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Perspective

Crystal Engineering for Creating Low Sensitivity and Highly Energetic Materials Chaoyang Zhang, Fangbao Jiao, and Hongzhen Li Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00929 • Publication Date (Web): 06 Sep 2018 Downloaded from http://pubs.acs.org on September 7, 2018

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Crystal Growth & Design

Crystal Engineering for Creating Low Sensitivity and Highly Energetic Materials Chaoyang Zhang,* †,‡ Fangbao Jiao, † and Hongzhen Li † †Institute

of Chemical Materials, China Academy of Engineering Physics (CAEP), P. O. Box 919-311, Mianyang, Sichuan 621900, China. ‡Beijing Computational Science Research Center, Beijing 100048, China.

Abstract: Energy and safety are the two most important concerns of energetic materials (EMs), while they usually contradict with each other: the high energy goes with the low safety together. Low sensitivity and highly energetic materials (LSHEMs) balance well the energy and safety, and thus are highly desired for extensive applications. Nevertheless, wholly, the energy-safety contradiction, the energy and component limits, and the insufficient knowledge about the relationships among components, structures, and properties and performances of EMs, make the LSHEMs, or even the entire EMs, evolved slowly. This Perspective focuses upon the current progress in the clarifications of the energy-safety contradiction and the crystal packing-impact sensitivity relationship of EMs. Also, we propose strategies for creating new LSHEMs or desensitized EMs through crystal engineering, covering traditional EMs composed of neutral single-component molecules, energetic cocrystals and energetic ionic salts. Two levels of intrinsic structures, molecule and crystal, are accounted for constructing LSHEMs: at the molecular level, it is proposed to make much chemical energy stored in bonds while avoid any too weak bond formed in an energetic molecule to intrinsically balance the energy and safety; at the level of crystal, it is suggested to enhance intermolecular interactions to increase packing compactness and energy density, and to strengthen the anisotropy of the intermolecular interactions to facilitate ready shear slide and low mechanical sensitivity; and overall, a big π-bonded energetic molecule with an oxygen balance close to zero and a hydrogen bond-aided face-to-face π-π molecular stacking is preferred to be a LSHEM. Hopefully, this Perspective will set a root for establishing a systematic theory for creating LSHEMs. / 34 ACS Paragon1Plus Environment

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■ INTRODUCTION Energetic materials (EMs) are a class of substances that can transiently release a large quantity of heat and gas through their self-decay, with extensive applications for civilian and military purposes. Typically, EMs are composed of C, H, N and O elements, and generally refer to propellants, explosives and pyrotechnics.1,2 Energy and safety are the two most important concerns of EMs, as the energy represents their efficiency and the safety guarantees their applicability. In practice, the energy has not clearly been defined yet; and it is usually denoted by multiple indexes, such as energy content/release of unit mass or volume, reaction heat, detonation velocity (D), detonation pressure, detonation heat, specific kinetic energy, and so forth. The safety is usually represented by various kinds of sensitivities, and the sensitivity kind is defined according to the stimulation style. The sensitivities are the response degrees of an EM against various styles of external stimuli: the lower sensitivity represents the higher safety.3,4 In this Perspective, the sensitivity is specified as impact sensitivity (represented by H50, the drop hammer height corresponding to the 50 % explosion), because impact is almost the most common stimulation style during manufacture, transportation, storage and application, and the impact sensitivity is universally concerned. With respect to these two most concerns of EMs, unfortunately and generally, they contradict with each other, i.e., the high energy goes with the low safety together, unsatisfactory to a common application requirement of high energy together with adequate safety. It is just the so-called energy-safety contradiction.5 Thus, an applied EM is in fact a balance between the energy and safety or a consequence of the energy-safety contradiction compromised, implying a challenge to create low sensitivity and highly energetic materials (LSHEMs). It is also a main reason for that there are only a small quantity of EMs applied in practice, relative to a huge quantity of energetic

/ 34 ACS Paragon2Plus Environment

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compounds synthesized in the past decades. For an application purpose, LSHEMs are highly desired. While, as pointed out above, owing to the energy-safety contradiction, it is still difficult to achieve such EMs. In principle, either low or reduced sensitivity represents the response of an EM against an external stimulation, and each is governed by its multiscale structures and physicochemical properties, the measurement conditions and the live stimulation styles. Among the multiscale structures, the molecular and crystal structures are intrinsic, as it should not change once a material given. In other words, it is intrinsic to improve the safety of EMs from the view of their molecular and crystal structures of points. Crystal engineering is the understandings of the relationship between molecular and crystal structures and the applications of such understandings to tailor materials with desired properties and performances.6,7 Accordingly, the crystal engineering of EMs is to understand the molecular-crystal structure relationship to construct new energetic crystals with desired properties and performances. Crystal engineering has grown and developed for over half of a century. 8-10 While, for the crystal engineering of EMs, it is a new thing of later years. It becomes thriving after the appearance of energetic cocrystals (ECCs), thereby, people can manufacture new EMs based on existing molecules, instead of organic syntheses.

