Thermal Decomposition Mechanism of CL-20 at Different

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A: Kinetics and Dynamics

Thermal Decomposition Mechanism of CL-20 at Different Temperatures by ReaxFF Reactive Molecular Dynamics Simulations Fuping Wang, Lang Chen, Deshen Geng, Junying Wu, Jianying Lu, and Chen Wang J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b01256 • Publication Date (Web): 05 Apr 2018 Downloaded from http://pubs.acs.org on April 8, 2018

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The Journal of Physical Chemistry

Thermal Decomposition Mechanism of CL-20 at Different Temperatures by ReaxFF Reactive Molecular Dynamics Simulations Fuping Wang, Lang Chen∗, Deshen Geng, Junying Wu, Jianying Lu, Chen Wang

State Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology, Beijing 100081, China

∗Corresponding author. Tel. and fax: +86 10 68914711. E-mail: [email protected] 1

ACS Paragon Plus Environment

The Journal of Physical Chemistry 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

ABSTRACT: Hexanitrohexaazaisowurtzitane (CL-20) has a high detonation velocity and pressure, but its sensitivity is also high, which somewhat limits its applications. Therefore, it is important to understand the mechanism and characteristics of thermal decomposition of CL-20. In this study, a ε-CL-20 supercell was constructed and ReaxFF-lg reactive molecular dynamics simulations were performed to investigate thermal decomposition of ε-CL-20 at various temperatures (2000, 2500, 2750, 3000, 3250, and 3500 K). The mechanism of thermal decomposition of CL-20 was analyzed from the aspects of potential energy evolution, the primary reactions, and the intermediate and final product species. The effect of temperature on thermal decomposition of CL-20 is also discussed. The initial reaction path of thermal decomposition of CL-20 is N–NO2 cleavage to form NO2, followed by C–N cleavage leading to destruction of the cage structure. A small number of clusters appear in the early reactions and disappear at the end of the reactions. The initial reaction path of CL-20 decomposition is the same at different temperatures. However, as the temperature increases, the decomposition rate of CL-20 increases and the cage structure is destroyed earlier. The temperature greatly affects the rate constants of H2O and N2, but it has little effect on the rate constants of CO2 and H2.

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1. INTRODUCTION Hexanitrohexaazaisowurtzitane (also known as CL-20) is one of the highest energy level and practical explosives with high detonation pressure, but it has relatively high sensitivity. Therefore, the mechanism and characteristics of CL-20 thermal decomposition are a matter of great concern. By performing reactive molecular dynamics simulations, the thermal decomposition process of explosives can be analyzed in depth. Among the many reactive force fields, ReaxFF proposed by Duin et al.1 is a commonly used reactive force field. It can be used to investigate bond breakage and formation based on the bond orders (BOs) of the atoms. Strachan et al.2 investigated the shock-induced chemistry of explosives using ReaxFF, which was the first time that ReaxFF was used in molecular dynamics simulations to investigate the initial chemical reactions of explosives under impact. Liu et al.3 revised ReaxFF and developed the ReaxFF-lg reactive force field considering the long-range interactions between molecules. Using this force field, the calculated crystal density is closer to the real situation. Recently, Wang et al.4 developed a ReaxFF reactive force field for hydrocarbons and investigated thermal decomposition of iso-octane. In recent years, many researchers have used ReaxFF and ReaxFF-lg to investigate the thermal decomposition reaction of explosives. Strachan et al.5 investigated thermal decomposition of explosives using ReaxFF and obtained the thermal decomposition reaction pathway of RDX (cyclotrimethylenetrinitramine). Zhang et al.6 investigated thermal decomposition

of

TATB

(1,3,5-triamino-2,4,6-trinitrobenzene)

and

HMX

(cyclotetramethylenete-tranitramine) at different temperatures and densities using ReaxFF, and they analyzed formation and evolution of carbon-rich clusters. Rom et al.7 used ReaxFF to investigate thermal decomposition of liquid nitromethane. When investigating the reaction, they divided thermal decomposition into three stages: the initial decomposition stage, the intermediate decomposition stage, and the final product evolution stage. Wen et al.8 used ReaxFF-lg to investigate thermal decomposition of TATB, and they tracked the chemical reaction path by processing a file containing the connections of the atoms. The lifetimes and 3



frequencies of the elementary reactions were determined, which provides an effective way to analyze the initial reaction path. Reactive molecular dynamics simulations of CL-20 thermal decomposition have also been performed. Isayev et al.9 used ab initio molecular dynamics to investigate thermal decomposition of a single molecule of gas phase CL-20. The results showed that the initial thermal decomposition reaction path of CL-20 is fracture of the N–NO2 bond. Zhang et al.10 used ReaxFF reactive molecular dynamics to investigate the decomposition mechanism of ε-CL-20, which laid the foundation for future study of the chemical reaction of CL-20 with ReaxFF. To study the de-sensing mechanism of the TNT/CL-20 co-crystal, Guo et al.12 analyzed the chemical reaction of γ-CL-20 at high temperature11 and compression shear using ReaxFF-lg. Xue et al.13 calculated the thermal decomposition reaction of ε-CL-20 using ReaxFF-lg. However, the calculation time was relatively short, so the main analysis was the initial reaction of CL-20. In this study, we investigated complete thermal decomposition of ε-CL-20, which is a relatively stable phase of CL-20. The crystal structure was constructed and ReaxFF-lg was used to simulate complete thermal decomposition of CL-20 at different temperatures. The thermal decomposition reactions and the influence of temperature were analyzed from evolution of the potential energy (PE), reaction frequencies of various elementary reactions, intermediate products, and final products, including clusters. Finally, we determined the reaction kinetic parameters of CL-20 thermal decomposition.

