CL-20 has five different conformations in crystal, namely α, β, γ, ε, and ζ . The β-, γ-, and ε-CL-20 showed in Fig. S1 of supporting information. The conformations in CL-20:DNP and CL-20:DNG co-crystals are different. The conformations of CL-20 molecules are taken from the optimization cell present γ form in CL-20:DNP co-crystal and β form in CL-20:DNG co-crystal, as seen in Fig. S2. Therefore, this difference in conformation will lead to different conformational changes of CL-20 at high temperatures.
At a temperature of 1000 K, no small molecules are formed in the decomposition of two co-crystals, only a conformational change occurs. CL-20:DNG co-crystal undergoes a conformational change, but CL-20:DNP co-crystal does not happen. In CL-20:DNP co-crystal, CL-20 presents γ form, which is more stable, so the conformation remains in γ form during 20 ps. However, in CL-20:DNG co-crystal, all CL-20 molecules change from β conformation to γ and then to ε conformation. The process of conformation change at 1000 K is showed in Fig. 2. The optimized CL-20 molecular conformation is β form and the stability is worst. Thereafter, as the temperature rises to 1000 K, the conformation of the β-CL-20 molecules gradually become to the γ form at 1.5 ps. Then, they change to the ε form at 6.5 ps. Finally, at 20 ps, the β-CL-20 molecules completely transform into the γ and ε forms. The conclusion is consistent to the order of conformational stability is ε > γ > β . Under the influence of temperature, the CL-20 molecules change from the unstable β form to the more stable γ and ε forms.
At 2000 and 3000 K, CL-20, DNP, and DNG molecules in CL-20:DNP and CL-20:DNG co-crystals begin to decompose. Figure 3 shows the initiation reactions of CL-20, DNP, and DNG molecules. At 2000 K, the N − NO2 bond in the five-member ring of CL-20 molecule is broken to release NO2 in CL-20:DNP co-crystal at 2.44 ps, while the bond length increases from 1.546 to 1.786 Å, as shown in Fig. 3(a). At 5.38 ps, the N − NO2 bond in the six-member ring of CL-20 molecule is broken, while its bond length increases from 1.683 to 1.724 Å, as shown in Fig. 3(b). The decomposition of DNP is much later than CL-20. At 14.11 ps, the C − N bond in DNP opens to form two small molecules and its bond length increases to 1.801 Å, as shown in Fig. 3(c). The main product of the C − N bond breakage in DNP is CH3N(NO2)CH2, which immediately decomposes into NO2. When the temperature rises to 3000 K, the initial decomposition of CL-20 is same as that at 2000 K, but occurs earlier. The release time of the nitro group in five-member ring and six-member ring is about 0.5 ps. In all, the increase in temperature will promote the decomposition of CL-20, and the initial decomposition of DNP is the cleavage of N − NO2 by releasing the nitro group. As shown in Fig. 3(d), the N − NO2 bond in the DNP molecule breaks at 0.79 ps and its bond length increases from 1.406 to 1.766 Å. This is different from the initial decomposition reaction at 2000 K.
In CL-20:DNG co-crystal, the decomposition of CL-20 at 2000 K is that N − NO2 bonds in five-member and six-member rings break at 2.5 ps to form NO2. When the time reaches to 6.56 ps, DNG molecule begins to decompose. The decomposition of DNG is N − NO2 bond breaking to produce NO2. The N − NO2 bond length increases from 1.430 to 1.962 Å, as seen in Fig. 3(e). At 3000 K, the initial decomposition reaction is the same as that at 2000 K, but it occurs earlier. The decomposition of CL-20 occurs at about 0.5 ps and DNG occurs at 2.5 ps.
In summary, there are five initial decomposition paths, including the rupture of the N − NO2 bond of five-member and six-member rings in CL-20, which is consistent with experimental speculation and theoretical calculated results [28, 45], and increasing temperature can promote decomposition. The initial decomposition reaction of DNP is C − N bond cleavage and N − NO2 bond cleavage, and in DNG is the N − NO2 bond cleavage. It can be found that different temperature could affect initial decomposition mechanism of DNP.
