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.

The regulation of high-energy insensitive compound 2,6-diamino-3,5-dinitropyrazine-1-oxide by external electric field.

CONTEXT: The influence of external electric fields (EEFs) on chemical substances has always been a hot topic in the field of theoretical chemistry research. 2,6-Diamino-3,5-dinitropyrazine-1-oxide (LLM-105) is an energetic material with excellent comprehensive properties and enormous potential for application. This article explores the molecular structure, electronic structure, energy change, frontier molecular orbitals (FMOs) and density of states (DOS), UV-Vis spectra, and infrared spectra of LLM-105 under various electric field conditions. The results indicate that negative EEF can improve the stability of LLM-105, reflected in the initiation of changes in bond length and HOMO-LOMO gap. EEF has a significant impact on the electronic structure of LLM-105. The polarization of the electronic structure brings about a change in total energy, which is reflected in the analysis of energy changes. In addition, the external electric field will cause the frequency of the infrared spectra and the UV-Vis spectra to have different degrees of blue shift. The results of the analysis are helpful to understand the changes of energetic materials under the applied electric field. METHODS: Based on the density functional theory (DFT), the structural optimization and energy calculation were carried out by using B3LYP/6-311G(d, p) and B3LYP/def2-TZVPP methods, respectively. After optimization convergence, vibration analysis was performed without imaginary frequencies to obtain stable configurations. Then, the molecular structure, electronic structure, energy changes, molecular orbital and density of states, UV-Vis spectra, and infrared spectra were analyzed.

Theoretical insight into different energetic groups on the performance of energetic materials 2,5,7,9-tetranitro-2,5,7,9-tetraazabicyclo[4,3,0]nonane-8-one.

CONTEXT: Energetic materials are a class of materials containing explosive groups or containing oxidants and combustibles. The optimization of energetic materials has a significant impact on the development of industry and national defense. For high-energy density compounds (HEDC) that have not been synthesized or are dangerous to experimental operation, it is of guiding significance to predict its energy level, physicochemical properties, and safety through molecular design and theoretical calculation. Cyclic urea nitramine series compounds are a type of energetic compounds with high density and excellent detonation performance. In this study, 2,5,7,9-tetranitro-2,5,7,9-tetraazabicyclo[4,3,0]nonane-8-one (K-56) was used as the parent structure, and 36 energetic derivatives were designed. The effects of introducing single and multiple substituents on the electronic structure, energy gap, heat of formation, detonation performance, thermal stability, thermodynamic parameters, and surface electrostatic potential of K-56 and its derivatives were discussed in detail. The results exhibit the following: (1) the single substitution of -C(NO2)3 (A6) can reduce the detonation velocity of K-56 by 11.9 % and the detonation pressure by 19.8 %, while the double substitution of -C(NO2)3 (B6) can increase the density of K-56 by 11.6 %, the detonation velocity by 10.9 %, and the detonation pressure by 31 %. (2) The heat of formation of K-56 (-110.0 kJ mol-1) increased by 324.18 % and 628.81 %, respectively, proving that -N3 is an extremely effective group to improve HOF. (3) The thermal stability of the derivatives generated by the monosubstitution of the target group on the six-membered ring is better than that of the parent compound. METHODS: Gaussian16 and Multiwfn 3.8 packages are the software for calculation. In this study, the parent structure K-56 and its derivatives were optimized at the B3LYP/6-311G (d,p) level to obtain the zero point energy and thermal correction data of all compounds. Then the vibration analysis of the optimized structure is carried out to confirm that its configuration is stable. Then the M06-2X-D3/def2-TZVPP basis set is used to calculate the single point energy.

Molecular design and theoretical study of oxadiazole-bifurazan derivatives.