2. COMPUTATIONAL METHODS 2.1 Simulation Method The large-scale atomic/molecular massively parallel simulator (LAMMPS) was used to investigate the CL-20 thermal decomposition reaction with ReaxFF-lg. First, we obtained the experimental unit cell of ε-CL-20 determined by Bolotina et al. 14 at room temperature. The cell was then enlarged 3 times, 3 times, and 2 times along the a, b, and c axes to construct a 3 × 3 × 2 supercell, which contains 18 unit cells, 72 CL-20 molecules, and 2592 atoms, as 4


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shown in Figure 1.

Supercell

Unit cell 3×3×2 supercell

Figure 1. Construction of the 3 × 3 × 2 CL-20 supercell. C, H, O, and N atoms are colored gray, white, red, and blue, respectively. These colors also apply to the following figures. The conjugate gradient algorithm was used to perform geometric relaxation of the CL-20 supercell at a force convergence tolerance of 10−7 (kcal/mol)/Å, giving the equilibrium structure at 0 K. According to the Maxwell–Boltzmann distribution, the initial speeds of all of the atoms were set at 298 K, and a MD simulation was performed with the canonical (NVT) ensemble and Berendsen thermostat at 298 K for 10 ps. We then performed an isobaric– isothermal (NPT) simulation using the Nose–Hoover thermostat and barostat at 298 K and 0 Pa for 15 ps to obtain the equilibrium structure. By comparing the lattice parameters, density, and radial distribution function of CL-20 with those obtained for the experimental crystal, the applicability of ReaxFF-lg to CL-20 was verified. The relaxed systems were then calculated at 2000, 2500, 2750, 3000, 3250, and 3500 K using the NVT ensemble with Berendsen thermostat until the PEs became nearly stable. The calculation times were 1200, 350, 150, 150, 150, and 150 ps at 2000, 2500, 2750, 3000, 3250, and 3500 K, respectively. Periodic boundary conditions were applied and the time step was set to 0.1 fs. Bonds with BO ≥ 0.3 were considered to be chemical bonds. The trajectories of the atoms, chemical bonds between atoms, and molecular species were recorded every 50 fs. These data were used to analyze the mechanism of CL-20 thermal decomposition.

5



3. RESULTS AND DISCUSSION 3.1 Feasibility Verification of ReaxFF-lg To verify the feasibility of using ReaxFF-lg to simulate the CL-20 crystal, the lattice parameters and density obtained at 298 K and 0 Pa were compared with the experimental values obtained by X-ray single crystal diffraction at 298 K14 (Table 1). We calculated the radial distribution functions of the experimental crystal structure14 and relaxed structure at 298 K and 0 Pa (Figure 2). From Table 1 and Figure 2, the unit cell parameters, density, and radial distribution function of the molecular mass center predicted by ReaxFF-lg are in good agreement with the experimental values. Therefore, it can be considered that ReaxFF-lg can be used to describe the crystal structure of CL-20. Table 1 Lattice Parameters of CL-20 Crystal ε-CL-20

b(Å)

c(Å)

ρ(g·cm-3)

Reference14 8.863

12.593

13.395

2.035

ReaxFF-lg

12.837

13.655

1.921

Method

a(Å)

9.035

0.16

RDF-experiment RDF-ReaxFF-lg

0.14 0.12

g(r)

0.10 0.08 0.06 0.04 0.02 0.00 0

5

10

Å

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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15

20

Distance/

Figure 2. Comparison of the radial distribution functions of for the ε-CL-20 experimental structure at 298 K and the ε-CL-20 structure obtained using ReaxFF-lg. 3.2 Evolution of the PE and Total Number of Species in the System Evolution of the PE and total number of species with time at various temperatures is shown in Figure 3. The molecular PE is related to the molecular interactions and relative position, so the evolution of total PE of the system determines whether the chemical reaction 6


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is balanced. At all three temperatures, the PE reaches a maximum during a surprisingly short time, which indicates heat absorption, and then gradually decreases, which means an exothermic reaction. The rate of decrease of the PE decreases with time and eventually tends to a stable value. In addition, when the temperature is higher, the maximum of the PE is higher, the rate of PE decrease is higher, and the stable value is higher. The change of the total number of species in the system can take into account the overall degree of decomposition fragmentation. As shown in Figure 3, the total number of species first increases and then decreases with time at all three temperatures. As the temperature increases, the maximum total number of species increases, which accelerates decay of CL-20. At 2000 K, the total number of species reaches a maximum of 120 at 53 ps, whereas the maximum number is 160 at 4.4 ps for 3500 K. This shows that the degree of fragmentation increases and the maximum occurs earlier at higher temperature, which indicates that the reactions are more violent. 180 2000K-PE 2750K-PE 3500K-PE 2000K-TS 2750K-TS 3500K-TS

-230000 -240000 -250000

160 140 120 100

-260000 80 -270000 60 -280000

Total Number of Species

-220000

Potential Energy/kcal⋅mol-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


40

-290000

20

-300000

0 0.1

1

10

100

1000

Time/ps

Figure 3. Evolution of the PE (solid lines) and total number of species (dotted lines) with time at various temperatures. Black, red and blue represent 2000, 2750, and 3500 K, respectively. 3.3 CL-20 Initial Decomposition Reaction Path To analyze the initial decomposition path of CL-20 thermal decomposition, a series of scripts were written to process the connection table of the atoms calculated by molecular dynamics. We obtained the main chemical reactions and their lifetimes, as well as the 7



frequencies of their occurrence in the first 10 ps at different temperatures (Table 2). The reactions in the table are arranged in chronological order based on when they first occur. The reactions with earliest appearance and highest frequencies indicate the main initial decomposition reaction of CL-20, which is shown in Figure 4. The reaction is consistent with the results of Isayev et al.9 obtained by ab initio molecular dynamics. Table 2 Primary Reactions with the Highest Frequencies T/K

