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Cite this: Phys. Chem. Chem. Phys., 2014, 16, 21620

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An ab initio molecular dynamics study of thermal decomposition of 3,6-di(azido)-1,2,4,5-tetrazine† Qiong Wu, Weihua Zhu* and Heming Xiao Ab initio molecular dynamics simulations were performed to study the thermal decomposition of isolated and crystal 3,6-di(azido)-1,2,4,5-tetrazine (DiAT). During unimolecular decomposition, the three different initiation mechanisms were observed to be N–N2 cleavage, ring opening, and isomerization, respectively. The preferential initial decomposition step is the homolysis of the N–N2 bond in the azido group. The release mechanisms of nitrogen gas are found to be very different in the early and later decomposition stages of crystal DiAT. In the early decomposition, DiAT decomposes very fast and drastically without forming any stable long-chains or heterocyclic clusters, and most of the nitrogen gases are released

Received 11th June 2014, Accepted 13th August 2014

through rapid rupture of nitrogen–nitrogen and carbon–nitrogen bonds. But in the later decomposition

DOI: 10.1039/c4cp02579b

strong carbon–nitrogen bonds. To overcome the obstacles, the nitrogen gases are released through slow formation and disintegration of polycyclic networks. Our simulations suggest a new decomposition

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mechanism for the organic polyazido initial explosive at the atomistic level.

stage, the release of nitrogen gas is inhibited due to low mobility, long distance from each other, and

Introduction The decomposition mechanisms of explosives are very important to understand their complicated behaviors, to control their risk during usage and storage, and to develop new high-energy materials. It was extremely difficult to obtain a clear molecular level picture of the decomposition of the explosives in experimental measurements due to their complex behaviors and danger. Therefore, their decomposition behaviors are not clear yet. The atomistic simulation is an effective way to model the physical and chemical properties of the solid explosives at the atomic level as a complement to experimental work. Recently, several studies1–5 on the decomposition of the explosives have been done by using ab initio molecular dynamics (AIMD). For example, Isayev et al.1 reported the thermal decomposition of gas and solid phases of 2,4,6,8,10,12-hexnitro-2,4,6,8,10,12hexaazaisowurtzitane (CL-20 or HNIW) in the temperature range 1500–3000 K. Wu et al.2 performed ab initio molecular dynamics simulations to study the decomposition mechanisms of pentaerythritol tetranitrate (PETN) at 3000 and 4000 K. Although these studies provided some useful information for understanding the behaviors of the explosives, the full picture of their decomposition mechanisms is complex and far from completion.

Institute for Computation in Molecular and Materials Science and School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China. E-mail: [email protected] † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c4cp02579b

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High-nitrogen organic polyazido compounds only containing C and N atoms like 3,6-di(azido)-1,2,4,5-tetrazine (DiAT, Fig. 1)6 and 4,4 0 ,6,60 -tetra(azido)azo-1,3,5-triazine (TAAT)7 are ideal potential candidates for initial explosives because of their high energy and clean and thermodynamically favorable decomposition, which extrude nitrogen gas, carbon nitrides, and carbon nanoparticles as the products under detonation. DiAT (Fig. 1) was prepared by Marcus and Remanick6 in 1963, but no crystal structure was available. Its crystal structure was reported by Huynh et al. in 20048 and they used it to prepare carbon nanospheres and nitrorich carbon nitrides. Its heat of formation, detonation velocity,

Fig. 1 A unit cell (a), a 1  2  2 supercell (b), and a 1  3  2 supercell (c) of DiAT and molecular numbering of unreacted DiAT (d). Gray and blue spheres stand for carbon and nitrogen atoms, respectively.

