Initial Decomposition Mechanisms Research Articles

Temperature and pressure effects on the decomposition mechanisms of 2,6-diamino-3,5-dinitropyrazine-1-oxide crystal: ab initio molecular dynamics study.

The decomposition process of 2,6-diamino-3,5-dinitropyrazine-1-oxide (LLM-105) crystal at high temperatures (2500 and 3390K) and detonation pressure of 33.4 GPa coupled with temperatures were studied by ab initio molecular dynamics simulations. The results show that the initial decomposition mechanism of LLM-105 is the same under different conditions. The product analysis indicates that high temperature is conducive to the formation of N2 and CO2, but inhibited the formation of H2O. It is found that the formation mechanism of H2O is the same under different conditions, which involves the reaction between OH radical and H radical. Although the detailed processes of the formation of N2 are different, they all involve the reaction between nitrogen-containing fragments, and its core is the formation of intermediates with R1-NN-R2 structure. The core of the formation of CO2 under different conditions is to form the intermediate R1-CO-R2 with carbonyl structure, and then generate the fragment with -OCO- structure, and finally generate CO2. This research may provide new insights into the initiation and subsequent decomposition mechanisms of energetic materials under extreme conditions. The LLM-105 supercell was constructed using the Materials Studio 7.0 package. AIMD simulations were performed in the CASTEP package. AIMD simulations adopted NVT and NPT ensemble, and the temperature was controlled by Nosé thermostat, while the pressure was controlled by Andersen barostat. Besides, DFT calculations were carried out at the B3LYP/6-311 + G(d,p) level using the Gaussian 09 package.

The impact of electronic effect on the dissociation mechanism of oxadiazole-based regioisomeric energetic materials

With the rapid development of synthesis methods for energetic materials, the regioisomerism effect is gaining increasing attention in the design of multifunctional energetic compounds and in the in-depth study of the relationship between structure and performance. Understanding regioisomerism in energetic materials and its impact on material performance can provide a theoretical basis for material improvement and design. In this paper, the influence of regioisomerism on the thermal stability of oxadiazole-based energetic materials 5,5′-bis(trinitromethyl)-3,3′-bi(1,2,4-oxadiazole) (BTBO) and trinitromethyl substituted β-bis(1,2,4-oxadiazole) (TSBO), which have a significant difference in the dissociation temperature, has been thoroughly investigated. The initial decomposition mechanism of the regioisomers is fully discussed, proving that the NO2-dissociation path is the main decomposition pathway, revealing that the difference in the energies of the main dissociative bonds of BTBO and TSBO is the direct cause of the different thermal stabilities. The regioisomerism on the thermal stability of BTBO and TSBO is investigated: the electronic effect increases the polarity and reduces the bond order of the main dissociative bond in BTBO, resulting in a lower dissociation temperature for BTBO.

Initial Decomposition Mechanism of NH3OH+N5- Crystal under Thermal and Shock Loading: A First-Principles Study.

NH3OH+N5- is a novel energetic material (EM) which has attracted much interest for its promising performances, including high energy density, high density, low sensitivity, and low toxicity. In this study, the initial decomposition mechanism of NH3OH+N5- crystal was investigated under thermal and shock loading by molecular dynamics simulation. First, programmed heating and constant temperature simulations were carried out by molecular dynamics simulation on the basis of density functional theory (DFT-MD). Results indicated that the initial decomposition reactions of NH3OH+N5- could be described by three reactions: proton transfer, ring-opening reaction, and cation decomposition and recombination, and three pathways of ring-opening reaction were found, including the ring-opening of N5-, HN5, and H2N6. The first two reactions are the main pathways that produce N2 molecules. Furthermore, we carried out DFT-MD simulations to study the shock decomposition behaviors of NH3OH+N5-, and three initial steps were proposed: N5-, HN5, and N6 ring-opening. The fewer N5- and HN5 ring-opening reactions were found during the shock simulation, accompanied by a significant change in the N5- bond angle. What's more, the transition states of decomposition reactions were investigated through quantum chemical calculations. The results revealed that the proton transfer reaction exhibits lower activation barriers compared to ring-opening reactions, and proton transfer would accelerate ring-opening reactions. In addition, the ring-opening reaction is the main energy-releasing reaction in the early stages of the decomposition. This work could promote the comprehension of the decomposition mechanism and energy release regularity of N5- ions.

