Crystal Packing of Impact-Sensitive High-Energy ... - ACS Publications

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Crystal Packing of Impact-Sensitive High-Energy Explosives Yu Ma,† Anbang Zhang,†,‡ Xianggui Xue,† Daojian Jiang,† Yuanqiang Zhu,‡ and Chaoyang Zhang*,† †

Institute of Chemical Materials, China Academy of Engineering Physics (CAEP), P.O. Box 919-327, Mianyang, Sichuan 621900, China ‡ College of Chemistry and Chemical Engineering, Southwest Petroleum University, Chengdu, Sichuan 610500, China S Supporting Information *

ABSTRACT: Molecular and crystal designs are crucial to the engineering of high-energy explosives, which are a class of substantial materials usually with high costs and high risks. Understanding their structures, properties, and performances, and the relationships among them is the basis for the design. As a continuation of a systemic analysis of the crystal packing of low-sensitivity and high-energy explosives (LSHEs) (Cryst. Growth Des. 2014, 14, 4703−4713), we present in this work another analysis of 10 existing impact-sensitive high-energy explosives (SHEs), which possess both velocities of detonation and impact sensitivity close to or higher than those of RDX. We find that SHE molecules are usually less stable than LSHE ones, due to the deficiencies of big π-conjugated molecular structures, and adequate and strong intramolecular hydrogen bonds (HBs) even though H atoms are contained. The intermolecular HBs cannot be formed sometimes in H-contained SHE crystals, and the noncovalent O···O interactions dominate the connection of SHE molecules to build a three-dimensional network and hold crystals, generally, with the strength above intermolecular HBs. The absence of single-atom-layered stacking in SHE crystals makes the intermolecular sliding difficult or even unallowed when against impact, which leads to inefficiency of energy buffering and ease of molecular decay, hot spot formation, and final combustion or detonation. In contrast to LSHEs, SHEs are disadvantageous on dual structural levels causing their high sensitivity: molecules with low stability and crystals without HB-aided single-atom-layered stacking. It re-verifies that the intermolecular HB-aided π−π stacking is necessary for crystal engineering of LSHEs, which are highly desired currently.

1. INTRODUCTION Nowadays, one of the aims of energetic crystal engineering is to obtain explosive crystals with both high energy and high safety. Energy and safety are the two most important properties of explosives, representing respectively their power and sensitivity to external stimuli, which are usually in conflict with each other, i.e., higher energy usually goes with lower safety. Therefore, a compromise or a balance of the energy−safety contradiction appears generally in practical applications. For instance, in most cases, the safety of an explosive is improved at the cost of energy decrease.1 Crystal packing is crucial to both the energy and the safety. In general, the energy is denoted by detonation properties like velocity of detonation (VOD), which is predominated by chemical component and density and can be well predicted using detonation equations.2,3 Because a given crystal structure gives ascertained chemical component and density, the energy of an explosive (VOD) can be exactly predicted if its crystal structure is provided. On the other hand, the safety is influenced by the structure of the explosive, and the response of the explosive is variable depending on the external stimuli.4 The structures are of multihierarchies including molecule, crystal, surface-interface, and bulk block. Molecular stacking modes in crystal structures can influence the safety largely. For © XXXX American Chemical Society

example, explosive crystals with intermolecular hydrogen bonds (HBs) aided π−π stacking are usually impact-insensitive.5−9 TATB, DAAF, and DAAzF possess this kind of stacking mode and are very insensitive to external mechanical stimuli, even though DAAF and DAAzF have molecular stability close to HMX with much higher sensitivity to mechanical stimuli.10 Also, various sensitivity measurement values of explosive polymorphs verify the influence of crystal packing on the safety. For instance, in the HMX polymorphs, there are two kinds of molecular conformers, chair for the β-form and chair− chair for the α- and δ-forms. A theoretical calculation showed a small molecular stability difference between these two kinds of conformers,11 while the impact energy of β-, α-, and δ-HMX are much different, 0.75, 0.2, and 0.1 kg/cm2, respectively.12 The CL-20 polymorphs possess molecular conformers different from one another, the so-called β-, ε-, and γ-forms in the β-, ε-, and γ-polymorphs correspondingly, in terms of the orientations of the NO2 groups in the molecules.13 The impact sensitivity of β-, ε-, and γ-formed CL-20 is different from one another too.14 Conclusively, crystal packing is one of the most important Received: August 25, 2014 Revised: September 23, 2014

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dx.doi.org/10.1021/cg501267f | Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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Table 1. VOD and Edr of TATB and SHEs

a

explosives

TATB

ONDO

PETN

TNAZ

RDX

β-HMX

BCHMX

BTF

HNB

ε-CL-20

ONC

VODa (m/s) VODb (m/s) Edr (J)

8177 8640 >78.44,33 120.234

9416 9075 6.084

8406 8228 3.4−4.933 3.2−3.933

9005 8891 13.724

8843 8789 7.8433 6.8633 8.0435 5.8836 7.4537

9153 9198 7.3533 7.8433

9030 8907 2.9838

8789 8855 5.154

9597 9148 2.7039,40

9374 9753 13.134

9760 9274 c

Calculated using the VLW method.3 bCalculated using EXPLO5, version 6.02.32 cONC cannot exist above zero degree.31

molecules lack intramolecular HBs and planar big π-conjugated molecular geometric structures, which causes their low molecular stability. Also, the intermolecular HB aided π−π stacking does not appear in the SHE crystals. It will, when against external mechanical stimuli like impact, make the intermolecular sliding forbidden, no energy dispersed by the slide, and the molecular decomposition, hot spot formation, and final combustion and detonation easy. This is a reason SHEs have a higher impact sensitivity than LSHEs. Conclusively, SHEs are more sensitive than LSHEs due to their structural inferiorities: inferior molecular stability and no intermolecular HB aided π−π crystal stacking.

