Heat-Induced Polymorphic Transformation Facilitating the Low Impact

Article Cite This: J. Phys. Chem. C 2019, 123, 16014−16022

pubs.acs.org/JPCC

Heat-Induced Polymorphic Transformation Facilitating the Low Impact-Sensitivity of 2,2-Dinitroethylene-1,1-diamine (FOX-7) Rupeng Bu, Weiyu Xie, and Chaoyang Zhang*

Downloaded via KEAN UNIV on July 29, 2019 at 10:53:13 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

Institute of Chemical Materials, China Academy of Engineering Physics (CAEP), P.O. Box 919-311, Mianyang, Sichuan 621900, China ABSTRACT: The intermediate structures for an energetic material (EM) loaded until the final decay are often inaccessible and overlooked, while they are a determining factor of property and performance, with a similar importance of the original structure under common conditions. The present work exemplifies the importance by revealing the low impact sensitivity of 2,2-dinitroethylene-1,1-diamine (FOX-7) with a consideration of heat-induced polymorphic transformation. Checking the packing structures of the polymorph at ambient conditions (α-form) and the two heat-induced ones (β- and γ-forms) of FOX-7, we confirm that the heating until the final decay makes the shear sliding increasingly ready. That is, from the α- to β- and γ-FOX-7, the crystal packing varies from a wavelike shape to a face-to-face one, with the increase of molecular planarity, as their maximal torsion angles of O−N− C−C decrease from 35.6 to 25.6 and 20.2°; and their shear-sliding barriers reduce and ready sliding ranges increase in the same order, verified by density functional theory calculations. This heat-induced polymorphic transformation of FOX-7 from wavelike to face-to-face π−π stacking is responsible for its low impact sensitivity, by remedying its disadvantage of relatively low thermal stability. Hardly, we will understand the low impact-sensitivity of FOX-7 if the original α-form is considered alone. This work presents an exact example to show the importance of intermediates produced by external stimuli loaded on an EM for understanding its performance. It also shows the complexity of the sensitivity mechanism of EMs and some possible deficiencies caused by considering the initial unloaded case alone.

1. INTRODUCTION Energetic materials (EMs) are a special class of metastable substances, which can release a large amount of heat and gaseous products once they suffer an enough external stimulation. In the field of EMs, energy and safety are the two most important and hottest topics. Energy determines the effectiveness of application, and safety guarantees application.1 Nevertheless, wholly, the high energy goes with the low safety of EMs. This is the so-called energy and safety contradiction of EMs. Thus, any EM applied in practice is a compromise or balance of the contradiction.2−5 In general, the safety of EMs is experimentally evaluated by many measurements, among while sensitivity is compulsory. Sensitivity is the degree of an EM in response to an external stimulation: the higher sensitivity represents the lower safety. As to the underlying mechanism of sensitivity, it is very complicated and is thought to be related to multiscale structures, such as molecular stability, molecular stacking mode, crystal perfection, crystal shape, crystal size, interfacial features, and so forth.6,7 In addition, it is well known that the safety has relation to stimulation style, testing condition, and sample status. In a word, from the aspect of science, technology, or engineering, the safety is full of challenges. Molecule and crystal are the two intrinsic structures of EMs, that is, for various samples of an EM, their basic molecular and © 2019 American Chemical Society

crystal structures are the same, even though their apparent characteristics like size, shape, defect, and density may be different. These two intrinsic structures have been verified to be strongly responsible for impact sensitivity, which is one of the most common kinds of sensitivity and has attracted extensive attention already. The insight into the dependence of the intrinsic structures and the impact sensitivity is continuously deepened and exhibits much progress. For energetic molecules, as building bricks of an EM, their high stability will contribute to low impact sensitivity.8,9 An energetic molecule with a strong big π-bond and strong intramolecular hydrogen bonds (HBs) are usually stable.10−12 With respect to crystal packing, the higher compactness suggests the less free volume in the crystal, a disadvantage to molecular degradation and hot spot formation, thus contributing to low sensitivity;13 moreover, it deems that the HBaided face-to-face π-stacking can efficiently buffer against external mechanism stimuli and facilitate the low impact sensitivity too.10−12 Despite the above progress, it is still difficult to exactly predict or understand the sensitivity from the intrinsic Received: April 26, 2019 Revised: May 20, 2019 Published: June 10, 2019 16014

DOI: 10.1021/acs.jpcc.9b03921 J. Phys. Chem. C 2019, 123, 16014−16022

Article

The Journal of Physical Chemistry C

FOX-7 possesses much less thermal stability than LLM-105, as its differential scanning calorimetry (DSC) decomposition temperature peak of ∼250 °C is much lower than that of LLM105, 342 °C, showing the easier thermal decomposition.34,35 With regard to the crystal packing, both FOX-7 and LLM-105 are wavelike stacked. In a word, it will be difficult to understand the sensitivity similarity of these two impactinsensitive EMs with a similar crystal packing mode whereas a big difference in thermal stability, if we only consider the original structure under common conditions and overlook the intermediate structures in the evolution until final decay. As a matter of fact, FOX-7 also suffers heat-induced polymorphic transformation. α-FOX-7 is the most stable form under common condition and undergoes a transformation with temperature increasing to β-FOX-7 at ∼113 °C and subsequently to γ-FOX-7 at 173 °C.36−38 Afterward, the molecular stacking modes change. We find in this work that the transformation of α- → β- → γ-FOX-7 facilitates the shear sliding. As demonstrated in Figure 1, during heating, the

