First-principles band gap criterion for impact sensitivity of energetic crystals: a review

Balancing the Trade-Off Between Detonation Power and Safety by Spatially Anchoring Copper Azide on Nitrogen-Doped Reduced Graphene Oxide

Developing an effective tailoring approach to overcome the intrinsic trade-off between detonation power and safety in energetic materials is crucial for micro-electromechanical detonation systems but remains challenging. Herein, the anchoring of the high-energy-density yet highly sensitive primary explosive copper azide (CA) onto an N-doped reduced graphene oxide (NrGO) shell (denoted as CA@NrGO) is reported via electronic interactions. This approach simultaneously achieves a three-fold enhancement in mechanical safety, a ≈36-fold improvement in electrostatic safety compared to pure CA, and high detonation capacity. Theoretical calculations reveal that the electronic interaction between NrGO and CA not only facilitate energy dissipation from mechanical forces acting on CA-via intralayer compression and slip, thereby enhancing mechanical safety-but also promote interfacial electron transfer from CA to NrGO, preventing charge accumulation in CA and improving electrostatic safety. Furthermore, the excellent detonation power of CA@NrGO is demonstrated in a micro-detonation device, where 6 mg of CA@NrGO reliably initiated 20 mg of the secondary explosive CL-20. This work highlights how manipulating electronic interactions between energetic materials and their supports contributes to the design of high-energy-density yet safe energetic materials for miniaturized detonation devices.

First-principles study on energetic cocrystals of CL-20/4,5-MDNI with two different stoichiometric ratios under high pressure

CONTEXT

This research determined the crystal structure, molecular structure, electronic structure, optical properties, mechanical properties, and Hirshfeld analysis of the CL-20/4,5-MDNI cocrystal at two distinct stoichiometric ratios under hydrostatic pressures varying from 0 to 100 GPa. The findings revealed that the CL-20/4,5-MDNI cocrystal with a 1:1 ratio experienced two structural transitions at pressures of 80 GPa and 90 GPa. Notably, new covalent bonds, C10-O13 and C9-O14, were established, whereas the C10-H10C bond was disrupted. In contrast, the CL-20/4,5-MDNI cocrystal with a 1:3 ratio underwent three structural transformations at pressures of 55 GPa, 63 GPa, and 95 GPa, leading to the creation of new covalent bonds such as C17-N35, C25-N43, C14-O9, C21-O7, and N27-H9. These transitions were corroborated through the examination of lattice parameters, variations in covalent bond lengths, density of states, and optical coefficients. Additionally, the study explored the similarities and differences between the two cocrystals in terms of their crystal structure, molecular structure, electronic properties, optical properties, mechanical properties, and Hirshfeld analysis.

METHOD

In this investigation, the CASTEP module from the Materials Studio software package was utilized to perform first-principles calculations based on density functional theory (DFT). Specifically, the Broyden-Fletcher-Goldfarb-Shanno (BFGS) optimization technique was applied to refine the geometric structures of the CL-20/4,5-MDNI cocrystals, which were prepared in the stoichiometric ratios of 1:1 and 1:3. These calculations were conducted under a range of hydrostatic pressures, varying from 0 to 100 GPa. To achieve a fully relaxed state at atmospheric pressure, the Perdew-Zunger local density approximation (LDA/CA-PZ) functional was employed. The plane wave cutoff energy was meticulously set at 489 eV to ensure the convergence of the total energy within the unit cell system. Additionally, the k-point mesh was configured as 1 × 1 × 1 to facilitate accurate calculations. Before each simulation, different hydrostatic pressures were systematically applied to analyze the structural changes under varying conditions.

Theoretical study on multi-perspective interaction analysis of ADN and ADN-H2O-CH3OH solutions

CONTEXT

Revealing the mechanism of intermolecular interactions in dinitroamine ammonium (ADN)-based liquid propellants and exploring the reasons for their performance changes, multi-perspective interaction analyses of ADN and ADN-water (H2O)-methanol (CH3OH) solutions have been conducted via theoretical methods. The band structure, density of states (DOS), surface electrostatic potential (ESP), Hirshfeld surface, reduced density gradient (RDG), AIM topological analysis, and detonation performance were studied and the results showed that both the ADN and ADN-H2O-CH3OH solutions had hydrogen bonds and van der Waals interactions. By introducing the small molecules H2O and CH3OH, the detonation performance of the ADN-H2O-CH3OH solution slightly decreased, but its sensitivity also decreased. Overall, the comprehensive performance of the ADN-H2O-CH3OH solution has improved, and the application range has expanded. These results are helpful for obtaining a deeper understanding of ADN-based liquid propellants at the atomic level and contribute to the development of new liquid propellants.

METHODS

The ADN and ADN-H2O-CH3OH solutions were constructed by Amorphous cell module and optimized via GGA with PBE methods in the Dmol3 module of the Materials Studio, and their electronic properties were calculated. Hirshfeld surfaces were generated with CrystalExplorer 3.0. A topological analysis of a variety of molecular clusters was performed via QTAIM. The QTAIM and RDG analyses in this work were generated by Multiwfn 3.0.

Boosting 2000-Fold Hypergolic Ignition Rate of Carborane by Substitutes Migration in Metal Clusters

Hypergolic propellants rely on fuel and oxidizer that spontaneously ignite upon contact, which fulfill a wide variety of mission roles in launch vehicles and spacecraft. Energy-rich carboranes are promising hypergolic fuels, but triggering their energy release is quite difficult because of their ultrastable aromatic cage structure. To steer the development of carborane-based high-performance hypergolic material, carboranylthiolated compounds integrated with atomically precise copper clusters are presented, yielding two distinct isomers, Cu14B-S and Cu14C-S, both possessing similar ligands and core structures. With the migration of thiolate groups from carbon atoms to boron atoms, the ignition delay (ID) time shortened from 6870 to 3 ms when contacted with environmentally benign oxidizer high-test peroxide (HTP, with a H2O2 concentration of 90%). The extraordinarily short ignition ID time of Cu14B-S is ranking among the best of HTP-active hypergolic materials. The experimental and theoretical findings reveal that benefitting from the migration of thiolate groups, Cu14B-S, characterized by an electron-rich metal kernel, displays enhanced reducibility and superior charge transfer efficiency. This results in exceptional activation rates with HTP, consequently inducing carborane combustion and the simultaneous release of energy. This fundamental investigation shed light on the development of advanced green hypergolic propulsion systems.

