HMX Cocrystal: A Case

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Initial Decay Mechanism of the Heated CL-20/HMX Co-Crystal: A Case of the Co-Crystal Mediating the Thermal Stability of the Two Pure Components Xianggui Xue, Yu Ma, Qun Zeng, and Chaoyang Zhang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b00698 • Publication Date (Web): 14 Feb 2017 Downloaded from http://pubs.acs.org on February 17, 2017

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The Journal of Physical Chemistry

Initial Decay Mechanism of the Heated CL-20/HMX Co-Crystal: A Case of the Co-Crystal Mediating the Thermal Stability of the Two Pure Components Xianggui Xue, Yu Ma, Qun Zeng, and Chaoyang Zhang* Institute of Chemical Materials, China Academy of Engineering Physics (CAEP), P. O. Box 919-311, Mianyang, Sichuan 621900, China.

P

Abstract: Energetic co-crystallization, by combining existing molecules together, is thought to be new strategy for creating energetic materials. Nevertheless, the underlying mechanism of its influences on properties and performances, in comparison with their pure components, remains unclear. The present work reveals the co-crystallization influence of a typical energetic co-crystal of CL-20/HMX on thermal stability, by ReaxFF molecular reactive dynamic simulations and kinetics calculations on the pure and co-crystals. As a result, we find that the co-crystal mediates the thermal stability of pure crystals, in agreement with experimental observations. The initial decay steps in pure crystals remain still in the co-crystal, i.e., the independent and intramolecular reactions of N-N bond cleavage governing the initial decay of the pure CL-20 and HMX crystals also dominate in the co-crystal of CL-20/HMX. Meanwhile, during the thermal decomposition of the co-crystal, CL-20 releases heat faster than HMX, thus the heat is transferred from CL-20 to HMX, and further the decay rate of HMX increases while that of CL-20 decreases, relative to the pure crystals. This leads to a moderate decay rate of the co-crystal and a small difference in decay barrier after co-crystallization. Besides, the moderated decay rate is also attributed to the small variation in intermolecular interactions after co-crystallization and the intrinsic weak stability of both component molecules of CL-20 and HMX. Thus, the intrinsic molecular stability of components and intermolecular interactions should be noted as two main factors in a strategy for increasing stability by energetic co-crystallization.

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1. INTRODUCTION Energetic materials (EMs) are a class of crucial substances with a large amount of stored chemical energy and widely applied in civil and military areas by releasing the stored chemical energy. EMs with high power while guaranteed safety are highly desired.1,2 Nevertheless, it is still difficult to achieve these desired EMs in reality, due to some main reasons as follows. This first is the limit of stored chemical energy. On one hand, more energy stored will cause the weaker chemical bonds and further the lower molecular stability; on the other hand, at least, EMs are required to exist stably at common conditions. Thereby, the stored chemical energy is limited by the so-called inherent and safety-power contradiction of EMs. In addition, the risk in the overall life period of EMs and the strict multiple requirements for applications prevent us from achieving them efficiently. Only a slight part of new energetic compounds can finally be applicable as EMs and most of synthesized compounds are deserted in practice. Thus, EMs evolve very slowly. For example, from 1863, in which a benchmark of modern EMs of 2,4,6-trinitrotoluene, TNT, was synthesized, to 1998, in which the currently most powerful EM, 2,4,6,8,10,12-hexanitro2,4,6,8,10,12-hexaazatetracyclododecane, CL-20 was produced, detonation velocity increases below one time only (~7300 vs. ~9400 m/s), unlike many electronic materials and energy materials, whose properties and performances can increase several orders in a short period. Now that most syntheses possess low success rates in practice, why not we develop new EMs by existing energetic compounds? This should be a strategy. Currently, energetic co-crystallization, co-crystallizing two or more kinds of neutral molecules to form new lattice structures, among which at least one kind is energetic, acts as such strategy. Energetic co-crystals (ECCs) provide a promising approach to high performance EMs and become thriving. Relative to pure components, ECCs vary in components and packing structures and thereby properties and performance. That is


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to say, properties and performances can be tuned through cocrystallization.3-7 As pointed out above, CL-20 features a high density (2.04 g/cm3), good oxygen balance (-10.95 %), large heat of formation (460 kJ/mol) and therefore high power