11

Besides, the crystal engineering can make a large

quantity of deserted and forgotten molecules alive. Obviously, to improve the safety of EMs from the intrinsic molecular and crystal structures is one of the main aims of the crystal engineering of EMs. Thus, two premises are required for it. One is the relationship between molecular structures and crystal packing. This case is the same as the other crystalline materials. The other is the relationship between the crystal packing structures, and the properties and performances of EMs, some of which like energy and sensitivity are specialized for EMs. This Perspective focuses upon the impact sensitivity of CHON contained EMs, therein,



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the crystal packing-impact sensitivity relationship will be paid special attention to. Currently, much progress has been made in the crystal engineering of EMs covering traditional ones composed of neutral single-component molecules, ECCs and energetic ionic salts (EISs), settling a base for promoting the evolution of EMs. Meanwhile, the energy-safety contradiction, as an inevitable topic of EMs, should be clarified beforehand, because it requires a clear theoretical base for the low or reduced sensitivity. Thereby, the remaining of this Perspective is organized as follows: sections 2 and 3 are the clarifications of the energy-safety contradiction and the crystal packing-impact sensitivity relationship, respectively; sections 4-6 provide the strategies for creating low sensitive or desensitized traditional EMs, ECCs and EISs by crystal engineering, respectively; and the conclusions are finally drawn. By the way, regarding LSHEMs, they were roughly defined as a special group of EMs, whose H50≥0.5 m and D≥8.5 km. These values of the impact sensitivity and D are close to those of TNT and RDX, respectively.12 Both TNT and RDX are the benchmarks of EMs. The sensitivity reduction refers to that after cocrystallization to form a cocrystal or after ionization to form a salt, relative to the more sensitive coformers or original neutral molecules. Because of much significance of the desensitization strategies, it is worth highlighting them, although the LSHEMs may not be achieved thereby, i.e., the desensitized EMs may not reach the energy and safety criterion of LSHEMs. Hopefully, this Perspective will set a base for designing new EMs together with high energy and sufficient safety to be applied possibly, and set a root for establishing a systematic theory for LSHEMs. Meanwhile, the crystal engineering should successfully be exemplified by this class of special materials.

■ THE ENERGY-SAFETY CONTRADICTION OF EMS Recently, we highlighted the alleviation of the energy-safety contradiction of EMs by crystal engineering to construct new LSHEMs.5,12 We reassert here that an EM can possess both high


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energy and low or reduced sensitivity. By means of the clarification of the energy-safety contradiction, someone won’t stick to that the contradiction is always inherent and can’t be alleviated anyway.

Figure 1. Micro- to macro-structures of a plastic bonded explosive (PBX) as an example of the structures of EMs and main concerns about the energy and safety (Cited from Ref 12).

Understanding the multiscale structures of EMs facilitates us to clarify the origin of the contradiction. As shown in Figure 1, for example, a plastic bonded explosive (PBX) block that is very close to a practical application form of EMs contains four-level structures, including molecule, crystal, PBX particle and PBX block. Usually, the energy is determined by the molecular composites and loading density. If we consider the intrinsic structures (molecular and lattice structures) alone, the energy is dominated by the heat and gas releases of the component molecules and the packing density (d). In contrast to the factors governing the energy, those determining safety are much more complex. As demonstrated in Figure 1, the molecular stability, the internal and external structures of the crystal particles, the interfacial structures and sizes of the PBX / 34 ACS Paragon5Plus Environment


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particles, the shapes and the linkage of the PBX block, and the response behaviors of the crystal and PBX particles against external stimuli all contribute to the sensitivity.

Figure 2. Plot showing the origins of the energy and safety of EMs, and factors influencing the safety (Cited from Ref 12).

As a matter of fact, the simple descriptions of the energy and the complex ones of the safety are largely attributed to their thermodynamic and kinetic nature, respectively.5,12 That is, the energy features more thermodynamic and is governed by the difference in energy between the original and final states alone; while, the safety features more kinetic and is strongly related to the intermediate details. As illustrated in Figure 2, for the energy, it can simply denoted by an energy difference, ΔE1; for the safety, it can also simply represented by an energy barrier, ΔE2, with numerous factors influencing it. When an external stimulation acts on an EM, a series of successive processes proceed to decay the EM, including energy transfer, energy absorption and accumulation, lattice vibration, molecular decomposition to release heat, hot spot formation and growth, and final combustion and/or detonation. If any process doesn’t occur sufficiently, the EMs will be failed in the final combustion and/or detonation. That is, the failure of any process will contribute to the low sensitivity. / 34 ACS Paragon6Plus Environment

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Reasonably, the more energy (ΔE1) stored in chemical bonds to enhance the energy of an EM is implemented at the cost of the more weakening of the related chemical bonds. These weakened bonds facilitate the decay of the energetic molecule, i.e., the reduced ΔE2. In other words, the energy enhancement causes the readier molecular decomposition. Consequently, the energy-safety contradiction, or the contradiction of the ΔE1 increasing with the ΔE2 reducing, occurs. It reveals that, only at the molecular level, the energy-safety contradiction, or the ΔE1-ΔE2 contradiction, takes place necessarily and remarkably as an inherent one; while, at the higher level of crystal, the energy-safety contradiction can be alleviated, as the above-mentioned failure of any intermediate process will contribute to the low sensitivity with maintaining an energy level. Because the energy and safety of EMs are originated from a sense of more thermodynamics and a sense of more kinetics of chemical reactions, respectively, the energy-safety contradiction is in fact a thermodynamic-kinetic one.5,12 Thereby, it sets a theoretical base for constructing LSHEMs. That is to say, the energy can be increased by enhancing the chemical energy stored in bonds and the packing density; meanwhile, by crystal engineering, we can make external stimuli insufficient to ignite EMs to enhance the safety, as described below.