2000

2500

2750

3000

Frequencies 72 20 42 192 52 159 76 68 55 36 259 220 105 74 111 56 36 273 100 235 168 163 73 44 31 187 163 50 69 57

Reaction time/ps 0.05-1.25 0.15-5.45 0.2-4.15 0.7-9.9 0.9-9.9 0.95-9.95 1.4-9.95 1.6-9.95 0.05-0.6 0.1-0.65 0.4-9.95 0.45-9.95 0.6-9.85 0.65-9.75 0.85-9.95 0.05-0.5 0.1-0.65 0.25-9.85 0.35-9.85 0.4-9.95 0.45-9.9 0.5-9.95 0.9-9.5 0.05-0.35 0.1-0.6 0.2-9.95 0.25-9.95 0.3-9.95 0.35-9.85 0.4-9.45

Primary reactions C6H6N12O12→NO2+C6H6N11O10 C6H6N10O8→NO2+C6H6N9O6 C6H6N11O10→NO2+C6H6N10O8 NO2+NO2→N2O4 N2O3→NO+NO2 N2O4→NO2+NO2 NO2+NO3→N2O5 N2O5→NO2+NO3 C6H6N12O12→ NO2+C6H6N11O10 C6H6N11O10→NO2+C6H6N10O8 NO2+NO2→ N2O4 N2O4→ NO2+NO2 N2O3→ NO+NO2 N2O4→ NO+NO3 NO+NO2→ N2O3 C6H6N12O12→NO2+C6H6N11O10 C6H6N11O10→NO2+C6H6N10O8 NO2+NO2→N2O4 N2O4→NO+NO3 N2O4→NO2+NO2 N2O3→NO+NO2 NO+NO2→N2O3 NO+NO3→N2O4 C6H6N12O12→NO2+C6H6N11O10 C6H6N11O10→NO2+C6H6N10O8 NO2+NO2→N2O4 N2O4→NO2+NO2 NO3→O2+NO N2O4→NO+NO3 NO+NO3→N2O4

8


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3250

3500

140 139 46 22 148 144 93 141 148 139 36 16 101 82 99 104 119 155

0.4-9.4 0.45-9.95 0-0.3 0.1-0.3 0.15-9.05 0.2-9.35 0.4-9.9 0.45-9.85 0.5-9.95 0.8-9.95 0-0.2 0.05-0.15 0.15-4.8 0.2-4.5 0.35-4.55 0.35-4.5 0.7-9.95 1.15-9.95

NO+NO2→N2O3 N2O3→NO+NO2 C6H6N12O12→NO2+C6H6N11O10 C6H6N11O10→NO2+C6H6N10O8 NO2+NO2→N2O4 N2O4→NO2+NO2 HNO2→NO+HO NO+NO2→N2O3 N2O3→NO+NO2 NO+N2→N3O C6H6N12O12→NO2+C6H6N11O10 C6H6N12O12→NO2+NO2+C6H6N10O8 NO2+NO2→N2O4 N2O4→NO2+NO2 N2O3→NO+NO2 NO+NO2→N2O3 HNO2→NO+HO N3O→NO+N2

C-N

N-O

N-N

C-H C-C

Figure 4. Main initial decomposition reaction of CL-20. After NO2 elimination, high-frequency reactions mainly occur between the free small molecules. With increasing temperature, the H atoms attached to the C-ring of CL-20 also participate in the reaction, which is proton transfer. The intermolecular reactions of small 9



molecules within the first 10 ps are 2NO2 ⇌ N2O4, N2O4 → NO + NO3, NO + NO2 ⇌ N2O3, and HNO2 → NO + HO. Among these reactions, the first reaction has the highest frequency. In addition, when the temperature is higher, the reaction occurs earlier. The reaction occurs at 0.7 ps for 2000 K, and 0.15 ps for 3500 K. To determine how CO2 is produced from a CL-20 molecule, we investigated the relevant elementary reactions in the first 10 ps at different temperatures (Table 3). We regarded CO2 as the final product, and the reactants were searched for as products. The reaction with the highest frequency was selected at each step. After tracing a few steps, there were many reaction paths with a frequency of 1. The main reaction pathway could not be identified, so we stopped the trace. From Table 3, CO2 is not found in the first 10 ps at 2000 and 2500 K. Since CO2 is the final product, it appears in the later stage of the reaction. CO2 is mainly formed from CN2O2. At 3000 K, CN2O2 is first converted to CNO and it then reacts with O2 to produce CNO3, which then generates CO2. When the temperature is greater than 3000 K, CN2O2 can directly generate CO2 and N2. Table 3 Elementary Reactions to Form CO2 T/K 2000 2500 2750

3000

3250

Frequencies — — 6 7 36 34 18 19 2 3 19 19 37 43 25 28 15

Reaction time/ps — — 5.6-9.55 6.35-9.9 4.3-9.95 3.7-9.9 1.8-9.85 4.15-9.8 4.95-5.35 3.7-5.4 2.6-9.95 2.7-9.9 2.7-9.95 2.65-9.95 2.05-9.85 3.3-9.9 1.9-9.7