factors influencing the energy and the safety, and for a polymorphic explosive, it seems that the more compactly stacked forms are welcome for high energy and high safety in most cases. It has been verified that a way to get new explosives with improved properties is by rearranging existing explosive molecules, i.e., the so-called energetic cocrystallization,15−22 and it has been confirmed there are small changes of former molecules after cocrystallization (for example, CL-20-based cocrystals),22 which suggests that the improved properties are mainly attributed to the changes of crystal packing, instead of those of former molecules. Therefore, a comprehensive study on the crystal packing of observed one-component explosives is requisite for the engineering. Our recent work showed that the crystal packing of low-sensitivity and high-energy explosives (LSHEs) features HB-aided π−π stacking. In LSHEs, big πbonded molecules are stacked in crystals in four modes: face-toface, wave-like, crossing and mixing, which cause different interlayer sliding features and therefore different impact sensitivities.10 As a continuation, we focus on the crystal packing of impact-sensitive high-energy explosives (SHE) in this work. Similar to LSHEs, there is no definition of SHEs. Considering that RDX has extensively been regarded as the representative of the second generation of modern explosives, we take it as a reference for defining SHEs roughly. At the same time, because impact is a common stimulation style, the impact sensitivity is a main indicator showing explosive safety.4 Usually, besides the nature of an explosive, different instruments and techniques can supply various impact sensitivity values.4 Therefore, it should be to unify these various values into the energy of drop (Edr) to compare the sensitivity (safety) conveniently. In this work, we will regard an explosive as a SHE when its VOD is close or superior to RDX and its Edr is close to or lower than RDX. Different from LSHEs, SHEs possess a common trend of the energy and the safety of whole explosives (i.e., the higher energy goes with the lower safety) and therefore have bigger yields. Here, we selected 10 SHE representatives in terms of molecular structures including chain (ONDO23 and PETN24), nonconjugated ring (TNAZ,25 RDX,26 β-HMX27 and BCHMX28), conjugated ring (BTF29 and HNB30), and cage (ε-CL-2013 and ONC31). Crystallographic information on these 10 SHEs is listed in Table s1 of Supporting Information. Table 1 contains the SHEs which possess energy close to or in excess of RDX and Edr close to or lower than RDX. Meanwhile, a most typical LSHE, TATB, is also selected to differentiate readily crystal packing of LSHEs and SHEs. After analyzing the crystal packing as well as molecular structure of the SHEs, we find that, in contrast to LSHEs, SHE

2. RESULTS AND DISCUSSION As noted before, energy and safety (sensitivity) are two main concerns of explosives. Numerous research has shown that

Figure 1. (a−k) Molecular structures of the SHEs, and sites (pointed to by black arrows) and dissociation energy (BDE, represented by a bolded number) of the weakest bond in a molecule. Gray, green, blue, and red balls denote carbon, hydrogen, nitrogen, and oxygen atoms, respectively. Similar representations of atoms are considered in the following figures.

many factors can influence them, including chemical energy storage in a molecular crystal, crystal packing and density, oxygen balance (OB) within the molecular structure, intermolecular hydrogen bonding in molecular crystals, longand short-range interactions, energy dissipation mechanisms, molecular motion within a crystal lattice, intramolecular and intermolecular bond breaking, voids, defects, dislocations, energy localization, decomposition, initiation, and so on. In B



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the seven hydrogen-containing SHE molecules, it is interesting to confirm only two weak intramolecular HBs in the ONDO molecules, each with EHB of about 5 kJ/mol. This is in accord with the previous confirmation of no intramolecular HB in the β-HMX crystal.41 In the ONDO molecule, there are eight hydrogen atoms, and only two of them take part in the HBs (see Figure s1 and Table s2 of Supporting Information). That is to say, only a small part of hydrogen atoms (1/4) can form the HBs. This case seldom appears in LSHEs. A careful examination of 11 existing LSHEs indicates only TNB without intramolecular HB.10 It shows that the intramolecular HBs are unfavorable in SHEs, which is disadvantageous to stabilize molecules. It should be a reason for their high sensitivity against mechanical stimuli like impact. Second, among all 10 SHE molecules, only BTF possesses a big conjugated bond π24 18, composed of all atoms in the entire molecule, and the remaining SHE molecules are without planarity. It seems that there is a plane consisting of all atoms of another SHE molecule HNB (Figure 1i). However, all its six nitro groups are in fact deviated from the benzene ring plane, which makes the HNB molecule like a scroll wheel as indicated in Figure 2. As stated previously,10 for LSHEs, like TATB, all non-hydrogen atoms in an entire molecule are π-bonded, exhibiting a molecular plane. The π-bonded molecular planes usually help LSHEs decrease shear stress when they undergo impact.5−8 On the other hand, from the dissociation energy (BDE) values42 of the weakest bonds43 of the SHEs in Figure 1, we can conclude that SHE molecules are usually less stable than LSHE ones. For example, a typical LSHE TATB molecule has a much higher BDE (314 kJ/mol) than all the SHE molecules. Even more, BTF possesses a very low BDE of 92 kJ/mol only. This should be a reason causing its high sensitivity despite its big conjugated molecular structure with a certain stability and its application as a primary explosive.4 HNB possesses the largest BDE among all SHE molecules, 230 kJ/mol, closest to that of