ReaxFF molecular dynamics simulation on the combustion mechanism of toluene/ethanol/n-heptane mixed fuel

The addition of ethanol and toluene to diesel fuel serves as an effective approach to mitigate engine knock. In this work, we employed ReaxFF molecular dynamics simulations to investigate the oxidation of toluene, ethanol, and n-heptane mixed fuels. Results show that toluene, ethanol, and 60.12 % of n-heptane molecules initiate dehydrogenation via active species such as O2, OH, HO2, etc. The initial decomposition mechanisms of the remaining n-heptane molecules involve the fission of CC bonds and direct dehydrogenation reactions. The formation of intermediate C2H4 primarily occurs via β-cleavage of saturated alkyl radicals with higher carbon atom numbers. The products CO and CO2 molecules strongly depend on the concentrations of C2H4 and CH3. Notably, the incorporation of ethanol/toluene in the system diminishes the presence of OH, thereby decreasing the consumption of C2H4 by OH. The activation energy values (43.36 ∼ 45.19 kcal mol−1) align well with experimental data.

Investigating the decomposition mechanism of CL-20/MTNI cocrystal explosive under high temperature and high pressure using ReaxFF/lg molecular dynamics simulations

The CL-20/MTNI cocrystal, as a novel energetic material (EMs), possesses the advantages of high energy content and low sensitivity, making it a promising substitute for traditional EMs. However, the thermal decomposition mechanism of this cocrystal has not been thoroughly investigated. Therefore, we utilized the ReaxFF/lg force field to study the thermal decomposition of CL-20/MTNI cocrystal at different temperatures (2500, 2750, 3000, 3250, and 3500 K) and pressures (3000 K/0.5 GPa, 3000 K/1.0 GPa, and 3000 K/1.5 GPa). The evolution of species, initial decomposition mechanisms, and changes in the number of final products were analyzed. The results indicate that hexanitrohexaazoisowuzane (CL-20) completely decomposes within 55 ps, while 1-methyl-2,4,5-trinitroimidazole (MTNI) decomposes completely within 110 ps, suggesting that CL-20 has lower thermal stability compared to MTNI. The high-temperature decomposition of CL-20/MTNI cocrystal exhibits three main possible reaction pathways. Firstly, the denitration of CL-20 occurs, followed by the denitration of MTNI, and finally, the nitro group isomerization of MTNI. This indicates that the nitro groups in CL-20 have lower thermal stability compared to those in MTNI within the CL-20/MTNI cocrystal. Analysis of the five final products reveals that the quantities of NO2 and NO are higher than other species, attributed to the higher nitrogen and oxygen content in the CL-20/MTNI cocrystal structure. As the temperature increases, the decomposition and conversion of NO2 are accelerated, leading to a sharp decrease in the quantity of NO2 and an increase in the content of NO. However, when the pressure increases, both the quantities of NO2 and NO exhibit a trend of initially increasing and then decreasing, with a noticeable increase in the quantity of N2. This indicates that high pressure accelerates the thermal decomposition of NO2 and NO.

Double-proton transfer triggering mechanism of nitromethane caused by intermolecular hydrogen bonding at low temperature

The initial triggering mechanism of nitromethane is investigated by first-principles molecular dynamics simulations and temperature-dependent Raman spectroscopy experiment. The Raman spectra at different temperature are obtained by Car-Parrinello molecular dynamics simulation to explore the activated vibration modes caused by low-temperature heat. The temperature-dependent Raman spectra experiment is constructed to compare with the theoretical data. Each possible initial reaction path caused by the activated vibration mode is analyzed to reveal the triggering mechanism of the initial chemical bond breaking for nitromethane. The results show that the intermolecular double-proton transfer is the first reaction step which is triggered by the intermolecular hydrogen bond reinforced with the active C–H stretching vibration (near 3000 cm−1), which is not reported before. The scanning energy barrier related to the increase of C–H bond lengths is about 60 kcal/mol, which is consistent with the calculated activation barrier (59.8 kcal/mol) of intermolecular proton transfer reaction. This proves the reliability of our conclusions. The results provide a new interpretation for the long-debated initial decomposition mechanism of nitromethane.

