BESIDES N-2, WHAT IS THE MOST STABLE-MOLECULE COMPOSED ONLY OF NITROGEN-ATOMS

Polynitrogen molecules have been studied systematically at high levels of ab initio and density functional theory (DFT). Besides N-2, the th ermodynamically most stable N-n molecules, located with the help of a newly developed energy increment system, are all based on pentazole un its. The geometric, energetic, and magnetic criteria establish pentazo le (2) and its anion (3) to be as aromatic as their isoelectronic anal ogues, e.g., furan, pyrrole, and the cyclopentadienyl anion. The bond lengths in 2 and 3 are equalized; both have large aromatic stabilizati on energies (ASE) and also substantial magnetic susceptibility exaltat ions (Lambda). The C-s symmetric azidopentazole (14), a candidate for experimental investigation, is the lowest energy N-8 isomer but is sti ll 196.7 kcal/mol higher in energy than four N-2 molecules. Octaazapen talene (12) with 10 pi electrons also is aromatic. The D-2d symmetric bispentazole (21) is the lowest energy N-10 minimum but is 260 kcal/mo l higher in energy than five N-2 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 inclement scheme also provid es estimates of the strain energies of polynitrogen compounds, e.g., t etraazatetrahedrane (8, 48.2 kcal/mol), octaazacubane (11, 192.6 kcal/ mol), and N-20 (27, 294.6 kcal/mol), and is useful in searching for ne w high-energy-high-density materials.