Azido-Nitrene Is Probably the N4 Molecule Observed in Mass Spectrometric Experiments

Abstract

Ab initio calculations determining structures and stabilities of the tetranitrogen N4•+/N4 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 N4•+ 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 (N3−N) featuring a triplet 3A‘ ‘ ground state and being 56 kJ/mol below the singlet tetrahedral Td isomer. The singlet azidonitrene has an open-shell 1A‘ ‘ state and the corresponding singlet−triplet energy gap amounts to 69 kJ/mol. In both states, fragmentation giving two N2 moieties needs to overcome a barrier height of about 55 kJ/mol. A remarkable difference between N4 isomers is that ionization of triplet azidonitrene leads to the linear 2Σ ground-state radical cation, whereas removal of an electron from singlet tetrahedrane (N4, Td) gives rise to a cyclic three-membered ring belonging to a Π-type excited state. The standard heats of formation are evaluated as follows: Δ (triplet azidonitrene) = 714 ± 20 kJ/mol, Δ (singlet azidonitrene) = 783 ± 20 kJ/mol, Δ (N4, Td) = 770 ± 20 kJ/mol, and Δ (N4•+) = 1398 ± 20 kJ/mol. The adiabatic ionization energies are estimated as IEa (triplet azidonitrene) = 7.3 ± 0.3 eV and IEa (N4, Td) = 10.4 ± 0.3 eV. When repeating the NRMS experiments using our tandem mass spectrometer and operating conditions, the collisional activation (CA) spectrum of N4•+ 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.