Initial Decomposition Mechanism of H2O2 at High Temperature and Pressure

In recent years, H2O2 as an excellent rocket propellant has been widely studied. Herein, the initial decomposition mechanism of H2O2 is studied in detail by molecular dynamics simulation based on density functional theory. It is found that when the energy input to H2O2 is low, the mechanism of intermolecular hydrogen transfer is dominant. With the increase of input energy, the breaking of O–O becomes the first step of the initial reaction, H–O fracture becomes the second step, and finally the transfer of hydrogen to produce H2O and HO2.

The initial reaction mechanism of FOX-7 under high temperature and high pressure

ABSTRACT In recent years, FOX-7 has attracted great interest due to its excellent performance. The initial decomposition mechanism of FOX-7 (ε phase) at high temperature and pressure is simulated by ab initio molecular dynamics. We mainly studied the initial reaction of FOX-7 under extreme conditions of 10 GPa and 700-3000 K. When the pressure is constant, FOX-7 shows different decomposition mechanisms as the temperature increases. At lower temperatures, hydrogen is transferred first. As the temperature increases, the transfer of hydrogen and the breakage of the C-NO2 bond leading to the generation of acid are the main initial decomposition pathways. The energy barrier for hydrogen transfer is lower than that of C-NO2 bond breaking, which was confirmed by a single-molecule transition state search. Thus, the correctness of the decomposition mechanism obtained by molecular dynamics is proved. Different from previous studies, this paper considers both temperature and higher pressure, providing a reference for the initial reaction mechanism of FOX-7 under extreme conditions.

Insights into the Reaction Kinetics of Hydrazine-Based Fuels: A Comprehensive Review of Theoretical and Experimental Methods

While researchers have extensively studied the initial decomposition mechanism of Monomethylhydrazine (MMH, CH3NHNH2) in the MMH/dinitrogen tetroxide (NTO) system, the investigation of Unsymmetrical Dimethylhydrazine (UDMH, (CH3)2NNH2) has been limited due to its high toxicity, corrosiveness, and deterioration rate. Hence, the effects of UDMH’s deterioration products on combustion performance and gas-phase combustion reaction mechanisms remain unclear. This comprehensive review examines the existing research on the reaction kinetics of the three widely used hydrazine-based self-ignition propellants: Hydrazine (HZ, N2H4): MMH: and UDMH, emphasizing the necessity for further investigation into the reaction kinetics and mechanisms of UDMH. It also discusses the implications of these findings for developing safer and more efficient rocket propulsion systems. Additionally, this review underscores the importance of utilizing computational chemistry theory to analyze hydrazine-based fuels’ combustion and decomposition properties, constructing detailed pyrolysis and combustion reaction mechanisms to optimize rocket engine fuel performance and environmental concerns.

Molecular Dynamics Simulations of the Thermal Decomposition of RDX/HTPB Explosives.

The addition of binders to energetic materials is known to complicate the thermal decomposition process of such materials. To assess this effect, the present work studied the thermal decomposition of cyclotrimethylene trinitramine (RDX)/hydroxy-terminated polybutadiene (HTPB) mixtures and of pure RDX over the temperature range of 2000-3500 K by combining the classical reaction and first-principles molecular dynamics methods. The incorporation of HTPB as a binder was found to significantly reduce the decomposition rate of RDX. At 3500 K, the decay rate constant of RDX in the RDX/HTPB system is 2.0141 × 1012 s-1, while it is 2.7723 × 1012 s-1 in the pure RDX system. However, the binder HTPB had little effect on the initial decomposition mechanism, which involved the rupture of N-NO2 bonds to produce NO2. The HTPB was predicted to undergo dehydrogenation and chain breaking. The free H resulting from these processes was predicted to react with low-molecular-weight intermediates generated by the RDX, resulting in greater equilibrium quantities of the final products H2O and H2 being obtained from the mixed system compared with pure RDX. HTPB-chain fragments were also found to combine with the primary RDX decomposition product NO2 to inhibit the formation of N2 and CO2.

