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Mar 30, 2017 - We study the intrinsic shock decomposition mechanism of 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) via quantum-based multiscale molecu...
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Initial Decomposition of Condensed-Phase 1,3,5-Triamino-2,4,6trinitrobenzene under Shock Loading Zheng-Hua He,†,‡ Jun Chen,*,‡,§ and Qiang Wu*,† †

National Key Laboratory of Shock Wave and Detonation Physics, Institute of Fluid Physics, China Academy of Engineering Physics, Mianyang 621900, Sichuan, China ‡ National Key Laboratory of Computational Physics, Institute of Applied Physics and Computational Mathematics, Beijing 100088, China § Center for Applied Physics and Technology, Peking University, Beijing 100871, China S Supporting Information *

ABSTRACT: We study the intrinsic shock decomposition mechanism of 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) via quantum-based multiscale molecular dynamics methods. Some distinctive and novel insights are obtained based on our simulation results. The reaction is always initiated by hydrogen transfer both in intra- and intermolecular conditions. The carbon atom is mostly oxidized by nitro O directly. The earliest gas product NO2 is produced with the cleavage of C-NO2. The initial N2 mainly derives from the interaction between NO and TATB molecular residues. More intriguingly, we detect 28 kinds of heterocyclic structures involved in the TATB decomposition process. Most of the newly formed C−N heterocycles possess high stability, which obviously inhibit the further reaction toward releasing C oxides and N2. These results reveal not only the early shock decomposition properties of TATB but also the different reaction mechanisms between sensitive and insensitive explosives.

1. INTRODUCTION 1,3,5-Triamino-2,4,6-trinitrobenzene (TATB), a typical nitro explosive, is famous as one of the most stable energetic materials. Owing to its extreme insensitivity toward external stimuli,1,2 such as heat, friction, impact, and shock, it has been widely applied in defense and industry fields during the past decades. TATB possesses both electron acceptor and donor groups (nitro and amino), which exhibit ortho distribution on the aromatic carbon ring. The unexpected insensitivity of TATB is associated with the strong hydrogen bonding interactions3,4 between these two groups within condensedphase TATB. In addition, the existence of special π-stacked5,6 configuration of aromatic rings can also contribute to their stability. Investigation of the microscopic properties and reaction process can provide essential insight about the chemical performance of energetic materials and promote better design and further application of them. In recent years, significant attention has been paid to the experimental study of the decomposition process of TATB under different temperatures and pressures. Several early dissociation mechanisms are proposed, such as C-NO2 bond cleavage,7,8 C-NH2 bond breaking,9 conversion of nitro to nitroso,10 molecular ring rupture,11 and hydrogen transfer.12 However, the existing experimental analysis methods cannot capture sufficient details to reveal how the first chemical bond breaking can induce such violent reactions because of extremely fast and complex © XXXX American Chemical Society

multibody reaction involved in explosion process. Theoretical numerical simulations just make up for this deficiency. Recently, enormous progress has been made, aiming at the microstructure feature of TATB under pressure.3−5,13−15 Manaa and Fried4 observe the nearly equivalent inter- and intramolecular hydrogen bonding within TATB crystal at high pressure. Gee et al.5 and Liu et al.15 suggest that the c axis is the most compressible and sensitive to stress. Meanwhile, the reaction energy barriers for different initial pathways have also been investigated broadly. For instance, C-NO2 cleavage with 64−77 kcal·mol−1,16−18 CONO isomerization with 55 kcal· mol−1,19 C-NH2 breaking with 103.4 kcal·mol−1, and hydrogen transfer with 47.5 kcal·mol−1.20 In more detail, Wu and Fried20 calculate many competing dissociation mechanisms of gasphase TATB; their results suggest that the early reaction prefers to form benzofurazan or its oxidized derivative by ring closure and intramolecular hydrogen transfer, which are the precursor for further decomposition products. Kuklja and co-workers18 propose that the NO2 loss is the dominant pathway at T > 500 K, while the CONO isomerization is the main one at low temperature. Wu et al.21 investigate the coupling effects of temperature and pressure on TATB thermal decomposition. They find that the initial decomposition step is intramolecular Received: October 13, 2016 Revised: March 14, 2017 Published: March 30, 2017 A