Figure 2. Front view (a) and side view (b) of the HNB molecule along its benzene ring plane.

this work, we will discuss the analyzed results by four sections: molecular structures, intermolecular hydrogen bonding, intermolecular O--O interactions, Hirshfeld surface analyses, and packing effect on impact sensitivity. Comparing these four contents of LSHEs and SHEs, we can understand qualitatively their differences in energy and sensitivity. 2.1. Molecular Structures. Molecular structures of the 10 SHEs, as well as TATB, are illustrated in Figure 1. In contrast to LSHE molecules like TATB, there are two obvious differences for SHE molecules. The first difference is whether hydrogen atoms are contained. For LSHE molecules, a previous careful examination confirmed that hydrogen atoms are always contained,10 whereas for SHEs, as shown in Figure 1, hydrogen atoms, for example, are not contained in BTF, HNB, and ONC. It suggests that in SHEs, intra- or intermolecular HBs exist unnecessarily, much different from LSHEs. In recent work, we found that intramolecular HBs are in most LSHEs and intermolecular HBs are in all LSHEs,10 and the intramolecular HBs are usually strong; for instance, in TATB, the total dissociation energy (EHB) of intramolecular HBs is up to 309 kJ/mol (about 52 kJ/mol per HB) in terms of AIM analyses,10 which leads to a very stable molecule with a high DTA decomposition temperature of 374 °C.4 After AIM analyzing

Figure 3. ESP of TATB (a) and the SHEs (b−k) mapped onto the molecular surfaces of ρ = 0.001 au and minimum ESP (MESP, unit in kcal/mol). C



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Figure 4. Face-to-face stacking (a) of TATB and intermolecular hydrogen bonds (b) represented by green dashes.

Figure 5. (a−f) Intermolecular hydrogen bonds in SHEs represented by green dashes.

D



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Table 2. Geometry and AIM Analyses of the Intermolecular HBs in Crystals explosive

D

H

A

D−H, Å

H···A, Å

A···D, Å

A−H···D,°

symmetry operation

ρ, e/Å3

EHB, kJ/mol

ΣEHB/2, kJ/mol

TATB

N4 N6 N2 N6 N2 N4 C2 C3 C1 C1 C1 C3 C2 C2 C3 C1 C1 C2 C1 C2 C2 C2 C3 C1 C5

H4 H6 H2 H5 H1 H3 H1 H3 H6 H7 H2 H6 H4 H4 H5 H1 H1 H4 H2 H3 H3 H2 H4 H1 H5

O1 O3 O5 O4 O6 O2 O7 O8 O5 O4 O1 O2 O1 O2 O5 O1 O2 O2 O4 O1 O4 O6 O5 O6 O1

1.054 0.954 0.849 0.868 0.965 0.757 0.927 0.901 1.140 1.14 1.091 1.075 1.087 1.087 1.087 1.110 1.110 1.094 1.091 1.101 1.101 0.970 0.981 0.937 0.933

2.239 2.348 2.381 2.371 2.392 2.396 2.554 2.590 2.325 2.443 2.483 2.530 2.589 2.462 2.523 2.528 2.588 2.360 2.594 2.479 2.566 2.431 2.531 2.439 2.517

2.929 2.933 2.951 2.991 2.991 2.99 3.402 3.495 3.456 3.468 3.455 3.280 3.582 3.327 3.190 3.223 3.639 3.369 3.521 3.464 3.429 3.356 3.017 3.177 3.360

121.3 119.1 125.0 128.7 126.7 136.3 152.3 173.2 179.5 148.8 147.8 126.1 151.5 135.7 118.6 119.4 157.4 152.6 142.3 148.3 134.6 159.6 110.4 135.7 150.4

−1+x, y, z; 1+x, y, z; −1+x, −1+y, z; 1+x, 1+y, z; x, −1+y, z; x, 1+y, z; −1+x, −1+y, z; 1+x, 1+y, z; x, −1+y, z; x, 1+y, z; −1+x, y, z; 1+x, y, z; −1+x, y, z; 1+x, y, z; −1+x, y, z; 1+x, y, z; 1/2−y, 1/2−x, 1/2+z; 1/2−x, 1/2−y, −1/2+z; −y, x, 1−z; y, −x, 1−z; 1/2−x, 1−y, −1/2+z; 1/2−x, 1−y, 1/2+z; 1/2−x, 1−y, −1/2+z; 1/2−x, 1−y, 1/2+z; 1/2−x, −1/2+y, z; 1/2−x, 1/2+y, z; 1/2−x, −1/2+y, z; 1/2−x, 1/2+y, z; −1/2−x, y, 1/2−z; 1/2−x, y, 1/2−z; −1−x, −y, −1−z; (2) −1+x, y, z; 1+x, y, z; −1−x, −y, −z; (2) x, 1/2−y, −1/2+z; (2) −1−x, −1/2+y, −1/2−z; −1−x, −1/2+y, −1/2−z; −1−x, −1/2+y, −1/2−z; −1−x, −1/2+y, −1/2−z; −1+x, y, z; 1+x, y, z; 1−x, −1/2+y, 1−z; 1−x, 1/2+y, 1−z; 1−x, 1−y, 2−z; (2) 1+x, y, z; −1+x, y, z;