Initial decomposition mechanisms of 2,4,6-triamino-1,3,5-trinitrobenzene (TATB) and their kinetic isotope effect

2,4,6-triamino-1,3,5-trinitrobenzene (TATB) is an insensitive High Explosive (HE) that is widely studied to better understand the physical properties of safety and sensitivity of HE. A dominant initial decomposition mechanism of TATB is believed to be a dehydration reaction that forms mono- and di-furazans, although other mechanisms have been reported. In this work, seven initial decomposition mechanisms were modeled with ab initio simulations to calculate its free energy barriers, decomposition rates, and kinetic isotope effects. The energy barrier for mono-benzofurazan mechanisms was found to be high, >61 kcal/mol in the gas phase; however, the reaction energy can decrease significantly in a disordered condensed state. The predicted kinetic isotope effect ratio of the furazan mechanism was found to be kh/kd≈ 1.41 at 600 K, in agreement with the experiment. The NO2 scission mechanism was found to be an entropy-driven mechanism because the free energy barrier decreased significantly with temperature, making it the most energetically favorable mechanism at high temperatures in the gas phase. The results provide a better understanding of the atomistic decomposition mechanisms of TATB and may be useful for improving models of safety and sensitivity.

Initial decomposition mechanisms and the inverse effects of temperature and PH2 on the thermodynamic stability of UH3.

The thermodynamic stability of uranium hydrides is of broad interest and fundamental importance for understanding the hydriding corrosion of uranium, and the storage and isotope separation of hydrogen. Based on the first-principles calculations, we reveal the initial decomposition mechanism, interpret the experimental pyrolysis results, and discuss the inverse effects of temperature and hydrogen pressure (PH2) on the thermodynamic stability of β-UH3. The decomposition mechanism of β-UH3 is found to be closely related to the changes of U-H bonding properties in UH12 cages. Specifically, at the beginning it is difficult to break the first U-H covalent bond in each UH12 cage, which brings in the existence of a concave region in the experimental PH2-C-T curve; however, it boosts the itinerant character of U-5f electrons. Thereafter, the formation energy of H-vacancies in the degraded UH11 cages is almost changeless when the H/U atom ratio decreases, resulting in the van't Hoff plateau of the PH2-C-T curve. Based on the above mechanisms, we propose a theoretical method to evaluate the thermodynamic stability of β-UH3. The calculated PH2-C-T curve is consistent with experiment, showing that temperature promotes β-UH3 decomposition and PH2 plays an opposite role. Moreover, this method is independent of experimental calibration and is applied to discuss the isotope effect of hydrogen in β-UH3. This work provides new insight and a practical method for the scientific studies of uranium hydride, which is also essential to industrial applications in hydrogen isotope separation.

Pressure, directional dependent mechanical anisotropies and phase transition studies of β-5-nitro-2,4-dihydro-3H-1,2,4-triazol-3-one (NTO) and 2,4,6-triamino-1,3,5-trinitrobenzene (TATB)