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and detonation pressure were calculated to be 1125.05 kJ mol1, 8280 m s1, and 29.35 GPa, respectively.9 This indicates that DiDA may be used as an initial explosive. Compared with organic nitro compounds, the studies on the thermal decomposition of organic azides10–18 are few, which may be due to the difficulty and danger arising from their extremely high sensitivity. Furthermore, these limited studies are mainly focused on the simple alkyl azides or azide polymers composed of C, H, N, and even O atoms. For example, the thermal decomposition of methyl azide in the gas phase under unimolecular conditions was investigated experimentally10,11 and theoretically.12,13 Bock et al.14 studied the gas-phase pyrolysis of vinyl azide by means of the PE spectrum. Kruglyakova et al.15 elucidated the kinetics of the thermal decomposition of a series of azido-1,2,4-triazoles experimentally. Chen et al.16 studied the mechanisms and kinetics of the thermal decomposition of 2-azido-N,N-dimethylethanamine theoretically. Overall, these studies10–18 pointed out that the initial decomposition step for most of the investigated organic azides is the cleavage of the N–N2 bond, whose activation energies are from 108.8 to 188.4 kJ mol1. The subsequent decomposition reactions are much more complicated due to the involvement of different competing reaction channels. However, the studies on the decomposition process of organic polyazido explosives containing only C and N atoms are few. Because of their unique structures without H and O atoms, their properties and decomposition mechanism are expected to be different from those of nitro explosives and other organic azides. Although Huynh et al.,8 Gillan,19 and Miller et al.20 reported that DiAT (C2N10), 2,4,6-tri(azido)-1,3,5-triazine (C3N12), and 2,5,8tri(azido)-s-heptazine (C6N16) can produce nitrogen-rich carbon nitrides during the thermal decomposition and stated that there is the release of N2 gas during the decomposition, the three studies did not report their decomposition mechanisms and processes. Fortunately, lately, Nedel’ko et al.21 studied the thermal decomposition of azidopyridines and reported that the only gaseous product of the thermal decomposition was nitrogen. During the first stage of thermal decomposition, the molecular nitrogen was released by the formation of nitrenes and the planar two-dimensional networks were formed through the subsequent intermolecular reactions. Among the studied azides, 2,4,6-triazido-3,5-dicyanopyridine and 2,3,5,6-tetraazido6-cyanopyridine only containing C and N atoms are similar to DiAT and also have similar structures to that of DiAT to some degree. Thus, their decomposition mechanisms may be helpful for studying and understanding the decomposition of DiAT. In this work, we performed ab initio MD simulations to investigate the initial chemical processes and decomposition reactions of isolated and crystal DiAT at about 3000 K.

Computational method Our AIMD simulations were performed within the framework of DFT based on the CASTEP code22 using Troullier–Martins normconserving pseudopotentials23 and a plane-wave expansion of

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the wave functions. The Perdew–Burke–Ernzerhof24 (PBE) exchange– correlation function and a single k-point were employed. We utilized a plane wave cutoff of 400 eV for MD simulations and 700 eV for geometry optimizations. We controlled the ionic temperature ´ thermostat.25 An NVT ensemble was employed. The using a Nose initial positions of the simulation supercell were taken from the experimentally determined X-ray crystal structure.8 We used bond-length criteria to identify the changes in geometry in the simulations. The bond-length tolerance specifies the deviation from the ideal bond length that is tolerated for forming a bond between two atoms. The ideal bond length is defined as the sum of the covalent radii of the two atoms forming the bond. The smallest distance between two atoms has to be larger than 0.6 times the ideal bond length for which a bond is formed; the two atoms are not bonded any more when the largest distance between them is greater than 1.15 times the ideal bond length. Isolated system The unimolecular decompositions of isolated DiAT were investigated by using NVT ensemble dynamics with a 18.0 Å cubic periodic cell, which is large enough to prevent interactions between periodic images. First, one optimized single DiAT molecule at 0 K was placed in the center of the 18.0 Å cubic periodic cell. Then, the system was heated to 300 K and was equilibrated at 300 K for 5 ps by using a time step of 1.0 fs. After that, ab initio MD simulations were carried out by using a time step of 2.0 fs at 3000 K for 10 ps. The ionic velocities are randomly initialized with respect to the simulation temperature. To provide appropriate statistical sampling and obtain more reliable results, we carried out 10 independent simulations of the unimolecular decomposition process. The Mulliken bond orders of DiAT were calculated based on the equilibrated single molecule in the 18.0 Å cubic periodic cell at 300 K. Crystal phase The multimolecular decomposition simulations were first performed on a 1  2  2 supercell (8 molecules and 96 atoms), as shown in Fig. 1. After the system was equilibrated at 300 K for 5 ps using a time step of 1.0 fs, one long simulation (130 ps) and two extra short simulations (10 ps) at 3000 K were carried out using a time step of 1.0 fs. Previous studies2 reported that a time step of 1.2 fs was used to study the thermal decomposition of PETN. A time step of 1.0 fs was also adopted in the studies of sodium26 and iron27 under extreme conditions of temperature and pressure by using AIMD. To verify system size dependence and perhaps obtain better sampling, we also performed one additional large-scale simulation of a bigger system (1  3  2 supercell, 12 molecules and 144 atoms, as shown in Fig. 1) at 3000 K for 15 ps using a 1.0 fs time step after the system was equilibrated at 300 K for 5 ps. Fig. S1–S3 (ESI†) displays the time dependencies of total potential energy, total kinetic energy, and temperature in the 5 ps of equilibration at 300 K for 1  2  2 and 1  3  2 supercells, respectively. It is found that the systems are stable and converged. To test the effects of time step on the results, we also performed three extra simulations on a 1  2  2 supercell and one additional

