphen;dimensional potential energy functions for the electronic ground stateXthinsp;2Pgr;gof N3have been generated and used in beyond Bornndash;Oppenheimer calculations of the rovibronic energy levels by a variational approach accounting for anharmonicity, rotationndash;vibration, electronic angular momenta, and electron spin coupling effects. The vibronic levels (J=P) for energies up to 4300 cmminus;1are given. The few experimentally known vibronic energy differences have been reproduced with an accuracy of better than 10 cmminus;1. For several vibronic levels also the rovibronic levels are reported. The electronndash;nuclear motion and anharmonic coupling effects have been analyzed. The strength of the Bornndash;Oppenheimer breakdown depends strongly on the rovibronic symmetry. Like in the isoelectronic CO+2ion, the anharmonic coupling in the vibronic states occurs mainly within the blocks of Fermi polyads for (2ngr;1+ngr;2) =constant. In low lying states the anharmonic coupling effects are weaker in N3than in CO+2. The presence of the Fermi polyad blocks of interacting states strongly alleviates the characterization of the excited rovibronic states in the N3radical.
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