0.01399 0.01173 0.01054 0.01047 0.01019 0.00956 0.00756 0.00624 0.00611 0.00890 0.00708 0.00667 0.00758 0.00985 0.00751 0.00840 0.00697 0.01128 0.00694 0.00795 0.00664 0.00749 0.01001 0.00851 0.00696

12.8 10.7 9.6 9.4 9.1 8.6 5.8 4.7 5.0 7.1 5.7 5.6 6.5 8.3 6.8 7.0 5.4 9.1 6.0 6.3 5.4 6.0 9.1 7.0 5.4

60.2

ONDO PETN RDX

β-HMX

BCHMX ε-CL-20

Vlattice

ONDO PETN TNAZ RDX HMX BCHMX BTF HNB CL-20 ONC

843.156 589.495 1371.253 1633.856 519.387 524.826 880.642 1162.984 1424.146 1558.173

TATB NQ DAAzF DAAF DATB DNDP NTO TNA FOX-7 LLM-105 TNB TNT

442.524 1570.616 368.573 403.758 439.136 741.145 902.06 854.426 519.47 748.155 3377.726 1823.555

Vmolecule SHE 312.325 223.905 131.510 153.803 205.178 194.027 159.524 221.087 274.337 285.773 LSHE 175.754 78.357 139.325 146.059 170.557 140.123 87.891 158.884 103.869 146.616 150.661 165.255

Nmolecule

PC

2 2 8 8 2 2 4 4 4 4

0.74 0.76 0.77 0.75 0.79 0.74 0.72 0.76 0.77 0.73

2 16 2 2 2 4 8 4 4 4 16 8

0.79 0.80 0.76 0.72 0.78 0.76 0.78 0.74 0.80 0.78 0.71 0.72

12.1 32.9

39.2

15.1 12.4

LSHE molecules like TATB. In the SHE molecules, because the nature of O atoms is electron rich, the more negative charges on them suggest a more stable molecule. This has been verified using nitro group charges to be indicative of the stability of nitro compounds.34 As illustrated in Figure 3, the negatively charged regions (in red) are located around the O atoms of SHE molecules, exhibiting the electron-rich nature as expected. Also, the minimum ESPs (MESPs)47 on the electron density isosurfaces give a qualitative tendency of the more negative MESP corresponding to the lower sensitivity: TATB with the most MESP, −26.5 kcal/mol, is much more stable than the SHEs, whereas ONC with the least MESP, −7.8 kcal/mol only, cannot exist above zero degree,31 and the remaining SHE molecules possess stability between ONC and TATB, as their MESPs are between those of ONC and TATB too. To summarize briefly, in contrast to LSHE molecules, SHE molecules are usually less stable, attributed to the deficiency of big π-conjugation, intramolecular HBs, and negative charges on O atoms. 2.2. Intermolecular Hydrogen Bonding. To compare the intermolecular HBs in SHEs and LSHEs conveniently, we first exhibit the molecular stacking and intermolecular HBs of insensitive TATB in Figure 4. As illustrated in the figure, the intermolecular HBs hold the single atomic layers of TATB.10,48 Or the intermolecular HBs of TATB exist in its layers. Among all seven hydrogen-contained SHEs, it is interesting to find there is no intermolecular HB in TNAZ crystal by AIM analysis. This case has not been found in the existing LSHEs.10 As shown in Figure 5, the intermolecular HBs link molecules three-dimensionally to form network and compose crystals, much different from TATB crystal in which the intermolecular HBs connect neighboring molecules to build layers parallel to one another. This connection mode will make the intermolecular slide very difficult, in that the slide along any orientation

Table 3. Packing Coefficients (PC) of SHEs and LSHEs explosives

10.5

TATB. But it is impact sensitive too, due to its scroll wheel-like molecular shape, which leads to a high energy barrier or even molecular decay when it suffers from impact and intermolecular slide occurs. This will be discussed later. Besides, from the viewpoint of electrostatic potentials (ESPs)44,45 on isosurfaces of electron density46 of the SHE molecules, we can also find that they are less stable than the E



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Figure 6. (a−f) Intermolecular O···O contacts represented by red dashes in SHEs with intermolecular hydrogen bonds.

2.3. Intermolecular O···O Interactions. Next, we are concerned about the intermolecular O···O interactions in the SHEs. OB can largely influence explosive performances.49 Generally, a deficient OB results in a smaller VOD (generally a poorer performance), while an excess oxygen balance also leads to nonoptimal performance. At the same time, it was stated that the oxygen balance increase leads to the sensitivity increase.49 Relative to LSHEs, it is true that there are more oxygen atoms exposed on the molecular external of SHEs. Obviously, the intermolecular O···O interactions become of interest, since the intermolecular contacts occur through the O···O contacts. In fact, they have been the objectives of much research on explosives.41,50−58 Presumably, as exhibited in Figure 3, the negative charges on the oxygen atoms will decrease the packing coefficients (PCs) of SHEs, due to electrostatic repulsion. That is to say, to a large extent, more O···O repulsion in SHEs will lead to the less PCs,59 whereas more intermolecular HBs in LSHEs will make the molecular more compact, or the higher PCs. However, it is not the case. In Table 3, as a whole, it is not

will change the intermolecular interactions much. It is a reason for the high sensitivity of SHEs, while for TATB, the interlayer slide will not make the intralayer HBs changeable, only changing the interlayer interactions, which are small. The geometries and AIM analyses of these HBs are listed in Table 2. Comparing with TATB, the strength of the intermolecular HBs in SHEs is usually weak even though they sometimes (for example, in RDX or β-HMX) possess a considerable amount. As shown in the table, the distances between the hydrogen bond donors (D) and acceptors (A) in the SHE crystals are always above 3 Å, longer than those in TATB crystal, and their EHB is usually less than 9 kJ/mol. These longer D···A distances and smaller EHB show the weaker intermolecular HBs in the SHE crystals, in contrast to LSHE crystals. At the same time, the two biggest values of total dissociation energy per molecule (ΣEHB/2) of the SHE crystals appear in RDX and β-HMX, 33 and 39 kJ/mol, respectively, due to their large amounts of the HBs. However, they are still less than that of TATB, 60 kJ/mol.10 F