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large-scale simulation on a 1  3  2 supercell with a time step of 0.1 fs for 1 ps after the system was equilibrated at 300 K for 3 ps.

Table 2 Energies (kJ mol1) and observed times of the three initiation reactions in unimolecular decomposition (UMD) and multimolecular decomposition (MMD) of DiAT

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Energies (DEr, kJ mol1)

Results and discussion

Initiation reactions

To validate the reliability of our results, the bond lengths and bond angles from the equilibrium structure of DiAT by our ab initio molecular dynamics simulations at 300 K are compared with experimental data at 293 K in Table 1. It is seen that the calculated bond lengths and bond angles compare well with the experimental values, indicating that our simulated method is reasonably satisfactory for studying the DiAT crystal.

N–N2 cleavage Ring opening Isomerization

Unimolecular decomposition From the ten independent simulations, three different initial decomposition reactions were observed and the corresponding snapshots are shown in Fig. 2. They involve N–N2 bond cleavage to release a nitrogen gas (Init1), ring opening through the breaking of one C–N bond (Init2), and isomerization of one azido group to form a new tetrazole ring (Init3). The energies and observed times of the three reactions are given in Table 2. To compare with the AIMD results, the bond dissociation energies (BDE) for several important bonds of DiAT were calculated by using B3LYP/6-311+G**, as shown in Table 2. It is found that the results obtained by B3LYP/6-311+G** present a similar variation Table 1 Experimental and calculated bond lengths (Å) and angles (1) for crystal DiAT

Bond lengths

Expt.8

Cal.

Bond angles

Expt.8

Cal.

N1–N2 N2–N3 N3–C4 C4–N5 N5–N6

1.109 1.240 1.375 1.328 1.311

1.163 1.241 1.364 1.337 1.317

N1–N2–N3 N3–C4–N5 C4–N5–N6 C4–N7–N8–N9 N5–C4–N7–N8

171.8 119.9 116.6 0.02 0.02

170.2 119.5 117.2 0.70 0.64

Fig. 2 Snapshots of the three different initiation reactions of isolated DiAT at 3000 K.