structures only. Apart from some factors like testing conditions and tester, two aspects of difficulties exist. On one hand, there are some structures beyond the molecular and crystal levels for an EM applied, which are also responsible for sensitivity;14 while, hardly we can understand sensitivity considering one level of structure alone. On the other hand, because the process of an EM against an external stimulation is a dynamic one, it is impossible to reveal the sensitivity mechanism if we are blind to the detailed dynamic evolution from a stimulation loading until the final combustion or detonation. It also suggests the importance of considering the detailed evolution to clarify the sensitivity mechanism. For example, compression and shear sliding will take place if the impact is loaded on an EM. Meanwhile, the external mechanical energy is stored as increased stress. Subsequently, one or more defects are produced, and the temperature increases with increasing strain, i.e., the hot spots are formed around the defects. When the hot spots grow to a certain size, combustion and/or detonation occur. In a word, the compression, shear sliding, and temperature and stress increasing inevitably occur during the evolution. As an important consequence of the temperature and stress increasing for impacted EMs, the polymorphic transformation that is universal for EMs may occur. For example, 1,3,5,7tetranitro-1,3,5,7-tetrazocane (HMX) has three pure forms α, β, and δ at ambient conditions.15 β-HMX that is the most stable under common conditions can change with increasing temperature to α-HMX at 102−104 °C and to δ-HMX at 160−164 °C.16 Besides, HMX possesses two polymorphs of εand φ-forms at high pressures above 12 and 27 GPa, respectively.17−19 Moreover, some similar cases occur for 2,4,6,8-hexanitro-2,4,6,8,10,12-hexa-azatetracyclododecane (CL-20), which possesses at least five polymorphs with mutual transformations.20−23 Besides these common EMs composed of neutral molecules, polymorphs and polymorphic transformation for the energetic ionic salts like dihydroxylammonium 5,5′-bistetrazole-1,1′-diolate (TKX-50) were also found.24−28 These heat- and stress-induced polymorphic transformations during the evolution until final combustion and/or detonation should be the important factors influencing the sensitivity; while they usually overlooked, i.e., only the most stable form under the common condition is mostly accounted. Besides the polymorphic transformation, some stable intermediates that can back to reactants can exhibit in the evolution to decay. They can also influence the sensitivity.29,30 Thus, it is fundamental to reveal the underlying mechanism of the evolution of an EM against external stimulation to understand the sensitivity mechanism.2,3 In the present work, we will exemplify the influence of the heat-induced polymorphic transformation on impact sensitivity, by uncovering the heat-induced variation of the shear-sliding characteristic of 1,1-diamino-2,2-dinitroethylene (FOX-7). FOX-7 is a relatively novel low impact-sensitive EM with an energy of drop of 30.9 J, close to that of another one, 2,6-diamino-3,5dinitropyrazine-1-oxide (LLM-105), 28.7 J, while higher than that of 2,4,6-trinitrotoluene, 15 J.31−33 From the aspect of the aforementioned intrinsic structures, molecular structure and crystal packing, we can hardly understand the similarity of impact sensitivity of FOX-7 and LLM-105. As to the molecular stability, similar to LLM-105, FOX-7 also possesses a big πboned molecular structure and strong intramolecular HBs, contributing to its high molecular stability.10−12 Despite this,

Figure 1. Crystal packing of (a) α-FOX-7, (b) β-FOX-7, (c) γ-FOX-7, and (d) TATB. The C, H, N, and O atoms are represented in gray, green, blue, and red, respectively. These representations are also employed in the following figures.

wavelike molecular stacking tends to be planar, close to the case of 2,4,6-trinitrobenzene-1,3,5-triamine (TATB), which is a very impact-insensitive EM. This should partly be responsible for the low impact-sensitivity of FOX-7, as it has already been confirmed that HB-aided face-to-face π-stacking can efficiently buffer against external mechanism stimuli and facilitate low impact sensitivity too.10−12 Therefore, we exemplify that the intermediate packing structure in the evolution can also be an important factor influencing sensitivity and cannot be overlooked, by means of revealing the variation of the shear-sliding characteristic that resulted from the variation of the heated-induced molecular stacking of FOX-7. In principle, structure determines property and performance, while deficiency will exhibit in understanding the sensitivity mechanism if the structure at ambient conditions is considered alone. That is, the intermediate structures involved in the evolution after loading should be seriously accounted for predicting, evaluating, and understanding the performances like impact sensitivity.

2. METHODOLOGIES During the evolution of an EM loaded mechanically until final combustion and/or detonation, the shear sliding has already been verified to be a key factor to trigger its decomposition.10−12,39−44 That is, ready shear sliding contributes to low impact sensitivity. The shear sliding induced by external 16015