1,8-10

, and becomes a

preferred coformer in ECCs. Presently, above 10 CL-20-based ECCs have been prepared.5,6,11 Relative to pure ε-CL-20 (the most stable form of all CL-20 polymorphs at common conditions), the impact sensitivity of these ECCs reduces; while, their thermal stability varies without a clear rule. In fact, as a whole, though it was well documented that energetic co-crystallization is an effective technique for improving the safety of EMs, most underlying mechanisms remains unclear. Thus, understanding these mechanisms becomes necessary, because it is a premise for guiding ECC preparation with desired properties and performances. The CL-20/HMX co-crystal (1) was reported to be more powerful than pure β-HMX while similar impact sensitive to pure β-HMX, severing as a successful sample of power enhanced while safety maintained, as a potential for practical application.6 However, the underlying mechanisms of 1 against external stimuli, or, the underlying mechanisms of the influence of co-crystallization, appear lacking. Thereby, the present work focuses on the decay mechanism when heating 1, as well as the most stable polymorphs of CL-20 and HMX at common conditions, ε-CL-20 and β-HMX, respectively, for comparison. In practice, the β-CL-20 and β-HMX configurations appear in 1. In contrast to the absence of understandings on the decay mechanism of 1, those on CL-20 and HMX are much richer. In the past decades, lots of experiments were carried out to reveal the decay mechanism of heated CL-20 and HMX.12-23 The homolyses of N-N and C-C bonds were generally considered to be the main primary steps for CL-20 decomposition.12-17 For instance, Naika et al proposed that CL-20 decay commences with the N-N bond cleavage, followed by the break of C-N and C-C bonds. For HMX, three initial steps were found, including N-N bond rupture, concerted



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breaking of ring C-N bonds and HONO elimination.20-23 Meanwhile, theoretical studies were also performed for the thermal decay of these two EMs.24-33 In general, the decay pathways were firstly proposed by experiments and then verified by the theoretical calculations. For instance, by an ab initio molecular dynamics (MD) simulation, the thermal decomposition of CL-20 was found to be trigged by N-N partition, followed by a ring opening.25 Such partition was also found to trigger the HMX decay.26-28 In addition, the structures, and energetic and mechanical properties of 1, as well as ε-CL-20 and β-HMX, were theoretically investigated to understand property changes after co-crystallization.34 Besides 1, we have to mention another CL-20-based ECC, TNT/CL-20 (2)5, serving as an analog to 1. Recently, the thermal decomposition mechanism and compressive shear reactive dynamics of 2 were studied by the ReaxFF molecular reactive dynamics method.35,36 From the dominant product of NO2 at the early stage, the N-N and C-N bond cleavage were deduced as the initial steps of CL-20 and TNT decay, respectively. And it was found that 2 possesses a lower decay rate than ε-CL-20, implying its reduced sensitivity compared to ε-CL-20.35,36 This work pays attention to the thermal behaviors of 1, by MD simulations with a reactive molecular force field of ReaxFF-lg37 and kinetic calculations. As results, we find that the thermal decay rate of 1 is between those of ε-CL-20 and β-HMX, in agreement with differential scanning calorimetry (DSC) examinations.6 The DSC decomposition points of 1, ε-CL-20 and β-HMX at a heating rate of 5 oC/min are 235, 210 and 279 oC, respectively. Also, it is found that the primary molecular decompositions of CL-20 and HMX in 1 are unimolecular and therefore independent, similar to those of pure component crystals, i.e., CL-20 and HMX in both pure crystals and the co-crystal of 1 are decomposed from the N-N break. There is only a difference in rate during thermal decomposition, which is remarkable at relatively low temperatures: compared with ε-CL-20, the decay rate of the CL-20 molecules in 1 is reduced; while, for HMX molecules, they are decayed


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faster in 1 than in β-HMX. By the present work, we can well understand the experimental observation when heating. Besides, from this work, we conclude that the intrinsic molecular stability of components and intermolecular interactions should be noted in a strategy for increasing stability by co-crystallization. 2. METHODOLOGY

Figure 1. Unit cells of 1, ε-CL-20 and β-HMX. The C, H, N and O atoms are represented in grey, green, blue and red, respectively. These representations are considered in following figures.

As mentioned above, we would focus on the behavior of 1 by heated at constant temperatures, as well as the pure ε-CL-20 and β-HMX for comparison. Thus, three crystal structures and heating conditions were considered for simulations. The experimental unit cells of 1 6, ε-CL-20 β-HMX

38

1

and

shown in Figure 1 were adopted to construct simulation structures, with various sizes

enlarged. As listed in Table s1 of Supporting Information (SI), all three simulation systems each contains above 3000 atoms, adequate for revealing complex reactions in solid. Table 1. Comparison of the Lattice Parameters and Packing Densities between Experimental Determinations and Relaxation by ReaxFF-lg Force Field. Crystal 1 ε-CL-20 β-HMX

ReaxFF-lg Exp.6 ReaxFF-lg Exp.1 ReaxFF-lg Exp.38

a, Å

b, Å

c, Å

ρ, g/cm3

Relative error, %

16.625 16.345 8.872 8.852 6.686 6.540

10.106 9.936 12.584 12.556 10.960 11.050

12.346 12.142 12.842 13.386 8.629 8.700

1.902 2.001 2.121 2.044 1.883 1.894

-5.37 3.76 -0.58

As to heating conditions, five temperatures, 1200, 1500, 1800, 2000 and 2500 K, were ACS Paragon Plus Environment 5


