Structures of six nitrogen atom species (N6) have been reconsidered using high-level ab initio calculations. Harmonic vibrational wavenumbers obtained at the MP2/6-31G (d) level show that both the benzene analogue (hexazine, D6h) and the transbent open-chain (C2h) structures are not energy minima as previously reported, but rather transition structures on the N6 potential energy surface. The corresponding equilibrium structures are a cyclic complex of the two azide radicals and a C2 open chain, respectively. The open-chain form lies lower in energy than the cylic isomer (almost-equal-to 11 kcal/mol at the MP4/6-311 + G(d) + ZPE). Calculated vertical excitation energies also suggest that the open-chain diazide (N3)2 is the best candidate for the species exhibiting a yellow color (lambda(max) = 380 nm) observed by Vogler, Wright and Kunkley in their flash photoloytic experiment.