Molecular Structures, Dipole Moments, and Electronic Properties of β-HMX under External Electric Field from First-Principles Calculations

In order to investigate the impact of an external electric field on the sensitivity of β-HMX explosives, we employ first-principles calculations to determine the molecular structure, dipole moment, and electronic properties of both β-HMX crystals and individual β-HMX molecules under varying electric fields. When the external electric field is increasing along the [100], [010], and [001] crystallographic directions of β-HMX, the calculation results indicate that an increase in the bond length (N1-N3/N1'-N3') of the triggering bond, an increase in the main Qnitro (N3, N3') value, an increase in the minimum surface electrostatic potential, and a decrease in band gap all contribute to a reduction in its stability. Among these directions, the [010] direction exhibits the highest sensitivity, which can be attributed to the significantly smaller effective mass along the [010] direction compared with the [001] and [100] directions. Moreover, the application of an external electric field along the Y direction of the coordinate system on individual β-HMX molecules reveals that the strong polarization effect induced by the electric field enhances the decomposition of the N1-N3 bonds. In addition, due to the periodic potential energy of β-HXM crystal, the polarization effect of β-HMX crystal caused by an external electric field is much smaller than that of a single β-HXM molecule.

Molecular dynamics simulation of sensitivity of HMX, FOX-7, and TATB crystals

METHODS

This study used molecular dynamics (MD) to simulate three materials (HMX, FOX-7, and TATB) under the NVT ensemble. Six temperatures (100 K, 200 K, 300 K, 400 K, 500 K, and 600 K) were simulated. In addition, the trigger bond lengths, energy bands, and density of states of three materials were obtained at different temperatures and compared with the calculated results at 0 K.

CONTEXT

The results indicate that the trigger bond lengths of the three materials are very close to the experimental values. Overall, the maximum and average bond lengths of the trigger bonds increase with increasing temperature. The band gap value decreases with increasing temperature. The changes in trigger bond length and band gap value are consistent with the experimental fact that sensitivity increases with increasing temperature. And Eg > 1 eV is consistently found within the temperature range of 0-600 K, indicating that all three materials are non-metallic compounds.

Study of the relationship between pressure and sensitivity of energetic materials based on first-principles calculation

CONTEXT

In order to study the relationship between the sensitivity and pressure of energetic materials, six kinds of energetic materials were selected as the research object. The crystal structure, electronic, and phonon properties under hydrostatic pressure of 0 ~ 45 GPa were calculated by first principles. The calculation results show that the lattice parameters and band gap values of these six energetic materials decrease with the increase of pressure. The peak of the density of states decreases and moves to the low energy direction, and the electrons become more active. Meanwhile, the effect of pressure on the sensitivity of the energetic materials is analyzed based on the multi-phonon up-pumping theory. The number of doorway modes and integral of projected phonon density of states under high pressure is calculated. The results show that both of them increase with the increase of pressure. And the smaller the value of the band gap, the larger the number of doorway modes and integral of projected phonon density of states, and the more sensitive the energetic material is.

METHODS

All calculations are performed using the Materials Studio software based on density functional theory. The Perdew-Burke-Ernzerhof (PBE) functional of the generalized gradient approximation (GGA) is used to calculate the exchange correlation function, and the Grimme dispersion correction method is used to deal with the weak intermolecular interaction. The structure of the compound was optimized by BFGS algorithm. The linear response is used to calculate the phonon properties of energetic materials. The plane wave cutoff energy was set to 830 eV. The K-point grids of TATB, FOX-7, TNX, RDX, TNT, and HMX were chosen as 2 × 2 × 2, 2 × 2 × 1, 2 × 1 × 1, 1 × 1 × 1, 1 × 2 × 1, and 2 × 1 × 2.

Periodic DFT study of structural transformations of cocrystal CL-20/MMI under high pressure

CONTEXT

The crystal and molecular structure, electronic properties, optical parameters, and elastic properties of a 1:2 hexanitrohexaazaisowurtzitane (CL-20)/2-mercapto-1-methylimidazole (MMI) cocrystal under 0 ~ 100 GPa hydrostatic pressure were calculated. The results show that the cocrystal CL-20/MMI undergoes three structural transitions at 72 GPa, 95 GPa, and 97 GPa, respectively, and the structural transition occurs in the part of the MMI compound. Structural mutations formed new bonds S1-S2, C2-C7, and N1C5 at 72GPa, 95 GPa, and 97 GPa, respectively. Similarly, the formation of new bonds is confirmed on the basis of an analysis of the changes in lattice constants, cell volumes, and partial densities of states (PDOS) for S1, S2, C2, C7, N1, and C3 at the corresponding pressures. The optical parameters show that the pressure makes the peaks of various optical parameters of CL-20/MMI larger, and the optical activity is enhanced. The optical parameters also confirm the structural mutation of CL-20/MMI under the corresponding pressure.

METHOD

CL-20/MMI was calculated by using the first-principles norm-conservative pseudopotential based on density functional theory (DFT) in the CASTEP software package. For the optimization results, the Broyden-Fletcher-Goldfarb-Shanno (BFGS) method is selected to optimize the geometry of the cocrystal in the range of 0-100 GPa. GGA/PBE (Perdew-Burke-Ernzerhof) was selected to relax the cocrystal CL-20/MMI fully without constraints at atmospheric pressure. The sampling scheme in the Brillouin zone [10] is the Monkhorst-Pack scheme, and the number of k-point grids was 2 × 2 × 2. By contrast, this study will use the LDA method to calculate.