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employed to constantly heat above crystals. The choice for the simulation temperatures was based on recent similar simulations on the decay of both CL-2034,35 and HMX39-41. All MD simulations were carried out using LAMMPS software package42, with a reactive force field of ReaxFF-lg, an improved version of ReaxFF. As to ReaxFF, its principle can be referred Refs 43-45. Briefly, ReaxFF is an improved force field derived from earlier reactive empirical force fields such as those by Brenner46,47, parameterized to reproduce the density functional theory (DFT) results for selected systems and properties. It is a molecular force field based on the DFT, in which the formation of a chemical bond is determined by its bond order. Relative to the old version, an additional term of London dispersion is added in ReaxFF-lg for providing a more accurate description of cell parameters of molecular crystals at low pressures. It has been successfully applied to energetic materials such as RDX,44,48 HMX,39-41 PETN,37,41 TATB,37,41,49,50 and CL-20.35,36 Prior to the MD simulations, energy minimizations were performed to relax all the crystals. Subsequently, we carried out isothermal-isobaric (NPT, T=298 K and P=1 atm) MD simulations each for 5 ps to relax internal stress and obtain relaxed structures at common conditions. Nose-Hoover barostat and thermostat were employed to control the pressure and temperature. As listed in Table 1, the relaxed lattice vectors and densities of all three crystals are comparable to the experimental determinations, with relative errors below 5.5 %, verifying the reliability of ReaxFF-lg applied. Afterwards, five NVT-MD simulations were performed each with a time step of 0.1 fs and a total time of 80 ps at the five temperatures. To analyze the chemical specie evolution in the our simulations, the atomic connectivity was determined by bond distance cutoffs and a lifetime criterion to avoid miscounting the short-term fluctuations in bonds above/within the cutoffs. Cutoffs do not actually break/form a bond. Two atoms were counted as bonded if their distance is less than 1.33 times of the normal covalent bond


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length51, and must survive at least 20 and 45 fs for bonds involving and not involving H atoms, respectively.52 These time scales were chosen based on the characteristic vibrational frequency of the related chemical bonds by considering that the bond between two atoms should sustain at least a vibrational period to complete one vibration. 3．RESULTS AND DISCUSSION Before discussion, we define the CL-20 and HMX molecules in 1 and related pure component crystals as c-CL-20 and c-HMX, and p-CL-20 and p-HMX, respectively, for convenience. Thereby, the decomposition mechanism of 1 is discussed from two aspects: apparent kinetics and details of initial reactions. 3.1 Apparent Kinetics of Initial Reactions. 1.0 p-CL-20 c-CL-20 c-HMX p-HMX

(b) T=1500 K

(a) T=1200 K 0.8

Reactant fraction

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


p-CL-20 c-CL-20 c-HMX p-HMX

0.6

p-CL-20 c-CL-20 c-HMX p-HMX

(c) T=2500 K 1.0

CL-20 co-CL-20 co-HMX HMX

0.8 0.6

0.4 0.4 0.2

0.2

0.0 0.0

0.0 0

20

40

Time,ps

60

80 0

20

40

60

80 0

Time, ps

0.5

20

1.0

40

1.5

60

80

Time, ps

Figure 2. Comparison of the decay of HMX and CL-20 molecules in 1 and pure crystals at 1200, 1500 and 2500 K.

We first pay attention to the decay of HMX and CL-20 molecules in different crystal circumstances. Figure 2 shows the decreases of HMX and CL-20 molecules when heated at 1200, 1500 and 2500 K. For the remaining temperatures of 1800 and 2000 K, the cases are between those of 1500 and 2500 K. As a whole, the relatively low temperatures distinguish the decay rates of a same kinds of molecules in different crystal circumstances. For example, at 1200 K (Figure 2(a)), we can readily clarify the differences in decay rate between p-HMX and c-HMX, and between



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p-CL-20 and c-CL-20. Within the simulation timescale of 80 ps, a small part of p-HMX molecules are decomposed, consistent with a recent simulation result also by ReaxFF force field.31 While, about 60 % c-HMX molecules are decayed this time. According to this, c-HMX molecules are decayed faster than p-HMX. Contrarily, c-CL-20 molecules are decomposed slower than p-CL-20 molecules. As illustrated in Figure 2(a), the decay of p-CL-20 and c-CL-20 molecules is finished about 27 and 45 ps, respectively. From Figure 2(a), we can conclude that the co-crystallization of HMX and CL-20 facilitates the HMX decay while slowers the CL-20 decay, in contrast to the pure component crystals. In addition, the CL-20 molecules are always decayed faster than the HMX molecules, in both 1 and pure crystals. This should be attributed to the lower energy barrier for triggering N–N bond dissociation in CL-20 than that in HMX, as 37.6 24 vs. 44.6 30 kcal/mol. With temperature increasing, for example, as illustrated in Figure 2(b), the decay rate differences between p-HMX and c-HMX, and between p-CL-20 and c-CL-20 decrease. And at 2500 K, it even cannot clarify such differences (Figure 2(c)). In practice, when heating an EM, the temperature increases gradually. That is to say, the above discussed thermal behavior of 1 shows that 1 compromises the thermal decomposition of HMX and CL-20. It agrees with the observed DSC decomposition temperatures: 1 (235 oC) mediates between HMX (279 oC) and CL-20 (210 oC).6 It is interesting to find that there is a flat on the decay curve of c-HMX molecules from the time of 45 to 70 ps, as demonstrated in Figure 2(a). This beginning time of 45 ps just corresponds to the time when the c-CL-20 molecules are completely decomposed. It implies that the rapid decomposition of c-HMX molecules at the earlier stage is mainly supported by the rapid heat release from CL-20 decay. Once the heat release becomes slow, the decay of c-HMX molecules slower. Conversely, it is just the heat adsorption of c-HMX molecules that makes CL-20 decay slower, in contrast to p-CL-20. Thereby, we think, that 1 mediates the thermal decomposition of