■ CRYSTAL PACKING-IMPACT SENSITIVITY RELATIONSHIP OF EMS As pointed out above, there are two intrinsic structures for EMs, i.e., molecule and crystal; and the energy-safety contradiction is inherent on the molecular level, while it can largely be alleviated through crystal engineering, i.e., to improve crystal packing to facilitate low sensitivity. In this case, it requires a crystal packing-sensitivity relationship to put the crystal engineering into practice; or, the relationship is a premise of the crystal engineering of EMs. It is in fact an issue of what kind of packing structure makes the ignition of EMs efficient or inefficient after impacted. Of course, roughly, increasing intermolecular interactions facilitates the ignition inefficiency or the low



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sensitivity, as the stronger intermolecular interactions consume first more stimulation energy to weaken themselves, with less energy remained to decompose energetic molecules. However, after all, the strong intermolecular interactions appear at the cost of the energy reduction. Thus, what kind of crystal packing structure favors the low ignition efficiency while keeping the energy level becomes crucial to the LSHEMs. Recently, we described the principle of the crystal packing-impact sensitivity relationship of EMs in a straightforward manner.13 Because multiple factors are responsible for the impact sensitivity and it is impossible to quantitatively correlate the lattice parameters with the impact sensitivity, the relationship is based on the molecular stacking styles and the impact sensitivity measured experimentally. That is, the relationship is just a qualitative one. Despite this, it is still worth establishing such relationship, as it becomes a straightforward tool to understand sensitivity mechanism and sets a base for designing new EMs by crystal engineering.

Figure 3. Plot showing the intra- and interlayered intermolecular interactions. The thickness of the lines represents the strength of intermolecular interactions: the thicker line suggests the stronger interactions (Cited from Ref 13).

The impact sensitivity mechanism facilitates us to describe the principle of the relationship. When an EM against impact, compression and shear are firstly induced; as time proceeds, the yields and defects are produced; and with strain and temperature increasing, the energetic molecules around the defects are activated to be decomposed to accumulate heat to form and grow hot spots, / 34 ACS Paragon8Plus Environment

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resulting in final combustion and detonation. Among this series of processes, the shear sliding characteristic is extensively thought to dominate the impact sensitivity mechanism.14-23 Thereby, it can be seen as a bridge of the packing structure-impact sensitivity relationship. First of all, the preferred sliding layer or the sliding face in crystals should be ascertained, as it is the base for discussing the relationship. Sliding occurs emergently for EMs against external mechanical stimuli. Microscopically and thermodynamically, the sliding difficulty along a direction can be evaluated by the strength of the intermolecular interactions along the direction.23 As illustrated in Figure 3, three possibly ready sliding faces or sliding layers (1, 2 and 3) exist in the crystal. Regarding thermodynamics, simply, a preferred sliding layer requires the strong intralayered intermolecular interactions to hold the layers, together with the weak interlayered intermolecular interactions to reduce the sliding resistance. For example, the intermolecular interactions along layer 1 are the strongest, while those between the neighboring layers are the weakest due to the largest interlayer distance; therefore, layer 1 should be a preferred sliding layer.

Figure 4. Demonstration of the principle of the crystal packing-impact sensitivity relationship by π-stacked energetic crystals. The upper plots show the Hirshfeld surfaces and impact sensitivity represented by H50 of some candidates. The shapes of the surfaces and the distributions of the red dots on the surfaces are applied as a straightforward tool to reflect packing characteristic. And the bottom plots show packing modes and the increased potential-sliding distance (d) dependences. α (black curves) and β (red curves) denote the cases when shear sliding occurs along front/back and right/left, respectively (Cited from Ref 13). / 34 ACS Paragon9Plus Environment


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Figure 4 illustrates the shear sliding characteristic and the principle of its dominance of impact sensitivity and of the crystal packing-impact sensitivity relationship, exemplified by some π-stacked crystals. All π-stacking are classified into four kinds: face-to-face, wavelike, crossing and mixed stacking. Straightforward, the interlayered sliding occurs more and more difficultly in an order of face-to-face, wavelike, crossing and mixed stacking: from left to right in the figure (Figures 4a to 4d), the potentials representing the sliding barriers increase successively on the bottom plots. This straightforwardness is from an aspect of sliding hindrance.23 Moreover, as demonstrated at the top of the figure, the shapes of the Hirshfeld surfaces and the red dot distributions on the surfaces are also able to well indicate the sliding characteristic. Besides, the impact sensitivity assessing by means of sliding characteristic agrees with that by the interplay between electronic and vibrational contributions to the detonation process24.

Figure 5. Hirshfeld surfaces as a straightforward tool to correlate the crystal packing and impact sensitivity. The values in brackets are of H50 in m, showing a threshold height of a drop hammer of 2.5 kg causing 50 % detonation of a sample (Cited from Ref 13).

We confirmed that the Hirshfeld surfaces can sever as a straightforward tool to reflect packing structures and sliding characteristic through their shapes and the distributions of the red dots on themselves, i.e., as illustrated in Figure 5, the more block-shaped the surfaces and the more red dots concentrated on the block edges suggest the more perfect face-to-face π-π stacking and the lower / 34 ACS Paragon10Plus Environment

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impact sensitivity.13 Besides, it should be stressed that the big π-bonded molecules are necessary but not adequate to produce either a perfectly block-shaped Hirshfeld surface or more red dots on the block edges. For example, BTF is a planar molecule, while, its Hirshfeld surface is not perfectly block-shaped, with some concaves on surfaces and red dots dispersed in many orientations in the figure, due to no face-to-face π-π stacking.25 As a disadvantage of molecular packing, it contributes to the high impact sensitivity of BTF, despite its relatively high molecular stability, i.e., it is thermally decomposed at 285 oC. It is also the reason for the high impact sensitivity of many other big π-bonded molecule-contained EMs, like ICM-101.26

Figure 6. Sliding barrier contours of TATB showing the electrostatic (a), vdW (b) and total interaction energy (c) (in kJ/mol) versus the a-b fractional coordinates of the centroid of a TATB molecule in the unit cell (Cited from Ref 14).

Figure 7. Designed TATB-graphene (G) sandwich complex (top) and the sliding contour plots of the complex and its pure components (bottom) (Cited from Ref 28).