Article


difference between the slid and original crystal structures (ΔE) of the three polymorphs of FOX-7. The self-consistent convergence criteria of energy were set to 1 × 10−4 and 0.03 eV for electronic and ionic relaxation, respectively. By the way, the methods should be adequate for comparing the three polymorphs, even though they may not provide higher accuracy relative to D3 or D4 corrections. As illustrated in Figure 1, straightforward, the most energetically favored shearsliding of α-FOX-7 or β-FOX-7 is parallel to the AOC plane and along the c-axis, whereas that of γ-FOX-7 is parallel to the BOC plane and along the c-axis. Therefore, the orientation of the FOX-7 molecules remained invariable in scanning the interfacial shear-sliding (Figure 3), which was performed with a step of a fraction of 0.1. Combining ΔE together, we obtained an energy contour to distinguish the shear-sliding characteristic of the three polymorphs of FOX-7. In other words, the heatinduced variation of the shear-sliding characteristic can be recovered. For comparison, the energy difference of TATB that slid parallel to the AOB plane and along the a-axis was calculated too. In the case of bulk shear-sliding, we set a series of slip systems along the most energetically favored sliding orientations of the three polymorphs for calculation. As demonstrated in Figure 4, the slip systems of (010)/[001], (010)/[101̅], (010)/[100], and (010)/[101] were adopted for α- and β-FOX-7, and those of (100)/[011], (100)/[010], (100)/[101̅], and (100)/[001] were employed for γ-FOX-7. Subsequently, a series of step-by-step calculations were performed for each polymorph to simulate a single cell against external mechanical stimuli.52 This step-by-step static process is to a certain extent close to a dynamic process, which should be much closer to practice. In these successive relaxation calculations, the relaxed structure of a stain-loaded cell in one step was used as the initial structure for the next one in which the shear slide was loaded again. In the relaxation, the

mechanical force produces no volume change with two extremes: bulk shear-sliding and interfacial shear-sliding, as shown in Figure 2.12,45,46 In general, the energy change after

Figure 2. Model showing the bulk and interfacial shear-sliding of an EM. The decreased size of the arrows denotes the dispersed mechanical energy due to shear sliding.

shear sliding per molar molecule is employed to denote the difficulty of sliding: the less the energy change suggests the readier shear-sliding to avoid the formation of hot spots, i.e., to facilitate the low impact sensitivity; and sliding is forbidden if the energy change is above the barrier for molecular decomposition.44−46 As to the case of interfacial shear-sliding, the density functional theory method with the pseudopotentials constructed by projected augmented waves,47−49 an exchange− correlation functional treated with the generalized gradient approximation following Perdew−Burke−Ernzerhof formulation,50 and D2 correction of Grimme51 in Vienna ab initio simulation package were employed to calculate the energy

Figure 3. Scanning models for calculating the energy variations of the interfacial shear-sliding of (a) α-, (b) β-, and (c) γ-forms of FOX-7. In each cell, the molecules involved are grouped into two layers according to the most energetically favored sliding shown in Figure 1: the layer in blue is fixed, while that in red is slid. The factional coordinates are of the centroids of the two layers. 16016


Article


Figure 4. Models for the bulk shear-sliding calculation of the three polymorphs of FOX-7. The sliding planes and orientations are shown by shadowed planes and dashes, respectively.

Figure 5. Intermolecular HBs represented by green dashes in the three polymorphs of FOX-7 and TATB.

averaged ∠A−H···D changes from 138.9 to 127.9 and 135.5°. Meanwhile, regarding the density of these intermolecular HBs, it also shows a first weakening and subsequent strengthening, as the total amount of the intermolecular HBs around one FOX-7 molecule in α- to β- and γ-FOX-7 are 12, 10, and 13, respectively. In a word, the intermolecular HBs in β-FOX-7 are weaker than those in the other polymorphs; and the heating makes the FOX-7 molecule increasingly planar. This can also be ascertained by the Hirshfeld surfaces method, which can straightly reflect the intermolecular interactions in the crystal and whose principle can be consulted elsewhere.54,55 In principle, the color mapping on a surface distinguishes the intensity of points, and the red and blue represent the high and low intensities, respectively. As demonstrated in Figure 6, wholly, the Hirshfeld surface of the FOX-7 molecule in each polymorph appears in a shape of block, whereas, a side view of each block exhibits a difference, that is, the less offset of the lower right red spot is found in γFOX-7, in contrast to α- and β-FOX-7, showing an increased molecular planarity of FOX-7 after being heated. Similar to TATB, γ-FOX-7 is much closer to face-to-face π−π stacking, contributing to a readier shear-sliding and low impactsensitivity. 3.2. Shear-Sliding Characteristics of the Three Polymorphs of FOX-7. Figure 7 shows the ΔE contours for the interfacial shear-sliding of the three polymorphs of FOX-7, as well as TATB. In the shear sliding of EMs, they will

geometry of the cell remained fixed. The same theoretical method applied in the above interfacial sliding calculations was employed for the relaxation. Linking all of the calculated ΔE against strain of one case, we obtained an energy profile correspondingly, showing the related bulk shear-sliding characteristic.

3. RESULTS AND DISCUSSION 3.1. Comparison of the Packing Structure of the Three Polymorphs of FOX-7. As a consequence of heating, FOX-7 changes in turn from α- to β- and γ-forms, and their molecular stacking, i.e., the π−π stacking, varies from a wave shape to a parallel one, similar to TATB (Figure 1). Moreover, we find that the FOX-7 molecule becomes increasingly planar with increasing temperature as the maximal torsion angle changes from 35.6 to 25.6 and 20.2°, facilitating the normal face-to-face π−π stacking. For EMs, the π−π stacking is usually supported by intermolecular HBs.10−12 It is so for all of the three polymorphs of FOX-7, as a dense HB network is exhibited in each layer of the polymorphs of FOX-7. This case is similar to that of TATB (Figure 5). By comparison, we find that these intermolecular HBs are weak or rather weak according to Jeffrey’s criterion.53 From the viewpoint of the length of single HB, it is observed in Table 1 that the intermolecular HBs are first weakened (α- to β-FOX-7) and then strengthened (β- to γ-FOX-7), as the averaged H···A distance varies from 2.293 to 2.558 and 2.391 Å, and the 16017