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The Journal of Physical Chemistry C Table 1. Comparison of Computational and Experimental Unit Cell Lattice Parameters of TATB Crystal4,29,30 ref 29 ref 4 ref 30 this work

method

a (Å)

b (Å)

c (Å)

α (deg)

β (deg)

γ (deg)

X-ray DFT-GGA DFT-LDA DFTB

9.010 9.219 8.875 9.131

9.028 9.181 8.892 9.147

6.812 7.161 6.450 6.862

108.58 107.7 109.36 109.92

91.82 92.0 91.90 92.02

119.97 120.2 119.97 119.60

fewer atoms and significantly reducing computational cost. The interatomic forces are calculated using quantum-based selfconsistent charge density functional tight-binding (SCCDFTB)26 method to give a reasonable prediction of the chemical reactions. This method is based on second-order expansion of the Kohn−Sham total energy in density functional theory (DFT) with respect to charge density fluctuations, which allows one to describe the total energies, atomic forces, and charge transfer in a self-consistent manner. It was already applied to the study of NM,27 HMX,28 and TATB23 under pressure, which gave the actually reliable description for their microscopic properties and reaction process. The initial crystal structure of TATB is obtained from the experimental X-ray data. Condensed-phase TATB is a typical triclinic molecular crystal, with the crystallographic constants of the unit cell a = 9.010 Å, b = 9.028 Å, c = 6.812 Å, α = 108.58°, β = 91.82°, and γ = 119.97°.29 In this study, we first carry out the unit cell optimization to obtain the structure parameters, which are compared with the X-ray experimental data and another two computational results (see Table 1). Our computed lattice parameters a, b, and c show very good agreement with the experimental data, with less than 1.35% deviation. It is even more precise than that of DFT calculations4,30 (with larger than 5% difference at c axes). The computational lattice angles α, β, and γ are strictly consistent with experimental and DFT calculation results, with less than 1.5° deviation. Thus, our simulation based on DFTB methods can give a reasonable and reliable prediction of microscopic features for condensed-phase TATB. After that, we construct a 2 × 2 × 3 supercell structure (see Figure 1b) and

hydrogen transfer, and it is independent of the various pressures. Tiwari and co-workers22 explore the shock-induced decomposition of TATB using Reaxff molecular dynamics method; they suggest that the main products are H2O, CO2, and N2. N2 is produced through three different intermolecular reactions. Manaa et al.23 report quantum-based multiscale simulations for TATB decomposition experiencing overdriven shock loading. They find that high concentrative nitrogen-rich heterocyclic clusters are formed during reaction proceeding, which obviously inhibit the further dissociation. Despite such extensive efforts having been devoted to study the reaction properties of TATB, some of which even focus on shock decomposition, the detailed reaction mechanism for TATB under shock loading, on account of the formation of main gas products and the stability of early intermediates, is still not sufficiently studied. In this study, we present a quantum-based multiscale molecular dynamics study for the shock decomposition of TATB. The early decomposition mechanism is discovered by analyzing the details of main chemical bond breaking and formation process. Our results reveal that the interlayer molecular interactions are efficiently enhanced by shock compression. The intermolecular hydrogen transfer becomes the main reaction pathway, which is almost as important as the intramolecular hydrogen transfer proposed in thermal decomposition of TATB.21 Different from the HMX dissociation,24 the oxygen transport from nitrogen to carbon mainly occurs by interaction of N−O−C. The direct NO2 loss is not a favorable reaction pathway, but it can be obviously promoted when the corresponding C atom adsorbs other substituents, such as OH, nitro O, or N-fragment. The initial N2 is primarily produced by collision of early product NO and TATB molecular residues. More intriguingly, we detect 28 kinds of heterocyclic structures involved in the TATB shock decomposition process. The formation mechanism and stability of the main species are further analyzed.

