New carbon-nitrogen-oxygen compounds as high energy density materials

Molecular crystals are complex systems exhibiting various crystal structures,and accurately modeling the crystal structures is essential for understanding their physical behaviors under high pressure.Here,we perform an extensive structure search of ternary carbon-nitrogen-oxygen(CNO)compound under high pressure with the CALYPSO method and first principles calculations,and successfully identify three polymeric CNO compounds with Pbam,C2/m and I4m2symmetries under 100 GPa.More interestingly,these structures are also dynamically stable at ambient pressure,and are potential high energy density materials(HEDMs).The energy densities of Pbam,C2/m and I4m2 phases of CNO are about2.30 kJ/g,1.37 kJ/g and 2.70 kJ/g,respectively,with the decompositions of graphitic carbon and molecular carbon dioxide andα-N(molecular N_(2))at ambient pressure.The present results provide in-depth insights into the structural evolution and physical properties of CNO compounds under high pressures,which offer crucial insights for designs and syntheses of novel HEDMs.

High-energy-density pentazolate salts:CaN_(10)and BaN_(10)

The search for high energy density materials(HEDMs)in polymeric nitrogen compounds has gained considerable attention.Previous theoretical predictions and experiments have revealed that metal ions can be used to stabilize the pentazolate(N-5)anion.In this work,by employing a machine learning-accelerated crystal structure searching method and first-principles calculations,we found that the new pentazolate salts,CaN(10)and BaN(10),are energetically favorable at high pressures.Phonon dispersion calculations reveal that they are quenchable at ambient pressure.Ab initio molecular dynamics simulations verify their dynamic stability at finite temperature.Bader charge and electron localization function illustrates that alkaline earth atoms serve as electron donors,contributing to the stability of N5 rings.Bonding calculations reveal covalent bonds between nitrogen atoms and weak interactions between N5 rings.Similar to other pentazolate salts,these polymeric nitrides have high energy densities of approximately 2.35 kJ/g for CaN(10)and 1.32 kJ/g for BaN(10).The predictions of CaN(10)and BaN(10)structures indicate that these salts are potential candidates for green nitrogen-rich HEDMs.

High energy and insensitive explosives based on energetic porous aromatic frameworks

The design and synthesis of energetic materials with a compatibility of high energy and insensitivity have always been the research fronts in military and civilian fields.Considering excellent performances of porous organic frameworks and the lack of research in the field of energetic materials,in this study,a new concept named energetic porous aromatic frameworks(EPAFs)is proposed.The strategy of coating high energy explosives such as 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane(CL-20)and 1,3,5,7-tetranitro-1,3,5,7-tetrazocane(HMX)in the EPAFs by wet-infiltration method has successfully realized the assembly of target energetic composite materials.The results show that the 75 wt.%CL-20@EPAF-1 possesses the safer impact sensitivity of 31.4 J than that of CL-20(4.0 J).Notably,for 75 wt.%CL-20@EPAF-1,in addition to the superior detonation performances of the detonation velocity(8,761 m·s^(-1))and detonation pressure(31.27 GPa),the synergistic effect of the nitrogen-rich EPAFs and the nitramines high energy explosives results in a higher heat of detonation that surpasses the most of pristine high explosives and reported novel energetic materials.In prospect,energetic porous aromatic frameworks could be a promising and inspiring strategy to build high energy insensitive energetic materials.

DFT Study on Two C_4N_(12)O_4 Isomers with Pagodane- and Isopagodane-like Structures

Geometries, energies, and vibrational frequencies for two C4N12O4 isomers with pagodane- and isopagodane-like structures have been calculated at the B3LYP/6-31G＊ level.Isomers 1 and 2 are of D2h and D2d symmetry, respectively. Heats of formation for the two C4N12O4 isomers have been estimated in this paper, indicating they would be reasonable candidates for high energy density materials.

Theoretical Studies on the Structure and Detonation Properties of a Furazanbased Energetic Macrocycle Compound

Based on the full optimized molecular geometric structure at 6-311＋＋G＊＊ level,the density（ρ）,detonation velocity（D）,and detonation pressure（P） for a new furazan-based energetic macrocycle compound,hexakis[1,2,5]oxadi-azole[3,4-c：3＇,4＇-e;3＇＇,4＇＇-g：3＇＇＇,4＇＇＇-k：3＇＇＇＇,4＇＇＇＇-m：3＇＇＇＇＇,4＇＇＇＇＇-o][1,2,9,10]-tetraazacyclohexadecine,were investigated to verify its capacity as high energy density material（HEDM）. The infrared spectrum was also predicted. The heat of formation（HOF） was calculated using designed isodesmic reaction. The calculation on the bond dissociation energies（BDEs） was done and the pyrolysis mechanism of the compound was studied. The result shows that the N3–O1 bond in the ring may be the weakest one and the ring cleavage is possible to happen in thermal decomposition. The condensed phase HOF and the crystal density were also calculated for the title compound. The detonation data show that it can be considered as a potential HEDM. These results would provide basic information for the molecular design of novel high energy materials.

