Stability of Polynitrogen Compounds: The Importance of Separating the σ and π Electron Systems

摘要

Planar N_x systems such as cyclo-N_5~- and N_5~+ tend to be more stable than nonplanar systems such as the neutral cyclo-N_6. It is proposed that the key to stabilization is the separation of the σ and π electron systems. In both cyclo-N_5~- and N_5~+, a six-π-electron system is created upon either adding to or removing from the cyclo-N_5 radical one electron. Judicious addition of oxygen atoms to polynitrogen ring compounds such as cyclo-N_4 and cyclo-N_6 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.