2. COMPUTATIONAL DETAILS The shock decomposition process of TATB is simulated in this study, using quantum-based moleculat dynamics (MD) methods in conjunction with a multiscale shock technique (MSST)25 implemented in the CP2K code. The MSST method is based on the Navier−Stokes equations, which combines molecular dynamics and the one-dimensional (1D) Euler equations to mimic the propagation of the shock wave for compressible flow. The Hugoniot relation and Rayleigh line derive from the conservation equations of mass, momentum, and energy across the shock front. This method is different from non-equilibrium molecular dynamics processes. It just constrains the system pressure and volume to comply with the Hugoniot state (corresponding to a state behind the shock wave plane). So actually, there is no real shock wave going through the simulation box. Besides, the computational cell of the multiscale technique follows a Lagrangian point through the shock wave, enabling a simulation of a shock system with many

Figure 1. (a) Molecular structure for TATB (C6H6N6O6), (b) 2 × 2 × 3 supercell, and (c) slab model of 2 × 2 × 3 supercell with (001) surface.

also perform the complete optimization to obtain the reasonable original configuration. The calculated lattice parameters are a = 17.993 Å, b = 18.015 Å, c = 20.162 Å, α = 110.01°, β = 91.89°, and γ = 120.01°. The 2 × 2 × 3 supercell model is further cut along the (001) surface, exposing the molecular layer to facing the shock wave (see Figure 1c). The surface is employed within the y−z plane, and an extra 6 Å of vacuum is placed on top of it. We perform the continuous molecular dynamics simulations with time step of 0.5 fs and the target accuracy for SCF convergence of 10−6 au. First, the B

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The Journal of Physical Chemistry C supercell 2 × 2 × 3-(001) model is equilibrated under 300 K for 3 ps using the MD-NVT method. Then, the uniaxial compression of shock wave is loaded along the x-axis with multiscale technique. Two different shock speeds of 9 and 10 km·s−1 are calculated for 10 ps, indicating the overdriven shock compression processes (the steady detonation speed of TATB is 7.7 km·s−1).31 The fictitious cell mass of 5 × 107 au is employed for MSST simulations. The initial temperature and pressure are 300 K and 0 GPa, respectively. The MD trajectory is analyzed stepwise. We implement a MD postprocessing procedure to identify stable molecular components based on bond length and lifetime criteria.32 If the distance of two atoms is less than a critical value Rc, and they meet this condition for more than 10 fs, these atoms are considered to be bonded. Here, Rc is determined by selecting a reasonable cutoff of Mulliken bond order. According to a previous study,33 the cutoff is set to 0.3 for all atom pairs, and the corresponding Rc of different chemical bonds are listed in Table S1 in the Supporting Information. Furthermore, any atoms connecting with each other through the criteria belong to the same molecule. To identify the reliability of our calculation model, we have added another extra and larger model, 3 × 3 × 3-(001) slab with 54 TATB molecules. The simulation results show that the population changes of main chemical bonds, initial intermediate products, and their evolution trends are greatly consistent with the condition of 2 × 2 × 3-(001) model (see Figures S1 and S2), which indicates that our simulation can reasonably and reliably reveal the reaction properties of TATB decomposition.

and pressure are obviously greater than the conditions of steady detonation of TATB (7.7 km·s−1 and 29 GPa),31 only a few chemical reactions and intermediates are observed (see Figure S3). Our calculation time scale is actually short to investigate the decomposition process of TATB under 9 km·s−1. The other hundreds of picoseconds are required to obtain more significant chemical behaviors according to previous work,23 which is very expensive for the huge system in our study. In addition, we note that the reaction temperature of 9 km·s−1 is similar to the thermal decomposition in the previous theoretical study,33 but the reaction rate is obviously slower than that of thermal conditions. This discrepancy mainly derives from the different reaction pressures. At shock loading, the condensed-phase TATB is compressed (with the relative volume of ∼0.6) and displays different reaction mechanism compared with thermal decomposition. More time steps are needed to get violent chemical reactions for the compressed system. A similar phenomenon is also observed by Giefers and Pravica.34 During their radiation-induced TATB decomposition experiment, they found that the decomposition rate greatly slows with pressure increase. Thus, in the following sections, we mainly focus on analyzing the decomposition process of TATB with 10 km·s−1 shock loading. 3.1. Population Evolution of Various Chemical Bonds. First, we reveal the reaction sketch of TATB decomposition by analyzing the population changes of chemical bonds. Figure 3a

