Structure and Stability of Aromatic Nitrogen Heterocycles Used in the Field of Energetic Materials

Abstract

Understanding the stabilization of nitrogen heterocycles is critical in the field of energetic materials and calls for innovative knowledge of nitrogen aromatics. Herewith, we report for the first time that nitrogen lone pair electron (NLPE) delocalization in five-membered nitrogen heterocycles creates a second σ-aromaticity in addition to the prototypical π-aromaticity. The NLPE delocalization and the attendant dual-aromaticity are enhanced as more carbon atoms in the ring are substituted by unsaturated nitrogen atoms. The presence of adjacent nitrogen atoms in the ring can enhance the aromaticity of the nitrogen heterocycles and improve in-crystal intermolecular binding strength but will decrease the firmness of the individual molecular architecture. Notably, such σ-aromaticity is not present in six-membered nitrogen heterocycles, probably due to the longer bonds and broader regions of their rings; therefore, six-membered heterocycles present overall lower aromaticity than five-membered heterocycles. This work brings new knowledge to nitrogen aromatics and is expected to inspire broad interest in the chemistry community.

Applying machine learning to balance performance and stability of high energy density materials

“…However, when the increase in the OB leads to better performance, it pays the price of higher sensitivity and lower stability of the resulting HEDM ( Wang et al., 2018 ; Li et al., 2020a , 2020b , 2020c ). Moreover, the increase of ρ N in various nitrogen-rich HEDMs inevitably introduces N–N bonds, the low bond strength of which would result in reduced stability of such molecules ( Tang et al., 2017 ; Zong et al., 2020 ).…”

Full-nitro-nitroamino cooperative action: Climbing the energy peak of benzenes with enhanced chemical stability

“…Consequently, these compounds play important roles in pyrotechnics, mining engineering, aerospace exploration, and most modern defense systems (9)(10)(11)(12)(13)(14)(15)(16). The most representative energetic polynitrobenzenes are 2,4,6-trinitrotoluene (TNT; 1863, the most famous explosive) and hexanitrobenzene (HNB; 1966, the only fully nitrated benzene) (14)(15)(16)(17)(18). Generally, the energy of these materials progressively increases with an increasing number of nitro groups in the benzene ring (Fig.…”

“…Particularly, benzene, which is fully substituted with six nitro groups (i.e., HNB), has exhibited the highest energy (Q: 6993 kJ kg −1 ) among the organic explosives since 1966. In addition, HNB has other excellent energetic properties, such as high density (d: 1.98 g cm −3 ) and high detonation velocity (D: 9277 m s −1 ) (17)(18)(19)(20).…”

“…However, an increase in the number of nitro groups in the benzene ring usually results in lower chemical stability (12)(13)(14)(15)(16)(17)(18). For instance, TNT (three nitro groups) is highly stable and is still widely used for national defense purposes (14)(15)(16).…”

Adaptive σ aromaticity in the rhenacyclopropene rings

“…Aromaticity is one of the most fundamental concepts in chemistry, 1,2 which has attracted considerable attention and discussion from experimental and theoretical chemists 3,4 since Kekulé proposed the benzene structure. 5 In addition, this concept has wide applications in materials, [6][7][8][9][10] reaction mechanism 11 and pharmaceutical science. 12 According to Hückel's rule, [13][14][15] the planar conjugated species with [4n + 2] π electrons in the lowest singlet state (S 0 ) show aromaticity whereas the ones with [4n] π electrons in the lowest triplet state (T 1 ) display aromaticity according to Baird's rule.…”