Article

The Journal of Physical Chemistry C Table 1. Comparison of Intermolecular HBs in the Three Polymorphs of FOX-7 and TATB polymorphs

D

H

A

D−H, Å

H···A, Å

A···D, Å

A−H···D, deg

symmetry operation

α-FOX-7

N3 N4 N4 N3 N4 N3

H1 H3 H4 H1 H3 H2

O1 O1 O2 O3 O4 O4

H1 H4 H1 H3 H2 H2 H3

O1 O2 O3 O4 O4 O1 O2

N12 N23 N12 N23 N26 N23 N26 N9 N26 N12 N9

H14 H24 H14 H24 H27 H25 H27 H11 H28 H13 H10

O1 O1 O2 O3 O3 O4 O4 O15 O16 O18 O17

N4 N6 N2 N6 N2 N4

H4 H6 H2 H5 H1 H3

O1 O3 O5 O4 O6 O2

2.145 2.390 2.341 2.446 2.142 2.291 2.293 2.532 2.483 2.398 2.209 2.703 2.796 2.782 2.558 2.535 2.382 2.669 2.242 2.536 2.522 2.236 2.538 2.338 2.238 2.062 2.391 2.239 2.348 2.381 2.371 2.392 2.396 2.351

3.015 3.192 3.027 3.008 2.920 3.013 3.029 2.989 2.946 3.158 2.980 3.413 2.989 2.946 3.060 2.974 2.899 2.932 3.081 3.303 3.278 3.075 2.927 2.841 3.114 2.935 3.033 2.929 2.933 2.951 2.991 2.991 2.990 2.964

152.6 148.0 138.1 118.1 143.7 132.9 138.9 118.1 114.5 160.9 163.8 148.2 96.0 94.0 127.9 98.7 117.6 98.4 158.6 146.4 144.1 159.6 107.7 116.6 170.4 172.9 135.5 121.3 119.1 125.0 128.7 126.7 136.3 126.2

−1/2 + x, 1/2 − y, −1/2 + z −1/2 + x, 1/2 − y, −1/2 + z −1 + x, y, z 1/2 + x, 1/2 − y, −1/2 + z −1/2 + x, 1/2 − y, −1/2 + z 1/2 + x, 1/2 − y, −1/2 + z

N3 N4 N3 N4 N3 N3 N4

0.945 0.903 0.849 0.945 0.903 0.943 0.915 0.793 0.862 0.793 0.795 0.805 0.805 0.795 0.807 0.885 0.884 0.885 0.884 0.878 0.882 0.878 0.879 0.878 0.886 0.878 0.881 1.054 0.954 0.849 0.868 0.965 0.757 0.908

average β-FOX-7

average γ-FOX-7

average TATB

average

1.5 − x, 1 − y, −1/2 + z −1 + x, y, z 1/2 − x, 1 − y, −1/2 + z 1/2 − x, 1 − y, −1/2 + z 1.5 − x, 1 − y, −1/2 + z 1.5 − x, 1 − y, −1/2 + z −1 + x, y, z x, −1 + y, z 1/2 + x, 1.5 − y, −1/2 + z x, 1 + y, z; x, −1 + y, z 1/2 + x, 1/2 − y, −1/2 + z 1/2 + x, 1/2 − y, −1/2 + z 1/2 + x, 1.5 − y, −1/2 + z 1/2 + x, 1/2 − y, −1/2 + z −1/2 + x, 1.5 − y, −1/2 + z x, −1 + y, z −1/2 + x, 1/2 − y, −1/2 + z −1/2 + x, 1/2 − y, −1/2 + z 1 + x, 1 + x, x, 1 + 1 + x, x, 1 + 1 + x,

y, z 1 + y, z y, z 1 + y, z y, z y, z

Figure 6. Hirshfeld surfaces of the FOX-7 molecules in different polymorphs and TATB molecules in the crystal. γ-FOX-7 possesses Z′ = 2, as illustrated by (c) and (d) separately.

be decayed once the increased internal energy is above the energy required to trigger its molecular decomposition, which is the dissociation energy of the weakest bond (BDE). For a FOX-7 molecule, the weakest bond is the C−NO2 bond, whose BDE is 65 kcal/mol on the aforementioned theory level. Thus, we adopt a close energy of 60 kcal/mol as a reference to define a feasible shear-sliding region, i.e., ΔE ≤ 60, 30, and 10 kcal/mol are adopted as the criteria for feasible, rather ready,

and ready sliding, as illustrated in Figure 8. Therefore, we can readily conclude that the regions of the rather ready and ready shear-sliding become increasingly wide from α- to β- and γFOX-7. When α-FOX-7 changes to β- and γ-FOX-7, the region width corresponding to the feasible shear-sliding increases from 0.48c to 0.62c and 1.0c. After the transformation to γFOX-7, the interfacial shear-sliding can occur almost freely, as ΔE is almost always below 10 kcal/mol, with only two small 16018


Article


Figure 7. ΔE (unit in kcal/mol) contours for the interfacial shear-sliding of the three polymorphs of FOX-7 and TATB.