The bond dissociation energy (BDE) of the initial reactions are calculated by DFT. As can be seen from calculation results in Fig. 4, the BDE of the N − NO2 bond is 149.05 kJ/mol in five-member ring, while 173.78 kJ/mol in six-member ring. This is why the N − NO2 bond in five-member ring is decomposed first, which is in agreement with the previous report . The BDE of the N − NO2 bond is 178.88 kJ/mol in DNP and 190.67 kJ/mol in DNG, both higher than that in CL-20. So CL-20 first decomposes in co-crystals. It is worth noting that the BDE of the C − N bond in DNP is much higher, but it only decomposes first at 2000 K. This may be effected by the temperature and surrounding molecules.
The detailed subsequent decomposition process of CL-20 in co-crystals after two initiation reactions are shown in Fig. 5. After CL-20 is triggered by two different initial decomposition reactions, the cage-opening reactions of CL-20 have four paths. It can be seen from Fig. 5 that INT1 is formed by the cleavage of N − NO2 in five-member ring, and then it is decomposed mainly by two ways: (a) Int1-1 intermediate is produced by the cleavage of N − NO2 and C − N bonds, and then the cage structure is changed to a ring structure. (b) Int1-2 is produced by the C-N bond cleavage. Int1-2 is further decomposed by the C − N bond cleavage and opens the cage to form a double five-member ring. The other decomposition is the cleavage of two C − N bonds and opening the cage to form a five-member ring. Int2 is formed by the N − NO2 breaking in six-member ring, and then they are mainly decomposed in two paths: (a) Through the cleavage of a C − C bond, the cage is broken to form Int2-1, and then Int2-1 opens the tricycle by two C − N bonds cleavage. (b) Int2-2 is produced by the cleavage of N − NO2 and C − C bonds. Then it breaks down by the C − N and C − C bonds cleavage. This is the step to produce seven- and nine-member heterocycles, which have also been reported in previous research .
Above all, these decomposition processes are the main decomposition pathways of CL-20 in CL-20-based co-crystals and CL-20 decomposes more thoroughly at high temperature. The decomposition mechanism of CL-20 in both co-crystals is consistent with previous studies .
Meanwhile, subsequent decomposition processes of DNP and DNG in two CL-20 co-crystals are shown in Fig. 6. From the DFTB-MD simulations, it is observed that DNP molecules have two decomposition channels: C − N bond cleavage and N − NO2 bond breakage. Also, there are cleavage of C − N bonds to form small radicals C2H5N, CH3N2O2, and NO2. Meantime, the decomposition mechanism of DNG molecule is roughly the same as that of DNP molecule. The decomposition of DNG is N − NO2 bond cleavage. After that, there are also C − N bonds break to form small segments. These decomposition substances are all highly reactive intermediates. They easily react with the decomposition species of CL-20 in co-crystals.
The time evolution of CL-20, DNP, DNG, and main decomposition species in co-crystals at 2000 and 3000 K are presented in Fig. 7. As shown in Fig. 7(a), both CL-20 and DNP molecules decompose at 2000 K, all CL-20 decompose at about 10 ps, and only one DNP decomposes. When the temperature rises to 3000 K, CL-20 and DNP are all decomposed. CL-20 first starts to decompose and lasts 1.2 ps. Then at about 2.0 ps, DNP takes place to decompose. As the temperature increases, the decomposition becomes more completely. From Fig. 7(b), it can be found that CL-20:DNG co-crystal situation is similar to CL-20:DNP co-crystal. All CL-20 decompose at 2000 K and only one DNG decomposes. After the temperature rises to 3000 K, all CL-20 and DNG are decomposed. The average decomposition duration of DNG is 10.2 ps, indicating that the stability of DNG at high temperatures is better than DNP.
The main decomposition products of two CL-20 co-crystal are NO2, NO, N2, CO, and H2O. The most important product is NO2, which is produced by N − NO2 bond cleavage during the initial decomposition. As the decomposition reaction occurs at 2000 K, the amount of NO2 gradually increases, but at 3000 K, the amount of NO2 increases rapidly and then decreases after reaching the maximum value. This is because NO2 will be consumed in subsequent reactions to form intermediates, such as NO, HONO etc. The subsequent production of NO at 3000 K is also higher than that at 2000 K. It is generally believed that H2O and CO are the final decomposition products of two co-crystals. It can be seen that the number of CO and H2O produced at 3000 K are the same as those at 2000 K, but the former is produced earlier. This indicates that the increasing temperature will promote the decomposition of the two co-crystals. The presence of N2 indicates that the decomposition is complete at 3000 K. For CL-20:DNG co-crystal, the changing trends of the products NO2, NO, and N2 are the same as those in CL-20:DNP co-crystal, the difference is that no H2O is produced. This indicates that CL-20:DNG co-crystal is completely decomposed, and the decomposition of CL-20:DNP co-crystal at high temperature is more thorough than that of CL-20:DNG co-crystal.