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Figure 7. (a−d) Intermolecular O···O contacts represented by red dashes in SHEs without intermolecular hydrogen bond.

intermolecular bonding in SHE crystals can be broken more readily than LSHE crystals once against external stimuli. In a common sense, the easier structural deconstruction suggests the higher sensitivity. It is a reason for higher sensitivity of SHEs relative to LSHEs. Maybe the nature of noncovalent O···O interactions is interesting as these interactions are so popular and predominant in both SHEs and LSHEs. To clarify it, we explore the dependence of EO−O and O···O distance (RO−O) by AIM analyses. The RO−O−EO−O dependence in Figure 8 exhibits an obvious tendency: the shorter RO−O, the bigger EO−O. This case is similar to covalent bonds and intermolecular C−H···F interactions.61 As indicated in the figure, RO−O is within 2.7 and 3.3 Å; therefore, we think that the noncovalent O···O interactions within the distances are mainly attributed to electron cloud overlap as in a covalent O−O bond, as stated previously, the intermolecular O···O interactions are bonding closed-shell interactions.41,50−58 2.4. Hirshfeld Surface Analyses. Hirshfeld surface62−64 is a useful tool to explore geometrically the intermolecular interactions in crystal. From Figure 9, we find some differences of Hirshfeld surfaces between the LSHE TATB molecule and the SHE molecules. The first is the shape of the surfaces. Figure 9a exhibits a plate-shaped surface of TATB, suggesting a planar molecule of TATB. And the red dots showing the intermolecular close contacts are not located on the front faces but on the side faces of the plate. It suggests that the intermolecular interactions in the TATB crystal occur through the hydrogen and nitrogen atoms encompassing the molecules, while for another big π-bonded molecule BTF in Figure 9h, its Hirshfeld surface looks like a plate too. But this plate is convex−concave. Furthermore, the red dots distribute on both front and side faces, which implies the interactions take place through both the external and internal atoms. The shapes of the

found that the PCs of the SHEs are less than those of LSHEs. It suggests that we should reconsider these O···O interactions. We show O···O contacts within 3.3 Å only in Figures 6 and 7, because too far interactions are usually overlooked. As pointed out above, in SHE molecules, some or all hydrogen atoms do not take part in forming HBs, while, for oxygen atoms, it is interesting to find that they play a role in the O···O contacts within 3.3 Å. In numerous research studies, the dissociation energy (E = (1/2)v, in which v is the potential energy density derived from AIM analysis)60 has been employed to evaluate the noncovalent interactions. Here, the dissociation energy of intermolecular O···O interactions, EO−O, was calculated and listed in Table 4. Comparing Tables 4 with 2, wholly, one can find the amount of the intermolecular O···O interactions is much larger than that of the intermolecular HBs per one SHE molecule. Individually, the case of RDX is reverse, and comparing EO−O with EHB in Table 4, we find O···O interactions dominate the intermolecular interactions among most SHE crystals. The RDX and BCHMX crystals are mainly held by intermolecular HBs, and in HMX, the intermolecular O···O interactions and HBs contribute almost equally to consolidate the crystal, in terms of its total EO−O and EHB, ∑EO−O /2 and ∑EH/2. Regarding the linkages of these differences between intermolecular O···O interactions and HBs in SHEs and LSHEs to their sensitivities, we think it is attributed to the difference of the interaction strength. Because per O···O interaction (average 6 kJ/mol of dissociation energy from Table 4) is usually weaker than per HB (average 6 kJ/mol of dissociation energy from Table 3 of ref 10), the intermolecular O···O interactions can be more readily dissociated than the intermolecular HBs. As analyzed above, in general, the intermolecular interactions in SHE crystals are governed by the O···O interactions, and those in LSHE crystal are mainly of HBs. That is to say, from this viewpoint, the G