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AIMD

B3LYP/ 6-311+G**

Observed times in UMD

Observed times in MMD

59.17 84.56 98.07

728.87 258.43 8.04

7 2 1

17 5 2

trend to those by AIMD. It is interesting to find that the reaction energy (DEr) for Init1 is positive (endothermic), while DEr for Init2 and Init3 are negative (exothermic). Init1 was observed 7 times, while Init2 and Init3 were only observed 2 times and 1 time, respectively. The Init1 was observed experimentally in the thermal decomposition of many organic azides,10,11,13,14,17,18,21 especially in two energetic compounds 2,4,6-triazido-3,5-dicyanopyridine and 2,3,5,6-tetraazido-6-cyanopyridine,21 which have the same element constitutions as and similar structures to DiAT. Init1 from the decomposition of DiAT has lower DEr (59.2 kJ mol1) than the two azidopyridines21(121.6–145.9 kJ mol1). This may be because the former was obtained at 3000 K while the latter was determined at 333–440 K. Compared with Init1, the incidences of Init 2 and Init3 are very low. Since the detection of subsequent generated intermediates is more difficult in the unimolecular decomposition of DiAT, Init 2 and Init3 were not reported in the decomposition of other organic azides. This should be confirmed by further experimental studies. In fact, the isomerization of an azido group to form a new tetrazole ring is easy to occur and was observed and implied to obtain new compounds experimentally in azido-triazines.28 More detailed decomposition processes after the three initiation reactions are shown in Fig. 3. It is seen that the processes were accompanied by the breaking of C–N and N–N bonds to form all kinds of radicals and the release of nitrogen gas. Most of the radicals are very unstable. For example, one DiAT molecule rapidly decomposes into three nitrogen gases and one C2N4 in 0.5 ps. C2N4 is much more stable than other radicals and can survive until the end of the unimolecular simulations (10 ps). To illustrate the energetics of the unimolecular decomposition of DiAT, Fig. 4 displays the relative energies for three of ten decomposition simulations (the reactant (unreacted DiAT) was selected as the reference state). It is seen that the unimolecular decomposition reactions are endothermic. In addition, Table S1 (ESI†) lists the calculated bond orders and BDE of unimolecular DiAT at 300 K. It is found that the N3–C4 bond has smaller bond order than any other bonds, suggesting that the N3–C4 bond is the weakest bond and its cleavage will trigger the decomposition of DiAT. The bond order of N2–N3 and C4–N5 bonds is slightly higher than that of the N3–C4 bond, but the cleavage of the former was observed while the cleavage of the latter was not found in the initial decomposition stage of DiAT. This indicates that the priority of the initial decomposition step is not strictly related to the strengths of the trigger bonds based on the bond orders. However, the C4–N5, N3–C4, and N5–N6 bonds have

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Fig. 3

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Three decomposition processes of the isolated DiAT at 3000 K.

lower BDE values than the N2–N3, indicating that the former three bonds are weaker than the latter. In fact, the observed times of the cleavage of the C4–N5 bond are smaller than that of the N2–N3 bond although the homolysis of N3–C4 and N5–N6 bonds was not observed. Multimolecular decomposition From the temporal dependence of the decomposed intermediates and products, it is found that the thermal decomposition of DiAT is a complicated process. During the whole molecular dynamics simulation, we observed the release of nitrogen gas through different kinds of cleavage, rearrangement, and combination reactions in the decomposition of DiAT. The release mechanisms of nitrogen gas are very different in the early and later decomposition stages, which experience rapid nitrogen–nitrogen rupture, carbon–nitrogen cleavage, and the slow formation and disintegration of polycyclic networks. Fig. 5 displays the time dependence of the concentration of nitrogen gas in the system. It is seen that thirty one

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(about 0.044 mol cm3) nitrogen gases are released fast until 4.0 ps, while only three and more are liberated in the following 126 ps. To facilitate the analysis, the former is defined as the early decomposition stage of DiAT, while the latter is defined as the later decomposition stage. The time dependence of the concentration of C2N4 is also displayed in Fig. 5. (a) N–N2 cleavage, ring opening, and isomerization. From the three independent simulations, Init1, Init2, and Init3 were observed for 17, 5, and 2 ps, respectively (as shown in Table 2). The ratio of observed times for Init1 : Init2 : Init3 is 8.5 : 2.5 : 1, which is close to that (7 : 2 : 1) observed in unimolecular decomposition. From the two ratios, it is concluded that the N–N2 bond homolysis of the azido as the initial decomposition step is predominant compared with other two initiation paths. This agrees with the conclusion that the N–N2 bond in azido is the weakest bond in DiAT.9 Both from the long simulation (130 ps) and two short simulations (10 ps), it could be observed that after the initiation decompositions, thirty one nitrogen gases (77.5% of the theoretical

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Fig. 4 Relative energies (kJ mol1) of the three pathways observed in unimolecular decomposition of DiAT at 3000 K.