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Previously, we revealed the origin of the energy barrier for the shear sliding of the face-to-face π-stacked TATB.14 That is, the sliding energy barrier is mainly attributed to the variation of the electrostatic interactions, verified by the high similarity between sliding contours in Figures 6a and 6c. As illustrated in Figure 6b, the vdW interactions vary in a range of 29 kJ/mol, much less than that of electrostatic interactions of 105 kJ/mol (Figure 6a), i.e., the total energy variation ranging within 125 kJ/mol (Figure 6a) is mainly resulted from that of the electrostatic interactions. This sliding barrier is much lower than that of the TATB decay, above 300 kJ/mol, suggesting the feasible sliding of TATB along the (001) face without molecular decomposition. Besides, we also found that the denser π-electrons lead to the stronger π-π interactions.27 Accordingly, we designed a TATB/graphene (G) complex with a sandwich structure and a more lubricating ability relative to its pure components.28 Figure 7 exhibits that the TATB/G complex possesses the smallest energy barrier among the TATB/G complex and the two pure components. Furthermore, by adjusting the number proportion and the overlaying order of the two kinds of layers, we can hopefully obtain a series of complexes with adjustable properties. It could be an approach to molecular monolayers of explosives in micro devices.28

Figure 8. Demonstration of the crystal packing-impact sensitivity relationship perfected by extending the perfect π-stacking structures to more general layers (Cited from Ref 13). / 34 ACS Paragon12Plus Environment

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Obviously, the perfect face-to-face π-stacking is highly desired for impact insensitive EMs, in combination with high molecular stability. Nevertheless, in practice, the perfect face-to-face π-stacked EMs appear rare. Interestingly, for many EMs without perfect face-to-face π-stacking, there still exists a packing mode-sensitivity correlation, as lots of cases have exemplified that the sensitivity can be improved through crystal engineering, and the weak interlayered interactions and the strong intralayered interactions can sever more generally as a premise for interlayered sliding. In this case, the perfect face-to-face π-stacking is not necessary but enough for ready sliding. Thus, we extended the perfect face-to-face π-stacking structures to more general ones, with the difference maintained in strength between the intra- and interlayered interactions, i.e., intralayered interactions are much stronger than interlayered ones, as demonstrated in Figure 8.13 It will be exemplified later.

Figure 9. Parameterization ranges for LESHEMs: (a) classification of EMs, (b) d, (c) OB, (d) BDE, and (e) QNitro.

Besides above-mentioned crystal packing mode, some other indexes like OB, dissociation



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energy of the weakest bond (BDE) and nitro charge (QNitro) can also influence the energy and safety of EMs1,2 and therefore are employed as parameters to delimit LSHEMs. Obviously, these delimitations of d and other indexes derived from energetic molecules facilitate the molecular design of LSHEMs. We collected related indexes of several tens of traditional CHON contained EMs in Figure 9. As illustrated in Figure 9a, as a whole, the entire EMs exhibit the higher energy (D) going with the lower safety (H50). When a LSHEM defined by double requirements of H50≥0.5 m and D≥8.5 km, only FOX-7, LLM-105 and NTO (in Region A) among these traditional EMs satisfy the requirements and are LSHEMs. By statistics, d should be above 1.78 g/cm3 (Figure 9b), and OB, BDE and QNitro should be delimited within -50 to -29 %, 56 to 69 kcal/mol and -0.365 to -0.23 e, respectively (Figures 9c to 9e).29 It should be noted that, these delimitations based on the data of several tens of traditional EMs are rather rough and the strict ones require more extensive statistics. Still, it provides an idea to consider both the energy and safety, which usually contradict with each other. Overall, if energetic molecules in a crystal are stacked as intralayered interactions are distinguished to be much stronger than interlayered ones to favor ready sliding, this crystal could be impact insensitive. It is just the crystal packing-impact sensitivity relationship. Even though the case of perfect face-to-face π-stacking is ideal and rare, we should pay much attention to it, as such stacking facilitates much the low sensitivity.

■ STRATEGY FOR CREATING LOW SENSITIVE TRADITIONAL EMS In this section, we focus upon the strategy for creating low sensitive traditional EMs. Rationally, analyses and induction from the existing EMs should facilitate to develop a strategy for creating new ones. Usually, traditional EMs denote the EMs composed of neutral single component CHON-contained molecules, relative to current ECCs and EISs, as well as other element-contained


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energetic molecules. Similar to the common small organic molecules, these traditional energetic molecules build molecular crystals with rather weak intermolecular interactions. The weak intermolecular interactions suggest rather low lattice energy, and further only a few heat releases when the molecules consolidated to crystal. It shows the dominance of energy release of the chemical decay of component molecules, with a little compensation for separating molecules one another. This is the reason for that the enthalpies of formation of gas phase can represent those of solid phase by simplification in energy calculations. Regarding an energetic molecule, as a block building the entire crystal, its heat release is in principle related to its all chemical bonds; while, its decomposition kinetics mainly attributed to the strength of its weakest bond. That is, for one thing, it is expected to reduce bond energy to enhance the energy release; while for another thing, it is expected to enhance the strength of the weakest bond and thereby the safety. Therefore, averaging the bond strength is proposed to consider the above two things well. Obviously, the energetic conjugation structures are preferred.

Figure 10. Molecular structures of the low sensitive EMs and big π-bonds involved. C, H, O and N atoms, and intramolecular HBs are represented in grey, green, red and blue balls, and green dash, respectively. These representations are also considered in following figures (Cited from Ref 15).



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Figure 11. Molecular structures of the impact sensitive EMs, and sites pointed to by black arrows and BDE, represented by a bolded number of the weakest bond in each molecule (Cited from Ref 16).