Elementary reactions — — CNO3→NO+CO2 NO2+CO2→CNO4 CNO3→NO+CO2 NO+CO2→CNO3 CNO4→NO2+CO2 NO2+CO2→CNO4 O2+CNO→CNO3 CNO3→O2+CNO CN2O2→NO+CNO NO+CNO→CN2O2 CN2O2→N2+CO2 N2+CO2→CN2O2 CNO3→NO+CO2 NO+CO2→CNO3 NO+CNO→CN2O2

10


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3500

3 20 20 68 66 51 51 23 43 41

1.9-8.95 2.15-9.95 2.05-9.9 2.8-9.95 3.7-9.95 3.7-9.95 3.65-9.9 1.6-4.35 4.5-9.95 2.8-9.9

O2+CNO→CNO3 CN3O→N2+CNO N2+CNO→CN3O CN2O2→N2+CO2 N2+CO2→CN2O2 CNO3→NO+CO2 NO+CO2→CNO3 NO+CNO→CN2O2 CN3O→N2+CNO N2+CNO→CN3O

The CL-20 decomposition reaction is the process of CL-20 molecules decomposing into small molecules, which means cleavage of various types of chemical bonds. There are C–C, C–N, N–N, N–O, and C–H bonds in the CL-20 molecule, and the numbers of each type of bond are 2, 12, 6, 12, and 6, respectively, as shown in Figure 4. Cleavage of the N–N bond generates NO2. Cleavage of the N–O bond gives NO, while cleavage of the C–N and C–C bonds leads to opening of the ring of CL-20 molecule, which further decomposes to small molecules. Cleavage of the C–H bond means migration of H atoms. Our program calculates the cleavage frequency of each type of bond. This frequency is then divided by the number of that type of bond in the CL-20 molecule, which is defined as the broken bond ratio. Figure 5 shows the broken bond ratios of the C–N, N–N, N–O, C–C, and C–H bonds at 2000 and 3500 K. The C–N, N–N, and N–O bonds are mainly broken, and the C–N and N–N bonds are more active than the N–O bond in the initial reaction stage at both temperatures. As the reaction proceeds, the broken bond ratios of the C–N and N–N bonds rapidly reach maximum values and then slowly decrease, while the broken bond ratio of the N–O bond slowly increases and remains relatively stable. At 2000 K, the three types of broken bonds are relatively small ( CO2 > H2O > CO > H2. Figure 9 shows the numbers of N2, H2O, CO2, H2, and CO molecules at the end of the simulations at different temperatures. The effect of temperature on the number of each type of product is not regular. However, at each temperature, the number of N2 molecules in the gas product is the highest, followed by numbers of CO2 and H2O molecules. The numbers of H2 and CO molecules are the lowest and there is almost no H2 generated, especially at 2000 K. The number of CO molecules is less than that of H2 molecules. However, the results generally agree with the experimental results of Simpson.15 Figure 10 shows a snapshot of the CL-20 supercell at 150 ps for 3000 K, in which formation of small molecules, such as N2, CO2, and H2O, can be observed. 400

2000 K 2500 K 2750 K 3000 K 3250 K 3500 K

350 300

Molecule Number

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


250 200 150 100 50 0

N2

H2O

CO2

H2

CO

Figure 9. Final numbers of N2, H2O, CO2, H2, and CO molecules for various temperatures. 15



Figure 10. Snapshot of an equilibrium configuration at 150 ps for 3000 K. 3.4.2 Clusters Table 4 shows the maximum number, molecular weight, and aggregation time of clusters during CL-20 decomposition at different temperatures. The number of clusters at 2500 K is the largest, while the molecular weight is the largest at 2000 K. We define the aggregation time of clusters as the time from the beginning of their appearance to the time of the last cluster with more than two carbon atoms. As shown in Table 4, with increasing temperature, the aggregation time decreases, and most of the clusters exist in the early reaction stage. At the end of the simulation, the clusters decompose to form small molecule products. Table 4 Maximum Number, Molecule Weight, and Aggregation Time of Clusters during CL-20 Decomposition at Different Temperatures Temperature/K

Maximum Maximum amount weight

Aggregation time/ps

2000

8

1582

0.2-813.15

2500

11

1386

0.1-37.65

2750

8

1526

0.1-31

3000

8

1294

0.1-20

3250

8

1176

0.1-8.8

3500

8

1960

0.1-6.6

16


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To investigate the maximum number of C atoms in a cluster, we determined the largest number of C atoms in a cluster during complete decomposition at different temperatures (Figure 11). The results show that the higher the temperature, the smaller the number of C atoms in the cluster below 3250K. However，when the temperature is higher than 3250K, the maximum number of C atoms in the cluster increases. The maximum number of C atoms in a cluster abruptly decreases in the range of 3000 and 3250 K. This means that most of the C atoms leave the clusters to form small molecules. 50

Cmax Maximum Number of C Atoms

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


40

30

20

10

0 2000

2500

3000

3500

Temperature/K

Figure 11. Maximum number of C atoms in a cluster for various temperatures. 3.5 Reaction Kinetics Parameter Analysis The reaction rate analysis method proposed by Rom et al.7 was used to analyze the reaction rate of CL-20 decomposition in the initial decomposition stage, intermediate decomposition stage, and final product evolution stage. From Figure 7, the number of CL-20 molecules rapidly decreases and all of the CL-20 molecules have disappeared within 3 ps at 2000 K, and the time required for CL-20 to completely disappear is shorter at 3500 K. Therefore, in this article, we do not analyze the endothermic reaction rate of CL-20, but we directly analyze the exothermic reaction stage after the PE reaches the maximum value. 3.5.1 Intermediate Decomposition Stage After initial equilibration and induction time, the gradual decrease of the PE of the supercell with time can be fitted by a single exponential function:6 U (t ; T ) = U 0 (T ) + ∆U (T ) × exp[− （t − tmax）/ τ (T )] 17