CONTEXT: The design and synthesis of new high energy density materials is an important part of the research in the field of high energy materials. However, the synthesis of high-energy materials is very difficult and dangerous. Therefore, it is necessary to design the compounds in advance and evaluate the performance of the designed compounds, so as to screen the high-energy candidate compounds with excellent performance and provide reference for future synthesis and application. 1,2,5-oxadiazole (furazan) and 1,2,4-oxadiazole are five-membered nitrogen-oxygen heterocycles. Because their structures contain high-energy N-O, C=N bonds, they can effectively improve the energy density and oxygen balance of compounds, which has attracted widespread attention. In this paper, 42 kinds of oxadiazole-bifurazan energetic derivatives were designed by inserting different functional groups and changing the parent bridging groups with 1,2,4-oxadiazole and furazan as the basic structural units. Their electronic structures, aromaticity, heats of formation (HOFs), detonation properties, thermodynamic properties and electrostatic potential were systematically studied by density functional than theory (DFT). The results show that -C (NO2)3 has the greatest improvement effect on HOFs among all the substituent groups. The detonation performance of -N=N- bridged oxadiazole-bifurazan derivatives is better than that of -NH-NH- bridged derivatives. And -C(NO2)3 is the most effective group to improve the detonation performance and density of compounds. Compared with the parent compounds, when a -C(NO2)3 was introduced, the density increased by about 5.5%. A6 (D = 10.30 km·s-1, P = 48.86 GPa) and D6 (D = 9.57 km·s-1, P = 42.31 GPa) are the compounds with the best D and P among the designed compounds, which are higher than RDX and HMX, and are potential candidates for new high-energy materials. METHODS: With the help of Gaussian16 software and Multiwfn 3.8 package, the B3LYP method in density functional theory was selected. The 6-311G (d, p) basis set was used to optimize the structure of the 42 derivatives, and the high-precision def2-TZVPP basis set was used to calculate the energy.

Triazene-bridged energetic materials based on nitrotriazole: synthesis, characterization, and laser-ignited combustion performance.

Compared to the widely concerned azo bridges (-NN-), triazene bridges (-NN-NH-) with longer nitrogen chains are also favorable linking units leading to novel energetic materials. In this work, a new family of nitrogen-rich nitrotriazolate-based energetic compounds with a triazene bridge were synthesized and well characterized. The experimental results indicated that most of these new compounds have good thermal stabilities and low sensitivities. Among these, ammonium 5,5'-dinitro-3,3'-triazene-1,2,4-triazolate (3) and potassium 5-nitro-3,3'-triazene-1,2,4-triazolate (7) decomposed at a relatively high temperature (240.6 °C for 3 and 286.9 °C for 7). The impact sensitivities of the obtained compounds ranged from 15 J to 45 J. They also have relatively high positive heats of formation between 667.5 to 817.3 kJ mol-1. The calculated detonation pressures (P) were located between 23.7 and 34.8 GPa, and the calculated detonation velocities (D) were between 8011 and 9044 m s-1. Interestingly, ammonium 5-nitro-3,3'-triazene-1,2,4-triazolate (8) and hydroxylammonium 5-nitro-3,3'-triazene-1,2,4-triazole (10) possessed excellent laser-ignited combustion performance.

Regulation of external electric field on sensitivity of ICM energetic materials.

CONTEXT: [2,2'-Bi(1,3,4-oxadiazole)]-5,5'-dinitramide (ICM-101), 2,4,6-triamino-5-nitropyrimidine-1,3-dioxide (ICM-102), and 6-nitro-7-azido-pyrazol[3,4-d][1,2,3]triazine-2-oxide (ICM-103) are excellent China-made explosives, but their performance under external electric fields (EEF) has never been explored, especially sensitivity. To study the induction effect of EEF on it, the chemical reactivity, electron localization function (ELF), spectrum, and other parameters were calculated by density functional theory. The results show that the increasing EEF can weaken the △EHOMO-LUMO (△EHOMO-LUMO = EHOMO-ELUMO) materials, making the stability worse and the sensitivity higher. The proportion of the positive electrostatic surface potential area is also smaller under the increasing EEF, indicating that ICM molecules are becoming more and more unstable. The ELF and localized orbital locator (LOL) decrease with the increase of EEF strength, which suggests that the trigger bond length increases, the EBDE decreases, and the molecular sensitivity increases. When the intensity of EEF increases, the absorption peak of the molecular spectrum gradually redshifts, and even a weak new absorption peak appears, indicating that the color of the material may change. Finally, EEF strength affects electron density, nitro charge, and chemical reactivity parameters. METHODS: Gaussian 16 software was used for calculation. The calculation levels are B3LYP/6-311G+ (d, p) and B3LYP/Def2-TZVPP. The optimized structure has a local true minimum energy on the potential energy surface and no imaginary frequency. Multiwfn 3.8 and VMD 1.9.3 were used in this work to analyze the ICM series of energetic material wave functions. The strength range of EEF is 0.000-0.016 a.u., and the increasing gradient is 0.002 a.u.