Recently, we carefully investigated and compared the molecular stability of traditional energetic molecules. As demonstrated in Figures 10 and 11, the insensitive molecules are distinguished from the sensitive ones by planar molecular structures, big π-bonds, strong intramolecular hydrogen bonds (HBs), and large BDEs, indicative of higher molecular stability. These characteristics responsible for high molecular stability are not above the common knowledge of the traditional organic molecules. In a word, the stable energetic molecules set a base for building insensitive energetic crystals. However, the high molecular stability is achieved necessarily at the cost of lowered energy. This is the inherent energy-safety contradiction. Fortunately, the high molecular stability is not necessary for the impact insensitivity. It is just to alleviate the energy-safety contradiction to achieve LSHEMs by crystal packing.12 In fact, as concluded in above section, the face-to-face π-stacking facilitates the low impact sensitivity. In this case, the energetic molecules don’t necessarily require excellent molecular stability to contribute to the insensitivity. For example, DAAF (Figure 12c) possesses impact sensitivity close to those of TATB (Figure 12a) and DAAzF (Figure 12b), despite its much lower molecular stability. As a matter of fact, DSC measurements show that DAAF and TATB are decomposed at 250 and 371 oC, / 34 ACS Paragon16Plus Environment

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respectively, implying the much lower molecular stability of DAAF. Such no high molecular stability that causes high energy, and crystal packing close to face-to-face π-stacking, as well as high compactness, make a LSHEM feasible.

Figure 12. Three impact insensitive crystals with HB-aided face-to-face π-stacking (Cited from Ref 15).

Figure 13. Three impact insensitive high energy crystals with HB-aided wavelike π-stacking (Cited from Ref 15).

Further, another case is of three LSHEMs, NTO, FOX-7 and LLM-105 in Figure 13, which are more energetic than RDX, while more impact insensitive than RDX and TNT. In these three energetic crystals, the component molecules are wavelike π-stacked, instead of face-to-face πstacked. Nevertheless, due to the high molecular stability, a certain sliding feature of the wavelike



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π-stacking, and high crystal compactness, they are impact insensitive with high energy. It shows that the face-to-face π-stacking is not necessary for the low impact sensitive EMs. It is also the reason for that we extended the perfect π-stacking structures to more general layers (Figure 8). Meanwhile, the role of intralayered HBs shouldn’t be overlooked, since they support and hold the integrity of each layer. Thus, HBs with rather high strength are required for the low sensitivity EMs.14-16 Overall, due to neither strong HB donor (HBD) nor acceptor (HBA) in the traditional EMs, the intermolecular HBs are rather, even very weak. Nevertheless, these HBs are generally abundant in crystal and make them strong enough to support the layers, as demonstrated in Figures 12 and 13. Thereby, the HB-aided face-to-face π-stacking is thought to be the most ideal packing favoring low impact sensitivity. As the EMs arrayed in Figure 14, those with typical π-stacking are generally impact insensitive than those without.

Figure 14. Array of EMs according to their H50 (Cited from Ref 14).

Summarily, we propose that averaging the bond strength, and, in particular, avoiding any too weak bond to form conjugated structures (e.g., all nonhydrogen atoms are in a same big π-bond) are a base for creating high safety and highly energetic molecules. Meanwhile, the crystal packing close to HB-aided face-to-face π-stacking is preferred to LSHEMs.

■ STRATEGY FOR CREATING LOW SENSITIVE OR DESENSITIZED ECCs / 34 ACS Paragon18Plus Environment

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O O

H

O

O

O

O

H

H

O

Energetic Non-hydrogen contained molecule

+

O

H

H

O

H H

H

O

O

H

H

H

O

H

Energetic Hydrogenous molecule

EECC

Figure 15. Scheme showing the strategy to stabilize hydrogen-free energetic molecules by cocrystallizing with hydrogenous energetic molecules to form stable EECCs (Cited from Ref 30).

Maybe, it possesses a most remarkable sense of crystal engineering to creating new EMs by energetic cocrystallization. These new EMs are called ECCs. To avoid too much energy dissipation, the energetic-energetic cocrystals (EECCs) attract much more attention. The first case is the cocrystallization of a H-free energetic molecule, BTF, separately with hydrogenous energetic molecules including CL-20, DNB, MATNB, TNA, TNAZ, TNB and TNT, by intermolecular HBs (Figure 15). Relative to pure BTF, these BTF-based EECCs are desensitized.30-33 As expected, intermolecular HBs are formed in these cocrystals, belonging to O···H and N···H contacts.

5000 CED 4000

3000 2000 1000

0

Figure 16. Coherent energy density (CED) of the BTF crystal and the seven BTF-based EECCs (Cited from Ref 30). / 34 ACS Paragon19Plus Environment


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By cocrystallization, the intermolecular interactions are expected to be enhanced. To verify this, we checked the coherent energy density (CED) and formation energy of these EECCs. Relative to the pure BTF crystal, the HBs strengthen the intermolecular interactions since that HB is usually energetically favored than other intermolecular interactions like O···O, C···O and N···O, which are dominant in the BTF crystal. As expected, the CEDs of the EECCs in Figure 16 are largely enhanced. It shows that the introduction of hydrogenous coformer molecules to cocrystallize with the H-free BTF can increase its stability by increasing the intermolecular interactions. Also, we paid attention to the thermodynamics driving the BTF-based EECCs formation, with considering the lattice energy change after cocrystallization, the formation energy of EECCs (ΔE), by Pixel calculations. As a result, it was found that an evident internal energy reduction drives the formation, different from most other EECCs driven by the entropy increase. 30,34 Most of energetic-energetic cocrystallization proceeds from the existing energetic molecules. It implies that we will profit from utilizing numerous energetic molecules that have been synthesized already but have not been thought to be useful owing to their poor compatibility or stability. Among these overlooked molecules, there are an important group of H-free molecules. Many of these molecules such as HNB, ONC, DNF and DNOF feature high energy, making them the promising components to form high EECCs. That is, the excellence in energy of these compounds could be displayed in practice by the stabilizing effect of cocrystallization. On the other hand, these molecules, with high O contents and low stability, require to be stabilized by increasing intermolecular interactions like HBs. Therefore, as illustrated in Figure 15, it is a strategy to select appropriate energetic hydrogenous coformer molecules to maintain energy and stabilize the energetic H-free molecules. That is, to a certain extent, the disadvantages of low molecular stability could be overcome through cocrystallization. Thereby, it could make the highly energetic


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molecules that have ever been thought to be useless useful this time.