3. RESULTS AND DISCUSSION In this work, we perform two multiscale calculations with different shock speeds of 9 and 10 km·s−1. The time evolution of system pressure, relative volume (the ratio of the compressed volume to the initial volume), and temperature are displayed in Figure 2. For the 10 km·s−1 shock simulation, the relative

Figure 3. Population evolution of main chemical bonds involved in decomposition of TATB at 10 km·s−1: (a) original bonds, (b) newly formed bonds, and (c) details for newly formed C−N bonds.

Figure 2. Time evolution of pressure (a), volume (b), and temperature (c) of 2 × 2 × 3-(001) condensed-phase TATB at shock speeds of 9 and 10 km·s−1.

displays the evolution of the original bonds within TATB molecules, while Figure 3b,c shows that of the newly formed chemical bonds along the reaction proceeding. The population of N−H bonds has the fastest decrease rate, illustrating that the hydrogen transfer is the main reaction pathway for the initial decomposition. This process commonly occurs with amino H transferred to nitro O, causing rapid increase of O−H bonds. It is worth noting that there exist obviously contrary rise and fall

volume of condensed-phase TATB rapidly decreases and finally remains about 0.52 with shock compression. The temperature gradually increases from 3100 to 3700 K, and the pressure increases from 62 to 82 GPa as a result of the initial chemical reactions. For the condition of 9 km·s−1, the temperature and pressure almost maintain their constant values of 2400 K and 42 GPa within our simulation period. Although the shock speed C

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The Journal of Physical Chemistry C of N−H and O−H bonds at about 3.5 and 6 ps (see Figure 3a,b). This result illustrates that some newly formed O−H bonds are not stable, which can interact with the adjacent nitrogen atom to transfer H back to N. While most of the O−H groups easily adsorb another H to eliminate H2O, resulting in the gradual decrease in the population of N−O bonds. The other consumption of N−O bond is carbon oxidation to form C−O bond. About 35% of C−C bonds still exists after 10 ps, which indicates that the cleavage of C−C bond is not energy favorable. More intriguingly, there are two types of C−N bond within the TATB molecule, C-nitro and C-amino, which display obviously different reaction behaviors during TATB decomposition. The C-NO2 bonds show obviously higher reaction activity. At the earliest reaction stage, we observed that their population has a dramatic decrease from 72 to 60 within the first 0.25 ps (see Figure 3a). While the population of CNH2 bonds decreases slowly, with ∼25% of bonds still exsiting at the end of the simulation. This result indicates that the interaction between C and NH2 is stronger than that of C and NO2, which is consistent with the results of energy barrier calculation18−20 and charge density analysis35 regarding these two kinds of bonds in previous theoretical studies. A contrary reaction phenomenon is observed when we analyze the newly formed C−N bonds. As shown in Figure 3b, the N coming from the original amino has the advantage of forming the new C−Namino bonds within the first 6 ps. In detail, Figure 3c reveals that most of the C−Namino bonds consist of CNO2-Namino (the carbon atom deriving from C-NO2). Although the nitro N can also interact with C to form C−Nnitro bonds, its formation rate is obviously slower than that of C−Namino at the early stage. After 6 ps, however, the population of C−Nnitro bonds reaches a level similar to that of C−Namino. This might be because the configuration of TATB molecules is completely destroyed and the distinctions between the original C−Nnitro and C−Namino groups are diminished. The formation of C− Nnitro bonds between amino-C and nitro N (CNH2-Nnitro) is more popular than that of nitro-C and nitro N (CNO2-Nnitro) (see Figure 3c). All the distinct phenomena denote that the C− N bonds prefer to generate between the different types of the C and N, such as nitro-C and amino N or amino-C and nitro N. As a result, many large molecular chains are formed through nitrogen atoms, which would be the precursor for the high concentration of the N-rich hetercycles reported in previous work.23 In addition, few N−N bonds are formed, with the really small population of 26 at the end of the simulation, illustrating that the gaseous N2 is not easily generated during early decomposition of TATB. 3.2. Details of the Main Reaction Pathways for TATB Decomposition. 3.2.1. Hydrogen Transfer to Form Water. We observe that 87.5%of TATB molecules initiate their dissociation with hydrogen transfers, which almost equivalently occurs with intra- and intermolecular inteactions. The intramolecular transfer has been reported in a previous study, which is the main pathway for TATB thermal decomposition.21 As seen in Figure 4, although this process from INT1 to INT2 has a low energy barrier (42.1 or 47.5 kcal·mol−1),18,20 elimination of water from INT2 within itself requires a very high activation energy of 68.6 kcal·mol−1 for HONO rotation.18 Thus, it is quite difficult for the intramolecular dehydrogen to form water. In addition, the elimilation of HONO from INT2 is also not energy favorable because of the strong interaction between C and NO2. Here, the hydroxyl O of HONO group prefers to