γ-FOX-7. Moreover, Figure 9d shows a remarkable advantage of easy sliding of TATB, which is partly responsible for its high impact insensitivity. Besides, the largest ΔE values that can be regarded as sliding barriers on all profiles are compared in Figure 10. As demonstrated in the figure, the sliding barriers along various orientations of the three polymorphs are 6.0, 5.9, 5.2, and 2.3 kcal/mol along the orientations of (010)/[101̅], (010)/[001], (010)/[101], and (010)/[100] of α-FOX-7, 3.8, 3.4, 3.4, and 2.0 kcal/mol along the orientations of (010)/[001], (010)/ [101̅], (010)/[101], and (010)/[100] of β-FOX-7, and 4.1, 3.2, 2.7, and 1.9 kcal/mol along the orientations of (100)/ [001], (100)/[010], (100)/[101̅], and (100)/[011] of γ-FOX7, respectively. This shows that the lowest sliding barrier of each polymorph of FOX-7 decreases from 2.3 kcal/mol (along the (010)/[100] of α-FOX-7) to 2.0 kcal/mol (along the (010)/[100] of β-FOX-7) and 1.9 kcal/mol (along (100)/ [011] of γ-FOX-7). That is, the orientation of the readiest bulk shear-sliding, represented by the lowest sliding barrier, appears in γ-FOX-7. Despite a small difference in the lowest barriers between β-FOX-7 and γ-FOX-7, the shear sliding is thought to be much readier in the latter, in combination with the above analysis of another extreme of interfacial sliding. 3.3. Correlation of the Low Impact Sensitivity of the Heat-Induced Polymorphic Transformation. We will exemplify the influence of heat-induced polymorphic transformation on the impact sensitivity by comparing the sensitivity mechanism of FOX-7 and LLM-105. As aforementioned, it has experimentally been confirmed that FOX-7 and LLM-105 possess similar impact sensitivity.10 Nevertheless, it would not be well understood if the structures under common conditions are considered alone. From the viewpoint of intrinsic structures (molecule and crystal), it has been confirmed that the factors responsible for impact sensitivity can be molecular stability, barrier for the decomposition

Figure 8. Comparison in width of the regions of feasible (ΔE < 60 kcal/mol), rather ready (ΔE < 30 kcal/mol), and ready (ΔE < 10 kcal/mol) interfacial shear-sliding of the three polymorphs of FOX-7 in Figure 7.

regions of 10 < ΔE < 20 kcal/mol on the whole sliding face (Figure 7c). This suggests the increasingly easy sliding during heating FOX-7, in agreement with the above analysis of molecular stacking, in which it demonstrates the approximate face-to-face π−π stacking of γ-FOX-7. The case of γ-FOX-7 is the closest to that of TATB. As to the case of bulk shear-sliding, wholly, the ΔE in Figure 9 is significantly lower than that in the case of the interfacial shear-sliding, suggesting the higher readiness of the former. Compared to the energy profiles in Figure 9a,c, a big difference in ΔE between α-FOX-7 with β- and γ-FOX-7 is observed, as ΔE values corresponding to many shear strains are above 4 kcal/mol in α-FOX-7, whereas those in β- and γ-FOX-7 are below 4 kcal/mol. This suggests a readier bulk sliding of β- or 16019


Article


Figure 9. Dependences of ΔE on bulk shear strain along various orientations of the three polymorphs of FOX-7 and TATB.

Figure 10. Predicted shear barriers of the three polymorphs of FOX-7.

than LLM-105, with a difference of 92 °C of the DSC decomposition temperature, i.e., 250 vs 342 °C at a heating rate of 10 °C/min, implying a much lower barrier for thermal decay. It shows a much readier thermal decay, or easier hot spot formation and growth of FOX-7, despite having similar molecular stability to LLM-105. Based on this point only, FOX-7 should be much more impact-sensitive than LLM-105, inconsistent with the experimental measurement. Therefore, other factors should remedy for the above disadvantage of the thermal stability of FOX-7. These factors could be the aforementioned molecular stacking mode and reversible reaction. In comparison, our recent molecular dynamics simulations and static calculations showed that the reversible reactions, i.e., the reversible hydrogen transfers, can

through intra- or intermolecular reactions, molecular stacking mode, and reversible reaction as the primary step for decomposition.56 That is, the high decomposition barrier, the face-to-face π−π stacking, and the existence of a reversible reaction as the initial step for decay all facilitate low impact sensitivity. Checking these factors of the two impact-insensitive EMs, we can hardly explain the similarity of their impact sensitivity, if we overlook the heat-induced variation of the packing structure. A quantum chemical calculation at the theoretical level of B3LYP/6-311++G(d,p) was performed and showed a small difference in BDE (the trigger bonds of the two molecules are both C−NO2) between FOX-7 and LLM-105, 62.5 vs 60.2 kcal/mol.57 However, FOX-7 is much less thermally stable 16020