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HMX and CL-20, in contrast to pure crystals, through the heat transfer from the CL-20 moieties (reactants and products of CL-20) to the HMX moieties (reactants and products of HMX). This transfer decreases heating on CL-20 further the decay rate, while increases heating on HMX further the decay rate. (b) T=1500 K

(a) T=1200 K Potential energy/mass, kcal/g

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(c) T=2500 K

0.0

-0.5

CL-20 1 HMX Formalized by 2CL-20/1HMX


-1.0

-1.5 0

20

40

Time, ps

60


80 0

20

40

60

80 0

Time, ps

20

40

60

80

Time, ps

Figure 3. Comparison of potential energy evolution of 1 and pure crystals at 1200, 1500 and 2500 K.

We are also concerned ourselves about the potential energy evolution of all three crystals, which can clearly show the reaction tendency. For EMs against heating, the first stage is always endothermal to break bonds, for example, the N-N bonds of HMX and CL-20 molecules. Thereby, the potential energy increases at this stage, and the reactions at this stage are thought to be primary reactions. Subsequently, the reactions become complex and produce a lot of small stable molecules with much energy release. Thus, the potential energy decreases. As illustrated in Figure 3(a), the pure HMX crystal almost doesn’t release potential energy, corresponding to few reactions occurred at a relatively low temperature of 1200 K in Figure 2(a). With the increasing of temperature, the pure HMX crystal becomes more and more active, as the decomposition is more and more deepened (Figures 2(b) and 2(c)) with more and more potential energy released (Figures 3(b) and 3(c)). Interestingly, a bump is found on the potential energy curve of the pure HMX in Figure 3(b), showing a delay period for activating HMX, much longer than those of pure CL-20 and 1; while, this period is much shortened at a high temperature of 2500 K (Figure 3(c)). The potential energy ACS Paragon Plus Environment 9


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increase is responsible for the N-N break, which is endothermal and corresponds to the rapid decrease of p-HMX molecules shown in Figure 2(b). In addition, we can find from the figure that the pure CL-20 crystal is always more sensitive to heating than the pure HMX crystal, as its potential energy decreases more quickly. The potential energy evolution of 1 mediates between those of pure component crystals, implying that the thermal stability of 1 does so. Besides, to verify whether the potential energy evolution of 1 is simply an addition of those of pure component crystals or not, we formalized them in unit mass and obtained a green curve in Figure 3(a), which is below a red curve of 1. It suggests that the co-crystallization of HMX and CL-20 makes the heat release slower compared with the pure component crystals, or the co-crystallization is more stable than a simple physical mixture in a same component proportion. When temperature increases to 1500 K, the red and green curves overlap with each other, showing no difference between them, as demonstrated in Figure 3(b). At the highest temperature of 2500 K, all four curves overlap one another, showing similar heat release (Figure 3(c)). Table 3. Decay Rate (k, ps-1) of CL-20 and HMX at Various Temperatures. T, K

p-CL-20

c-CL-20

c-HMX

p-HMX

1200 1500 1800 2000 2500

0.225 0.768 1.869 3.448 8.264

0.164 0.654 1.677 2.994 7.463

0.025 0.092 0.496 1.092 5.376

0.063 0.369 1.009 4.717

To evaluate the influence of crystal circumstances on thermal stability, we study the decay kinetics of the reactions in Figure 2 with an assumption that these reactions follow the first-order. In fact, the heat and shock induced decay of explosives40,48,53,54 and the thermal decay of fossil fuel55 are usually assumed to follow this order as below, c(t ) = c0 (1 − exp( − kt ))

(1)

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where t , c0 and c(t ) are time, initial molecular amount and molecular amount at t , respectively. Equation 1 can be converted into Eq. (2) 1 c0 k = ln t c0 − c(t )

(2)