H

HNB

BTF

BCHMX

β-HMX

RDX

TNAZ

O4B O7 O6 O3 O4 O3 O5 O6 O2 O4 O5

O2 O1 O4 O1 O1 O2 O4 O4 O1 O1 O1

O8B O4

O8 O3

O4

O7 O8 O7B

O5 O5 O6

O2

O8

O4

O1 O4 O3 O5 O3 O6 O3

O7 O8

O3 O3

O1 O1 O3 O4 O2 O2 O1

O5B

O3

PETN

O4B O5 O4 O6B O8

O1 O1 O2 O2 O2

ONDO

Atom2

Atom1

explosive

3.063 3.020 3.266 3.197 3.088 3.070 2.954 3.245 3.106 3.036 3.066

3.061

2.934 3.041 3.215 3.290 2.944 3.116 3.093

2.929 3.130

3.278 3.208 3.182

3.179

2.944 3.085

3.234

3.248 3.152 3.248 2.958 3.063

length, Å x, −1+y, z; (2) −x, 2−y, −z; (2) −1+x, y, z; 1+x, y, z; −1−x, 1−y, −z; 1−x, 1−y, −z; −1+x, y, z; 1+x, y, z; −1−x, 1−y, −z; 1−x, 1−y, −z; x, −1+y, z; (2) −x, 2−y, −z; (2) 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; −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; 1/2−x, −1/2+y, 1/2−z; 1/2+x, 1.5−y, 1/2+z; −1+x, y, z; 1+x, y, z; −1−x, 1−y, −z; 1−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, 1/2−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, −1/2+y, 1/2−z; −1/2+x, 1/2−y, −1/2+z; −1/2+x, 1.5−y, −1/2+z; −1+x, y, z; 1+x, y, z; −1−x, 1−y, −z; 1−x, 1−y, −z; −1+x, y, z; 1+x, y, z; −1−x, 1−y, −z; 1−x, 1−y, −z; −1/2+x, 1.5−y, −1/2+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+x, y, z; 1−x, 1−y, −z; −1/2+x, −1/2+y, −1/2−z; −1/2+x,−1/2+y,1/2−z; −1/2−y, −1/2−x, −1/2+z; −1/2−y, 1/2−x, 1/2+z; 1/2+y, 1/2+x, 1/2+z; 1/2+y, −1/2+x, 1/2+z; 1/2−x, 1/2+y, −1/2−z; 1/2−x, 1/2+y, 1/2−z; 1/2+x, y, 1/2−z; −1/2+x, y, 1/2−z; −1/2+x, y, 1/2−z; 1/2+x, y, 1/2−z; −x, 1−y, −z; 1/2−x, −1/2+y, z; 1/2−x, 1/2+y, z; −x, 1−y, 1−z; (2) −1/2+x, y, 1/2−z; 1/2+x, y, 1/2−z; 1+x, 1/2−y, 1/2+z; −1+x, 1/2−y, −1/2+z; 1−x, −1/2+y, 1/2−z; −1−x, −1/2+y, −1/2−z; −1−x, −1/2+y, −1/2−z; −1−x, 1/2+y, −1/2−z; 1+x, −1/2−y, 1/2+z; 1+x, 1/2−y, 1/2+z; −1−x, −y, −z; (2) 1+x, y, z; (2) −x, −1/2+y, −z; −x, 1/2+y, −z; 1−x, −1/2+y, 1−z; 1−x, 1/2+y, 1−z; −1+x, y, z; 1+x, y, z; −1+x, y, 1+z; 1+x, y, −1+z; 2−x, −y, 1/2+z; 2−x, −y, −1/2+z; 1/2+x, 1/2−y, z; −1/2+x, 1/2−y, z; 1+x, y, −1+z; −1+x, y, 1+z; x, −y, −1/2+z; −x, −y, −z; x, −y, 1/2+z; −x, −y, 1−z; x, −y, −1/2+z; x, −y, 1/2+z; −x, −y, −z; −x, −y, 1−z; 1/2−x, 1/2−y, 1/2−z; (2) −1/2−x, 1/2−y, z; (2)

symmetry operations

Table 4. Geometry and AIM Analyses of the Intermolecular O···O Interactions in Crystalsa

0.00613 0.00706 0.00405 0.00531 0.00457 0.00551 0.00763 0.00351 0.00562 0.00512 0.00492

0.00646

0.00977 0.00551 0.00267 0.00306 0.00825 0.00603 0.00708

0.00726 0.00506

6.2 7.2 3.8 5.6 4.5 5.5 9.7 3.4 5.3 5.1 4.9

6.2

10.5 5.4 2.7 2.9 8.4 5.9 7.0

7.2 5.3

4.6 4.7 4.7

5.5

0.00526 0.00479 0.00472 0.00503

7.2 5.8

3.8

0.00404 0.00735 0.00564

3.2 4.7 4.2 6.2 6.6

EO−O, kJ/mol

0.00326 0.00483 0.00431 0.00617 0.00663

ρ, e/Å3

110 (0)

28.7 (0)

11 (15.1)

38.8 (39.2)

14.3 (32.9)

20.2 (0)

21.2 (12.1)

129.6 (10.5)

ΣEO−O/2, kJ/mol

Crystal Growth & Design Article


a

I

Atom2

O6 O2 O4 O3 O5B O5 O6 O6 O5 O7 O10 O11 O3 O10 O12 O8 O12 O7 O11 O8 O9 O12 O3F O4 O5F O7 O2F O6 O8F O5F O7 O4F O6 O8F O8F O8 O8F

Atom1

O1 O2 O2 O3 O3 O4 O4 O6 O1 O1 O1 O1 O3 O3 O3 O4 O4 O5 O5 O6 O8 O10 O1 O1 O1 O1 O2 O2 O2 O3 O3 O4 O4 O4 O5 O6 O8

3.031 3.036 3.095 3.057 3.079 3.056 3.081 3.118 2.894 3.257 3.188 2.715 2.791 3.212 2.998 3.209 2.927 2.876 2.756 3.061 3.086 3.028 3.251 3.178 2.832 3.161 3.154 3.261 3.170 3.132 3.140 3.167 3.082 3.297 3.014 3.002 2.916

length, Å

ρ, e/Å3 0.00710 0.00726 0.00610 0.00738 0.00605 0.00480 0.00598 0.00690 0.00938 0.00476 0.00424 0.01160 0.01267 0.00470 0.00754 0.00522 0.00791 0.00816 0.01299 0.00547 0.00641 0.00489 0.00316 0.00451 0.00896 0.00418 0.00464 0.00340 0.00535 0.00447 0.00551 0.00542 0.00503 0.00374 0.00623 0.00612 0.00888