Infrared spectra and electronic structural changes of DNTF under high pressure: experimental and theoretical studies

3,4-Bis(3-nitrofurazan-4-yl)furoxan (DNTF) is a novel energetic material with an excellent performance and has attracted considerable attention. Motivated by recent theories and experiments, we had carried out experimental and theoretical studies on the high-pressure responses of vibrational characteristics, in conjunction with structural and electronic characteristics. It is found that all observed infrared spectra peaks seem to shift towards higher frequencies. And the peaks attributed to N-Oc (coordinated oxygen atom) stretching vibrations become broader due to the decrease of lattice constants and the free region of DNTF crystals with the increase of pressure, where the a-direction is more sensitive to pressure. In addition, the non-covalent interaction between adjacent DNTF molecules in the same layer changes from the van der Waals interaction to the steric effect with the increase of pressure, and that between layers also changes from the van der Waals interaction to the π-π stacking interaction. More importantly, these results highlight that the increase of pressure may lead to the stability decrease and impact the sensitivity increase of DNTF. This study can deepen the understanding of the energetic material DNTF under high pressure and is of great significance for blasting and detonation applications of DNTF.

Bistriazolotriazole-tetramine: commendable energetic moiety and cation

CONTEXT

Various 7H,7'H-[6,6'-bi[1,2,4]triazolo[4,3-b][1,2,4]triazole]-3,3',7,7'-tetramine (A) based nitrogen-rich energetic salts were designed and their properties explored. All energetic salts possess relatively high nitrogen content (> 48%), positive heats of formation (> 429 kJ/mol) and stability owing to a significant contribution from fused backbone. The cationic component shows a very high heat of formation (2516 kJ/mol); therefore, it is highly suitable for enthalpy enhancement in new energetic salts. The cation was paired with the energetic anions nitrate (NO3-), perchlorate (ClO4-), dinitromethanide (CH(NO2)2-), trinitromethanide (C(NO2)3-), nitroamide (NHNO2-), and dinitroamide (N(NO2)2-) to improve oxygen balance and detonation performance. Designed salts show moderate detonation velocities (7.9-8.7 km/s) and pressures (23.8 - 33.1 GPa). The distribution of frontier molecular orbitals, molecular electrostatic surface potentials, QTAIM topological properties, and noncovalent interactions of designed salts were simulated to understand the electronic structures, charge distribution in molecules, hydrogen bonding, and other nonbond interactions. The predicted safety factor (SF) and impact sensitivity (H50) of designed salts suggest their insensitivity to mechanical stimuli. This work explored the 7H,7'H-[6,6'-bi[1,2,4]triazolo[4,3-b][1,2,4]triazole]-3,3',7,7'-tetramine as a suitable cationic component which could be promising and serve exemplarily in energetic materials.

METHODS

The optimization and energy calculations of all the designed compounds were carried out at the B3LYP/6-311 +  + G(d,p) and M06-2X/def2-TZVPP levels, utilizing the Gaussian software package. The molecular surface electrostatic potential, quantum theory of atoms in molecules (QTAIM), reduced density gradient (RDG), and noncovalent interaction (NCI) analysis were performed by employing Multiwfn software. The EXPLO5 (v 7.01) thermochemical code and PILEM web application were used to predict the detonation properties.

The structural and electronic properties of (001) surface of 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) with first-principles calculations

CONTEXT

1,3,5-triamino-2,4,6-trinitrobenzene (TATB) is a typical insensitive energetic material. It can be used in explosive formulations, such as PBX-9502 and LX-17-0. TATB is an intriguing and unusual explosive for another reason: it crystallizes into a wide array of planar hydrogen bonds, forming a graphite-like layered structure. Therefore, TATB is one of the important research objects, and its surface structure needs to be deeply understood. In this research work, the electronic and energetic properties of TATB (001) surface are explored.

METHODS

In this paper, the structural, electronic, energetic properties and impact sensitivity of TATB (001) surface structure at 0 and -3 GPa along with x-axis were calculated in this study using the first-principles calculations. The calculations in this paper are performed in the CASTEP code, which is based on the density functional theory with the first-principles calculation method using the plan-wave pseudopotential approach. The exchange-correlation interaction was adopted by the generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) functional. The DFT-D method with the Grimme correction accurately models van der Waals interactions. To model the surface structures of TATB, the planar slab method was employed. We constructed TATB (001) periodic slabs including three layers with a 15-Å vacuum layer.

Azo-Bridged Triazole Macrocycles: Computational Design, Energy Content, Performance, and Stability Assessment

Oxadiazole and triazole are extensively investigated heterocyclic scaffolds in the development of energetic materials. New energetic molecules were designed by replacing 1,2,5-oxadiazole with 2H-1,2,3-triazole in the reported conjugated macrocyclic systems to assess the influence on the energetic properties and stability. In addition, nitro groups were introduced in triazole units (N-functionalization) to improve the energetic performance. Energetic properties, including heat of formation, oxygen balance, density, detonation pressure and velocity, and impact sensitivity, were estimated for these triazole-based macrocycles. The replacement of 1,2,5-oxadiazole with 2H-1,2,3-triazole and 2-nitro-1,2,3-triazole significantly enhances the energy content, detonation performance, and noncovalent interactions. The theoretically computed energetic properties of triazole-based macrocycles reveal high positive heats of formation (1507-2761 kJ/mol), oxygen balance (-88.8 to -22.8%), high densities (1.87-1.90 g/cm3), superior detonation velocities (8.41-9.52 km/s), pressures (26.64-40.55 GPa), acceptable impact sensitivity (27-40 cm), and safety factor (51-290). The overall energetic assessment highlights triazole-based macrocycles as a potential framework that will be useful for developing advanced energetic materials.