Regulation of external electric field on the high-energy polynitrogen compound 1,5-diaminotetrazole-4 N-oxide.

BACKGROUND: The external electric field (EEF) tends to have a significant impact on chemicals, especially energetic materials. METHODS: Molecular structure, electrostatic potential (ESP), electron density difference, density of states (DOS), and frontier molecular orbitals (FMOs) of 1,5-diaminotetrazole-4N-oxide (SYX-9) are calculated by density functional theory (DFT) at B3LYP/6-311G+(d, p) and M062X/def2-TZVP under external electric field. RESULTS: Calculated results reveal that EEF has definite influence on the trigger bond of SYX-9, especially in positive direction, and the shortening of the trigger bond caused by it can effectively reduce the sensitive of SYX-9. In addition, EEF has an effect on the electron density of SYX-9. The positive EEF can reduce the HOMO-LUMO gap. From the perspective of components of energy variation and the force on atoms, the factors of structural deformation are specifically investigated. The aromaticity of SYX-9 makes its structure stable under the influence of EEF, which is verified by the method of the iso-chemical shielding surface (ICSS).

Structural transformation of methyl urotropine perchlorate under high pressure.

Based on the first-principles calculations of density functional theory (DFT), the crystal structure, molecular structure, electronic properties, and optical absorption properties of methyl urotropine perchlorate under hydrostatic compression in the range of 0 ~ 100 GPa were calculated. The results show that the crystal structure of methyl urotropine perchlorate undergoes two structural transformations under hydrostatic compression. The H1A-H1B bond breaks at 25 GPa, generating two new covalent bonds N3-H1A and O1-H1B. The covalent bonds of O2A-C1 and Cl1-H3A are formed at 85 GPa. The compression ratio of lattice constants (a, b, c) and unit cell volume change abruptly at 25 GPa and 85 GPa, respectively. The conclusion that new bonds are formed under high pressure is further demonstrated by analyzing the partial density of states (PDOS) of N3, H1A, O1, H1B, O2A, C1, Cl1, and H3A atoms. The absorption spectrum showed that the absorption peak of methyl urotropine perchlorate gradually enhanced with the increase of pressure and the highest absorption peak shifted to high frequency.

Theoretical study on BTF-based cocrystals: effect of external electric field.

The external electric field plays an important role in the sensitivity of cocrystal energetic materials. To reveal the influence of external electric field on benzotrifuroxan/2,4,6-trinitroaniline (BTF/TNA), benzotrifuroxan/trinitroazetidine (BTF/TNAZ), benzotrifuroxan/1,3,5-trinitrobenzene (BTF/TNB), and benzotrifuroxan/trinitrotoluene (BTF/TNT) cocrystals' sensitivity, atoms in molecules (AIM), frontier molecular orbitals, nitro group charges (QNO2), electron density values (ρ), electrostatic surface potentials (ESPs), bond dissociation energy (EBDE), and interaction energy (Eint) of the C-NO2 bond were calculated by density functional theory at M062X-D3/ma-def2 TZVPP and B3LYP-D3/6-311 + G (d, p) levels in this article. The results indicate that both negative and positive electric fields reduce the energy gap of the BTF-based cocrystals, and BTF/TNAZ is the most sensitive cocrystal among the four cocrystals. For BTF/TNA and BTF/TNB, the EBDE and the negative charge of the nitro group decreases with increasing positive electric field strength, the Vs max increases with positive electric field strength, and the sensitivity of cocrystal eventually tends to increase under the positive electric field. For BTF/TNAZ and BTF/TNT, the EBDE and the negative charge of the nitro group decrease with increasing negative electric field strength, the Vs max increases with negative electric field strength, and the sensitivity of cocrystal eventually tends to increase under the negative electric field. Finally, the variation in bond length, nitro charge, and AIM electron density values are well correlated with the strengths of the external electric field.

