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 Σ 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 Π-type excited state. The standard heats of formation are evaluated as follows: ∆H°f (triplet azidonitrene) ) 714 ( 20 kJ/mol, ∆H°f (singlet azidonitrene) ) 783 ( 20 kJ/mol, ∆H°f (N 4 , T d ) ) 770 ( 20 kJ/mol, and ∆H°f (N 4 •+ ) ) 1398 ( 20 kJ/mol. The adiabatic ionization energies are estimated as IE a (triplet azidonitrene) ) 7.3 ( 0.3 eV and IE a (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 neutralizationreionization 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 N 4 neutral could effectively be reionized, and the resulting spectra could not be reproduced easily, when changing even slightly the experimental conditions.