Confirmation of the “long-lived” tetra-nitrogen (N4) molecule using neutralization-reionization mass spectrometry and ab initio calculations.

Tetra-nitrogen (N4), which has been the subject of recent controversy [Cacace, d. Petris, and Troiani, Science 295, 480 (2002); Cacace, Chem. Eur. J. 8, 3839 (2002); Nguyen et al., J. Phys. Chem. A 107, 5452 (2003); Nguyen, Coord. Chem. Rev. 244, 93 (2003)] as well as of great theoretical interest, has been prepared from the N4 +· cation and then detected as a reionized gaseous metastable molecule with a lifetime exceeding 0.8 μs in experiments based on neutralization-reionization mass spectrometry. Moreover, we have used the nature of the charge-transfer reaction which occurs between a beam of fast N4 +· ions (8 keV translational energy) and various stationary gas targets to identify the vertical neutralization energy of the N4 +· ion. The measured value, 10.3±0.5, most closely matches that of the lowest energy azidonitrene 4N4 +· Cs(4A′) ion, resulting in the formation of the neutral bound azidonitrene 3N4Cs(3A″). Neutralization of the global minimum 2N4 +· D ∞h (2∑u +) ion leads to a structure 166 kJ mol-1 above the dissociation products [N2(1∑g +) + N2(1∑g +)]; moreover, it was not possible to find a minimum on the 1N4 neutral potential energy surface for a covalently bonded structure. Ab initio calculations at the G3, QCISD/6-31G(d), and MP2/AUG-cc-pVTZ levels of theory have been used to determine geometries and both vertical neutralization energies of ions (doublet and quartet) and ionization energies of neutrals (singlet and triplet). In addition, we have also described in detail the EI ion source for the Ottawa VG ZAB mass spectrometer [Holmes and Mayer, J. Phys. Chem. A 99, 1366 (1995)] which was modified for high-pressure use, i.e., for the production of dimer and higher number cluster ions. [ABSTRACT FROM AUTHOR]