Figure 17. Numbering of the isowurtzitane cage of the CL-20 molecule (a), design of CL-20 based EECs built by hydrogen bonded synthons (b), and two newly-synthesized CL-20-based cocrystals based on the synthons, (c) and (d). The ground yellow and blue represent chains composed of CL-20 and coformers alternately, and CL-20, respectively. The HBs are denoted by purple dash (Cited from Ref 39).

Another strategy is to design and synthesize ECCs by means of the concept of synthons. Due to an advantage of high energy while a disadvantage of high sensitivity required for improvement, CL-20 is usually considered as a coformer of ECCs. As illustrated in Figure 17, the CL-20 molecule consists of a rigid isowurtzitane cage with six NO2 attached to each of the bridging N atoms. Six C atoms on the cage are usually weak HBDs in observed CL-20 polymorphs35 and CL-20-based cocrystals

36-39

. Four H atoms 3,5,9,11-sited on the cage are almost on a plane (Figure 17a). These

four H atoms are pairwise (3,11-sited or 5,9-sited) close to each other, for instance, with a distance of 2.27 Å in β-CL-20, suggesting that they will share one HBA once the HBA positions moderately between them, as an R21 (5) HB formed. That is to say, for all the four H atoms, two R21 (5) HBs can be formed. Therefore, from the viewpoint of constructing H-bonded synthons in binary ECCs and considering the two R21 (5) HBs, a coformer with two HBAs is required. And a supramolecular chain will be extended in crystal by the C-H…A interactions, in the case of a coformer molecule containing double HBAs with a bilateral symmetry (Figure 17b). According to this, two cocrystals,



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CL-20/para-benzoquinone and CL-20/1,4-naphthoquinone, were designed and synthesized successfully.39 As demonstrated in Figures 17c and 17d, they both possess the synthons as expected. Due to the non-energetics characteristic, both cocrystals are largely desensitized, relative to the pure ε-CL-20.35

Figure 18. Comparison in impact sensitivity among the DADP-based cocrystals and their pure components, cited from Refs 40 and 41.

Figure 19. Crystal packing of the cocrystals of DADP/TXTNB (X=Cl, Br and I): from top to bottom, overall crystal packing, intralayered structures and interlayered structures. The dashes show the intermolecular interactions and are distinguished by blue and green for intra- and inter-layered ones (Cited from Ref 42).


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As a strategy for promoting EMs, it is necessary to reveal the underlying mechanisms of ECCs responsible for the varied packing structures and properties, relative to their pure components, in particular for some exceptions. For example, as illustrated Figure 18, the cocrystal of DADP/TITNB (3) 40 appears to be much desensitized in contrast to its both pure components, much different from a usual case that the impact sensitivity is mediated by the cocrystllization. Recently, we focused on the underlying mechanism responsible for the remarkably decreased impact sensitivity of 3, as well as impact sensitivity of its cocrystal analogs of DADP/TCTNB (1) and DADP/TBTNB (2) 41 for comparison42. The reason for the much impact sensitivity reduction of DADP/TITNB is attributed to that the crystal packing is much improved to facilitate ready slide.42 That is, in contrast to pure components, the enhanced intermolecular interactions and their anisotropy are the main reason for the reduced impact sensitivity of DADP/TITNB. We employed this result to stress the concepts of intra- and interlayered interactions for discussing shear sliding characteristic, because the π-π stacking is not evident in this case as in traditional EMs.14,15 Obviously, these concepts are more general than the π-π stacking. As illustrated in Figure 19, for DADP/TCTNB, it seems that the intermolecular interactions along the sliding layers, i.e., intralayered interactions (along the molecular plane of TCTNB) are not evidently stronger than the interlayered ones, implying the difficult shear sliding. The case of DADP/TBTNB is the same as that of DADP/TCTNB. While, for DADP/TITNB, its crystal packing is much different from that of DADP/TCTNB or DADP/TBTNB. Comparing Figures 19a and 19b with 19c, it is ready to find the evident layered packing of DADP/TITNB along the molecular plane of TITNB, and the intralayered interactions (among the TITNB and DADP molecules) remarkably stronger than the interlayered ones (among the DADP molecules themselves), with a strength ratio of 2.3:1, much above that of DADP/TCTNB (1.3) or



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DADP/TBTNB (1.4).42

Figure 20. Crystal packing (a) and intralayered HBs (b) of BTO·H2O.

During preparing TKX-50, 43 it was found that its neutral precursor BTO can be solvated by water to form graphite-like stacking. Interestingly, Figure 20 exhibits the single atomic thickened layers (SATLs) of BTO·H2O, a solvate, also a cocrystal. Due to such SATLs with ready sliding characteristic, BTO·H2O is impact insensitive. A recent H50 measurement of BTO·H2O of above 126 cm (hammer weight, 2 kg; sample, 30 mg) shows the impact insensitivity as expected, much different from a neutral tetrazole-contained molecule like BTO that is generally impact sensitive.12 The crystal packing of BTO·H2O richens our knowledge of constructing low sensitive high EMs with different components. Even though the enhanced intermolecular interactions, or the elevated lattice energy, can decrease ΔE1 a little, the impact sensitivity can remarkably be improved. This is a part of the crystal engineering of EMs. Summarily, stacking different kinds of molecules together to enhance intermolecular interactions and their anisotropy is proposed as a strategy for building low impact sensitivity EMs, as a main objective of the crystal engineering of EMs. In particular, the SATLs is preferred for impact insensitivity. / 34 ACS Paragon24Plus Environment

