Ab initio calculations determining structures and stabilities of the tetranitrogen N-4(.+)/N-4 system and mass spectrometric experiments were carried out in an attempt to understand the processes occurring in a recent neutralization-reionization mass spectrometric (NRMS) experiment starting from a linear N-4(.+) radical cation (Cacace et al. Science, 2002, 295, 480). Calculations were performed using RCCSD(T) and MRCISD+Q methods with the 6-311+G(3df) basis set. The most stable bound tetranitrogen molecule is an azidonitrene (N-3-N) featuring a triplet (3)A" ground state and being 56 kJ/mol below the singlet tetrahedral T-d isomer. The singlet azidonitrene has an open-shell (1)A" state and the corresponding singlet-triplet energy gap amounts to 69 kJ/mol. In both states, fragmentation giving two N-2 moieties needs to overcome a barrier height of about 55 kJ/mol. A remarkable difference between N-4 isomers is that ionization of triplet azidonitrene leads to the linear (2)Sigma ground-state radical cation, whereas removal of an electron from singlet tetrahedrane (N-4, T-d) gives rise to a cyclic three-membered ring belonging to a Pi-type excited state. The standard heats of formation are evaluated as follows: DeltaH(f)(o)(triplet azidonitrene) = 714 +/- 20 kJ/mol, DeltaH(f)(o)(singlet azidonitrene) = 783 +/- 20 kJ/mol, AH' (N-4, T-d) = 770 20 kJ/mol, and AHf' (N-4(.+)) = 1398 20 kJ/mol. The adiabatic ionization energies are estimated as IEa (triplet azidonitrene) = 7.3 +/- 0.3 eV and IEa (N-4, T-d) = 10.4 +/- 0.3 eV. When repeating the NRMS experiments using our tandem mass spectrometer and operating conditions, the collisional activation (CA) spectrum of N-4(.+) could be recorded, whereas we could not reproduce the neutralization-reionization spectrum reported by Cacace et al. These results suggest that although azido-nitrene was apparently generated in NRMS experiments, only a very small fraction of the N4 neutral could effectively be reionized, and the resulting spectra could not be reproduced easily, when changing even slightly the experimental conditions.