Figure 4. Hydrogen transfer to form water molecule.

adsorb another H from the adjacent molecule (see INT3 in Figure 4) to further form H2O. The intermolecular hydrogen transfer was reported to require a 59.2 kcal·mol−1 activation energy,20 which is obviously higher than that of the intramolecular condition. In this study, however, the intermolecular hydrogen transfer also plays an important role during the initial shock decomposition of TATB. Differing from the thermal decomposition, the intermolecular interaction is obviously enhanced by shock compression, especially for the interlayer molecules. As shown by INT4 in Figure 4, the nitro and amino groups rotate out of the molecular plane to form an electrophilic adsorption structure, enabling a hydrogen atom to be transferred between interlayer molecules. The HONO within INT5 can easily abstract one H from amino to produce water, in both intramolecular (INT6) and intermolecular (INT7) conditions. The other bimolecular hydrogen transfers are also observed, which are commonly induced by other electrophilic groups, such as hydroxyl and carboxyl. 3.2.2. Oxygen Transfer from Nitrogen to Carbon and Elimination of NO2. As for formation of water molecules, many nitro O are transferred to H, while most of the others are transferred to carbon (see Figure 3b). It is worth noting that the oxygen transport is obviously different from that in HMX24 and PETN36 decomposition. In those two types of sensitive explosives, intermediate water acts as a special catalyst, efficiently promoting the oxygen transport from nitrogen to carbon. The special catalytic effects of water are responsible for the rapid decomposition process of these two explosives. For TATB dissociation, we observe that most of (more than 65%) the oxygen atoms are transferred from N to C by direct interaction of N−O−C between different TATB molecules or molecular residues. No significant promoting effect of water is observed. As shown in Figure 5, there are two typical oxygentransfer pathways. For the first one, the intermolecular interaction occurs within adjacent molecular layers. The nitro group rotates to connect with aromatic C atom to form INT8. Then, the corresponding N−O bond ruptures to transfer oxygen to carbon, forming the carboxyl or C-hydroxyl structure (INT10). The following carbon ring-breaking produces the carbon oxides finally, which requires a few picosecond. For the second pathway, the aromatic carbon ring ruptures to form the terminal C groups (C-R of INT11 in Figure 5), which can easily abstract oxygen from the nitro group (from INT11 to INT12). The newly formed oxidized-C group would further convert into small C oxides. Unlike the cleavage of N-NO 2 involved in HMX decomposition,33 the direct elimination of NO2 is not energy favorable for decomposition of TATB. The cleavage of C-NO2 requires a 64−77 kcal·mol−1 activation energy,16−18 indicating a D

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Figure 5. Oxygen transfer from nitro to carbon and elimination of NO2. (R represents the TATB molecular residue.)