Stability,detonation properties and pyrolysis mechanisms of polynitrotriprismanes C_6H_(6-n)(NO_2)_n (n=1-6)

To further test whether polynitriprismanes are capable of being potential high energy density materials （HEDMs）, extensive theoretical calculations were carried out to investigate on a series of polynitrotriprismanes （PNNPs）： C6H6-.（NO2）. （n=1-6） Heats of formation （HOFs）, strain energies （SE）, and disproportionation energy （DE） were obtained using B3LYP/6-311＋G（2df, 2p）//B3LYP/6-31G＊ method by designing different isodesmic reactions, respectively. Detonation properties of PNNPs were obtained by the well-known KAMLET-JACOBS equations, using the predicted densities （p） obtained by Monte Carlo method and HOFs. It is found that they increase as the number of nitro groups n varies from 1 to 6, and PNNPs with n〉4 have excellent detonation properties The relative stability and the pyrolysis mechanism of PNNPs were evaluated by the calculated bond dissociation energy （BDE）. The comparison of BDE suggests that rupturing the C--C bond is the trigger for thermolysis of PNNPs. The computed BDE for cleavage of C--C bond （88.5 kJ/mol） further demonstrates that only the hexa-nitrotriprismane can be considered to be the target of HEDMs.

Structures and Stability of Th(N_(6)),Pa(N_(6)),and U(N_(6))

The cyclo-N_(6)anion is a total nitrogen unit with higher nitrogen content than cyclo-N^(-)_(5).However,the low decomposition barrier of cyclo-N_(6)anions hinders its application as a high energy density material(HEDM).Using first-principles calculations,we reveal that the covalent components that enhance the interaction between the cyclo-N_(6)anion and the cation can effectively improve the stability of cyclo-N_(6)anions.The actinide metals(Th,Pa,U)are selected as suitable cations.Further electronic structure analysis showed that the charge transfer from the actinide metal to cyclo-N_(6)anions resulted in a strong covalent bond,which promoted the stability of the cyclo-N_(6)anion in the Th(N_(6)),Pa(N_(6)),and U(N_(6))structure.This discovery is helpful for the rational design and synthesis of new HEDMs.

Theoretical Study on the Nitrogen-rich Derivatives Based on 1,2,4-Triazole and 1,2,3-Triazole Rings:an Extended Family of Power Performance Energetic Materials

The geometric and electronic structures of the derivatives of 4-nitro-5-(5-nitroimino-1,2,4-triazol-3-yl)-2H-1,2,3-triazolate(named A~J)are explored employing density functional theory(DFT)calculations at the B3LYP/6-311G^(**)level of theory.Based on the optimized molecular structures,the heats of formation(HOF)are obtained,and the electronic properties,density and molecular sensitivity by characteristic heights(H_(50))are discussed.Besides,the detonation performances(detonation velocity,detonation pressure)are estimated via Kamlet-Jacobs(K-J)formula.Compounds B(H50=29.4 cm,ρ=1.91 g/cm^(3),Q=1563.04 cal/g,P=36.05 GPa,D=8.95 km/s)and H(H_(50)=31.9 cm,ρ=1.80 g/cm^(3),Q=1610.09 cal/g,P=37.31 GPa,D=9.12 km/s)have positive HOFs and remarkable insensitivity and good detonation performance,strongly suggesting them as the acceptable new-type explosive.The initiating power surpasses conventional primary explosives,such as HMX.The outstanding detonation power of compounds B and H contributes to its future prospects as a promising green primary explosive.

DFT Studies on Detonation Properties, Pyrolysis Mechanism, and Stability of the Nitro and Hydroxyl Derivatives of Benzene

Ninety-one nitro and hydroxyl derivatives of benzene were studied at the B3LYP/6-31G＊ level of density functional theory. Detonation properties were calculated using the Kamlet-Jacobs equation. Three candidates （pentanitrophenol, pentanitrobenzene, and hexanitrobenzene） were recommended as potential high energy density compounds for their perfect detonation performances and reasonable stability. The pyrolysis mechanism was studied by analyzing the bond dissociation energy （BDE） and the activation energy （Ea） of hydrogen transfer （H--T） reaction for those with adjacent nitro and hydroxyl groups. The results show that Ea is much lower than BDEs of all bonds, so when there are adjacent nitro and hydroxyl groups in a molecule, the stability of the compound will decrease and the pyrolysis will be initiated by the H--T process. Otherwise, the pyrolysis will start from the breaking of the weakest C--NO2 bond, and only under such condition, the Mulliken population or BDE of the C--NO2 bond can be used to assess the relative stability of the compound.

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Density functional theory (DFT) method has been employed to study the effect of nitroamino group as a substituent in cyclopentane and cyclohexane, which usually construct the polycyclic or caged nitra-mines. Molecular structures were investigated at the B3LYP/6-31G** level, and isodesmic reactions were designed for calculating the group interactions. The results show that the group interactions ac-cord with the group additivity, increasing with the increasing number of nitroamino groups. The dis-tance between substituents influences the interactions. Detonation performances were evaluated by the Kamlet-Jacobs equations based on the predicted densities and heats of formation, while thermal stability and pyrolysis mechanism were studied by the computations of bond dissociation energy (BDE). It is found that the contributions of nitroamino groups to the detonation heat, detonation velocity, detonation pressure, and stability all deviate from the group additivity. Only 3a, 3b, and 9a-9c may be novel potential candidates of high energy density materials (HEDMs) according to the quantitative cri-teria of HEDM (ρ ≈ 1.9 g/cm3, D ≈ 9.0 km/s, P ≈ 40.0 GPa). Stability decreases with the increasing number of N-NO2 groups, and homolysis of N-NO2 bond is the initial step in the thermolysis of the title com-pounds. Coupled with the demand of thermal stability (BDE > 20 kcal/mol), only 1,2,4-trinitrotriazacy-clohexane and 1,2,4,5-tetranitrotetraazacyclohexane are suggested as feasible energetic materials. These results may provide basic information for the molecular design of HEDMs.