Article


(6) Li, H.; Xu, R.; Kang, B.; Li, J.; Zhou, X.; Zhang, C.; Nie, F. Influence of Crystal Characteristics on the Shock Sensitivities of Cyclotrimethylene Trinitramine, Cyclotetramethylene Tetranitramine, and 2, 4, 6, 8, 10, 12-hexanitro-2, 4, 6, 8, 10, 12hexaazatetra-cyclo [5, 5, 0, 03, 1105, 9] dodecane Immersed in Liquid. J. Appl. Phys. 2013, 113, No. 203519. (7) Nomura, K.; Kalia, R. K.; Nakano, A.; Vashishta, P. Reactive Nanojets: Nanostructure-enhanced Chemical Reactions in a Defected Energetic Crystal. Appl. Phys. Lett. 2007, 91, No. 183109. (8) Politzer, P.; Alper, H. E. Computational Chemistry: Reviews of Current Trends; Leszczynski, J., Ed.; World Scientific: River Edge, NJ, 1999; pp 271−286. (9) Zeman, S. Energetic Materials, Part 2. Detonation, Combusion; Politzer, P.; Murray, J. S., Eds.; Elsevier: Amsterdam, 2003; pp 25−52. (10) Ma, Y.; Zhang, A.; Xue, X.; Jiang, D.; Zhu, Y.; Zhang, C. Crystal Packing of Low-Sensitivity and High-Energy Explosives. Cryst. Growth Des. 2014, 14, 4703−4713. (11) Zhang, C.; Wang, X.; Huang, H. π-stacked Interactions in Explosive Crystals: Buffers against External Mechanical Stimuli. J. Am. Chem. Soc. 2008, 130, 8359−8365. (12) Tian, B.; Xiong, Y.; Chen, L.; Zhang, C. Relationship between the Crystal Packing and Impact Sensitivity of Energetic Materials. CrystEngComm 2018, 20, 837−848. (13) Pospíšil, M.; Vávra, P.; Concha, M. C.; Murray, J. S.; Politzer, P. A possible Crystal Volume Factor in the Impact Sensitivities of Some Energetic Compounds. J. Mol. Model. 2010, 16, 895−901. (14) Huang, B.; Qiao, Z.; Liu, J.; Li, H.; Yang, G. In Huang Construction of Core-shell HMX@nano-TATBComposites with Improved Mechanical Sensitivity, Proceedings of 49th International ICT Conference, Karlsruhe, 2018; p 79. (15) Cady, H. H.; Smith, L. C. Studies on the Polymorphs of HMX, Los Alamos Scientific Laboratory Report LAMS-2652 TID-4500; Los Alamos National Laboratory: Los Alamos, NM, 1961. (16) Gibbs, T. R.; Popolato, A. LASL Explosive Property Data; University of California Press: Berkeley, CA, 1980. (17) Yoo, C. S.; Cynn, H. Equation of State, Phase Transition, Decomposition of β-HMX (Octahydro-1,3,5,7-Tetranitro-1,3,5,7Tetrazocine) at High Pressures. J. Chem. Phys. 1999, 111, 10229− 20135. (18) Hare, D. E.; Forbes, J. W.; Reisman, D. B.; Dick, J. J. Isentropic Compression Loading of Octahydro-1,3,5,7-Tetranitro-1,3,5,7-Tetrazocine (HMX) and the Pressure-induced Phase Transition at 27 GPa. Appl. Phys. Lett. 2004, 85, 949−951. (19) Pravica, M.; Galley, M.; Kim, E.; Weck, P.; Liu, Z. A Far- and Mid-infrared Study of HMX (Octahydro-1,3,5,7-Tetranitro-1,3,5,7Tetrazocine) under High Pressure. Chem. Phys. Lett. 2010, 500, 28− 34. (20) Russell, T. P.; Miller, P. J.; Piermarini, G. J.; Block, S. Pressuretemperature Phase Diagram of Hexanitrohexaazaisowurtzitane. J. Phys. Chem. 1993, 97, 1993−1997. (21) Russell, T. P.; Miller, P. J.; Piermarini, G. J.; Block, S. High Pressure Phase Transition in γ-Hexanitrohexaazaisowurtzitane. J. Phys. Chem. 1992, 96, 5509−5512. (22) Foltz, M. F.; Coon, C. L.; Garcia, F.; Nichols, A. L., III The Thermal Stability of the Polymorphs of Hexanitrohexaazaisowurtzitane, Part I. Propellants, Explos., Pyrotech. 1994, 19, 19−25. (23) Foltz, M. F.; Coon, C. L.; Garcia, F.; Nichols, A. L., III The Thermal Stability of the Polymorphs of Hexanitrohexaazaisowurtzitane, Part II. Propellants, Explos., Pyrotech. 1994, 19, 133−144. (24) Fischer, N.; Fischer, D.; Klapötke, T. M.; Piercey, D. G.; Stierstorfer, J. Pushing the Limits of Energetic Materials, the Synthesis and Characterization of Dihydroxylammonium 5,5′-bistetrazole-1,1′Diolate. J. Mater. Chem. 2012, 22, 20418−20422. (25) Dreger, Z. A.; Tao, Y.; Averkiev, B. B.; Gupta, Y. M.; Klapötke, T. M. High-Pressure Stability of Energetic Crystal of Dihydroxylammonium 5,5′-Bistetrazole-1,1′-Diolate: Raman Spectroscopy and DFT Calculations. J. Phys. Chem. B 2015, 119, 6836−6847. (26) Meng, L.; Lu, Z.; Wei, X.; Xue, X.; Ma, Y.; Zeng, Q.; Fan, G.; Nie, F.; Zhang, C. Two-sided Effects of Strong Hydrogen Bonding on

take place in these two EMs with similar thermodynamics and kinetics.58,59 As to the molecular stacking mode of the most stable polymorph under common conditions, it is wavelike for the both EMs. Interestingly, the molecular stacking mode of FOX-7 is not invariable after being heated as discussed above, whereas that of LLM-105 is invariable until thermal decay.34 Thus, the varied molecular stacking mode of FOX-7, from a wavelike shape to a planar one, compensates its much lower thermal stability and contributes to similar impact sensitivity to LLM-105.