This work highlights the effect of pressure ranging from 0 to 9 GPa on structural, directional dependent mechanical properties and unravel the previously unknown phase transitions of two important high energy molecular solids namely monoclinic-β-Nitrotriazole (NTO) and 2,4,6-triamino-1,3,5-trinitrobenzene (TATB). The projected augmented plane wave method with generalized gradient approximation Perdew–Burke–Ernzerhof functional with the D2 van der Waals corrections method of Grimme is used to reproduce the experimental data within ∼1% error. The structural optimization results reveal that β-NTO undergoes a previously unknown structural phase transition at 9 GPa which is evident from the abrupt change of calculated lattice vectors, volume (V), lattice angle β at 9 GPa. The single crystal elastic properties analysis also supports these findings and NTO voilate the Born’s mechanical stability criteria at 9 GPa. Besides to it, all the calculated volumetric and directional dependent shear modulus (G), bulk modulus (B), compressibility results of β-NTO in (100), (010), (001) planes also suggest a possible phase transition around 9 GPa. The directional dependent polycrystalline compressibility anisotropy analysis of TATB with pressure in (100), (010), (001) planes unreveal the origin of experimentally reported new phase transition around 4 GPa. The calculated Pugh B/G ratio suggests that, both the materials found to be brittle in the studied pressure range except NTO at 9 GPa. The degree of mechanical anisotropy of β-NTO found to increase with increasing pressure from (100)–(010)–(001) planes, while the TATB anisotropy results were found to be relatively small and stable. The Young’s modulus (E), Poisson’s ratio (σ), P-wave modulus, universal elastic anisotropy (A ), Chung-Buessen anisotropy (A ), Vickers hardness coefficient (H ), sound velocities ((V ) average, (V ) longitudinal, (V ) transverse) and (θ D ) Debye temperature are also predicted. The calculated intermolecular interaction strength contribution to the total Hirsh Field Surface at different pressures confirms the initial decomposition mechanism of NTO, TATB and the results are good in agreement with previous observations. Thus our work has accentuated the reasons behind the impact and friction sensitivity differences of β-NTO, TATB through the two new phase transitions.

Flash pyrolysis vacuum ultraviolet photoionization mass spectrometry of cycloheptane: A study of the initial decomposition mechanism.

Thermal decomposition of cycloheptane was studied using flash pyrolysis coupled with vacuum ultraviolet (118.2 nm) single photon ionization time-of-flight mass spectrometry at temperatures ranging from 295 K to 1380 K. C-C bond breaking of cycloheptane leading to the 1,7-heptyl diradical was considered as the initiation step. The 1,7-heptyl diradical could readily isomerize to 1-heptene and decompose into several fragments, with dissociation to •C4H9 and •C3H5 as the predominant product channel. The 1,7-heptyl diradical could undergo direct dissociation, as evidenced by the production of the C5H10 species. Quantum chemistry calculations at UCCSD(T)/cc-pVDZ//UB3LYP/cc-pVDZ level of theory on the initial reaction pathways of cycloheptane were also carried out to support the experimental observations. Other possible initiation channels, as well as some secondary reaction products, were also identified.

Theoretical study on the quick thermal decomposition pathways for MTNI(1-Methyl-2,4,5-Trinitroimidazole)

In this study, we investigated the initial and subsequent decomposition mechanisms for MTNI(1-Methyl-2,4,5-trinitroimidazole) by DFT calculations. In the second stage of the nitro isomerization, a stable loose intermediate –ONO isomer is not generated but an unstable tight three-membered ring structure is generated, resulting in the direct production of NO. In the initial and subsequent process of H transfer, under certain conditions, a synergistic reaction occurs. The occurrence of the synergistic reaction is determined by the orientation of the –OH group on the hydroxymethyl group. Compared with the common decomposition mechanism, less energy is needed absorbed in above reaction.

Theoretical insights into the role of regiochemistry in thermal stability regulation of energetic materials

In this study, the regioisomerism impact on the thermal stability of two pairs of regioisomers with obvious deviation in decomposition temperature has been thoroughly investigated. The initial decomposition mechanism results indicate that the large gaps in decomposition temperature of both pairs are caused by the change of primary initial decomposition paths. Despite with same molecular formula, the regioisomers may decompose through totally different way, and lead to a sharp change in decomposition temperature, hence, making the regioisomerism be capable of improving the thermal stability of energetic materials.

High-temperature thermal decomposition of iso-octane based on reactive molecular dynamics simulations.

