EXTENDED ABSTRACT: All-nitrogen crystals are one of the most promising candidates for energetic materials that are potential to drive revolutionary advancements in aerospace, defense, mineral exploration, and other fields. Despite centurylong persistent endeavours, no environmentally stable all-nitrogen crystal has been successfully produced. In this work, we designed a flexible all-nitrogen N8 molecule utilizing cyclic N5 and linear N3 as building blocks, which are able to combine into N8 in an oxidizing agent. The molecule adopts a lollipop shaped conformation, with two distinct delocalized systems within nitrogen’s L-shell: 10 π-electrons above and below the N8 plane and 14 lone pair σ-electrons surrounding the N8 at the molecule’s equatorial plane. The fluidity of the delocalized electrons allows the linear N3 component (the “stick”) to easily bend at various angles relative to the cyclic N5 component (the “candy”), with a negligible energy barrier of 0.06 eV. Utilizing the lollipop-shaped N8 molecule as the fundamental unit, we predicted six environmentally stable crystal structures. These structures showcase abundant intermolecular interactions, further delocalizing electrons surrounding the N8 molecules. Their stability was confirmed by calculating lattice vibrations and equilibrium bond strength under equilibrium state, atomic motions at finite temperatures, structure complement under static high-pressure loading. These crystals exhibit a high density (with ρ up to 1.91 g/cm3), significantly surpassing the nitrogen density of traditional organic energetic materials (0.32 to 0.77 g/cm³) and even exceeding the densities of previously reported crystals based on free N8 molecules (1.67 to 1.72 g/cm³). The comprehensive properties, with detonation velocity and pressure up to 9.59 km/s and 43.12 GPa and with h50% up to 46.4 cm outperform existing energetic materials. The remarkable properties of these designed materials are attributed to two distinct delocalized systems within nitrogen’s L-shell: π- and lone pair σ-electrons, which not only stabilize the molecular structure but also facilitate interconnected 3D networks of intermolecular interactions. Findings challenge the experimental synthesis of environmentally stable N8 all-nitrogen solids.
Keywords: all-nitrogen; electron delocalization; dual-aromaticity; property regulation
REFERENCES:
[1] C. Yao, L. Zhang*, S. Pang, et al. J. Phys. Chem. Lett. 2024, 15, 1507-1514.