symmetry operations x, −1+y, z; x, −1+y, z;−x, −1+y, 1/2−z; −x, 1+y, 1/2−z; −x, −y, 1−z; x, −y, −1/2+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−z; 1/2−x, 1/2+y, 1−z; 1/2−x, 1/2−y, 1/2−z; −1/2+x, 1/2−y, z; 1/2−x, −1/2+y, 1−z; 1/2−x, 1/2+y, 1−z;−1/2+x, −1/2+y, −1/2+z; −1/2+x, 1/2+y, −1/2+z; x, 1−y, −1/2+z; −x,1−y,1−z;x, 1−y, 1/2+z; −x, 1−y, −z; x, 1−y, −1/2+z; (2) −x, 1−y, 1−z; (2) x, 1−y, −1/2+z; −x, 1−y, 1−z; 1/2−x, 1/2+y, 1.5−z; 1/2−x, −1/2+y, 1.5−z; −1+x, y, z; 1+x, y, z; −1+x, y, z; 1+x, y, z; 1/2−x, −1/2+y, 1.5−z; 1/2−x, 1/2+y, 1.5−z; 1−x, 1−y, 1−z; 1−x, 1−y, 1−z; (2) 1/2−x, −1/2+y, 1.5−z; 1/2−x, 1/2+y, 1.5−z; −1/2+x, 1.5−y, −1/2+z; 1/2+x, 1.5−y, 1/2+z; −1/2+x, 1.5−y, −1/2+z; 1/2+x, 1.5−y, 1/2+z; 1.5−x, −1/2+y, 1.5−z; 1.5−x, 1/2+y, 1.5−z; 1/2−x, −1/2+y, 1.5−z; 1/2−x, 1/2+y, 1.5−z; 1−x, 1−y, 2−z; (2) 1/2+x, 1.5−y, 1/2+z; −1/2+x, 1.5−y, −1/2+z; 1/2+x, 1.5−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, −1/2+y, 1/2−z;1/2+x, −1/2+y, z; 1/2+x, 1/2+y, z; −1/2+x, 1/2+y, z; 1/2+x, −1/2+y, z;1/2−x, −1/2+y, 1/2−z; 1.5−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;1/2+x, 1/2+y, z; 1/2+x, −1/2+y, z; 1/2−x, 1/2+y, 1/2−z; −1/2+x, −1/2+y, z;1.5−x, −1/2+y, 1/2−z; 1/2+x,1/2+y,z 1/2−x, 1/2−y, 1−z; 1/2+x, 1/2−y, −1/2+z; x, 1−y, −1/2+z; x, 1−y, 1/2+z;1−x, 1−y, −z; 1−x, 1−y, 1−z; x, 1−y, −1/2+z; (2) 1−x, 1−y, 1−z; (2) 1/2−x, 1/2+y, 1/2−z; 1/2−x, −1/2+y, 1/2−z;1/2+x, −1/2+y, z; 1/2+x, 1/2+y, z; x, −1+y, z; 1−x, −1+y, 1/2−z;x, 1+y, z; 1−x, 1+y, 1/2−z; x, −y, −1/2+z; 1−x, −y, 1−z; 1.5−x, 1/2−y, 1−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, −z; x, 1−y, −1/2+z; (2)1−x, 1−y, 1−z; (2) 1/2−x, 1/2+y, 1/2−z; 1/2−x, −1/2+y, 1/2−z;1/2+x, −1/2+y, z; 1/2+x, 1/2+y, z; x, 1−y, 1/2+z; x, 1−y, −1/2+z;1−x, 1−y, −z; 1−x, 1−y, 1−z; x, 1−y, −1/2+z; 1−x, 1−y, 1−z;

Values in brackets are the dissociation energy of intermolecular H-bonds (ΣEHB/2).

ONC

ε-CL-20

explosive

Table 4. continued 7.0 7.0 6.1 7.1 5.8 4.8 5.7 6.5 9.3 4.5 4.1 12.1 13.1 4.5 7.6 4.9 8.4 8.2 13.5 5.3 6.3 5.0 3.0 4.4 8.9 4.3 4.4 3.4 5.1 4.5 5.4 5.4 4.9 3.3 6.1 6.0 8.7

EO−O, kJ/mol

137.1 (0)

100.3 (12.4)

ΣEO−O/2, kJ/mol

Crystal Growth & Design Article



Article

intermolecular HB in TNAZ. This disagreement comes from no consideration of angles in HB formation by Hirshfeld surfaces; that is, only distance is accounted for. In a word, in SHE crystals, there exists weak intermolecular O···H, O···O, O···N, and O···C interactions with variable weights. And before molecular decays, the break of these weak interactions, for example, by lattice deformation under impact, would sufficiently destabilize the crystal to initiate the molecular reaction. 2.5. Packing Effect on Impact Sensitivity. In a previous study, we divided HB-aided π−π stacking of LSHEs into four types and found the face-to-face type in favor of interlayer sliding with neglectable energy barriers, which corresponds to low impact sensitivity.10 Examining crystal packing of all 10 SHEs, we find that HNB possesses similar face-to-face stacking because all benzene rings in the HNB crystal stack in a plane or in parallel. The interlayer distance of HNB is 4.1 Å, much larger than that of TATB, 3.1 Å. Presumably, this larger interlayer distance of HNB would help it slide more readily than TATB. However, it is not the case. As demonstrated in Figure 12, when the interlayer sliding occurs, the interlayer interaction changes in TATB crystal can be overlooked, due to a constant and big interlayer distance, 3.1 Å. It suggests the easy interlayer sliding in TATB, which is an important reason for its very low sensitivity (Edr > 78.4 even 120.2 J in Table 1). Whereas for HNB, owing to much distortion of nitro groups from the benzene, the intermolecular O···O distances will be shortened even though the interlayer distance is constant 4.1 Å. This will change dramatically the interlayer interactions, in fact increase the interlayer repulsion, which suggests the unallowed interlayer sliding. This is the reason for its high sensitivity (Edr = 2.7 J in Table 1) even though it is a rather stable molecule with the highest BDE of 240 kJ/mol among all 10 SHE molecules. In addition, a sliding barrier calculation67 was carried out to verify the big difference in sliding properties of TATB and HNB. As shown in Figure 13, the sliding barriers of TATB and HNB are predicted to be about 30 and 330 kcal/mol (126 and 1386 kJ/mol), respectively. These two kinds of sliding will lead to remarkably different results. For TATB, the barrier is much