Fig. 5 Time dependence of the concentration of nitrogen gas and C2N4 during the decomposition of crystal DiAT at 3000 K.

maximum quantity) are released fast in 4.0 ps without forming any stable long-chain or heterocyclic clusters. During this process,

Fig. 6

the decomposition reactions experience rapid rupture of the N–N and C–N bonds. The decomposition reactions are similar to those in unimolecular decomposition but take place much more quickly and are complicated. For instance, C2N4 can only survive 2.3 ps (Fig. 5) and then further decompose into more short radicals through two possible ways (Fig. 6), which are two NCN radicals or one CN radical and one NNCN radical. The NNCN radical can further decompose into one CN radical and one nitrogen gas. The short radicals are very active and tend to combine with each other or with other more long radicals to form different kinds of intermediates. Most of the intermediates are very unstable and will transform into others immediately except for NCNCNCN (C3N4), NCNCN (C2N3), and NCCN (C2N2) (Fig. 5), which can survive in about 2.4 ps (from time 1.4 to 3.8 ps), 1.2 ps (from 4.0 to 5.2 ps), and 0.6 ps (from 3.4 to 4.0 ps), respectively. (b) Formation and disintegration of polycyclic networks. After the fast early decomposition stage and the release of thirty one nitrogen gases in the initial 4.0 ps, DiAT decomposes much more slowly because of the formation of a long-chain and complicated carbon-rich heterocyclic ring and networks in the following 126 ps (defined as the later decomposition stage). In the latter decomposition stage of DiAT, the release of nitrogen gas is very slow because of the long distances between the rest of the nitrogen atoms and their low mobility. But the release mechanism of nitrogen gas is found to be different to that in the early decomposition stage. Many active radicals are formed after most of the nitrogen gases are released. Then the radicals will combine with each other to form various sizes of long-chains. The long-chains are also unstable and will decompose or rearrange. In fact, it is found that they tend to form polycyclic intermediates through a series of complicated decomposition and rearranging reactions. These polycyclic intermediates (C3N2.25–3.375) are similar to carbon nitride (C3N4), which was observed experimentally in the decomposition of DiAT at 423 K8 and TAT (2,4,6-tri(azido)1,3,5-triazine) at 358 K.19 It is seen in Fig. 7 that at 4.7 ps, there are thirty one nitrogen gases and two long-chains in the system. Then, through the breaking and rearranging of the chains, a six-membered ring is

Further decomposition of C2N4 and the formation path of C2N2, C2N3, and C3N4 in the early decomposition stage of crystal DiAT at 3000 K.

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Fig. 7 Release mechanism (slow formation and disintegration of polycyclic networks) of nitrogen gas in the later decomposition stage of crystal DiAT. Gray spheres stand for carbon atoms. Blue lines or spheres stand for nitrogen atoms.

formed at 9.1 ps and a ten-membered ring is at 10.0 ps. At 23.0 ps, it is observed that the first bicyclic intermediate is formed. Then it transforms into a tricyclic network at 25.0 ps. Next, a tetracyclic network is formed at 45.5 ps. However, at 46.5 ps, this tetracyclic network is disintegrated and one nitrogen gas (the 32nd nitrogen gas) is released. After the 31st nitrogen gas is released, the formation and release of the 32nd one (about 0.0454 mol cm3) consumed about 42.5 ps. This formation process can be summarized as follows: first, a relatively small ring is formed through the breaking and rearranging of long-chains. Second, this small ring enlarges to a much bigger one. Next, this big ring rearranges to form a polycyclic network

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with more and more linked rings step by step. Finally, this polycyclic network disintegrates and one nitrogen gas is released. The reason why nitrogen gas is released through this process may be explained by the observation that the nitrogen atoms are difficult to link together to form and release nitrogen gas due to the long distance between the nitrogen atoms and the strong C–N bond in the system. Thus, the formation of a polycyclic network can cause two nitrogen atoms to come closer till they are linked directly. Then, this polycyclic network disintegrates and carbon– nitrogen bonds break to release one nitrogen gas. This process is also observed both in the release of the 33rd (about 0.0469 mol cm3) and the 34th (about 0.0483 mol cm3)

