Polynitrogen clusters encapsulated inside B₂₄N₂₄fullerene-like nanocages: Nanoscale high energy materials studied by density functional theory

The complexes of polynitrogen clusters encapsulated inside o-B₂₄N₂₄and s-B₂₄N₂₄fullerene-like nanocages, Nn@o-B₂₄N₂₄and Nn@s-B₂₄N₂₄, are predicted as nanoscale high energy materials and optimized at ωB97X-D theoretical levels with the basis set of 6-31G(d). It is found that the maximum of 10 N atoms can be encapsulated inside o-B₂₄N₂₄and 9 N atoms inside s-B₂₄N₂₄. The encapsulated Nn clusters in Nn@o-B₂₄N₂₄have the similar configurations for n = 1–5 and 7, and the different configurations for n = 6, 8 and 9 to those in Nn@s-B₂₄N₂₄. The total energy E_Nn@B24N24analysis and percentage relative elongation (ε = (R/R₀− 1) × 100%), all indicate that s-B₂₄N₂₄is better for encapsulating the Nn clusters (n ⩽ 5), and worse for the Nn clusters (n ⩾ 6) than o-B₂₄N₂₄. The HOMO-LUMO energy gap of Nn@B₂₄N₂₄generally show a gradual decrease with the increase of the number of nitrogen atom of the encaged Nn in odd or even number, respectively. And different from the pure B₂₄N₂₄, the electron density patterns indicate that for complexes Nn@B₂₄N₂₄, the HOMO for n = 6–9 and the LUMO for n = 2–9 mainly concentrate in the encaged Nn. The NBO analysis, the Mulliken population analysis and the topological analysis of the electron localization function (ELF) all demonstrate that there exist some covalent interactions between the encapsulated Nn clusters and the nanocages for the Nn@o-B₂₄N₂₄(n = 7,9) and Nn@s-B₂₄N₂₄(n = 5,7,9), while other complexes only have Van der Waals interactions.