Theoretical study on the correlation between the structure, excess energy, surface energy, electronic structure, nitro charge, and friction sensitivity of N,N'-dinitroethylenediamine (EDNA)

The sensitivity of energetic materials along different crystal directions is not the same and is anisotropic. In order to explore the difference in friction sensitivity of different surfaces, we calculated the structure, excess energy, surface energy, electronic structure, and the nitro group along (1 1 1), (1 1 0), (1 0 1), (0 1 1), (0 0 1), (0 1 0), and (1 0 0) surfaces of EDNA based on density functional theory. The analysis results showed that relative to other surfaces, the (0 0 1) surface has the shortest N-N average bond length, largest N-N average bond population, smallest excess energy and surface energy, widest band gap, and the largest nitro group charge value, which indicates that the (0 0 1) surface has the lowest friction sensitivity compared to other surfaces. Furthermore, the conclusions obtained by analyzing the excess energy are consistent with the results of the N-N bond length and bond population, band gap, and nitro charge. Therefore, we conclude that the friction sensitivity of different surfaces of EDNA can be evaluated using excess energy.

Structural, vibrational and electronic properties of nitrogen-rich 2,4,6-triazide-1,3,5-triazine under high pressure

CONTEXT AND RESULTS

2,4,6-triazide-1,3,5-triazine (TAT) has received widespread attention for its great potential to synthesize or convert to nitrogen-rich high energy density materials (HEDMs). The TAT structure alteration in the compression process up to 30 GPa has characteristics as follows: (a) [N₃] groups straighten; (b) [N₃] groups gather toward the six-membered C-N heterocycles. At about 5 GPa, Raman peak split at 700 cm^-1 was observed both in calculation and in-situ Raman experiment, which is caused by pressure-induced intramolecular stress. Besides, the broad band of the amorphous two-dimensional C=N network (centered at 1630 cm^-1) occurred at about 12 GPa. Meantime, the study on electronic features suggests the pressure-induced deformation in TAT molecular structure cause the discontinuous change of band gap at about 4.5 GPa and 8.0 GPa, respectively.

COMPUTATIONAL AND THEORETICAL TECHNIQUES

The static compression process of TAT was explored in the range of 0-30 GPa by using dispersion corrected density functional theory (DFT-D) calculations combined with in-situ Raman experiment. The GGA/PBE+G06 method that has less errors than other calculation methods was used to predict the geometry structure, vibrational properties and electronic structure of TAT under pressure.

Initial Decomposition of DATB Induced by an External Electric Field

1,3-Diamino-2,4,6-trinitrobenzene (DATB), a nitro aromatic explosive with excellent properties, can be detonated by an electric field. Using first-principles calculation, we have investigated the initial decomposition of DATB under an electric field. In the realm of electric fields, the rotation of the nitro group around the benzene ring will cause deformation of the DATB structure. Furthermore, when an electric field is applied along the [100] or [001] direction, the C4-N10/C2-N8 bonds initiate decomposition due to electron excitation. On the contrary, the electric field along the [010] direction has a weak influence on DATB. These, together with electronic structures and infrared spectroscopy, give us a visual perspective of the energy transfer and the decomposition caused by C-N bond breaking.

Theoretical study on the structure, electronic properties, intermolecular interactions, and detonation performance of DAF:ADNP co-crystal

CONTEXT

Explosives have a wide range of applications in many fields due to their high energy and high density. Recently, a new synthesized co-crystal explosive DAF:ADNP presents high detonation performance and low sensitivity. This work is aimed to understand how the structure and intermolecular interactions affect the performance of the DAF:ADNP co-crystal. The results indicate that the formed π-π interactions and stronger hydrogen bonds in the co-crystal enhance its stability and its impact sensitivity is reduced. The strong intralayer H···N and H···O interactions and interlayer π-π stacking are the main driving force for the formation of the co-crystal. Compared with the pure crystals, the detonation performance of the co-crystal slightly decreases, while its sensitivity reduces.

METHODS

All calculations were used the DFT-PBE-D method with Vanderbilt-type ultrasoft pseudopotentials and plane wave (340.0 eV) in the CASTEP package. Radial distribution function were calculated by NVT-MD simulations for 100 ps with a time step of 1 fs at 298 K. Hirshfeld surfaces were generated by CrystalExplorer 3.0 and reduced density gradient analyses were performed by Multiwfn 3.0.

Pressure-dependent structure and electronic properties of energetic NTO crystals dominated by hydrogen-bonding interactions

3-Nitro-1,2,4-trihydroxy-5-one (NTO), a highly potential high-performance explosive with good thermal stability and low sensitivity, has attracted much attention for its physicochemical properties in recent years. In this work, the pressure effect of the vibrational and electronic properties is investigated to understand the intermolecular interaction of NTO under hydrostatic compression. From the pressure-dependent Raman and infrared spectra, we found that the red-shifts of high-wavenumber N-H stretching modes and the discontinuous shifts of all Raman modes occur at 3 and 6 GPa, indicating an evident change of molecular configuration and intermolecular interaction upon compression. Based on structural analysis, the changes of intra- and intermolecular hydrogen bonds (HBs) are closely relevant to the anomalous rotation of the nitro group and the lengthening of N-H bonds, which can be treated as an important step of a potential structural transformation of NTO. Moreover, intermolecular hydrogen-bonding interaction leads to the shrinkage of the band gap at 6 GPa, caused by the fast charge transfer of 0.07 e from the nitrogen heterocycle to the nitro group. These results manifest a non-covalent interaction mechanism for modulating the molecular configuration of EMs under pressure loading and provide vital insights into understanding the pressure effects for energetic molecular crystals.

