Computational studies on the infrared vibrational spectra, thermodynamic properties, detonation properties, and pyrolysis mechanism of octanitrocubane

Crystal engineering of high explosives through lone pair-π interactions: Insights for improving thermal safety

In this high-risk/high-reward study, we prepared complexes of a high explosive anion (picrate) with potentially explosive s-tetrazine-based ligands with the sole purpose of advancing the understanding of one of the weakest supramolecular forces: the lone pair-π interaction. This is a proof-of-concept study showing how lone pair-π contacts can be effectively used in crystal engineering, even of high explosives, and how the supramolecular architecture of the resulting crystalline phases influences their experimental thermokinetic properties. Herein we present XRD structures of 4 novel detonating compounds, all showcasing lone pair-π interactions, their thermal characterization (DSC, TGA), including the correlation of experimental thermokinetic parameters with crystal packing, and in silico explosion properties. This last aspect is relevant for improving the safety of high-energy materials.

Thermal decomposition mechanisms of energetic CL-20-based co-crystals: quantum molecular dynamics simulations

The decomposition mechanisms of energetic CL-20:2,4-dinitro-2,4-diazapentane (DNP) and CL-20:2,4-dinitro-2,4-diazaheptane (DNG) co-crystals at high temperatures (1000, 2000, and 3000 K) were studied by density functional tight-binding molecular dynamics (DFTB-MD) simulations. At different temperatures, their decomposition mechanisms are very different. At 1000 K, conformational changes are observed only for the CL-20:DNG co-crystal, in which the CL-20 changes from β-CL-20 to γ-CL-20. When the temperature is increased to 2000 K, CL-20, DNP, and DNG begin to decompose, and there are five paths for the main initial mechanisms. Further increasing the temperature to 3000 K promotes a more complete decomposition. The initial reactions of CL-20 in the two co-crystals have two channels. There are two initial decomposition channels in the DNP molecule and only one channel in the DNG molecule. As the temperature increases, the decomposition products of the two co-crystals are different. Our work may provide the in-depth understanding of the decomposition mechanisms of high-energy CL-20-based co-crystals at high temperatures.

Combination multi-nitrogen with high heat of formation: theoretical studies on the performance of bridged 1,2,4,5-tetrazine derivatives

A series of bridged tetrazine derivatives (BDDT) were designed by using different bridges to connect two molecules of 1,2,4, 5-tetrazine oxides and then combining different substituents. At the same time, we used DFT-wB97/6-31 + G** method to regularly predict the HOMO-LUMO, heats of formation (HOF), detonation properties, thermal stability, and thermodynamic property orbitals of BDDT compounds. By studying the comprehensive relationship between different substituents and bridging and performance, it is shown that -N(NO₂)₂ and -C(NO₂)₃ are not only excellent groups to improve the heat of formation and detonation properties, but also can cause the compound to have a superior oxygen balance. And that the incorporation of the -N = N- and -NH-N = N- is helpful to enhance their thermal stabilities and HOF. -CH₂-CH₂- and -CH₂-NH- are good for improving the HOMO-LUMO energy gaps. Performances with positive HOF (1170-1590 kJ mol^-1), remarkable density (1.88-1.93 g cm^-3), outstanding detonation properties (D = 9.15-9.80 km s^-1, P = 38.24-44.40 GPa), and acceptable impact sensitivity lead C5, D8, E5, E7, F5, and F7 to be the potential candidates of HEDMs.

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%.

Structural, elastic, vibrational and optical properties of energetic material octanitrocubane studied from first-principles theory

We present a thorough density functional theory based computational study of crystalline properties of cubane caged potential energetic material octanitrocubane (ONC). As expected for a layered molecular solid, van der Waals corrections are inevitable and the same has been incorporated to capture the ground state properties more accurately. Study of Born effective charge and zone centered phonon frequencies using density functional perturbation theory reveals the important role of N2, N4 type nitrogen and associated oxygen atoms in contributing to the high intensity infrared modes. From the calculated electronic band structure we can conclude that ONC is an insulator with a band gap of 5.31 eV. The optical properties of ONC are found to be nearly isotropic in low energy region in spite of strong anisotropic crystal structure.