dissociates to form the N2 molecule at 3.9 ps. This mechnism is similar to the results reported by Manaa et al.37 They proposed that the amino first eliminated hydrogen to form water, which then combined with each other to produce N2. At 0.75 ps, the second N−N bond is produced by interacting between nitroso N and imino N to form the INT17 structure. Tiwari and coworkers22 suggested that this reaction pattern was the main pathway for the N2 formation. However, the initial two reaction pathways are actually not favorable in this study. As in TATB decomposition, some key intermediate products are produced, especially for NO (the detailed formation process will be analyzed in the following section), and a novel formation mechanism of N2 induced by newly formed NO is observed. More than 60% of the initial N2 is produced through this reaction pathway. As shown in Figure 6, NO can adsorb on the nitroso N or imino N to form the −N−NO structures, such as the INT18 and INT19. Most of them prefer to combine with the terminal N within TATB decomposition residues (see INT20). The −N−NO group easily converts into N2 by abstracting a H atom to eliminate the O atom. 3.3. Evolution of Main Components Involved in TATB Decomposition. After analyzing the initial reaction mechanism, we further study the components of decomposition products. The evolution of the main gas products and the stability of intermediate heterocycles are systematically analyzed. 3.3.1. Gas Decomposition Products. Figure 7 displays the time evolution of the population of the main gas products involved in TATB decomposition under 10 km·s−1 shock loading. The typical gas products, such as NO2, NO, N2, H2O, and CO, are observed. Some of them are also reported in other theoretical studies.23,33 TATB molecules are gradually depleted within 1 ps, which mostly convert into C6H5N6O6 H, C6H5N6O6, and C6H6N6O6H, through intramolecular and intermolecular hydrogen transfer (see Figure 4). Although the C-NO2 bond is very strong, NO2 still is the first gas product for TATB decomposition. It derives from the C−N breaking induced by other active groups as analyzing above. Like the HMX decomposition, NO2 is rapidly consumed by the second reactions. Water and NO molecules are produced at almost the same time. Water is produced through further hydrogen abstraction of the hydroxyl structure formed by hydrogen transfer. NO primarily comes from the reaction of NO2 + H = NO + OH (see Figure 5). It also can be produced through

strong interaction between C and NO2. In this study, the initial NO2 molecules are mostly produced by multibody reaction. The C-NO2 bond is efficiently activated by the oxidation of the corresponding carbon atom. As seen in Figure 5, NO2 is easily elimilated from the oxidized carbon through the intermidate of INT9 and INT10. In addition, the adsorption of other substituents, such as hydroxyl and C−N fragment (see INT14), forming extra C−O, C−C, or C−N bonds, also can promote the NO2 elimilation. The newly formed NO2 has high oxidation activity, which is rapidly depleted by abstracting H atom to produce NO and OH. In comparison, an interaction similar to that of INT8 also occurs at the C-NH2 group, but no significant dissociation behavior of the C-NH2 bond is observed during the early stage of TATB decomposition. This is consistent with the previous experimental study on TATB detonation carried out by Sharma and Owens.7 Their XPS spectra data also illustrated that the C-NO2 bond cleavage to elimination nitro groups is more popular and important than that of the C-NH2 bond during the shock-induced detonation processs of TATB. 3.2.3. N−N Bond Formation to Further Produce Initial N2. Differing from the structure of HMX, there is no original N−N bonds within the TATB molecule. The formation of gaseous N2 may be delayed by first forming their precursors, N−N structures. By tracking the formation process of N−N bonds, we find that the first N−N bond appears at about 0.55 ps, which comes from the interaction of imino N atoms of interlayer molecules (see INT16 in Figure 6). It further