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■ STRATEGY FOR CREATING LOW SENSITIVE OR DESENSITIZED EISs Besides above-mentioned traditional EMs and ECCs that are composed of neutral molecules, the salts that consist of ions can also be EMs. Currently, ionic liquids are thriving, due to their unique properties such as low vapor pressure, liquidity over a wide temperature range, ionic conductivity, structural designability, high thermal stability, the ability to dissolve a wide range of chemical species, and so forth. Ionic liquids are one of the most exciting scientific discoveries in chemical science and are prevalent in nearly every branch of chemistry and material science. Among the ionic liquids, some of them are energetic and therefore are called as energetic ionic liquids (EILs). EILs have emerged as a new class of energetic materials, with environmental friendliness, low-melting points, and thermally stability. 44 There are two main advantages of using EILs for energetic applications, that is, extremely low vapor pressure, and structural designability and property’s modularity. EILs may be a good energetic source as propellants 45

Figure 21. Comparison in decomposition temperature (Td) of pure HA and HA-based EISs.

In this Perspective, we focus upon the solid EISs under common conditions, which are seldom molten below 100 oC. Relative to neutral molecules, ionization can benefit EMs, as it can / 34 ACS Paragon25Plus Environment


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increase molecular stability, enhance intermolecular interactions and packing densities. Also, it can increase quickly the quantity of EMs by combining various cations and anions in pairs. Thereby, the population of EISs becomes larger and larger. For the stability enhancement by the ionization, it can be exemplified by that of HA. As shown in Figure 21, the HA-based EISs each features a higher decomposition temperature (Td) than the pure HA, i.e., in contrast to the Td of pure HA, 33 oC, those of the HA-based EISs are significantly enhanced, and even some of them, like BT-HA, DNBTO-HA, BTO-HA, AFTA-HA and BNT-HA, possess Td above 200 oC.

Figure 22. Comparison in energy barriers for decaying isolated and condensed HA and HA-based EISs. The dashed barriers show the various possibilities of the barriers (Cited from Ref 46).

We clarified the underlying mechanism of the significant Td enhancement of HA-based EISs.46 As illustrated in Figure 22, the pure HA is decayed with the lowest energy barrier; while, the decay of both an isolated HA molecule and an isolated H-HA+ (NH3OH+) requires much more energy to break their weakest bonds of N-O; and in the most HA-based EIS crystals, it is a prerequisite to overcome an barrier much above that for decaying pure HA to implement proton transfer, followed by the decay of the H-transferred products. Therefore, relative to the pure HA, the thermal stability of the HA-based EIS crystals increases. Based on this finding, we proposed a strategy for stabilizing unstable chemical species like HA, by the ionization of unstable neutral


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molecules and the separation of homogenous molecules. A currently synthesized cycle-N5 can exist above 100 oC, also due to the ionization and separation. That is, there are two determining factors for stabilizing cycle-N5: N5- is more stable than HN5, and N5- is separated and supported by intermolecular HBs.47

Figure 23. Populations of the close interatomic contacts of HA+ in HA-based EIS crystals and of neutral energetic molecules in their crystals (Cited from Ref 48).

As mentioned above, ionization can enhance intermolecular interactions like HBs. Figure 23 confirms the higher close interatomic contact populations of HBs in the HA-based EISs than in traditional energetic crystals. As illustrated in the figure, the sum contact populations of HBs of O…H and N…H in the HA-based EISs are mostly above 80%, even close to 90 %, whereas those in traditional energetic crystals are below 70%, even below 40% for ε-CL-20. In contrast to the traditional energetic crystals and ECCs, the contribution of N…H contacts is more evident in HA-based EISs than in traditional energetic crystals because of the higher N-contents of the anions in EISs. These enhanced HBs facilitate to increase packing density and further the energy of EMs.48



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Figure 24. Crystal packing (a) and intralayered HBs (b) of HA-AFTA. And the double arrows pointing to the preferred sliding.

Besides, ionization can improve molecular stacking to be close to face-to-face π-π stacking, to favor ready sliding and further impact sensitivity. As illustrated in Figure 24a, the AFTA molecules in HA-AFTA are a little wavelike π-stacked with the aid of intermolecular HBs (Figure 24b).49 Even though the interlayered HBs exist, they are much more spatially sparse than the intralayered ones, suggesting the much weaker interlayered HBs and the ready interlayered shear sliding. It contributes to the low impact sensitivity of HA-AFTA of above 50 J, close to that of TATB, even though it can thermally be analyzed at a common temperature of 213 oC. Besides HA-AFTA, similar crystal packing leading to low impact sensitivity is also found in other EIS.50,51 Overall, the ionization of energetic molecules facilitates to increase molecular stability, and enhance intermolecular HBs to lower impact sensitivity and increase the energy. It can also improve crystal packing mode to favor lower sensitivity. Thus, the energetic ionization is also a kind of crystal engineering of EMs.