Computational study of energetic derivatives of 3,3′-bridged ditriazoles

A series of energetic bridged ditriazole was designed by incorporating different bridges and substituents into 4H-1,2,4-triazole ring. The geometrical structures, heats of formation, detonation properties, electronic structures, thermodynamic properties, free spaces, impact sensitivities, and thermal stabilities of the designed compounds were evaluated by employing density functional theory. The results elucidate that the –N3 substituent and –N=N– bridge can sufficiently increase their heats of formation. The calculated values of detonation properties show that –NF2, –ONO2, –O–, and –N=N(O)– are useful structural fragments to improve their detonation performance. The incorporation of the oxy (–O–) bridge increases their HOMO–LUMO energy gaps. An analysis of h50 values indicate that most of the designed compounds are less sensitive. The N(ring)-NO2 bond in the majority of the derivatives may be a possible trigger bond in thermal decomposition process. The incorporation of –CH2–CH2– and –O– is helpful to enhance their thermal stabilities. Based on appropriate thermal stabilities and superb detonation properties, six compounds were screened as promising high energy density compounds.

Unraveling the kinetics and molecular mechanism of gas phase pyrolysis of cubane to [8]annulene

The kinetic and electron density flows are studied theoretically for the gas phase pyrolysis of cubane via its cage opening to reach bicyclooctatriene and then thermal rearrangement of bicyclooctatriene to produce [8]annulene which is the experimentally observed major product. The observed kinetic data at the MN15-L/maug-cc-pVTZ level of theory were in good agreement with the experimental results as compared to the CBS-QB3 method. The cage opening and the thermal rearrangement steps at the experimentally employed temperature of 520 K were exergonic and exothermic. The atmospheric rate constants calculated by means of the RRKM theory show that the cage opening is the rate-determining step. The temperature dependence of the rate constant for the cage opening step at the MN15-L level can be expressed as log(k/s⁻¹)^1bar_MN15-L = (15.63) − (48.99 kcal mol⁻¹)/RT ln 10. The molecular mechanism of the reactions has been investigated by means of the bonding evolution theory (BET) at the B3LYP/6-311G (d,p) level of theory. The cage opening course is described topologically by cleaving of C1–C2, C4–C8, and C5–C6 single bonds and electron saturation of the C1–C4, C2–C6, and C5–C8 bonds, while the rearrangement of bicyclooctatriene is described by C3–C7 bond rupture, depopulation of C1–C4 and C5–C8 double bonds, and electron saturation of C1–C5, C3–C4, and C7–C8 bonds. Electron density rearrangement along the two successive steps are asynchronous and the sequence of catastrophes can be represented as: η-1-13-C^†C^†FFFC^†C^†FFFC^†C^†-2-6-[C]₂C^†[F]₂[C^†]₂C^†-0.

Kinetic and electron density flows are studied theoretically for the gas phase pyrolysis of cubane via its cage opening to bicyclooctatriene and then thermal rearrangement of bicyclooctatriene to produce [8]annulene which is the experimentally observed major product.

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.

A series of bridged pyridine-based energetic compounds were designed and their geometrical/electronic structures, ΔH_f, detonation properties, thermal stabilities, thermodynamic properties and electrostatic potential were fully investigated using DFT.

Thermal and Sensitiveness Determination of Cubanes: Towards Cubane-Based Fuels for Infrared Countermeasures

As infrared seeking technology evolves, threats are better able to distinguish defensive infrared (IR) flares from true targets. Spectrally matched flares, which generally employ carbon-based fuels, are better able to decoy some advanced missiles by more closely mimicking the IR emission of the target. Cubane is a high-energy carbon-based scaffold which may be suitable for use as a fuel in spectrally matched flares. The enthalpy of formation and strain energy of a series of cubanes was predicted in silico, and their thermal and impact stability examined. All were found to undergo highly exothermic decomposition in sealed cell differential scanning calorimetry, and two cubanes subsequently underwent quantitative sensitiveness testing. Despite their F of I values being in the secondary explosive range, cubane-1,4-dicarboxylic acid (F of I=70) and 4-carbamoylcubane-1-carboxylic acid (F of I=90) were identified as potentially useful fuels for pyrotechnic infrared countermeasure flare formulations.