Theoretical investigation of vibrational and electronic properties of HMX crystal under uniaxial compression

Octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) is a typical energetic molecular crystal with excellent detonation performance and good thermal stability, has been widely used in military and civilian purposes. In this work, the vibrational properties of HMX combined with structure and electronic properties are studied to understand its pressure response against uniaxial compressions. The calculated eigenvalues of stress tensors show significant anisotropy of intermolecular interactions. Especially, the direction of shear stressτ_xyandτ_xzin [100] compression have an abrupt change nearV/V₀= 0.84. Further, Raman spectra under each uniaxial compression are simulated to inspect the molecular configuration of HMX. Compared to the blue shifts of [010] and [001] orientations, the discontinuous Raman shifts of atV/V₀= 0.86-0.84 in [100] orientation suggest that HMX would undergoes a possible structural transformation at the pressure of 6.82-9.15 GPa. Structural analysis implies that the subtle rotation of NO₂group is changed by intermolecular interactions of HMX. Moreover, the abnormal evolution of band gap is observed atV/V₀= 0.84 in [100] orientation, which is associated with the structure modification of HMX. Overall, the compression behaviors of HMX under uniaxial compressions would provide a useful insight for the actual shock compression conditions.

Single-Crystal Diffraction, Raman Spectroscopy, and Density Functional Theory of DTO [N-(1,7-Dinitro-1,2,6,7-tetrahydro-[1,3,5]triazino[1,2-c][1,3,5]oxadiazin-8(4H)-ylidene)nitramide]

A combined experimental and modeling study of energetic compound N-(1,7-dinitro-1,2,6,7-tetrahydro-[1,3,5]triazino[1,2-c][1,3,5]oxadiazin-8(4H)-ylidene)nitramide [C₅H₆N₈O₇, (DTO)] has been performed. We report its crystal structure, solid-phase heat of formation, and its vibrational and electronic structure obtained by single-crystal X-ray diffractometry, Raman spectroscopy, and density functional theory (DFT). DTO exhibits two adjoining six-membered rings, a triazine ring (C₃N₃) and an oxadiazine ring (C₃N₂O) ring containing two nitro functional groups and one nitroamino group. DTO crystallizes with four molecules in its unit cell and presents a density of 1.862 kg/m³ at 298 K, in excellent agreement with both DFT calculations performed both at the molecular level using the B3LYP with the 6-311+G** basis set and the solid-state level using the hybrid functional HSE6 optimized with norm-conserving pseudopotentials. The calculated vibrational structure allows for the symmetry assignment of key Raman modes in terms of atomic movements, and the calculated frequency values are in good agreement with experiment. The solid-phase DFT calculations reveal that the N atoms of the triazine ring contribute mostly to the density of states at the Fermi level. In addition, we present and discuss the computed solid-phase heat of formation (215.9 kJ/mol) and molecular electrostatic potential surface of DTO and compare them to complementary materials.

First-principles study on the mechanical and electronic properties of energetic molecular perovskites AM(ClO4)3 (A = C6H14N2 2+, C4H12N2 2+, C6H14N2O2+; M = Na+, K+)

Density functional theory (DFT) simulations were conducted to study the crystal structures, and mechanical and electronic properties of a series of new energetic molecular perovskites, including (C6H14N2)[Na(ClO4)3], (C6H14N2)[K(ClO4)3], (C4H12N2)[Na(ClO4)3] and (C6H14N2O)[K(ClO4)3], abbreviated as DAP-1, DAP-2, PAP-1, and DAP-O2, respectively. By calculating the elastic constants, moduli (Young's modulus E, bulk modulus B, and shear modulus G), Poisson ratio ν and Pugh's ratio B/G, we found that the four energetic molecular perovskites not only possessed good mechanical stability but excellent mechanical flexibility and ductility. In addition, DFT calculations were used to investigate the electronic properties of all of the perovskite compounds. The band gaps of DAP-1 and DAP-2 were comparable, and the band gap of PAP-1 was the smallest and that of DAP-O2 was the largest. A comprehensive analysis of the density of states and the M-O bonding characteristics provided a good explanation for the band gap characteristics. Besides, we found that the modulus properties of these molecular perovskite energetic compounds are also tightly bound to the strength of M-O bonding.

Anisotropic Impact Sensitivity of Metal-Free Molecular Perovskite High-Energetic Material (C6H14N2)(NH2NH3)(ClO4)3 by First-Principles Study

Density functional theory simulations were carried out to investigate energetic molecular perovskite (C₆H₁₄N₂)(NH₂NH₃)(ClO₄)₃ which was a new type energetic material promising for future application. The electronic properties, surface energy, and hydrogen bonding of (100), (010), (011), (101), (111) surfaces were studied, and the anisotropic impact sensitivity of these surfaces were reported. By comparing the values of the band gaps for different surface structures, we found that the (100) surface has the lowest sensitivity, while the (101) surface was considered to be much more sensitive than the others. The results for the total density of states further validated the previous conclusion obtained from the band gap. Additionally, the calculated surface energy indicated that surface energy was positively correlated with impact sensitivity. Hydrogen bond content of the surface structures showed distinct variability according to the two-dimensional fingerprint plots. In particular, the hydrogen bond content of (100) surface was higher than that of other surfaces, indicating that the impact sensitivity of (100) surface is the lowest.

Programming a Metal-Organic Framework toward Excellent Hypergolicity

Exploring novel hypergolic fuels for modern space propulsion is highly desired. However, the analysis and understanding of the structure and hypergolic performance at the molecular level are still insufficient. To understand the factors that dictate hypergolicity, we conducted a comparative study on a series of metal-organic frameworks (MOFs) characterized by the same topology but with varied ligand structures. The ignition delay (ID) time trend was found to be imidazole < triazole < tetrazole, and the rapid ID time was 8 ms. By combining experimental studies and density functional theory (DFT) calculations, we found that propargyl and cyanoborohydride groups that functioned as dual hypergolic triggers contributed to the hypergolicity, and a distinct electronic structure was detrimental to ID time. The structure-performance relationships presented herein can potentially provide some fundamental insights into the field of developing high-performance hypergolic fuels.