Study on regulation effect of external electric field on energetic material 1-methyl-2,4,5-Trinitroimidazole.

In this work, in order to explore the regulation effect of external electric field (EEF) on various properties of 1-methyl-2,4,5-trinitroimidazole (MTNI), density functional theory (DFT) was used to investigate the molecular structure, electronic structure, frontier molecular orbital (FMO) and ultraviolet-visible (UV-Vis) spectroscopy of MTNI under EEF. The results show that: (1) EEF significantly affects the geometric and electronic structure of MTNI, which in turn affects the sensitivity of MTNI. (2) When the EEF in the positive direction increases, the energy gap of MTNI decreases significantly and the total density of states (TDOS) turns to be dominated by the nitro groups. (3) EEF makes MTNI appear new absorption peak in visible light region and thus makes the MTNI display color. The generation of new absorption peaks is also dominated by the nitro groups. We also carefully analyzed how EEF deforms structure of the MTNI from the perspective of atomic forces and decomposition of energy variation.

Thermodynamic properties, decomposition kinetics of 2-(5-amino-2H-tetrazol-1-yl)-4-amine-3,5-dinitropyridine.

A novel energetic material 2-(5-amino-2H-tetrazol-1-yl)-4-amine-3,5-dinitropyridine (ATDP) was synthesized and characterized by 1H NMR, 13C NMR, mass spectroscopy, and elemental analysis. The research by differential scanning calorimetry (DSC) shows that ATDP decomposed about 290 °C. The calculating results of kinetic parameters using Ozawa method, Kissinger method, and Starink method were quite consistent. Self-accelerated decomposition temperature (TSADT), thermal ignition temperature (TTIT), and critical temperature of thermal explosion (Tb) were 272.55 °C, 121.71 °C, and 137.67 °C, respectively. Geometric optimization, heat of formation, detonation velocity (D), detonation pressure (P), bond dissociation energy (BDE), and electrostatic potential (ESP) were explored using Gaussian 16. The results show that ATDP has a much larger ΔHf,gas value than HMX(272.6 kJ mol-1). The D and P are predicted with the value of 7.50 km s-1 and 24.47 GPa, respectively. The relatively high BDE value (270.77 kJ mol-1) indicates that ATDP has moderate thermal stability.

Theoretical studies of novel high energy density materials based on oxadiazoles.

In this study, 32 energetic compounds were designed using oxadiazoles (1,2,5-oxadiazole, 1,3,4-oxadiazole) as the parent by inserting different groups as well as changing the bridge between the parent. These compounds had high density and excellent detonation properties. The electrostatic potentials of the designed compounds were analyzed using density functional theory (DFT). The structure, heat of formation (HOF), density, detonation performances (detonation pressure P, detonation velocity D, detonation heat Q), and thermal stability of each compound were systematically studied based on molecular dynamics. The results showed that the -N3 group has the greatest improvement in HOF. For the detonation performances, the directly linked -N=N- and -NH-NH- were beneficial when used as a bridge between 1,2,5-oxadiazole and 1,3,4-oxadiazole, and it can also be found that bridge changing had little effect on the trend of detonation performance, while energetic groups changing influenced differently. In general, the introduction of nitro groups contributes to the improvement of the detonation performance of the compounds. In this study, the compounds containing the highest amount of nitro groups were found to have better detonation performance than their counterparts and were not significantly different from RDX and HMX.