Molecule design and properties of bridged 2,2-bi(1,3,4-oxadiazole) energetic derivatives

A series of bridged 2,2-bi(1,3,4-oxadiazole) energetic derivatives were designed and their geometrical structures, electronic structures, heats of formation, detonation properties, thermal stabilities and thermodynamic properties were fully investigated by density functional theory. The results showed that the –N₃ group and the –N– bridge play an important role in improving heats of formation of these 2,2-bi(1,3,4-oxadiazole) derivatives. The calculated detonation properties indicated that the –NF₂ group and the –N– bridge were very useful for enhancing the heats of detonation, detonation velocities and detonation pressures. Twenty-four compounds were found to possess equal or higher detonation properties than those of RDX, while 14 compounds had equal or higher detonation properties than those of HMX. The analysis of the bond-dissociation energies suggested that the –CN group was the effective structural unit for increasing the thermal stabilities while the –NHNH₂ group decreased these values. Overall, taking both the detonation properties and thermal stabilities into consideration, 22 compounds (A4, A6, A8, A9, B4, B9, C2, C3, C4, C5, C7, C, C9 D4, D8, D9, E9, F4, F9, G9, H4 and H9) were selected as the potential candidates for high-energy-density materials.

A series of bridged 2,2-bi(1,3,4-oxadiazole) energetic derivatives were theoretically designed and investigated.

Exploration of High-Energy-Density Materials: Computational Insight into Energetic Derivatives Based on 1,2,4,5-Tetrahydro-1,2,4,5-tetrazine

Density functional theory was employed to investigate ten 1,2,4,5-tetrahydro-1,2,4,5-tetrazine-based energetic materials. The heats of formation and detonation properties were calculated by isodesmic reactions and Kamlet-Jacobs equations. The thermal stabilities and impact sensitivities were also estimated to give a better understanding of their decomposition mechanism. The results indicate that all of the designed compounds have high positive heats of formation ranging from 525.1 to 1639.1 kJ mol-1, moderate detonation properties (heats of detonation of 536.6 to 2187.6 cal g-1, theoretical densities of 1.48 to 2.32 g cm-3, detonation velocities of 7.02 to 12.18 km s-1, and detonation pressures of 19.8 to 75.1 GPa), and acceptable stabilities (bond dissociation energies of 0.8 to 104.9 kJ mol-1). Taking both the detonation properties and the stabilities into consideration, compounds A4 and B4 were finally selected as promising candidates of high-energy-density materials, as their detonation properties and impact sensitivities were superior to those of HMX. Additionally, the frontier molecular orbitals, electronic densities, electrostatic potentials, and thermal dynamic parameters of compounds A4 and B4 were also investigated.

QSPR modeling of detonation parameters and sensitivity of some energetic materials: DFT vs. PM3 calculations

The quantitative structure-property relationship (QSPR) methodology was applied to describe and seek the relationship between the structures and energetic properties (and sensitivity) for some common energy compounds. An extended series of structural and energetic descriptors was obtained with density functional theory (DFT) B3LYP and semi-empirical PM3 approaches. Results indicate that QSPR model constructed using quantum descriptors can be applied to verify the confidence of calculation results compared with experimental data. It can be extended to predict the properties of similar compounds.

Theoretical design of highly energetic poly-nitro cage compounds

The designed energetic compound, TNOAP, has a superior detonation performance to CL-20 and RDX, as well as satisfactory sensitivity.

Design and theoretical study of 15 novel high energy density compounds

In order to seek the potential high energy density compounds (HEDCs) with excellent performance and satisfactory safety, some combination rules are presented and 15 HEDCs are designed and sifted, and followed by the properties predicting. From the results, HEDC-3, HEDC-4, HEDC-9, HEDC-10, HEDC-11, HEDC-12, HEDC-13, and HEDC-14 have good comprehensive properties. They are furoxan, fused ring or cage-type compounds, whose frame is composed of some single ring by single (double or multi) point addition. Their densities are over 1.95 g cm(-3), and detonation velocities are over 9500 m s(-1). Their BDEs are over 85 kJ mol(-1), and the values of available free space (∆V) are lower than the ∆V of β-CL20 (∆V = 86). In view of the synthesis feasibility, the synthesis routes of HEDC-4, HEDC-9, HEDC-10, HEDC-12, HEDC-13, and HEDC-14 have been designed.