Unimolecular Decomposition Reactions of Picric Acid and Its Methylated Derivatives─A DFT Study

To handle energetic materials safely, it is important to have knowledge about their sensitivity. Density functional theory (DFT) has proven a valuable tool in the study of energetic materials, and in the current work, DFT is employed to study the thermal unimolecular decomposition of 2,4,6-trinitrophenol (picric acid, PA), 3-methyl-2,4,6-trinitrophenol (methyl picric acid, mPA), and 3,5-dimethyl-2,4,6-trinitrophenol (dimethyl picric acid, dmPA). These compounds have similar molecular structures, but according to the literature, mPA is far less sensitive to impact than the other two compounds. Three pathways believed important for the initiation reactions are investigated at 0 and 298.15 K. We compare the computed energetics of the reaction pathways with the objective of rationalizing the unexpected sensitivity behavior. Our results reveal a few if any significant differences in the energetics of the three molecules, and thus do not reflect the sensitivity deviations observed in experiments. These findings point toward the potential importance of crystal structure, crystal morphology, bimolecular reactions, or combinations thereof on the impact sensitivity of nitroaromatics.

Pressure-induced phase transitions and mechanical properties of insensitive high explosive 1,1-diamino-2,2-dinitroethylene

The structural and mechanical properties of an insensitive high-explosive 1,1-diamino-2,2-dinitroethylene (FOX-7) polymorphs were studied using dispersion-corrected density functional theory calculations. The predicted lattice parameters of FOX-7 polymorphs agree well with the available single-crystal X-ray diffraction data. From our elastic modulus calculations, we found that the ε phase has the highest shear modulus G, Young’s modulus E, longitudinal speed C L, and shear speed C S, respectively. Moreover, both α and α′ phase are brittle, ε phase is ductile nature. The results of Hirshfeld surfaces and fingerprint plots indicate that the α and α′ phase possess similar molecular packing modes. Meanwhile, the ε phase is found to have the strongest π…π interactions because of the nearly planer molecules formed a planar layer in the crystal. The pressure effects on the α and α′ phase presented an obvious anisotropy, a pressure-induced phase transition from phase α′ (P21/n) to ε phase (P1) was studied. And we also analyze the influence of pressure on the electronic structure.

Are HOMO-LUMO gaps reliable indicators of explosive impact sensitivity?

A high priority in designing and evaluating proposed explosives is to minimize sensitivity, i.e., vulnerability to unintended detonation due to an accidental stimulus, such as impact. In order to establish a capability for predicting impact sensitivity, there have been numerous attempts to correlate it with some molecular or crystal property or properties. One common approach has been to relate impact sensitivity to the difference between the energies of the highest-occupied and lowest-unoccupied molecular orbitals of the explosive molecule, the "HOMO-LUMO gap." In the present study, we tested this approach for a series of twelve explosive nitroaromatics, using four different computational methods. We found that the HOMO-LUMO gap does not appear to be a reliable indicator of relative impact sensitivity. Since detonation initiation involves a series of steps, all of which influence sensitivity; it seems more realistic to try to identify fundamental factors and general trends related to sensitivity ‒ an approach that has already had some success ‒ rather than to seek correlations with one or two specific properties.

Decomposition mechanism on different surfaces of copper azide

Copper azide, a potential primary explosives that may replace traditional primers such as lead azide, mercury fulminate and silver azide, has received widespread attention, but its decomposition mechanism remains unclear. Here, based on first-principles calculations, (010)_N3, (100)_N3and (001) facets with a copper/nitrogen atom ratio of 1/6 are found to be the most stable surfaces of copper azide crystal. Through transition state (TS) calculations, we find that during the decomposition process on the surface, there is a synergy effect between two Cu-N1-N2-N3 chains, where the terminal N2-N3 bonds on two chains break simultaneously, and the dissociated N3 atom bonds with another N3' atom of adjacent chain to form a N₂ molecule. Next, the Cu-N bond will rupture, and two more N₂molecules (N1-N2, N1'-N2') desorb from the surface. The overall reaction releases above 4 eV energy at a barrier of 1.23 eV on (001) surface. Electronic structure calculations reveal that the TS of N2-N3 rupture is more stabilized than that of N1-N2. According to the above results, we propose a new decomposition mechanism based on simulations of N-N bond breaking on different surfaces of copper azide. The results underscore the surface effect in decomposition of energetic materials.

Investigation of electronic and vibrational properties of dihydroxylammonium 5,5'-bistetrazole-1,1'-diolate under high-pressure conditions

Dihydroxylammonium 5,5'-bistetrazole-1,1'-diolate (TKX-50), a newly found ionic energetic material with excellent performance and low sensitivity, has attracted much attention. In this work, the high-pressure response of vibrational properties in conjunction with structural and electronic properties are investigated to understand its stability under hydrostatic and uniaxial compressions. From our calculations, the band gap of TKX-50 broadens up to 0.5 GPa, then gradually shrinks at 0.5-10 GPa due to the unusual evolution of the a-axis. Analysis of the Mulliken population implies that the pressure dependence of the band gap is weak due to the inhibition of charge transfer under hydrostatic pressure. The Raman spectra of TKX-50 under uniaxial and hydrostatic compressions were simulated. The results suggest that TKX-50 undergoes multiple possible structural transformations under uniaxial compressions through discontinuous modifications of bond lengths in the cation moieties, whereas it maintain structural stability up to 10 GPa under hydrostatic conditions. Overall, the investigation of the electronic properties and uniaxial compression responses increases knowledge of TKX-50 under high-pressure conditions.

