Cyclo‐N₉⁻: a Novel 5/6 Fused Polynitrogen Anion for High Energy Density Materials

Abstract After cyclo‐pentazolate anion, a 5/6 fused structure of N₉⁻ is constructed, and four novel nitrogen‐rich ionic compounds are assembled on its basis. The results of the quantum calculations revealed an uneven distribution of electrons on cyclo‐N₉⁻, with significant charge density near the N5/N9 atoms and an ADCH charge of −0.425. The relative strength of chemical bonds is assessed through bond order analysis, which is further supplemented by transition state theory and ab initio molecular dynamics, ultimately leading to the identification of the decomposition pathways of cyclo‐N₉⁻. The aromaticity of cyclo‐N₉⁻ and its individual sub‐rings is cleverly validated through a combination of NICS_ZZ and ICSS methods. Among the eight systems, cyclo‐N₉⁻ forms hydrogen bonds with other cations, and IGMH analysis revealed significant LP‐π and π–π stacking interactions between [N₇H₄⁺] and cyclo‐N₉⁻, both of which enhance system stability. The theoretical energy densities in all systems are at the forefront in the currently emerging nitrogen‐rich compounds. Attributed to its extraordinarily high enthalpy of formation, the detonation performance of [N₇H₄⁺] [N₉⁻] is particularly excellent. However, [NH₃OH⁺] [N₉⁻] exhibits better stability and most exciting performance, making it a highly promising candidate with application potential.