■ CONCLUSIONS / 34 ACS Paragon28Plus Environment

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It has been made great progress in the crystal engineering of EMs, despite many challenges. The progress includes the clarifications of the energy-safety contradiction and the crystal packing-impact sensitivity relationship, and strategies for creating highly stable and highly energetic molecules, low sensitive traditional EMs, ECCs and EISs. Besides, the crystal engineering of EMs for other purposes are also going ahead. 52-55 By means of crystal engineering, the rationality and efficiency of developing EMs are expected to largely increase. Nevertheless, some aspects requires further insight, such as accurate descriptions of molecular-crystal structure relationships, intermolecular interactions, accurate predictions of crystal structures, high-quality crystal manufactures, and so on. Overall, the crystal engineering of EMs is on the way. Symbols AFTA AG

Full name 4-amino-furazan-3-yl-tetrazol-1-olate aminoguanidinium

BNT

bis[3-(5-nitroimino-1,2,4-triazolate)]

BT

5,5′-bistetrazolate

BTO

5,5'-bistetrazole-1,1'-diolate

BT2O

5,5′-bis(tetrazole-2-oxide)

BTF CL-20

benzotrifuroxan 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane

DAAF DAAzF DADP DAG

trans-(d,d)-3,3′-diamino-4,4′-azofurazan trans-(p,p)-3,3′-diamino-4,4′-azofurazan diacetone diperoxide diaminoguanidinium

DATB DBO DNAAF

1,3-diamino-2,4,6-trinitrobrnzene 5,5′-dinitromethyl-3,3′-bis(1,2,4-oxadiazolate) 3,3′-dinitroamino-4,4′-azoxyfurazanate

DNABF DNBTO

3,3′-dinitramino-4,4′-bifurazane 3,3′-dinitro-5,5′-bis-1,2,4-triazole-1,1-diolate

DNDP DNP DPNA

4,6-dinitro-1,3-diphenol 2,4-dinitrophenol N-(3,4-dinitro-1H-pyrazol-5-yl)nitramidate

FOX-7 G

2,2-dinitroethylene-1,1-diamine guanidinium

HA

hydroxylammonium



HMX HNAB HNB ICM-101 LLM-105 NG NQ NTO NTX

1,3,5,7-tetranitro-1,3,5,7-tetrazocane 2,2´,4,4´,6,6´-hexanitro-trans-azobenzene hexanitrobenzene [2,2′-bi(1,3,4-oxadiazole)]-5,5′-dinitramide 2,6-diamino-3,5-dinitro-1,4-pyrazine 1-oxide nitroglycerin 2-nitroguanidine 5-nitro-2,4-dihydro-3h-1,2,4-triazol-3-one 5-nitrotetrazolate-2N-oxide

ONC ONDO PA PETN PNA RDX TAG

octanitrocubane 1,1,1,3,6,8,8,8-octanitro-3,6-diazaoctane picric acid pentaerythritol tetranitrate pentanitroaniline 1,3,5-trinitro-1,3,5-triazinane triaminoguanidinium

TATB TBTNB TCTNB TETNA TETNB Tetryl TITNB TKX-50 TNA TNAP TNAZ TNB TNDP TNT TNTP

1,3,5-triamino-2,4,6-trinitrobenzene 1,3,5-tribromo-2,4,6-trinitrobenzene 1,3,5-trichloro-2,4,6-trinitrobenzene 2,3,4,6-tetranitroaniline 1,2,3,5-tetranitrobenzene 2,4,6-trinitro-n-methyl-n-nitroaniline 1,3,5-triiodo-2,4,6-trinitrobenzene hydroxylammonium-5,5′-bistetrazolate 2,3,4,6-tetranitroaniline 4-amino-2,3,5-trinitrophenol 1,3,3-trinitroazetidine 1,3,5-trinitrobenzene 2,4,6-trinitro-1,3-diphenol 2,4,6-trinitrotoluene 2,4,6-trinitro-1,3,5-triphenol

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■ Information of Authors Corresponding Author C. Y. Zhang, email: [email protected]; Tel: 86-816-2493506. Notes The authors declare no competing financial interest.

■ ACKNOWLEDGMENT The authors thank a lot for the support of the Science Challenge Project (TZ-2018004) and the / 34 ACS Paragon30Plus Environment

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National Natural Science Foundation of China (21673210).

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(23) Zhang, C.; Xue, X.; Cao, Y.; Zhou, Y.; Li, H.; Zhou, J.; Gao, T. Intermolecular Friction Symbol Derived from Crystal Information. CrystEngComm, 2013, 15, 6837-6844. (24) Adam A.L. Michalchuk, Colin R. Pulham and Carole A. Morrison. The Big Bang Theory: Towards Predicting Impact Sensitivity of Energetic Materials. In: Proc. 49th Int. Conference of ICT, Karlsruhe, v17, 2018. (25) Cady, H. H.; Larson, A. C.; Cromer, D. T. The Crystal Structure of Benzotrifuroxan (Hexanitrosobenzene). Acta Crystallogr, 1966, 20, 336. (26) Zhang, W.; Zhang, J.; Deng, M.; Qi, X.; Nie, F.; Zhang, Q. A Promising High-Energy-Density Material. Nat. Communications, 2017, 8, 181-187 (27) Zhang C. Shape and Size Effects in π-π Interactions: Face-to-Face Dimers. J. Comput. Chem., 2011, 32, 152-160. (28) Zhang C.; Xia C.; Bin X. Sandwich Complex of TATB/Graphene: an Approach to Molecular Monolayers of Explosives. J. Phys. Chem. C, 2010, 114, 22684-22687. (29) OB, BDE and -QNitro were calculated using the equation of OB = z − (2 x + 1 / 2 y )  100% for an energetic 2x + 1/ 2 y

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For Table of Contents Use Only Crystal Engineering for Creating Low Sensitivity and Highly Energetic Materials Chaoyang Zhang, Fangbao Jiang, and Hongzhen Li

This Perspective focuses upon the current progress in the clarifications of the energy-safety contradiction and the crystal packing-impact sensitivity relationship, and proposes strategies for creating new LSHEMs or desensitized EMs, including traditional EMs composed of neutral simple-component molecules, energetic cocrystals (ECCs) and energetic ionic salts (EISs), by crystal engineering.


Crystal Engineering for Creating Low Sensitivity ... - ACS Publications

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