The main product N2 has an important feature in the decomposition process. Figures 8 and 9 present the release mechanisms of the nitrogen gas of CL-20:DNP and CL-20:DNG co-crystals, respectively. After the initial decomposition, CL-20 molecules gradually decompose into rings and some long chains with carbon-rich heterocycles. There are two N2 generation mechanisms in CL-20:DNP co-crystal, as shown in Fig. 8. At 1.875 ps, N − N−O radical is formed by C − N bond breaking. Then, N − O of N − N−O radical is broke rapidly to release N2 at 1.890 ps. As the simulation continues, at 15.050 ps, the carbon-rich heterocyclic chain breaks to form N − N−C − N, which is further decomposed to produce N2 and a fragment C − N.
However, the release mechanism of the nitrogen gas in CL-20:DNG co-crystal is different with CL-20:DNP co-crystal, as shown in Fig. 9. In CL-20:DNG co-crystal, N2 is generated by the C − N bond breaking from the long chains at 2.880 and 15.115 ps, respectively. The release mechanism of the nitrogen gas in CL-20:DNG co-crystal is the same as that in the thermal decomposition of high-energy crystal TEX . Due to the difference in structures of two CL-20 co-crystals, their release mechanisms of the nitrogen gas are different.
Radial distribution function
In the DFTB-MD simulation process, co-crystals begin to decompose at high temperatures and structures change from ordered to disordered. The radial distribution function (RDF) is used to check the structural properties of CL-20:DNP and CL-20:DNG co-crystals. The formula for calculating RDF is as follows :
where g(r) is the probability that another particle exists in a spherical space of radius r to r+δr. N is the number of atoms, V is the volume of the system and rij is the distance between i and j atoms. Figure 10 shows the RDFs of N···O atom pair in CL-20:DNP and CL-20:DNG co-crystals at different temperatures.
As seen from Fig. 10(a), at 0 ps, the maximum peak of the N···O atom pair in CL-20:DNP co-crystal is at 1.2 Å, it is the length of N − O bond in NO2. The peaks at three temperatures are the coincide because there is no decomposition at 0 ps and the co-crystal structures are the same. At 5 ps, the peaks at 1000 and 2000 K have little change, the intensity of the RDF peak at 3000 K becomes smaller, and the peak shape is smoothed from sharp peaks, indicating that co-crystal transitions from an ordered structure to a disorder state at a temperature of 3000 K . At 2000 K, the RDF peak intensity becomes weaker at 15 ps, indicating that the decomposition begins at 15 ps at 2000 K. With the increase of the time, the shape of the RDF peaks at other temperatures except for 1000 K become smoother and smaller, indicating that the co-crystal does not decomposed at 1000 K. As the temperature rises, the molecules decomposition occurs earlier, and the generated molecules move irregularly, making the system disorderly. The higher the temperature is, the more violent decomposition of CL-20:DNP co-crystal and the lower the peak intensity is.
For the RDF of N···O atom pairs in CL-20:DNG co-crystal, as shown in Fig. 10(b), a trend similar to that of CL-20:DNP co-crystal can be observed. At 0 ps, the RDFs of N···O atom pairs at three temperatures are in a co-crystal line state, the peak shapes are sharp and strong. The shape and intensity of the RDF peaks start to differ at 5 ps. The peak shape at 3000 K at 5 ps is smoother than that at 1000 K. The peak shape of RDF at 2000 K begins to decrease at 15 ps. Over time, the shape of the RDF peak becomes smoother and smaller. Comparing the RDF of N···O atom pair in two co-crystals at the same time, it can be seen that the peak positions are the same, but the shape and intensity of these peaks are different.