4. CONCLUSIONS In summary, we understand the low impact sensitivity of FOX7 similar to that of LLM-105, even though the former possesses much lower thermal stability. That is, heating, as a consequence of the loading impact, induces the polymorphic transformation of FOX-7, with a variation of the molecular stacking mode from a wavelike shape to a planar one. Nevertheless, it does not occur for LLM-105. The calculations on the shear sliding of the three polymorphs of FOX-7 are performed by considering two extremes, interfacial and bulk shear-sliding, and show that, for each shear sliding, it becomes increasingly ready when heating FOX-7 induces the transformation from α- to β- and γ-forms. This work exemplifies that it is important to clarify the intermediate structures and properties to understand the apparent performances of EMs. Because it is usually difficult to confirm these intermediates, they are overlooked and lack sufficient insights. This is also a root for the difficulty in predicting the sensitivity of EMs, which features more dynamically and depends much on the details of evolution.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: 86-816-2493506. ORCID

Rupeng Bu: 0000-0001-9215-7715 Chaoyang Zhang: 0000-0003-3634-7324 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors gratefully acknowledge the support of the Science Challenge Project (TZ-2018004) and the National Natural Science Foundation of China (21875231, 21875227, and U1530262).

■

REFERENCES

(1) Dong, H.; Zhou, F. Properties of High Energetic Explosives and Relatives; Science Press: Beijing, 1989. (2) Teipel, U. Energetic Materials. In Particle Processing and Characterization; WILEY-VCH Verlag GmbH & Co. KGaA, 2005. (3) Jiao, F.; Xiong, Y.; Li, H.; Zhang, C. Alleviating the Energy & Safety Contradiction to Construct New Low Sensitivity and Highly Energetic Materials through Crystal Engineering. CrystEngComm 2018, 20, 1757−1768. (4) Zhang, C. On the Energy & Safety Contradiction of Energetic Materials and the Strategy for Developing Low-sensitive Highenergetic Materials. Chin. J. Energy Mater. 2018, 26, 2−11. (5) Fried, L. E.; Manaa, M. R.; Pagoria, P. F.; Simpson, R. L. Design and Synthesis of Energetic Materials. Annu. Rev. Mater. Res. 2001, 31, 291−321. 16021


Article

The Journal of Physical Chemistry C the Stability of Dihydroxylammonium 5,5′-bistetrazole-1,1′-diolate (TKX-50). CrystEngComm 2016, 18, 2258−2267. (27) Klapötke, T. M.; Chapman, R. D. Progress in the Area of High Energy Density Materials. In 50 Years of Structure and Bonding; Springer: Berlin, 2015; Vol. 172, pp 1−15. (28) Lu, Z.; Xue, X.; Meng, L.; Zeng, Q.; Chi, Y.; Fan, G.; Li, H.; Zhang, Z.; Nie, F.; Zhang, C. Heat-Induced Solid−Solid Phase Transformation of TKX-50. J. Phys. Chem. C 2017, 121, 8262−8271. (29) Zeman, S.; Elbeih, A. E.; Hussein, A. K.; Elshenawy, T.; Pelikán, V.; Yan, Q.-L. Concerning the Shock Sensitivities of Certain Plastic Bonded Explosives Based on Attractive Cyclic Nitramines. Cent. Eur. J. Energ. Mater. 2017, 14, 775−787. (30) Pelikán, V.; Zeman, S.; Yan, Q.-L.; Erben, M.; Elbeih, A.; Akštein, Z. Concerning the Shock Sensitivity of Cyclic Nitramines Incorporated into a Polyisobutylene Matrix. Cent. Eur. J. Energ. Mater. 2014, 11, 219−235. (31) Latypov, N. V.; Bergman, J.; et al. Synthesis and Reactions of 1,1-Diamino-2,2-Dinitroethylene. Tetrahedron 1998, 54, 11525− 11536. (32) Ö stmark, H.; Langlet, A.; Bergman, H.; Wingborg, N.; Wellmar, U.; Bemm, U.In FOX-7 - a New Explosive with Low Sensitivity and High Performance, Proceedings of the Eleventh International Detonation Symposium; Office of Naval Research: Snowmass, Colorado, 2000; p 807. (33) Sorescu, D. C.; Boatz, J. A.; Thompson, D. L. Classical and Quantum-mechanical Studies of Crystalline FOX-7 (1,1-diamino-2,2dinitroethylene). J. Phys. Chem. A 2001, 105, 5010−5021. (34) Pagoria, P. F.; Mitchell, A. R.; Schmidt, R. D.; Simpson, R. L.; Garcia, F.; Forbes, J. W.; Swansiger, R.; Hoffman, D. M. Synthesis, Scale-up, and Characterization of 2,6-Diamino-3,5-Dinitropyrazine-1Oxide(LLM-105), Technical Report No. UCRL-JC-130518; Lawrence Livermore National Laboratory, 1998. (35) Boddu, V. M.; Viswanath, D. S.; Ghosh, T. K.; Damavarapu, R. 2,4,6-Triamino-1,3,5-Trinitrobenzene (TATB) and TATB-Based Formulations-A Review. J. Hazard. Mater. 2010, 181, 1−8. (36) Crawford, M. J.; Evers, J.; Goebel, M.; Klapoetke, T. M.; Mayer, P.; Oehlinger, G.; Welch, J. M. Gamma-FOX-7: Structure of a High Energy Density Material Immediately Prior to Decomposition. Propellants, Explos., Pyrotech. 2007, 32, 478−495. (37) Evers, J.; Klapötke, T. M.; Mayer, P.; Oehlinger, G.; Welch, J. α and β-FOX-7, Polymorphs of a High Energy Density Material, Studied by X-ray Single Crystal and Powder Investigations in the Temperature Range from 200 to 423 K. Inorg. Chem. 2006, 45, 4996− 5007. (38) Kempa, P. B.; Herrmann, M. Temperature Resolved X-ray Diffraction for the Investigation of the Phase Transitions of FOX-7. Part. Part. Syst. Charact. 2005, 22, 418−422. (39) Dick, J. J. Effect of Crystal Orientation on Shock Initiation Sensitivity of Pentaerythritol Tetranitrate Explosive. Appl. Phys. Lett. 1984, 44, 859−861. (40) Dick, J. J.; Mulford, R. N.; Spencer, W. J.; Pettit, D. R.; Garcia, E.; Shaw, D. C. Shock Response of Pentaerythritol Tetranitrate Single Crystals. J. Appl. Phys. 1991, 70, 3572−3587. (41) Dick, J. J.; Ritchie, J. P. Molecular Mechanics Modeling of Shear and the Crystal Orientation Dependence of the Elastic Precursor Shock Strength in Pentaerythritol Tetranitrate. J. Appl. Phys. 1994, 76, 2726−2737. (42) Kuklja, M. M.; Rashkeev, S. N.; Zerilli, F. J. Shear-strain Induced Decomposition of 1,1-diamino-2,2-dinitroethylene. Appl. Phys. Lett. 2006, 89, No. 071904. (43) Kuklja, M. M.; Rashkeev, S. N. Shear-strain-induced Chemical Reactivity of Layered Molecular Crystals. Appl. Phys. Lett. 2007, 90, No. 151913. (44) Kuklja, M. M.; Rashkeev, S. N. Interplay of Decomposition Mechanisms at Shear-Strain Interface. J. Phys. Chem. C 2009, 113, 17−20. (45) Zhang, C.; Cao, X.; Xiang, B. Understanding the Desensitizing Mechanism of Olefin in Explosives: Shear Slide of Mixed HMX-olefin Systems. J. Mol. Model. 2012, 18, 1503−1512.