Table 2 shows the k of p-HMX, p-CL-20, c-HMX and c-CL-20 at various temperatures. Clearly, we can see from Table 2 that, at any temperature, HMX possesses a smaller k than CL-20, due to the above-mentioned higher decay barrier; p-HMX possesses a smaller k than c-HMX; and p-CL-20 possesses a bigger k than c- CL-20. These k agree well with the decreased rates of HMX and CL-20 in Figure 2. Table 3. Ea (in kcal/mol) for HMX and CL-20 Molecules in Pure and Co-crystals. p-CL-20

c-CL-20

c-HMX

p-HMX

This work

16.5±0.6

17.5±0.3

30.3±0.1

32.3±0.6

Theoretical results

32.7±5.725 10.9±0.456

Experimental results

10.5±0.857,58 10.7±0.0259

32.759 35.660 52.761

As a most important index of kinetics, the activation barrier, Ea, is also concerned. According to the Arrhenius law, ln k = ln A − [

Ea 1 ] R T

(3)

where A is the exponential prefactor, we fitted the data in Table 2 and obtained the lnk~1/T dependences of the HMX and CL-20 molecules. The good linear lnk~1/T correlations in Figure s1 of SI suggest typical Arrhenius behavior of the thermal decay of HMX and CL-20, allowing us to estimate Ea. As a whole, the Ea of p-HMX and p-CL-20 in Table 3 are comparable to existing results of Refs 25 and 56-61. As listed in the table, CL-20 possesses a much lower Ea than HMX, in agreement with a big gap between their DSC decomposition temperatures, ~69 oC. In addition, p-HMX possesses a little lower Ea than c-HMX; while p-CL-20 possesses a little higher Ea than



c-CL-20. It also suggests that the co-crystallization of HMX and CL-20 mediates their activity, as mentioned above. Overall, the kinetics from the present work is reasonable. 3.2 Details of Initial Reactions. 8 Fragments/volume nm

-3

(a) 1500 K CL-20

(b) 1500 K 1

(c) 1500 K HMX

NO2

6

4

NO2

NO2

HONO NO HO H2O

HONO NO HO H2O

N2

CO2

N2

N2O

N2

N2O

HONO NO HO H2O CO2

N2O

2

80

(d) 1800 K CL-20 -3 Fragments/volume nm

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6

4

(e) 1800 K 1

(f) 1800 K HMX

NO2

NO2

HONO NO HO H2O

NO2

HONO NO HO H2O

HONO NO HO H2O

CO2

CO2

N2

CO2

N2

N2O

N2

N2O

N2O

2

0 0

20

40 Time, ps

60

80 0

20

40 Time, ps

60

80 0

20

40 Time, ps

60

80

Figure 4. Evolution of key chemical species of the pure- and co-crystals heated at 1500 and 1800 K.

As to the details of chemical reactions, the evolution of related chemical species should be paid attention to. In the present work, a total of fifteen cases are involved. Nevertheless, as pointed out above, in the simulation timescale of 80 ps, a relatively low temperature of 1200 K can only cause a small part of p-HMX molecules decayed, while, a relatively high temperature of 2500 K can lead to instantaneous decay of both HMX and CL-20. These two cases both make it difficult to understand the decay mechanism comprehensively: for the former, most reactions aren’t finished; for the latter, it is difficult to distinguish the related reactions due to their quickness. Thus, the cases at two moderate temperatures, 1500 and 1800 K, are considered. In the present work, we focus on the main


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small species including NO2, HONO, NO, CO, CO2, HO, H2O and N2, which were also the objects of previous experimental and theoretical work.12,16,17,25,35,36,62 Overall, with respect to the evolution of these small species, the case of 1 mediates between those of pure HMX and CL-20 at any temperature, as illustrated in Figure 4. In all case, NO2 is firstly richened and reaches a maximum. Afterwards, it decreases and reaches equilibrium. Because the N-N bond rapture is responsible for the NO2 appearance, the N-N bond rapture initiates the thermal decay of all three crystals. As well as, a small amount of HONO are observed in our simulations. Previous studies 12,16,17, 24,25,34,35 have shown these mechanism for pure HMX and CL-20 crystals. Thus, it suggests the similar mechanism in 1: the NO2 partition dominates the initial decay, accompanied by a little hydrogen transfers. After reaching maximums, NO2 decreases due to its high activity and reactions with other species to form more stable molecules. Meanwhile, a final stable product N2 is formed and becomes abundant subsequently. Afterwards, another product of water is produced. As to the higher contents of water and HONO in HMX, it should be attributed to its higher hydrogen content. This is also the reason for that the contents of water and HONO in 1 mediate between those in two pure crystals. CO2, as a final product, appears rarely at the early stage of thermal decay. It should be attributed to that the most of C atoms are still bonded in larger species at the early stage. This will be discussed later. Table 4. Numbers of Reaction Events (Nr) and Types (Nt) in the First 10 ps at Various Temperatures.