Figure 6. Formation of N−N bonds. (R‴ represents TATB molecular residuum.) E

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note that the population of N2 is lower than that of H2O, which is contrary to the result of thermal decomposition.33 This discrepancy can be ascribed to the formation of C−N heterocyclic structures during shock loading.23 As a result, the formation of N2 is impeded and obviously later than that of NO2, H2O, and NO. All the results are exactly in agreement with the formation mechanism of N2 illustrated in Figure 6. Although many C−O bonds are formed at an early reaction stage (see Figure 3b), only a few carbon oxides are produced in our study. These initial carbon oxide molecules are not the stable and final gas products, which can easily collide with other fragments to produce new carbon chain or molecular cluster, resulting in the dramatic oscillation for their populations. This is consistent with the results of TATB thermal decomposition.33 3.3.2. Main Stable Intermediate Heterocycles. Following the formation of small gas products, many larger molecular structures are produced by aggregation of TATB molecular residues. Most of the aggregations occur through the combination of carbon and nitrogen atoms, as shown in Figure 3b, which can convert into the nitrogen-rich heteroatomic rings like the structures reported by Manaa and Reed.23 At the end of the simulation, most of the heterocycles and other C−N chains aggregate into one large cluster, with the chemical components of C128N103O77H64. About 89% C atoms and 72% N atoms are

Figure 7. Population evolutions of main components involved in decomposition of TATB at 10 km·s−1.

direct disociation of the C−N bond within the C-nitroso group. The population of NO gradually increases to reach their maximum of 15 and then decreases with the further reactions. Correspondingly, the population of N2 rapid increases, indicating that the N within NO molecules is depleted to produce N2. The gaseous N2 gradually appears after 3 ps as the reaction proceeds to reach its maximum of 16 at ∼8 ps. We

Table 2. Main Stable Intermediate Heterocycles and Their Lifetimesa

a

Cyan, blue, and red balls represent carbon, nitrogen, and oxygen atoms, respectively. F

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two times, some of them are quite stable. For instance, C3N3-2 has an average lifetime of 2.193 ps; C5N2 and C5 have lifetimes of 1.865 and 1.88 ps, respectively. It seems that the C−N heterocyclic structures without oxygen always possess long lifetime and high stability. Most of them can exist more than 1 ps (except for C6N and C2N3-2). While the heterocycles containing oxygen atom display weaker stability. Their lifetimes are all shorter than 1 ps (except for C2N3O). The stable C−N heteroatomic rings, especially for C4N2-1, C3N2-1, C5N, and C4N, act as the nitrogen trap, efficiently impeding the elimination of N atoms to further produce N2. More importantly, the active C-fragments and radicals are also locked within the stable heterocyclic groups, which significantly weakens the reaction activity of the system and inhibits the further decomposition of TATB. Now, we can illustrate the slow reaction process of TATB more clearly, based on a deep understanding of its initial decomposition mechanism. Although many water molecules are produced, different from the HMX decomposition, no obvious self-catalytic effect of intermediate water is observed during TATB dissociation. The carbon oxidation relies on only the direct interaction between nitro O and aromatic C, resulting in a slower oxidation rate for carbon. Furthermore, most of the active C and N groups are locked within the following formed C−N heterocycles, greatly diminishing the reaction activity of system, which is also responsible for the slow reaction process.