Theoretical Studies on the Performance of HMX with Different Energetic Groups.

Forty nitramines by incorporating -C=O, -NH2, -N3, -NF2, -NHNO2, -NHNH2, -NO2, -ONO2, -C(NO2)3, and -CH(NO2)2 groups based on a 1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane (HMX) framework were designed. Their electronic structures, heats of formation (HOFs), detonation properties, thermal stabilities, electrostatic potential, and thermodynamic properties were systematically investigated by density functional theory. The comprehensive relationships between the structures and performance of different substituents were studied. Results indicate that -C(NO2)3 has the greatest effect on improvement of HOFs among the whole substituted groups. Thermodynamic parameters, such as standard molar heat capacity (C p,m θ), standard molar entropy (S m θ), and standard molar enthalpy (H m θ), of all designed compounds increase with the increasing number of energetic groups, and the volumes of energetic groups have a great influence on standard molar enthalpy. Except for -NH2(R1), -NHNH2(R5), and B3, all of the designed compounds have higher detonation velocities and pressures than HMX. Among them, E7 exhibits an extraordinarily high detonation performance (D = 10.89 km s-1, P = 57.3 GPa), E1 exhibits a relatively poor detonation performance (D = 8.93 km s-1, P = 35.5 GPa), and -NF2 and -C(NO2)3 are the best ones in increasing the density by more or less 12%.

Theoretical Study on CL-20-Based Cocrystal Energetic Compounds in an External Electric Field.

An external electric field has great effects on the sensitivity of cocrystal energetic materials. In order to find out the relationship between the external electric field and sensitivity of cocrystals 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane/benzotrifuroxan (CL-20/BTF), 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane/3,4-dinitropyrazole (CL-20/DNP), and 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane/1-methyl-3,5-dinitro-1,2,4-triazole (CL-20/MDNT), density functional theory at B3LYP-D3/6-311+G(d,p) and M062X-D3/ma-def2 TZVPP levels was employed to calculate frontier molecular orbitals, atoms in molecules (AIM) electron density values, bond dissociation energies (BDEs) of the N-NO2 bond, impact sensitivity (H 50), electrostatic potentials (ESPs), and nitro group charges (Q NO2 ) in this work. The results show that a smaller highest occupied molecular orbital-lowest unoccupied molecular orbital gap and the BDEs, as well as H 50, tend to have a larger sensitivity along with the positive directions in the external electric field. Moreover, a smaller local positive ESP (V s max) leads to better stability in the negative electric field. The sensitivity of cocrystal molecules decreases gradually in the negative external electric field with the increase of negative nitro group charges. Finally, the change in the bond lengths, AIM electron density values, and nitro group charges correlate well with the external electric field strengths.

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.

Molecular design and properties of bridged energetic pyridines derivatives.

A series of bridged pyridine-based energetic derivatives were designed and their geometrical structures, electronic structures, heats of formation, detonation properties, thermal stabilities, thermodynamic properties and electrostatic potential were fully investigated using density functional theory. The results show that the steric hindrance effect is a decisive factor for structural stability, and the formation of intramolecular or intermolecular hydrogen bonds doesn't provide advantages to stabilize molecular structure, which was demonstrated by insertion of 3,4,5-trinitro-1H-pyrazole, 3,4-dinitro-1H-pyrazol-5-amine, 3,5-dinitro-1H-pyrazol-4-amine and 3-nitro-1H-1,2,4-triazol-5-amine. The azide group and azo bridge play an important role in improving the heats of formation of energetic pyridine-based materials. All designed molecules were found to have values of density ranging from 1.70 g cm-3 (E6, F6) to 2.11 g cm-3 (D3), values of detonation velocity ranging from 7.1 km s-1 (F1) to 9.77 km s-1 (D8), and values of detonation pressure ranging from 21.5 GPa (F1) to 46.0 GPa (D8). When a p-π conjugation formed between the nitrogen atom and pyridine ring, the bond between nitrogen and hydrogen atoms may be broken as the trigger bond.