Computational studies on the energetic properties of polynitroxanthines

Density function theory calculations were performed to find comprehensive relationships between the structures and properties of a series of highly energetic polynitroxanthines. The isodesmic reaction method was employed to estimate the gas-phase heat of formation. The solid-state heats of formation for the designed compounds were calculated by the Politzer approach using heats of sublimation. All of the designed compounds were found to possess solid-state heats of formation of >100 kJ mol⁻¹. Detonation performances were evaluated by the Kamlet-Jacobs equations, based on the predicted densities and solid-state heats of formation. The results indicate that all of the compounds have excellent detonation velocities and pressures. The stabilities of the derivatives were calculated by evaluating their energy gaps, bond dissociation energies, and characteristic heights. The results indicate that all of the compounds have large bond dissociation energies and energy gaps. The characteristic height values of the compounds are more than or close to those of HMX and RDX. Thus, the polynitroxanthine derivatives show good thermodynamic and dynamic stability. Further, the present study may provide useful information on the structure-property relationships of these compounds, and for the development of novel high-energy materials.

Stability and electronic structures of substituted tetrahedranes, silicon and germanium parents – a DFT, ADMP, QTAIM and GVB study

The electronic structures and the stability of tetrahedrane, substituted tetrahedranes and silicon and germanium parents have been studied at ωB97XD/6-311++G(d,p) level of theory.

Computational studies on a high density cage compound hexanitrohexaazaisowurtzitane derivative

A newly designed polynitro cage compound with a framework of hexanitrohexaazaisowurtzitane (HNIW) was investigated by density functional theory (DFT) calculations. The molecular structure was optimized at the B3LYP/6-31G** level. IR spectrum, heat of formation (HOF), and thermodynamic properties were also predicted. The detonation velocity and pressure were evaluated by using the Kamlet–Jacobs equations, based on the theoretical density and condensed HOF. The bond dissociation energies (BDEs) and bond orders for the weakest bonds were analyzed to investigate the thermal stability of the title compound. The results show that the first step of pyrolysis is the rupture of the N8–NO2 bond. The crystal structure obtained by molecular mechanics belongs to the P21 space group, with the following lattice parameters: Z = 2, a = 11.10 Å, b = 15.15 Å, c = 10.77 Å, ρ = 1.872 g cm−3. The designed compound has high thermal stability and good detonation properties, and is a promising high-energy-density compound.

A THEORETICAL STUDY ON THE INFRARED SPECTRA, THERMODYNAMIC FUNCTIONS, AND DETONATION PARAMETERS FOR THE –CN, –NC, –NNO2and –ONO2DERIVATIVES OF HNS

The cyano (–CN), isocyano (–NC), nitramine (–NNO2), and nitrate (–ONO2) derivatives of HNS has been studied in this work at the B3LYP/6-31G* level of density functional theory. Their IR spectra were predicted and assigned by vibrational analysis. Based on the frequencies scaled by 0.96 and the principle of statistic thermodynamics, the thermodynamic functions were evaluated. It is found that the thermodynamic functions linearly increase with the number of – CN , – NC , – NNO2, and – ONO2groups, as well as the temperature. The contribution of various substitutents to the thermodynamic functions has the order of – ONO2> –NNO2> –NC > –CN . Detonation properties were evaluated using the modified Kamlet–Jacobs equations based on the calculated densities and heats of formation. Compared with the commonly used explosives (RDX and HMX), 3,3′,5-trinitramine-2,2′,4,4′,6,6′-Hexanitrostilbene, 3,3′,5,5′-tetranitramine-2,2′, 4,4′,6,6′-Hexanitrostilbene, 3,3′,5-trinitrate-2,2′,4,4′,6,6′-Hexanitrostilbene, and 3,3′,5,5′-tetranitrate-2,2′, 4,4′,6,6′-Hexanitrostilbene have better detonation performance and may be potential candidates of high energy density compounds.

The spectroscopic (FT-IR, FT-Raman, UV) and first order hyperpolarizability, HOMO and LUMO analysis of 3-aminobenzophenone by density functional method

In this work, experimental and theoretical study on the molecular structure and the vibrational spectra of 3-aminobenzophenone (3-ABP) is presented. The vibrational frequencies of the title compound were obtained theoretically by DFT/B3LYP calculations employing the standard 6-311++G(d,p) basis set for optimized geometry and were compared with Fourier transform infrared spectrum (FTIR) in the region of 400-4000 cm(-1) and with Fourier Transform Raman spectrum in the region of 50-4000 cm(-1). Complete vibrational assignments, analysis and correlation of the fundamental modes for the title compound were carried out. The vibrational harmonic frequencies were scaled using scale factor, yielding a good agreement between the experimentally recorded and the theoretically calculated values.