locked within this cluster. It is in great agreement with previous work.23,33,38 In this study, we focus on analyzing the detailed structures and components of these heterocycles, to illustrate their effects on further decomposition of TATB. During the whole MD simulation process, we detect 28 kinds of heteroatomic rings. The main heterocyclic structures and their lifetimes are listed in Table 2, according to their formation sequence. The others with lifetime longer than 0.1 ps are also considered (see Tables S2 and S3). There are various substituents and molecular fragments combined with these molecular rings, constructing long C−N chain and net structures. As a result, many of the carbon atoms have four-coordination configuration, indicating a sp3-hybridization of these C atoms. Carter et al.39 also observed the similar phenomenon when they analyzed the intermediate decomposition products of TATB, using X-ray photoelectron spectroscopy, infrared spectroscopy, and Raman spectroscopy. They suggested that the sp3 hybridized carbon was the dominate configuration for the early amorphous carbon structure. Here, for clarity, we display only the molecular rings themselves. As shown in Table 2, the first intermediate heterocycle, C4N2-1, is formed at 0.75 ps by directly combining carbon and nitrogen atoms within adjacent molecules. The early formed C4N2-1 always constructs bicyclic or tricyclic structures with the original benzene ring, as in eq 1 in the Supporting Information. This heterocycle is considerably more stable and can exist for an average of 1.822 ps. The N atoms are locked within this ring, inhibiting their further reaction to the final gas product of N2. The second ring, C2N2O-1, appears at 0.8 ps, which is formed by interaction of the adjacent nitro O and amino N within the TATB molecule. This is a famous intermedate configuration for decomposition of TATB and widely reported in previous studies.18,20,21 The C2N2O-1 groups have a short average lifetime of 0.801 ps, indicating a high activity and low stability. The frequent formations of C2N2O-1 promote the nitro O transferred to amino N. The thrid heterocycle, C3N2-1, is first formed at 0.85 ps by interaction of carbon atom with imino N (see eq 2 in the Supporting Information). It is one of the most popular structures and is continuously produced during the whole simulation period. All the existing C3N2-1 accumulates up to 20.9 ps, with average lifetime of 1.493 ps. Obviously, the terminal active C and N groups are consumed by forming this stable heterocyclic structure, resulting in a slow decomposition of TATB. Meanwhile, it worth noting that the opened aromatic ring prefers to convert into one nitrogen heterocycle, such as C5N, C6N, and C4N. All the C6N is formed within the initial 2.6 ps and derives from the molecular isomerization of TATB. C6N is not stable and has the shortest average lifetime of 0.634 ps among these three single-N rings. The C5N and C4N rings are produced through intra- or intermolecular interaction of carbon and nitrogen. C5N is the most stable heteroatomic ring, with the longest average lifetime of 2.029 ps. C4N is another popular intermediate heterocycle, which is produced during almost the whole decomposition process. It exists for 19.595 ps in all, with the average lifetime of 1.4 ps. There are another two heterocyclic structures containing oxygen atom, C3N2O-1 and C3NO. They exist for only 1.515 and 2.46 ps, with average lifetimes of 0.505 and 0.41 ps, respectively. The other heterocyclic structures are also considered and dispalyed in Tables S2 and S3. Many of them are the isomers of the main molecular rings analyzed above, like C2N2O-2 and C3N2-2. Although the formation of them occurs only one or

4. CONCLUSION In summary, we have revealed the shock decomposition process of TATB by means of quantum-based multiscale MD calculations. Compared with thermal conditions, the interlayer molecular interaction is obviously enhanced by shock compression; many different reaction mechansims and novel insights are obtained. The intermolecular hydrogen transfer becomes one of the most important reaction pathways for the initial decomposition of TATB. Unlike HMX decomposition,24 the carbon oxidation mostly occurs by direct interaction between nitro O and aromatic C. The initial N2 mainly derives from the collision between the early formed NO and C−N fragments. The cleavage of C−N bonds is much more favorable than that of C-NH2 bonds, which is efficiently promoted by adsorbing other constitutents on the carbon atom, such as hydroxyl, nitro O, or N-fragment. The early formed C−N bonds are mostly produced through the interaction between nitro-C and amino N, or amino-C and nitro N, which further connect into intermediate heterocycles. In particular, C4N2-1, C3N2-1, C4N, and C5N are the most popular and stable components of the heterocyclic sturctures. The further decompositions of TATB are impeded by formation of these stable heterocycles.

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b10354. Additional experimental data and eqs 1 and 2 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel.: 0086-010-61935175. *E-mail: [email protected]. Tel.: 0086-0816-2491024. G

DOI: 10.1021/acs.jpcc.6b10354 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C ORCID

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Zheng-Hua He: 0000-0002-5764-5972 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Science Challenging Program, National Natural Science Foundation of China (11572053), Development Foundation of China Academy of Engineering Physics (2014A0101004).

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DOI: 10.1021/acs.jpcc.6b10354 J. Phys. Chem. C XXXX, XXX, XXX−XXX