Besides N2, What Is the Most Stable Molecule Composed Only of Nitrogen Atoms?

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Abstract
Polynitrogen molecules have been studied systematically at high levels of ab initio and density functional theory (DFT). Besides N2, the thermodynamically most stable Nn molecules, located with the help of a newly developed energy increment system, are all based on pentazole units. The geometric, energetic, and magnetic criteria establish pentazole (2) and its anion (3) to be as aromatic as their isoelectronic analogues, e.g., furan, pyrrole, and the cyclopentadienyl anion. The bond lengths in 2 and 3 are equalized; both have large aromatic stabilization energies (ASE) and also substantial magnetic susceptibility exaltations (Λ). The Cs symmetric azidopentazole (14), a candidate for experimental investigation, is the lowest energy N8 isomer but is still 196.7 kcal/mol higher in energy than four N2 molecules. Octaazapentalene (12) with 10 π electrons also is aromatic. The D2d symmetric bispentazole (21) is the lowest energy N10 minimum but is 260 kcal/mol higher in energy than five N2 molecules. For strain-free molecules, the average deviation is ±2.6 kcal/mol between the DFT energies and those based on the increment scheme. The increment scheme also provides estimates of the strain energies of polynitrogen compounds, e.g., tetraazatetrahedrane (8, 48.2 kcal/mol), octaazacubane (11, 192.6 kcal/mol), and N20 (27, 294.6 kcal/mol), and is useful in searching for new high-energy−high-density materials.

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