(1)


where U 0 is the PE of the products, ∆U is the reaction heat released, t max is the time of the maximum PE, and τ is the characteristic time of the reaction. All three parameters are functions of the temperature. In Figure 12, the dotted lines are the fitted curves and the solid lines are the changes of the PE calculated by ReaxFF-lg reactive molecular dynamics simulations. At low temperature, CL-20 decomposition is faster than a first-order reaction. At high temperature, it is in complete agreement with the first-order reaction rate equation. At low temperature, it is likely that the reaction rate is not related to the number of CL-20 molecules (or concentration), indicating that formation of the primary decomposition products catalyzes and accelerates decomposition of CL-20. However, at high temperature, because the time required for CL-20 decomposition is reduced and the initial products are not yet ready to catalyze dissociation of unreacted CL-20, the reaction rate is proportional to the number of CL-20 molecules (concentration), which is consistent with the first-order reaction rate equation. The parameters obtained by fitting the molecular dynamics data with Equation (1) are listed in Table 5. As the temperature increases, U0 gradually increases and τ gradually decreases. This shows that increasing the temperature leads to a higher reaction rate and a shorter characteristic time. At all six temperatures, the ∆U value fluctuates around 45,000 kcal/mol. Therefore, the temperature has little effect on the amount of heat released when CL-20 decomposes to the final state, and it only affects the speed at which the final state is reached. -230000 -240000

Potential Energy/kcal⋅mol-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

-250000 -260000 -270000

3500K

-280000

2750K

2000K

-290000 -300000 0.1

1

10

100

1000

Time/ps

18


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Figure 12. Changes of the PE (solid lines) and the corresponding exponential function fitting curves (dotted lines) at various temperatures. Table 5 Parameters Describing the Exponential Behavior of the Change of the PE with Time T/K

tmax /ps

U0 /kcal·mol-1

∆U /kcal·mol-1

τ /ps

2000

0.25

-292424.597

45735.597

140.25

2500

0.45

-288446.559

45239.659

38.64

2750

0. 4

-286111.274

44657.494

23.46

3000

0.45

-284226.94

44594.72

14.67

3250

0.4

-282133.236

44486.926

10.68

3500

0.4

-279658.858

43764.988

8.18

Figure 13 shows the relationship between the characteristic time and the inverse temperature during CL-20 decomposition. The relationship is linear, indicating that CL-20 thermal decomposition complies with the Arrhenius law in the range 2000–3500 K. The activation energy in the exothermic reaction stage obtained by linear fitting is 128.2 kJ/mol. Zhu16 obtained an experimental activation energy of 170 kJ/mol, which is consistent with the cleavage energy of the N–N bond. Isayev9 calculated an activation energy of 137 ± 24 kJ/mol for N–NO2 cleavage by ab initio molecular dynamics simulations. Therefore, the activation energy of CL-20 thermal decomposition obtained in this work is slightly lower than those obtained by experiments and ab initio molecular dynamics. 403.42879

148.41316

τ Arrhenius behavior fit

54.59815

τ/ps

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


20.08554

7.38906

2.71828

1 0.25

0.30

0.35

0.40

0.45

0.50

1000/T(K-1)

19


0.55


Figure 13. Characteristic time versus inverse temperature (1/T) plot in the range 2000–3500 K. 3.5.2 Final Product Rate Constant The numbers of N2, CO2, H2O, and H2 molecules generated by CL-20 thermal decomposition at different temperatures were fitted by the following equation:7

C (t ) = C∞ {1 − exp[−k pro (t − ti )]}

(2)

where C ∞ is the stable number of product molecules, ti is the appearance time of the product, and kpro is the formation rate of the product. In Figure 14, the solid lines are the changes of the numbers of N2, CO2, H2O, and H2 molecules with time obtained by reactive molecular dynamics simulations, and the dotted lines are the curves fitted by Equation (2). The formation rates of N2, CO2, H2O, and H2 at different temperatures are shown in Figure 15. The formation rates of N2, CO2, H2O, and H2 increase with increasing temperature. The formation rates of H2O and N2 are greatly affected by the temperature, while the formation rates of CO2 and H2 are only slight affected by the temperature. 400

3000 K

350

Population of fragments

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

Page 20 of 37

300

N2

250

CO2

200 150 100

H2O H2

50 0 0

30

60

90

120

150

Time/ps

Figure 14. Changes of the numbers of N2, CO2, H2O, and H2 molecules (solid lines) and the corresponding fitted curves (dotted lines) at 3000 K.

20


Page 21 of 37

N2

0.20

H2O CO2

0.15

kpro/ps-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


H2

0.10

0.05

0.00 2000

2500

3000

3500

Temperature/K

Figure 15. Rate constants (kpro) of N2, CO2, H2O, and H2 at various temperatures.

4. CONCLUSIONS We have performed ReaxFF-lg reactive molecular dynamics simulations to systematically investigate the complete process of ε-CL-20 thermal decomposition at six different temperatures. The reaction path, changes of the numbers of the species, and kinetic parameters of the decomposition stages were obtained, and the influence of temperature on CL-20 thermal decomposition is discussed in detail. The initial reaction step of ε-CL-20 thermal decomposition is formation of NO2 by N– NO2 bond cleavage, followed by dissociation of the C–N bond, resulting in destruction of the cage structure. A small number of clusters appear during decomposition. In addition, NO2, NO, NO3, N2O, and other small molecules of the intermediate products form. The final products are mainly N2, H2O, CO2, and H2. The thermal decomposition reaction of CL-20 agrees with the Arrhenius law, and the activation energy is 128.2 kJ/mol. The temperature does not affect the initial reaction path of CL-20 thermal decomposition. However, with increasing temperature, the cage structure is destroyed earlier and the rate of decomposition increases. The temperature has a large effect on the formation rates of H2O and N2, but little effect on the formation rates of CO2 and H2.

