Stability of polynitrogen compounds: The importance of separating the σ and π electron systems

Planar N_x systems such as cyclo-N₅^- and N₅⁺ tend to be more stable than nonplanar systems such as the neutral cyclo-N₆. It is proposed that the key to stabilization is the separation of the σ and π electron systems. In both cyclo-N ₅^- and N₅⁺, a six-π-electron system is created upon either adding to or removing from the cyclo-N₅ radical one electron. Judicious addition of oxygen atoms to polynitrogen ring compounds such as cyclo-N₄ and cyclo-N₆ can increase their thermodynamic and kinetic stabilities, accompanied by only a small reduction in their efficiency as high energy density materials (HEDMs). The properties of some of these compounds are calculated and compared with the parent all-nitrogen compounds. Coordination of one or more oxygen atoms to the ring leads to effective separation of the σ and π electron systems helping to stabilize the systems. Natural bond analysis indicates that the exocyclic NO bonds can assume a single or double bond character, depending on the ring system.