Predicting the impact sensitivities of energetic materials through zone-center phonon up-pumping

The development of new energetic materials (EMs) is accompanied by significant hazards, prompting interest in their computational design. Before reliable in silico design strategies can be realized, however, approaches to understand and predict EM response to mechanical impact must be developed. We present here a fully ab initio model based on phonon up-pumping that successfully ranks the relative impact sensitivity of a series of organic EMs. The methodology depends only on the crystallographic unit cell and Brillouin zone center vibrational frequencies. We, therefore, expect this approach to become an integral tool in the large-scale screening of potential EMs.

Structure, intermolecular interactions, and dynamic properties of NTO crystals with impurity defects: a computational study

Frontier orbitals distribute in the position of impurity molecules, whose adjacent NTO molecules begin to decompose first.

Review of the molecular and crystal correlations on sensitivities of energetic materials

Highly efficient design on the levels of molecule and crystal, as well as formulation, is highly desired for accelerating the development of energetic materials (EMs). Sensitivity is one of the most important characteristics of EMs and should be compulsorily considered in the design. However, owing to multiple factors responsible for the sensitivity, it usually undergoes a low predictability. Thus, it becomes urgent to clarify which factors govern the sensitivity and what is the importance of these factors. The present article focuses upon the progress of the molecular and crystal correlations on the sensitivity, and the molecule-based numerical models for sensitivity prediction in the past decades. On the molecular level, composition, geometric structure, electronic structure, energy and reactivity can be correlated with the sensitivity; while the sensitivity can be also related with molecular packing pattern, intermolecular interaction, crystal morphology, crystal size and distribution, crystal surface/interface and crystal defect on the crystal level. And most of these factors, in particle on the crystal level, have been employed as variables in numerical models for predicting sensitivity of categorized EMs. Besides, we stress that more attention should be paid to the sensitivity correlations on the inherent structures of EMs, molecule and crystal packing, because they can be readily dealt by molecular simulations nowadays, facilitating to reveal the physical nature of sensitivity.

Effects of molecular vacancy and ethylenediamine on structural and electronic properties of CH3NO2 surfaces

The structural and electronic properties of (100) surface for nitromethane (NM) are studied using density functional theory (DFT) with the generalized gradient approximation and Perdew-Burke-Ernzerhof functional (GGA-PBE). Molecular vacancy and ethylenediamine (C₂H₈N₂) substitution are considered in this work. We find that ethylenediamine substitution significantly decreases the band gap, while molecular vacancy increases the band gap slightly. It indicates that ethylenediamine substitution has a positive effect on the impact sensitivity of NM. Also, the formation energies are calculated and the reasons for the decrease of band gap for ethylenediamine substitution and the increase of band gap for CH₃NO₂ vacancy are explained.

Pressure induced structural behavior of energetic cocrystal TNT/TNB: a density functional theory study

In order to find out the relationship between external pressures and properties of energetic materials, we used the density functional theory (DFT) method to investigate the structural, electronic, and absorption properties of crystalline 2,4,6-trinitrotoluene (TNT)/2,4,6-trinitrotoluene (TNB) under hydrostatic compression of 0-100 GPa. By analyzing the change of lattice constants (a, b, and c) of TNT/TNB under compression conditions, we found that variation tendency of the lattice constants was anisotropic. The b-axis is much stiffer than that along the a- and c-axes, which indicates that the TNT/TNB crystal is anisotropic within a certain pressure region. The pressure-induced structure transformation results in the new covalent bonds O11-C13, O12-C11, O8-C4, and O1-C12 at 60 GPa, and O4-C5 at 80 GPa, respectively. By analyzing the band structure and density of states of TNT/TNB in the pressure range over 40 GPa, the electronic structure of TNT/TNB changed to metallic system, which indicated it becomes more sensitivity under high pressures. The pressure-induced structure transformation of TNT/TNB also contributed to the relatively high optical activity of TNT/TNB at 70 GPa.

Stabilization of the High-Energy-Density CuN5 Salts under Ambient Conditions by a Ligand Effect

A series of excellent works have demonstrated that high-nitrogen-content metal pentazolate (cyclo-N5 -) compounds could be stabilized by high pressure. However, under ambient conditions, low stability precludes their synthesis and application in the field of high-energy-density material. In this work, by using a constrained structure search method, we predicted two new structures as P212121-CuN5 and P21/c-CuN5 containing cyclo-N5 - with strong N-N and Cu-N bonds. In both structures, cyclo-N5 - form four coordination with the Cu+ ligand, which increases the structural stability by lowering the disturbance to the aromaticity of cyclo-N5 -. The calculated results show that the P212121-CuN5 and P21/c-CuN5 structures exhibit high dynamic and thermal stability up to 400 K, indicating that they can be stabilized under ambient conditions. The decomposing energy of P212121-CuN5 and P21/c-CuN5 can reach up to 2.40 and 2.42 kJ/g, respectively. Strikingly, the detonation velocity and the pressure of P212121-CuN5 is predicted to be up to 10.42 km/s and 617.46 kbar, respectively, indicating that they are promising high-energy candidates in the field of explosive combustion.