(46) Zhang, C.; Cao, X.; Xiang, B. Sandwich Complex of TATB/ Graphene: An Approach to Molecular Monolayers of Explosives. J. Phys. Chem. C 2010, 114, 22684−22687. (47) Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for Ab Initio Total-energy Calculations Using a Plane-wave Basis Set. Phys. Rev. B 1996, 54, No. 11169. (48) Blöchl, P. E. Projector Augmented-wave Method. Phys. Rev. B 1994, 50, No. 17953. (49) Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to the Projector Augmented-wave Method. Phys. Rev. B 1999, 59, No. 1758. (50) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, No. 3865. (51) Grimme, S. Semiempirical GGA-Type Density Functional Constructed with a Long-Range Dispersion Correction. J. Comput. Chem. 2006, 27, 1787−1799. (52) Zhang, C. Understanding the Desensitizing Mechanism of Olefin in Explosives versus External Mechanical Stimuli. J. Phys. Chem. C 2010, 114, 5068−5072. (53) Jeffrey, G. A. An Introduction to Hydrogen Bonding; Oxford University Press: Oxford, 1997. (54) Spackman, M. A.; McKinnon, J. J. Fingerprinting Intermolecular Interactions in Molecular Crystals. CrystEngComm 2002, 4, 378−392. (55) McKinnon, J. J.; Jayatilaka, D.; Spackman, M. A. Towards Quantitative Analysis of Intermolecular Interactions with Hirshfeld Surfaces. Chem. Commun. 2007, 3814−3816. (56) Xiong, Y.; Ma, Y.; He, X.; Xue, X.; Zhang, C. Reversible Intramolecular Hydrogen Transfer: A Completely New Mechanism for Low Impact Sensitivity of Energetic Materials. Phys. Chem. Chem. Phys. 2019, 21, 2397−2409. (57) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A.; Vreven, T.; Kudin, K. N.; Burant, J. C.; et al. Gaussian 09, revision B.01; Gaussian, Inc.: Pittsburgh, PA, 2009. (58) Wang, J.; Xiong, Y.; Li, H.; Zhang, C. Reversible Hydrogen Transfer as New Sensitivity Mechanism for Energetic Materials against External Stimuli: A Case of the Insensitive 2,6-Diamino-3,5dinitropyrazine-1-oxide. J. Phys. Chem. C 2018, 122, 1109−1118. (59) Jiang, H.; Jiao, Q.; Zhang, C. Early Events When Heating 1,1Diamino-2,2-dinitroethylene: Self-consistent Charge Density-functional Tight Binding Molecular Dynamics Simulations. J. Phys. Chem. C 2018, 122, 15125−15132.

16022


Heat-Induced Polymorphic Transformation Facilitating the Low Impact

Recommend Documents