References (1) van Duin, A. C. T.; Dasgupta, S.; Lorant, F.; Goddard, W. A., III. ReaxFF: A Reactive Force Field for Hydrocarbons. J. Phys. Chem. A 2001, 105, 9396-9409. 21



Page 22 of 37

(2) Strachan, A.; van Duin, A. C. T.; Chakraborty, D.; Dasgupta, S.; Goddard, W. A., III. Shock Waves in High-energy Materials: The Initial Chemical Events in Nitramine RDX.

Phys. Rev. Lett. 2003, 91, 098301. (3) Liu, L.; Liu, Y.; Zybin, S.V.; Sun, H.; Goddard, W. A., III. ReaxFF-lg: Correction of The ReaxFF Reactive Force Field for London Dispersion, with Applications to The Equations of State for Energetic Materials. J. Phys. Chem. A 2011, 115, 11016-11022. (4) Wang, E.; Ding, J.; Qu, Z.; Han, K. Development of a Reactive Force Field for Hydrocarbons and Application to Iso-octane Thermal Decomposition. Energy Fuels. 2018,

32, 901-907. (5) Strachan, A.; Kober, E. M.; van Duin, A. C. T.; Oxgaard, J.; Goddard, W. A., III. Thermal Decomposition of RDX From Reactive Molecular Dynamics. J. Chem. Phys. 2005, 122, 54502. (6) Zhang, L.; Zybin, S. V.; van Duin, A. C. T.; Dasgupta, S.; Goddard, W. A., III.; Kober, E. M.

Carbon

Cluster

Formation

During

Thermal

Decomposition

of

Octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine and 1,3,5-Triamino-2,4,6- trinitrobenzene High Explosives from ReaxFF Reactive Molecular Dynamics Simulations. J. Phys. Chem. A 2009, 113, 10619- 10640. (7) Rom, N.; Zybin, S. V.; van Duin, A. C. T.; Goddard, W. A., III.; Zeiri, Y.; Katz, G.; Kosloff, R. Density-dependent Liquid Nitromethane Decomposition: Molecular Dynamics Simulations Based on ReaxFF. J. Phys. Chem. A 2011, 115, 10181-10202. (8) Wen, Y.; Xue, X.; Long, X.; Zhang, C.. Clusters Evolution at Early Stages of 1,3,5-Triamino-2,4,6-trinitrobenzene under Various Heating Conditions: A Molecular Reactive Force Field Study. J. Phys. Chem. A 2016, 120, 3929-3937. (9) Isayev, O.; Gorb, L.; Qasim, M.; Leszczynski, J. Ab Initio Molecular Dynamics Study on the Initial Chemical Events in Nitramines: Thermal Decomposition of CL-20. J. Phys. Chem.

B 2008, 112, 11005-11013. (10) Zhang, L.; Chen, L.; Wang, C.; Wu J. Mechanism of the Initial Thermal Decomposition of CL-20 via Molecular Dynamics Simulation. Chin. J. Expl. Propell. 2012, 35, 5-9. (11) Guo, D; An, Q; Zybin, S. V.; Goddard, W. A., III.; Huang, F.; Tang, B. The Co-crystal of TNT/CL-20 Leads to Decreased Sensitivity toward Thermal Decomposition from First Principles Based Reactive Molecular Dynamics. J. Mater. Chem. A 2015, 3, 5409-5419. (12) Guo, D.; An, Q.; Goddard, W. A., III.; Zybin, S. V.; Huang, F. Compressive Shear Reactive Molecular Dynamics Studies Indicating That Cocrystals of TNT/CL-20 Decrease Sensitivity. J. Phys. Chem. C 2014, 118, 30202-30208. 22


Page 23 of 37 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


(13) Xue, X.; Ma, Y.; Zeng, Q.; Zhang, C. Initial Decay Mechanism of the Heated CL-20/HMX Cocrystal: A Case of the Cocrystal Mediating the Thermal Stability of the Two Pure Components. J. Phys. Chem. C 2017, 121, 4899-4908. (14) Bolotina, N. B.; Hardie, M. J.; Jr, R. L. S.; Pinkerton, A. A. Energetic Materials: Variable-temperature Crystal Structures of γ- and ɛ-HNIW Polymorphs. J. Appl. Crystallogr. 2004, 37, 808-814. (15) 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. Propell. Explos.

Pyrot. 1997, 22, 249-255. (16) Zhu, Y. L.; Shan, M. X.; Xiao, Z. X.; Wang, J. S.; Jiao, Q. J. Kinetics of Thermal Decomposition of ε-hexanitrohexaazaisowurtzitane by TG-DSC-MS-FTIR. Korean J. Chem. Eng. 2015, 32, 1164-1169.

23



TOC Graphic Potential Energy/kcal⋅mol-1 3500K 3250K -240000 3000K 2500K 2750K

2000K

-300000 0.1

1

10 Time/ps

100

1000

Thermal decomposition of CL-20

24


Page 24 of 37

Page 25 of 37 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


Thermal Decomposition Mechanism of ε-CL-20 at Different Temperatures via ReaxFF Reactive Molecular Dynamics Simulations Fuping Wang, Lang Chen, Deshen Geng, Junying Wu, Jianying Lu, Chen Wang

15 Figures and 5 Tables

Supercell

Unit cell

3×3×2 supercell

Figure 1. Construction of the 3 × 3 × 2 CL-20 supercell. C, H, O, and N atoms are colored gray, white, red, and blue, respectively. These colors also apply to the following figures.

