Design and density functional theoretical study of three novel pyrazine-based high-energy density compounds

Molecular design of energetic tetrazine-triazole derivatives

Nitrogen-rich compounds are promising candidates for preparing high energetic density materials (HEDMs) and show the potential in the application of propellants, explosives, and pyrotechnics. Two kinds of typical nitrogen-rich compounds, such as tetrazine and triazole, have attracted the attentions in recent years owing to their high densities, good thermal stabilities, and excellent energetic performances. In this work, four series of innovative energetic compounds based on the conjugates of tetrazine and triazole bearing various substituents (-NH₂, -NO₂, and -NHNO₂) were designed. The optimized structures, crystal densities, heats of formation (HOFs) in gas phase and in condensed phase, detonation properties, bond dissociation energies (BDEs), and impact sensitivity (h₅₀) of these compounds were studied systematically via density functional theory (DFT) method. The detonation velocities of four series of compounds are in the range between 7.03 and 8.59 km s^-1 and their detonation pressures are in the range between 20.6 and 33.1 GPa. Results indicated that the linkage of -N=N- bond contributed significantly to HOFs and energy density of the energetic molecules, and 1,2,3-triazole showed better performances than 1,2,4-triazole slightly. As for the same series compounds with different substituents, the compounds with -NHNO₂ possessed the highest HOFs (such as A6, B6, C6, D6). In terms of the energetic properties (D and P), four compounds (A7, B7, C7, and D7) exhibited the comparable performance with the widely used hexa-hydro-1,3,5-trinitro-1,3,5-triazine (RDX) and in the meanwhile displayed superior thermal stability and sensitivity to RDX, which indicated their potential application in the insensitive energetic materials.

DFT studies on nitrogen-rich pyrazino [2, 3-e] [1, 2, 3, 4] tetrazine-based high-energy density compounds

By using the density functional theory method, we investigated the heats of formation (HOFs), electronic structure, detonation properties, thermal stability and sensitivity for a set of pyrazino [2, 3-e] [1, 2, 3, 4] tetrazine derivatives with different substituents and different numbers of N-oxides. Our findings reveal that the HOFs of the derivatives decrease dramatically with the increasing number of N-oxides. The effects of the substituents on the HOMO-LUMO gaps are coupled with those of the N-oxides. The calculated detonation properties point out that -NF₂, -ONO₂ and an increasing number of N-oxides are very helpful for improving the detonation performance of the designed derivatives. The bond dissociation energies of the weakest bonds indicate that a majority of our designed compounds have better thermal stability. The -NH₂ group is very useful to decrease the free space value. Most of the derivatives have higher h₅₀ values compared with parent molecules. Considering the sensitivity, thermal stability and detonation performance, four compounds could be considered as potential candidates of high-energy density compounds.

Computational study of the structure and properties of bicyclo[3.1.1]heptane derivatives for new high-energy density compounds with low impact sensitivity

To design new high-energy density compounds (HEDCs), a series of new bicyclo[2.2.1]heptane derivatives containing an aza nitrogen atom and nitro substituent were designed and studied theoretically. The density, heat of sublimation and impact sensitivity were estimated by electrostatic potential analysis of the molecular surface. Based on the designed isodesmic reaction, and the reliable heat of formation (HOF) of the reference compounds, HOFs were calculated and compared at B3LYP/6-311G(d,p) and B3P86/6-311G(d,p), respectively. The detonation performances, bond dissociation energies (BDE) and impact sensitivity were calculated to evaluate the designed compounds. The calculated results show that the number of aza nitrogen atoms and NO₂ groups are two important factors for improving HOF, density and detonation properties. Thermal stability generally decreases with increasing nitro groups. And the N-NO₂ bond is the trigger bond for all designed compounds except B8, whose trigger bond is C-NO₂. Importantly, the BDE values are between 86.95 and 179.71 kJ mol^-1 and meet the requirement for HEDCs. Detonation velocity and detonation pressure were found to be 5.77-9.65 km s^-1 and 12.30-43.64 GPa, respectively. After comprehensive consideration of thermal stability, impact sensitivity and detonation properties, A7, A8, B8, C8, D7, E7, F7 and G6 may be considered as potential HEDCs. Especially, A8, B8, C8, and D7 have better detonation properties than the famous caged nitramine CL-20 (D = 9.40 km/s, P = 42.00GPa). Besides, all the designed potential HEDCs have reasonable impact sensitivity. Graphical abstract New high-energy density compounds (HEDCs) with low impact sensitivity (A8, B8, C8 and D7 have better detonation properties than CL-20).

Impact sensitivity and the maximum heat of detonation

We demonstrate that a large heat of detonation is undesirable from the standpoint of the impact sensitivity of an explosive and also unnecessary from the standpoints of its detonation velocity and detonation pressure. High values of the latter properties can be achieved even with a moderate heat of detonation, and this in turn enhances the likelihood of relatively low sensitivity.