CL-20 1 HMX CL-20 1 HMX CL-20 1 HMX CL-20 1

T, K

Full Nr

Full Nt

1500 1500 1500 1800 1800 1800 2000 2000 2000 2500 2500

807 570 94 1405 1152 793 2352 1979 1779 7234 3988

574 360 39 1057 842 497 1856 1607 1298 6379 3376

Reduced Nr (N>4)

Reduced Nt （N>4）

189（23.4%） 176（30.9%） 48（51.1%） 265（18.9%） 248（21.5%） 259（32.6%） 341（14.5%） 258（13.1%） 383（21.5%） 546（7.5%） 393（9.8%）

22（3.8%） 16（9.1%） 3（7.6%） 28（2.6%） 25（2.9%） 20（4.02%） 42（2.3%） 31（1.9%） 37（2.8%） 61（0.95%） 37（1.1%）



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

HMX

2500

3685

3087

447（12.1%）

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49（1.6%）

Next, we pay attention to the detailed reactions. Detailed description of the chemical reaction mechanisms of condensed EMs under the extreme conditions is essential for understanding their detonation and safety properties. Reactions in such systems are quite complex with numerous intermediates interacting with one another simultaneously. Therefore, the analysis of the chemical reactions from a statistical point of view to obtain a complete picture of reaction events is necessary.63 To analyze the detailed reactions, a series of FORTRAN scripts were implemented. By searching and comparing the IDs of the atoms of all species between each two steps, the reaction path can be obtained. In this work, the numbers of the reactions and the types of these reactions during the first 10 ps were counted. Table 4 shows the number of reaction events (Nr) and the number of reaction types (Nt) in the first 10 ps at various temperatures. We find in Table 4 that Nr decreases in an order of CL-20, 1 and HMX, suggesting the decreasing of reaction complexity. With temperature increasing, both Nr and Nt increase rapidly, showing the increasing of the complexity. Meanwhile, a careful checking shows that most of these reactions possess low frequencies. That is, as listed in Table 4, the reactions with frequencies above 4 possess a small part, and percentage of these reactions decreases to the total with temperature increasing, suggesting that the reaction multicity, in particular, for low frequency reactions, increases when temperature increases. Table 5. Highest Frequent Primary Reactions during the First 10 ps at 2000 K. Crystals

(Frequencies) Primary reactions

CL-20

(57) C6H6O12N12→C6H6O10N11 + NO2 (A1) (35) NO2 + NO2→N2O4 (22) NO2 + NO3→ N2O5 (21) C6H6O10N11→ C6H6O8N10 + NO2 (B1) (13) C6H6O12N12→ C6H6O8N10 + NO2 + NO2 (11) N2O3→ NO2 + NO (10) NO2 + N2O4→N3O6 (9) NO2H + NO2→NO3H + NO


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(9) C2H2O4N4 + NO2→C2H2O6N5 (7) N3O6→NO2 + NO2 + NO2 (51) C6H6O12N12→C6H6O10N11 + NO2 (A1) (29) C6H6O10N11→ C6H6O8N10 + NO2 (B1) (24) C4H8O8N8 →C4H8O6N7 + NO2 (A2) (14) NO2 + NO2→NO3NO (14) C6H6O8N10→ C6H6O6N9 + NO2 (8) C2H2O4N4 + NO2→C2H2O6N5 (7) NO2 + NO2→NO3 + NO (7) C6H6O12N12→C6H6O8N10 + NO2 + NO2 (7) NO2 + N2O → N3O3 (6) C4H4O4N5 → C4H4O2N4 + NO2 (92) C4H8O8N8→ C4H8O6N7 + NO2 (A2) (46) C4H8O6N7→ C4H8O4N6 + NO2 (B2) (31) NO2 + NO2→ N2O4 (12) NO2 + NO3→ N2O5 (12) HO + NO2→ NO3H (11) C4H6O2N5→ C4H6N4 + NO2 (11) NO2H + NO3→ NO3H + NO2 (10) C4H7O6N7→ C4H7O4N6 + NO2 (10) C4H8O5N7→ C4H8O3N6 + NO2 (10) C4H8O4N6 + NO2→ C4H7O4N6 + HO2N

1

HMX

In addition, we summarize the 10 highest frequent primary reactions for each system at a moderate temperature of 2000 K in Table 5. As demonstrated in the table, the early chemical reactions are dominated by the N-N bond rupture to produce an intermediate of NO2. Besides, interestingly,

we

find

that

HONO(HO2N)

is

formed

by

the

reaction

of

C4H8O4N6+NO2→C4H7O4N6+HO2N in the HMX decay, i.e., by an intermolecular H transfer, instead of an intramolecular one, much different from reported observations. The initial decompositions of all three EMs are intramolecular NO2 partition, or, the unimolecular reactions, for example, Reactions A1, B1 and A2. The high abundance of NO2 is consistent with an experiment of HMX by Piermarini et al.64 Due to the abundance, it is usually found that lots of NO2 bonded in pair. The reactions that appear in the pure crystals appear in 1 with highest frequencies, suggesting that the co-crystallization of HMX and CL-20 does not change the initial steps of thermal decay. This should be a dominant reason for the 1 mediates between HMX and CL-20,