 Corresponding author. Tel. and fax: +86 10 68914711. E-mail: [email protected]



0.16

RDF-experiment RDF-ReaxFF-lg

0.14 0.12

g(r)

0.10 0.08 0.06 0.04 0.02 0.00 0

5

10

15

20

Distance/Å

Figure 2. Comparison of the radial distribution functions of for the ε-CL-20 experimental structure at 298 K and the ε-CL-20 structure obtained using ReaxFF-lg.

180 2000K-PE 2750K-PE 3500K-PE 2000K-TS 2750K-TS 3500K-TS

-230000 -240000

160 140 120

-250000 100 -260000 80 -270000 60 -280000

Total Number of Species

-220000

Potential Energy/kcalmol-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

Page 26 of 37

40

-290000

20

-300000

0 0.1

1

10

100

1000

Time/ps

Figure 3. Evolution of the PE (solid lines) and total number of species (dotted lines) with time at various temperatures. Black, red and blue represent 2000, 2750, and 3500


Page 27 of 37

K, respectively.

C-N

N-O

N-N

C-H C-C

Figure 4. Main initial decomposition reaction of CL-20.

(a)

2.5

C-N N-N N-O C-C C-H

2000 K 2.0

Broken Bond Ratio

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


1.5

1.0

0.5

0.0 0

2

4

6

Time/ps


8

10


(b)

12

C-N N-N N-O C-C C-H

3500 K

Broken Bond Ratio

10

8

6

4

2

0 0

2

4

6

8

10

Time/ps

Figure 5. Changes of broken bond ratios of C–N, N–N, N–O, C–C, and C–H bonds with time at 2000 and 3500 K.

(a)

300

2000 K 250

Molecule Number of C_count

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

Page 28 of 37

200

C1

150

100

0 0.1

C3

C2

50

1

10

100

Time/ps


1000

Page 29 of 37

(b)

300

3500 K

Molecule Number of C_count

250

200

C1 150

C3 100

C2 50

0 0.1

1

10

100

Time/ps

Figure 6. Changes of the numbers of C1, C2, and C3 fragments at 2000 and 3500 K.

(a)

160 140

2000K NO2

120


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


100 80

C6H6N12O12

60

NO3

40

N2O

20

NO

0 0.1

1

10

Time/ps


100

1000


(b)

220

3500K

200


180 160

NO2

140 120 100 80

NO

60

C6H6N12O12

40

NO3

20

N 2O

0 0.1

1

10

100

Time/ps

Figure 7. Changes of the numbers of CL-20 (C6H6N12O12), NO2, NO, NO3, and N2O molecules with time at 2000 and 3500 K.

(a)

400

2000K

N2

300


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

Page 30 of 37

CO2 200

H2O

100

H2 0 0

200

400

600

800

Time/ps


1000

1200

Page 31 of 37

(b)

400

3500K

N2


300

CO2

200

H2O 100

H2 0 0

20

40

60

80

100

120

140

Time/ps Figure 8. Changes of the numbers of N2, H2O, CO2, and H2 molecules with time at 2000 and 3500 K.

400

2000 K 2500 K 2750 K 3000 K 3250 K 3500 K

350 300

Molecule Number

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


250 200 150 100 50 0

N2

H2O

CO2

H2

CO

Figure 9. Final numbers of N2, H2O, CO2, H2, and CO molecules for various temperatures.



Figure 10. Snapshot of an equilibrium configuration at 150 ps for 3000 K.

50

Cmax

Maximum Number of C Atoms

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

Page 32 of 37

40

30

20

10

0 2000

2500

3000

3500

Temperature/K

Figure 11. Maximum number of C atoms in a cluster for various temperatures.


Page 33 of 37

-230000

Potential Energy/kcalmol-1

-240000 -250000 -260000 -270000

3500K

-280000

2750K

2000K

-290000 -300000 0.1

1

10

100

1000

Time/ps

Figure 12. Changes of the PE (solid lines) and the corresponding exponential function fitting curves (dotted lines) at various temperatures.

403.42879

148.41316

 Arrhenius behavior fit

54.59815

/ps

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


20.08554

7.38906

2.71828

1 0.25

0.30

0.35

0.40

0.45

0.50

0.55

-1

1000/T(K )

Figure 13. Characteristic time versus inverse temperature (1/T) plot in the range 2000–3500 K.



400

3000 K Population of fragments

350 300

N2

250

CO2

200 150 100

H 2O H2

50 0 0

30

60

90

120

150

Time/ps

Figure 14. Changes of the numbers of N2, CO2, H2O, and H2 molecules (solid lines) and the corresponding fitted curves (dotted lines) at 3000 K.

N2

0.20

H 2O CO2

0.15

kpro/ps-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

Page 34 of 37

H2

0.10

0.05

0.00 2000

2500

3000

3500

Temperature/K

Figure 15. Rate constants (kpro) of N2, CO2, H2O, and H2 at various temperatures.