Branched alkanes are the major components of endothermic fuels used for advanced aircrafts. Reactive molecular dynamics (RMD) simulations are carried out to explore the detailed kinetic mechanism for the thermal decomposition of iso-octane widely used as the primary reference fuel of branched alkanes. The RMD calculations indicate that the initial decomposition mechanism of iso-octane is mainly through two pathways: (1) the C - C bond cleavage to produce smaller hydrocarbon radicals and (2) the hydrogen-abstraction reactions by small radicals including •H and •CH3. Most of the alkenes which are associated with the endothermic capacities in the iso-octane pyrolysis are produced from the C-C β-scission reactions of alkyl radicals. Propylene and ethylene are observed to be formed in large amounts. Kinetic parameters with the activation energy of 52.1kcalmol-1 and pre-exponential factor of 7.2 × 1014s-1, based on the first-order kinetic analysis, are in good agreement with previous work.

The role of regioisomerism on thermal stability of furoxan based energetic materials

Development of high-performance energetic materials with good stabilities has been a great challenge, since such materials essentially suffer from high sensitivities. Regioisomerism, as an effective modification strategy for molecular properties, presents impacts on thermal stabilities of energetic materials. In this study, the effects of regioisomerism on thermal stabilities of a pair of furoxan based energetic materials (M3 and M4) have been thoroughly investigated. The initial decomposition mechanisms of the regioisomers are fully discussed, the NO2-dissociation path is proved as the primary decomposition path, instead of the previously reported ring-open channel. An unusual transition state is found for the NO2-dissociation path, due to the existence of Van der Waals potential well, which attributes to the strong weak interaction between the dissociation products. The regioisomerism has impacts on both molecular structures and decomposition mechanisms of M3 and M4. The existence of intramolecular weak interactions and the electron effects contribute to the molecular stability of M4. The elevation in the energy barrier of initial decomposition path of M4, and the decline in the energy of dissociation products of M3, result in different thermal stabilities of M3 and M4.

Impact of regiochemistry on thermal stability of trifuroxan based energetic materials: A theoretical perspective

Improving the stability of energetic materials with high energy density has been an issue over the past decades. Regioisomerism, as an effective molecular modification strategy, shows the potential to improve the thermal stability of energetic materials without a compromise in energy density. In this study, the impacts of regioisomerism on thermal stabilities of two trifuroxan based energetic regioisomers (BFTF-1 and BFTF-2) have been thoroughly investigated. The initial decomposition mechanism has been studied to reveal the decomposition process. According to the Gibbs free energy profiles, NO2-dissociation path is proved as the primary decomposition path for both regioisomers, and the difference in energy also well explains their distinct thermal stabilities. The regioisomerism impacts on molecular structure and NO2 connection have been evaluated through intramolecular interactions and electron effects. The structure of BFTF-2 has been stabilized by an intramolecular electrostatic attraction, the connection of the leaving NO2 group in BFTF-1 has been weakened by intramolecular steric effect, both of which result in different thermal stabilities of BFTF-1 and BFTF-2.

Mechanistic Investigation on the Initial Thermal Decomposition of Energetic Materials FOX-7 and RDX in the Crystal and Gas Phase: An MM/DFT-Based ONIOM Calculation.

Interpreting the initial decomposition mechanism is important for evaluating the thermal stability of explosives. In this study, we theoretically investigated the initial thermal decomposition reactions for two typical energetic materials, FOX-7 and RDX, in both the gas phase and crystal phase. Single molecular decomposition pathways in the gas phase are calculated using the density functional theory (DFT) method, and the crystal phase reactions are simulated through the MM/DFT-based ONIOM method. The calculation results indicate that the crystal environment has a significant influence on the initial thermal decomposition mechanism of FOX-7 and RDX. The cage effect induced by the crystal environment greatly confines molecular mobility and diffusion, rendering the generated small molecules to react with the remaining fragment and yield new decomposition channels compared with the gas phase condition. The crystal packing structures and intermolecular interactions (hydrogen bonds/π-π stacking) significantly increase the reaction barriers of FOX-7 and RDX, leading to the crystal phase reactions being more difficult to occur than in the gas phase. Since the practical application of explosives is mostly in the crystal state, it is important to consider the environmental effects on the initial decomposition reactions. The same insight can also be relevant for other energetic materials.