regarding above apparent kinetics and chemical specie evolution. Recently, a MD simulation described well the condensed TNT is more thermal sensitive than the gaseous TNT, as the Ea are 35 vs. 62 kcal/mol. With respect to decay mechanisms, they are intra- and intermolecular reactions for the gaseous and condensed TNT, respectively.65 For HMX and CL-20 in both pure and co-crystals, the N-N bonds are much weaker than the C-N bonds in TNT. The weakness or low stability of HMX and CL-20 makes a relatively weak influence of intermolecular interactions on molecular decay, much different from TNT, which is much stable. That is, the influence of interaction interactions on molecular will be more remarkable when the component molecules are more stable. Thus, the intrinsic molecular stability of components and the intermolecular interactions should be noted in a strategy for increasing stability by co-crystallization. As a matter of fact, Figure s2 of SI shows a small difference in two-dimensional fingerprint plot between p- and c-CL-20, or between p- and c-HMX, implying a small difference in intermolecular interaction. 1600

1600 CL-20 1 HMX

(a) 1200 K 1200

Mmax

1600 CL-20 1 HMX

(b) 1500 K 1200

1200

800

800

400

400

400

0

0

20

40

60

0

80

0

20

Time, ps

40

Time, ps

1600

0

80

0

20

40

Time, ps

1600 CL-20 1 HMX

(d) 2000 K 1200

1200

800

400

400

0

20

40

Time, ps

60

CL-20 1 HMX

(e) 2500 K

800

0

60

CL-20 1 HMX

(c) 1800 K

800

Mmax

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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80

0

0

20

40

60

80

Time, ps

Figure 5. Evolution of the molecular weight of maximal clusters (Mmax) at various temperatures.


60

80

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As a group of special species, clusters, usually possessing larger molecular weight (M) than reactant molecules, have been verified to be formed in the detonation process of EMs and to play an important role in influencing their reaction duration and final products formation, by experiments66-68 and simulations30,35,40,41,48-50,62. In the present work, we first pay attention the evolution of the molecular weight of maximal clusters (Mmax) at various temperatures in Figure 5. As illustrated in Figure 5(a), at a relatively low temperature of 1200 K, pure CL-20 produces more clusters with higher Mmax; pure HMX produces a few clusters with lower Mmax; and the case of 1 mediates between those of CL-20 and HMX. These results are derived from the difference in reaction activity among the three crystals. As pointed out above, at 1200 K, as shown in Figure 2(a), the p-HMX molecules react little, both p- and c-CL-20 molecules react completely, and only a part of c-HMX molecules begin decay. Because the initial N-N bond break is a premise for the next cluster formation, the complete initial reactions of CL-20 lead to high cluster abundance in Figure 5(a). With temperature increasing, the number of the clusters caused by HMX increases, as shown in Figures 5(b) to 5(e); while, at a relatively high temperature of 2500 K, the cluster amount tends to a decrease, as more and more clusters tend to form final small product molecules. From Figure 5, we can find that most clusters are provisionally formed and subsequently go to small species. This case is much different from our recent result of TATB, which can form graphite finally.50

Figure 6. Typical snapshots of clusters formed at an early stage. From left to right, the clusters belong to CL-20, 1 and HMX at 2000K, respectively.

Furthermore, the components of the clusters are concerned about. As demonstrated in Figure 6, ACS Paragon Plus 17 Environment


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the C-N bonds in which the CL-20 and HMX molecules are abundant remain still in the clusters, showing that the cluster formation occurs after the N-N bond break (the NO2 partition). In these clusters, the N-N bonds appear usually. It is just that the N-N bonds link different species with nitrogen radicals to form big clusters. Also, NO2 are usually found on the clusters, indicating their incomplete partition and stability, because the next partition becomes difficult once the former partition occurs. Besides, we checked the number of C atoms in clusters, and found that the clusters with 7-11 C atoms are dominant and the cluster number decreases with increasing C atoms, as shown in Figure s3 of SI. And we also found from the figure there is no evident difference in C content as time proceeded. The cluster formation and enlargement, or dissociation to small species, are strongly related with the original components of reactants.41 Because CL-20 and HMX possess very close components, the contents of the clusters produced from them and their co-crystal are evolved similarly. As a matter of fact, the structures in Figure 6 seem very similar to one another. These cluster structures are much different from those derived from TATB41,49,50 and TNT35, due to the different original structures and contents of C atoms. 4. CONCLUSIONS In summary, the thermal decomposition mechanisms of 1 and pure CL-20 and HMX crystals are revealed by ReaxFF molecular reactive dynamic simulations and kinetic calculations. As a result, we find that the N-N bond cleavage dominates all the initial decay steps of c-HMX, p-HMX, c-CL-20 and p-CL-20, as demonstrated in Figure 7. That is to say, in this case, relative to the pure crystals, the co-crystal does not change the primary decay steps: independent and intramolecular reactions of the NO2 partition. This causes a small difference in Ea after co-crystallization. Because the difference in heat release rate between CL-20 and HMX, the co-crystal mediates the decay rates of pure component crystals, i.e., c-HMX decays faster than p-HMX, while, c-CL-20 decays slower


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than p-CL-20. From these results, we have well understood the experimental observations: the DSC decomposition temperature of 1 is moderate between those of pure HMX and CL-20 crystals.