Page 35 of 37 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


Table 1 Lattice Parameters of CL-20 Crystal ε-CL-20

b(Å)

c(Å)

ρ(g·cm-3)

Reference14 8.863

12.593

13.395

2.035

ReaxFF-lg

12.837

13.655

1.921

Method

a(Å)

9.035

Table 2 Primary Reactions with the Highest Frequencies

72 20 42 192 2000 52 159 76 68 55 36 259 2500 220 105 74 111 56 36 273 100 2750 235 168 163 73

Reaction time/ps 0.05-1.25 0.15-5.45 0.2-4.15 0.7-9.9 0.9-9.9 0.95-9.95 1.4-9.95 1.6-9.95 0.05-0.6 0.1-0.65 0.4-9.95 0.45-9.95 0.6-9.85 0.65-9.75 0.85-9.95 0.05-0.5 0.1-0.65 0.25-9.85 0.35-9.85 0.4-9.95 0.45-9.9 0.5-9.95 0.9-9.5

44 31 187 163 3000 50 69 57 140

0.05-0.35 0.1-0.6 0.2-9.95 0.25-9.95 0.3-9.95 0.35-9.85 0.4-9.45 0.4-9.4

T/K

Frequencies

Primary reactions C6H6N12O12→NO2+C6H6N11O10 C6H6N10O8→NO2+C6H6N9O6 C6H6N11O10→NO2+C6H6N10O8 NO2+NO2→N2O4 N2O3→NO+NO2 N2O4→NO2+NO2 NO2+NO3→N2O5 N2O5→NO2+NO3 C6H6N12O12→ NO2+C6H6N11O10 C6H6N11O10→NO2+C6H6N10O8 NO2+NO2→ N2O4 N2O4→ NO2+NO2 N2O3→ NO+NO2 N2O4→ NO+NO3 NO+NO2→ N2O3 C6H6N12O12→NO2+C6H6N11O10 C6H6N11O10→NO2+C6H6N10O8 NO2+NO2→N2O4 N2O4→NO+NO3 N2O4→NO2+NO2 N2O3→NO+NO2 NO+NO2→N2O3 NO+NO3→N2O4 C6H6N12O12→NO2+C6H6N11O10 C6H6N11O10→NO2+C6H6N10O8 NO2+NO2→N2O4 N2O4→NO2+NO2 NO3→O2+NO N2O4→NO+NO3 NO+NO3→N2O4 NO+NO2→N2O3



139 46 22 148 144 3250 93 141 148 139 36 16 101 82 3500 99 104 119 155

0.45-9.95 0-0.3 0.1-0.3 0.15-9.05 0.2-9.35 0.4-9.9 0.45-9.85 0.5-9.95 0.8-9.95 0-0.2 0.05-0.15 0.15-4.8 0.2-4.5 0.35-4.55 0.35-4.5 0.7-9.95 1.15-9.95

N2O3→NO+NO2 C6H6N12O12→NO2+C6H6N11O10 C6H6N11O10→NO2+C6H6N10O8 NO2+NO2→N2O4 N2O4→NO2+NO2 HNO2→NO+HO NO+NO2→N2O3 N2O3→NO+NO2 NO+N2→N3O C6H6N12O12→NO2+C6H6N11O10 C6H6N12O12→NO2+NO2+C6H6N10O8 NO2+NO2→N2O4 N2O4→NO2+NO2 N2O3→NO+NO2 NO+NO2→N2O3 HNO2→NO+HO N3O→NO+N2

Table 3 Elementary Reactions to Form CO2 T/K 2000 2500 2750

3000

3250

Frequencies — — 6 7 36 34 18 19 2 3 19 19 37 43 25 28 15 3

Reaction time/ps — — 5.6-9.55 6.35-9.9 4.3-9.95 3.7-9.9 1.8-9.85 4.15-9.8 4.95-5.35 3.7-5.4 2.6-9.95 2.7-9.9 2.7-9.95 2.65-9.95 2.05-9.85 3.3-9.9 1.9-9.7 1.9-8.95

Elementary reactions — — CNO3→NO+CO2 NO2+CO2→CNO4 CNO3→NO+CO2 NO+CO2→CNO3 CNO4→NO2+CO2 NO2+CO2→CNO4 O2+CNO→CNO3 CNO3→O2+CNO CN2O2→NO+CNO NO+CNO→CN2O2 CN2O2→N2+CO2 N2+CO2→CN2O2 CNO3→NO+CO2 NO+CO2→CNO3 NO+CNO→CN2O2 O2+CNO→CNO3


Page 36 of 37

Page 37 of 37 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


20 20 68 66 51 51 23 43 41

3500

2.15-9.95 2.05-9.9 2.8-9.95 3.7-9.95 3.7-9.95 3.65-9.9 1.6-4.35 4.5-9.95 2.8-9.9

CN3O→N2+CNO N2+CNO→CN3O CN2O2→N2+CO2 N2+CO2→CN2O2 CNO3→NO+CO2 NO+CO2→CNO3 NO+CNO→CN2O2 CN3O→N2+CNO N2+CNO→CN3O

Table 4 Maximum Number, Molecule Weight, and Aggregation Time of Clusters during CL-20 Decomposition at Different Temperatures Temperature/K

Maximum Maximum amount weight

Aggregation time/ps

2000

8

1582

0.2-813.15

2500

11

1386

0.1-37.65

2750

8

1526

0.1-31

3000

8

1294

0.1-20

3250

8

1176

0.1-8.8

3500

8

1960

0.1-6.6

Table 5 Parameters Describing the Exponential Behavior of the Change of the PE with Time T/K

tmax /ps

U 0 /kcal·mol-1

U /kcal·mol-1

 /ps

2000

0.25

-292424.597

45735.597

140.25

2500

0.45

-288446.559

45239.659

38.64

2750

0. 4

-286111.274

44657.494

23.46

3000

0.45

-284226.94

44594.72

14.67

3250

0.4

-282133.236

44486.926

10.68

3500

0.4

-279658.858

43764.988

8.18


Thermal Decomposition Mechanism of CL-20 at Different

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