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nitrogen gases, as shown in Fig. 7. For instance, at 47.3 ps, a twenty-two-membered ring is formed. This big ring rearranges to form a hexacyclic network at 58.0 ps. This hexacyclic network opens to form a tetracyclic one at 66.4 ps. But it disintegrates completely at 68.0 ps and the 33rd nitrogen gas is formed. The release of the 33rd nitrogen gas spent about 21.5 ps, while that of the 34th nitrogen gas consumed about 8.0 ps. However, from 80.0 to 130 ps, no nitrogen gas is released, suggesting that the formation of 35th nitrogen gas needs more time. This is because the number of the nitrogen atoms becomes fewer and the distances between them become larger and this makes the linking of two nitrogen atoms to form the next nitrogen gas more and more difficult. To test the repeatability of our simulations, the snapshots of typical structures in the formation of polycyclic networks were taken from the two extra short simulations and are shown in Fig. 8. Fig. 8a is the snapshot of DiAT at 9.15 ps taken from the first short simulation, while Fig. 8b is the snapshot of DiAT at 9.9 ps taken from the other one. It is found that the structures in the two snapshots are similar to those at 9.1 ps and 10.0 ps taken from the long simulation, respectively. The two structures also present the formation trend of polycyclic networks. This indicates that our simulations are reproducible. We did not observe significant differences compared with the results from simulations for the 1  2  2 supercell and 1  3  2 supercell. For example, the ratio of observed times for Init1 : Init2 : Init3 is 8 : 3 : 1, which is similar to those in the decomposition of the 1  2  2 supercell and in unimolecular decompositions. Also, we observed that 36 nitrogen gases (60% of the theoretical maximum quantity) were released rapidly in the first 4.0 ps, and no nitrogen gas was formed in the following 11 ps because the formation and disintegration of polycyclic

Fig. 8 Snapshots of decomposition intermediates at (a) 9.15 ps and (b) 9.9 ps taken from the first short simulation. Snapshots of decomposition intermediates at 9.1 ps (c) and 15 ps (d) from the simulations performed on a 1  3  2 supercell.

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networks need a long time. The formation process of polycyclic networks can be observed in Fig. 8c and d. The seven-membered single ring at 9.1 ps further expands to form a big tricyclic network at 15.0 ps, which is the typical feature of the formation of polycyclic networks. Although the enlarging of the system size can produce more decomposition and combination reactions and lead to form bigger and more complicated polycyclic networks, the mechanisms of initiation and release of nitrogen gases will not change essentially. Therefore, a 1  2  2 supercell is enough to observe the most important features of the decomposition process. However, it should be noted that a larger system size is needed for obtaining a more accurate time evolution of decomposition products, which is due to the larger temperature fluctuations in the smaller system. Compared with the simulations using a time step of 1.0 fs, no obviously different results were observed from the simulations using a time step of 0.1 fs. For example, Table S2 (ESI†) lists the observed times of the three initiation reactions from the simulations using a time step of 0.1 fs for 1  2  2 and 1  3  2 supercells. It is found that the results are very similar to those from the simulations using a time step of 1.0 fs. Table S3 (ESI†) lists the number of released nitrogen gases in 0.5 and 1.0 ps for 1  2  2 and 1  3  2 supercells using a time step of 0.1 and 1.0 fs. It is found that the results are close to each other. In addition, Mcwilliams et al.29 reported that the pressure can induce the polymerization of energetic materials. Fig. S4 (ESI†) displays the pressures of DiAT as a function of time with the 1  2  2 supercell. It is seen that the pressure increases dramatically with the rapid release of nitrogen gas in 4.0 ps, which should have a great influence on the decomposition of DiAT. Thus, the generated enormous pressure may be responsible for the formation of polymers in DiAT here. (c) Release mechanism of energy. Fig. 9 displays the time dependence of the total energy of DiAT. It is seen that most of the energy is released in the initial 4.0 ps, while the decrease of energy is not obvious in the following 126.0 ps. This shows that the release mechanism of the energy is in agreement with that of nitrogen gas in the system. As an initial explosive, DiAT needs to

Fig. 9

Time dependence of the total energy of crystal DiAT at 3000 K.