Effects of boron doping on structural, electronic, elastic, and optical properties of energetic crystal 2,6-diamino-3,5-dinitropyrazine-1-oxide: a theoretical study using the first principles calculation and Hirshfeld surface analysis

Boron-contained compounds are one kind of new energetic materials, and have been synthesized successfully lately. However, the effects of introduced boron atoms into the energetic system are unclear. In this work, using the known insensitive energy crystal 2,6-diamino-3,5-dinitropyrazine-l-oxide (LLM-105) as the model compound, boron doping effects on its crystal structure, band gap and structure, intermolecular contacts, sensitivity, elastic property, optical absorption behavior, and dielectric function were studied by the first principles calculations and Hirshfeld surface analysis. One B atom was doped at four different doping sites in the ring (two kinds of nitrogen N1/N2 and carbon atoms C3/C4), respectively, and formed four new crystals LLM-105-B1/B2/B3/B4. The results showed that the B atom and its doping site both make great influence on the structure and properties. The B doping obviously decreased the band gap and weakened the strength of intermolecular contacts, giving rise to higher sensitivity and worse safety. Especially for LLM-105-B4 which has a 0 eV value of band gap, the doped B atom made great contributions to the density of states around the Fermi level, leading to the suddenly move down of lowest unoccupied molecular orbital and directly link of total density of states at the Fermi level. Doping the B atom at the site C3 improved the ductility and plasticity of LLM-105, while LLM-105-B2 was found to be the most brittle and anisotropic crystal. Doping B atoms at sites N2 and C4 increased the absorption to green, orange, and red lights, while the absorption strength to the infrared light was enhanced in most cases. The dielectric constant and polarity were significantly increased by doping boron atoms at sites C3 and C4.

Insight into the roles of small molecules in CL-20 based host–guest crystals: a comparative DFT-D study

The host–guest inclusion strategy has emerged as a promising method for developing advanced energetic materials and has been successfully applied to a CL-20 crystal.

Possible pre-phase transition of the α-HMX crystal observed by the variation of hydrogen-bonding network under high pressures

The variation of non-covalent interactions in the HMX crystal under high pressures was investigated through disassembling the hydrogen-bonding network.

Prediction of stable energetic beryllium pentazolate salt under ambient conditions

The most stable BeN₁₀ salt was directly predicted at atmospheric pressure, and the energy density is up to 5.36 kJ g⁻¹.

Theoretical design of novel high energy metal complexes based on two complementary oxygen-rich mixed ligands of 4-amino-4H-1,2,4-triazole-3,5-diol and 1,1'-dinitramino-5,5'-bistetrazole

In this study, 16 new energetic metal complexes [M(DNABT)(ATDO), M=Cu, Ni] were designed using the mixed complex construct strategy, which was based on two complementary oxygen-rich high-energy ligands of 1,1'-dinitramino-5,5'-bistetrazole (DNABT) and 4-amino-4H-1,2,4-triazole-3,5-diol (ATDO), then combined with metals Cu and Ni, and further adjusted by the introduction of NO₂ and NH₂. The molecular and electronic structures, heat of formation (HOF), density, detonation velocity, detonation pressure, and sensitivity were investigated by the density functional theory method. The results showed that in metals, the position and amount of NO₂/NH₂ have great effects on the structure and property of metal complexes, and these effects coupled with each other. N-NO₂ bond is the relatively weak bond, and its max length is related with the sensitivity closely. The designed metal complexes all have high HOF (673~868 kJ mol^-1), high density (2.06~2.14 g cm^-3), and ideal oxygen balance (- 19.2~- 6.7%), which further make them have higher detonation velocity (8.76~9.84 km s^-1) and detonation pressure (37.4~46.6 GPa) than three famous high-energy compounds 1,3,5-trinitro-1,3,5-triazine (RDX); 1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX); or even 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20). At the same time, they are less sensitive than RDX, HMX, and CL-20, making them potential candidates for high-energy density compounds.

Hypergolic Triggers as Co-crystal Formers: Co-crystallization for Creating New Hypergolic Materials with Tunable Energy Content

We demonstrate a co-crystal-based strategy to create new solid hypergols, that is, materials exhibiting spontaneous ignition when in contact with an oxidant, from typically non-hypergolic fuel molecules. In these materials, the energy content and density can be changed without affecting the ignition delay. The use of an imidazole-substituted decaborane as a hypergolic "trigger" component in combination with energy-rich but non-hypergolic nitrobenzene or pyrazine yielded hypergolic co-crystals that combine improved combustion properties with ultrashort ignition delays as low as 1 ms.

Interaction Mechanisms of Insensitive Explosive FOX-7 and Graphene Oxides from Ab Initio Calculations

Energetic material–graphene oxide (EM–GO) composites exhibit excellent thermal stability and insensitivity to mechanical stimuli. The interfacial interactions play an important role in affecting the structural and electrical properties of EM–GO composites. FOX-7 crystal with a wave-shaped layer structure is an ideal prototype system for matching with oxygen-rich GO monolayers to form FOX-7–GO composites. Here, we conducted a systematic investigation on FOX-7–GO composites by dispersion-corrected density functional approach. Our results revealed that there exists relatively strong interaction in the FOX-7–GO interface, which stems from the synergistic effect of interfacial charge transfer and hydrogen bonds. The electronic structure analyses demonstrated that GO can hybridize with FOX-7 to reduce charge accumulation on the FOX-7 surface. These theoretical results are useful for clarifying the interfacial effects on the sensitivity of FOX-7–GO composites.

A systematic study of the surface structures and energetics of CH3NO2 surfaces by first-principles calculations

Density functional theory (DFT) has been employed within the generalized gradient approximation and Perdew-Burke-Ernzerhof functional (GGA-PBE) to study the structural and electronic properties of nitromethane (NM) surface models. Different surfaces, including (100), (001), (101), (110), and (111), are considered in this work. The corresponding properties of bulk crystal for NM were also calculated to form a contrast to the slab models. Results with anisotropic characteristics of different surfaces have been observed in this study. There was an obviously great anisotropy in electronic parameters, especially the band gaps of different surfaces, indicating the anisotropic impact sensitivity along different directions of NM. The band gap value for (111) surface, 2.687 eV, was smaller than that of other surfaces, showing a higher impact sensitivity for NM. The estimated anisotropy has been revealed in surface energies for different surfaces. Graphical Abstract The valence band minimum (VBM) and conduction band maximum (CBM) of the nitromethane (100), (001), (101), (110) and (111) surface models.