Figure 7. Plot showing the main conclusions of this work: NO2 partition dominates the first decay steps of all three crystals, 1 increases a little decay barrier of CL-20 while decreases a little that of HMX, and 1 mediates the thermal stability between CL-20 and HMX.

Furthermore, the moderated decay rate of 1 is also attributed to the weak molecular stability of its components of CL-20 and HMX, and the small variation of intermolecular interactions after co-crystallization. Thus, the intrinsic molecular stability of components and intermolecular interaction should be noted as two important factors of a strategy for increasing stability by energetic co-crystallization. ■ ASSOCIATED CONTENT Supporting Information Detailed information of the simulation systems, lnk~1/T dependences, two-dimensional fingerprint plots and evolution of the C contents in clusters. This material is available free of charge via the Internet at http://pubs.acs.org.



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■ AUTHOR INFORMATION Corresponding Author C. Y. Zhang, email: [email protected]; Tel: 86-816-2493506. Notes The authors declare no competing financial interest. ■ ACKNOWLEDGMENT The authors gratefully acknowledge the support of Science Challenge Project, the National Natural Science Foundation of China (U1530262, 11602241 and 21673210). ■ REFERENCES (1) Nielsen, A. T.; Chafin, A. P.; Christian, S. L.; et al. Synthesis of Polyazapolycyclic Caged Polynitramines. Tetrahedron 1998, 54, 11793−11812. (2) Klapotke, T. M. Eds., Chemistry of High-energy Materials. Walter de Gruyter GmbH & Co. KG: Berlin/New York, 2011. (3) Landenberger, K. B.; Matzger, A. Cocrystal Engineering of a Prototype Energetic Material:Supramolecular Chemistry of 2,4,6-Trinitrotoluene. Cryst. Growth Des. 2010, 10, 5341-5347. (4) Landenberger, K. B.; Matzger, A. Cocrystals of 1,3,5,7-Tetranitro-1,3,5,7-tetrazacyclooctane (HMX). Cryst. Growth Des. 2012, 12, 3603-3609. (5) Bolton, O.; Matzger, A. J. Improved Stability and Smart-Material Functionality Realized in an Energetic Cocrystal, Angew. Chem., Int. Ed. 2011, 50, 8960–8963. (6) Bolton, O.; Simke, L. R.; Pagoria, P. F.; Matzger, A. J. High Power Explosive with Good Sensitivity: A 2: 1 Cocrystal of CL-20: HMX, Cryst. Growth Des. 2012, 12, 4311–4314. (7) Millar, D. I. A.; Maynard-Casely, H. E.; Allan, D. R.; Cumming, A. S.; Lennie, A. R.; Mackay, A. J.; Oswald, I. D. H.; Tang, C. C.; Pulham, C. R. Crystal Engineering of Energetic Materials: Co-crystals of CL-20. CrystEngComm 2012, 14, 3742-3749. (8) Simpson, R. L.; Urtiew, P. A.; Ornellas, D. L.; Moody, G. L.; Scribner, K. J.; Hoffman, D. M. CL-20 Performance Exceeds that of HMX and its Sensitivity is Moderate. Propellants, Explos.,Pyrotech. 1997, 22, 249–255. (9) Lobbecke S; Bohn M A; Pfeil A; et al. Thermal Behavior and Stability of HNIW( CL-20). Proceedings of 29th International Conference of ICT, Karlsruhe, 1998. (10) Paval Vávra. Procedure for Selection of Molecular Structures of Explosives having High Performance. Proceedings of 30th International Conference of ICT, Karlsruhe, 1999. (11) Yang, Z.; Li, H.; Zhou, X.; Zhang, C.; Huang, H.; Li, J.; Nie F. Characterization and Properties of a Novel Energetic−Energetic Cocrystal Explosive Composed of HNIW and BTF. Cryst. Growth Des. 2012, 12, 5155–5158. (12) Patil, D. G.; Brill, T. B. Thermal Decomposition of Energetic Materials 53. Kinetics and Mechanism of Thermolysis of Hexanitrohexaazaisowurtzitane. Combust. Flame 1991, 87, 145-151.


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Table of Contents Graphic Initial Decay Mechanism of the Heated CL-20/HMX Co-crystal: A Case of the Co-crystal Mediating the Thermal Stability of the Two Pure Components


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HMX Cocrystal: A Case

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