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release most of the energy before the formation of detonation. This requires that the energy release reactions must be fast and drastic, which are different from insensitive explosives. Although the long-chain or heterocyclic clusters were formed during the decomposition of DiAT, this fast energy release was not limited. Our simulations also suggest that in the early decomposition of DiAT, stable long-chain or heterocyclic clusters were not formed largely before most of the nitrogen gases are released. Therefore, our calculated results revealed the essence of explosive characteristics of DiAT as an initial explosive. From Fig. 9, we can also observe that the first stage in the decomposition process is endothermic. To extract the rate constant (tendo, ps) of the global endothermic reaction, a firstorder rate model (eqn (1)) was fitted to the decomposition curve of DiAT from t = 0 up to t = tmax, where N0 is the initial amount of DiAT molecules and tmax is the time when the maximum in total energy is obtained. The parameters obtained in these fits for systems of 1  2  2 supercell and 1  3  2 supercell using a time step of 1.0 fs and 0.1 fs are presented in Table S4 (ESI†). It is found that the results are close to each other.   t NðtÞ ¼ N0 exp  (1) tendo In addition, the rate constant of the release of nitrogen gas in the early decomposition stage was evaluated from the first-order expression a(t) = a0(1  exp(kt)), where a(t) is the conversion factor at time t, a0 is the equilibrium conversion (which usually equals 1) and k is the rate constant. For a given reaction, a(t) is determined by the ratio of generated N2 to the half of the number of nitrogen atoms. According to the linear fit, the estimated values of ln(k) for 1  2  2 and 1  3  2 supercells are 26.6 and 26.2 s1, respectively. This means that the generation and release of energy are extremely fast at 3000 K, which agrees with the feature of initial explosives. Furthermore, these values are obviously higher than that of NO2 fission in CL-20 (the calculated value of 21.5 s1) at 3000 K,1 indicating that DiAT decomposes much faster than CL-20, which is in line with the differences between the decomposition characteristics of initial explosives and secondary explosives.

Conclusions In this work, the thermal decomposition of isolated and crystal DiAT has been investigated by using ab initio MD. It is found that there are three different initiation mechanisms during unimolecular decomposition. They involve N–N2 cleavage, ring opening, and isomerization. The preferential initial decomposition step is the homolysis of the N–N2 bond in the azido group. The release mechanisms of nitrogen gas are found to be very different in the early and later decomposition stages of crystal DiAT. In the early decomposition, DiAT decomposes very fast and drastically without forming any stable long-chains or heterocyclic clusters, and most of the nitrogen gases are released through rapid rupture of nitrogen–nitrogen and carbon–nitrogen bonds in 4.0 ps. While in the later decomposition stage, because of low mobility, long distance from each other, and strong

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carbon–nitrogen bonds, the nitrogen atoms are difficult to link directly and the release of nitrogen gas is very slow. To overcome the obstacles, the nitrogen gases are released through slow formation and disintegration of polycyclic networks. Overall, enlarging the system size of the simulation box and reducing the time step did not change the mechanisms of initiation and release of nitrogen gases. Accordingly, a 1  2  2 supercell and a time step of 1.0 fs are enough to observe the most important features of the decomposition process. This work may provide useful information for understanding the decomposition mechanisms of organic polyazido initial explosives and for developing new green organic initial explosives.

Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 21273115) and A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions. Q. Wu would like to thank the Innovation Project for Postgraduates in Universities of Jiangsu Province (Grant No. CXZZ13_0199) for partial financial support.

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An ab initio molecular dynamics study of thermal decomposition of 3,6-di(azido)-1,2,4,5-tetrazine.

Ab initio molecular dynamics simulations were performed to study the thermal decomposition of isolated and crystal 3,6-di(azido)-1,2,4,5-tetrazine (Di...
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