Figure 8. Dependence of the distance (RO−O) and the dissociation energy (EO−O) of intermolecular O···O contacts in SHEs.

remaining SHE molecules are irregular, more uneven, and less planar, and the red dots distribute on these irregular surfaces. The second difference is the interaction type or the type of intermolecular interatomic contacts. From the figure, the intermolecular interatomic contacts of TATB mainly belong to O···H interactions. Apart from HMX, O···O contacts become dominant in other SHE molecules. This agrees with the fact that, in contrast to LSHE molecules, more oxygen atoms are exposed on the externals of SHE molecules. Two-dimensional plots of these intermolecular contacts are shown in Figure 10. The signal indicators of intermolecular HBs, two spikes at the left bottom of the plots of SHEs, become blunt or nothing, related to TATB. At the same time, a narrow belt denoting O···O contacts becomes more obvious in orange, which suggests the increase of the O···O contacts. In Figure 11, the population of the O···O contacts of TATB is less than any SHE molecules. For some molecules like ONDO, PETN, TNAZ, and ε-CL-20, the high populations of H···O contacts are apparent. AIM analyses show there are weak intermolecular HBs in ONDO, PETN, and ε-CL-20, and no

Figure 9. Hirshfeld surfaces65,66 of 10 SHE molecules in crystal stacking; each shows two plots with a torsion of 180°. Panels a−d denote contacts of O···H, O···O, O···N, and O···C, respectively. J



Article

Figure 10. Two-dimensional fingerprint plots in crystal stacking.

Figure 11. Populations of close contacts of 10 SHE molecules in crystal stacking.

sufficient to cause ignition. That is, these factors can play crucial roles in sensitivity mechanism. Obviously, we draw the above conclusion in the case that all crystals are perfect.

lower than its BDE (320 kJ/mol). It suggests the sliding will not destroy the molecular integrity and the sliding is allowed, whereas for HNB, the predicted barrier is greatly above its BDE, which implies it is a catastrophic sliding or the sliding is unallowed. For the remaining SHEs, the intermolecular sliding in any crystal is prohibited. From the crystal packing shown in Figures 6 and 7, in a straightforward way, we cannot find a sliding path with a low barrier as in TATB. As a matter of fact, a small interlayer sliding along the b axis of β-HMX will increase the intermolecular potential much more than 300 kJ/mol, largely above its BDE 210 kJ/mol, in terms of previous predictions.5 It also suggests the unallowed intermolecular or interlayer sliding. Certainly, voids, defects, dislocations, and other discontinuities can interrupt the energy flow and delocalization leading to hotspots (initiation sites), which accumulate energy

3. CONCLUSIONS Summarily, we accomplished a systemic analysis of crystal packing of 10 typical existing SHEs and find two structural reasons for their high sensitivity: one is the unstable molecule and the other is no HB-aided π−π molecular stacking in the crystal. It should be stressed the assistant role of intermolecular bonds. In some SHE molecules like BTF and HNB, big πbonded conjugation structures exist (e.g., all atoms in the entire molecule of BTF and all carbon atoms of HNB form the πconjugated structures individually), but, owing to no intermolecular bond, the π−π stacking cannot buffer external K



Article

Figure 12. Comparison of interlayer sliding of TATB (a) and HNB (b). The green and scarlet dashes represent the original and changed intermolecular interatomic contacts.

Figure 13. Comparison of interlayer sliding barriers of TATB and HNB. Panels (a) and (b) are the front and side views of the layers, respectively. Panel (c) is the sliding barriers in kcal/mol in a period.

π−π stacking is necessary for crystal engineering of LSHEs. Besides, we find that the O···O interactions dominate the intermolecular interactions in most SHEs, and these

mechanical stimuli efficiently, and they are still sensitive despite the π−π stacking. Therefore, combining the analyses of crystal packing of LSHEs and SHEs, we re-verified that the HB-aided L



Article

(4) Dong, H.; Zhou, F. High Energetic Explosives and Relatives; Science Press: Beijing, 1994; and its revision, Science City, Mianyang, Sichuan, 2005 (in Chinese). (5) Zhang, C.; Wang, X.; Huang, H. J. Am. Chem. Soc. 2008, 130, 8359−8365. (6) Zhang, C.; Cao, X.; Xiang, B. J. Phys. Chem. C 2010, 114, 22684− 22687. (7) Kuklja, M. M.; Rashkeev, S. N. Appl. Phys. Lett. 2007, 90, 151913. (8) Kuklja, M.; Rashkeev, S. Phys. Rev. B 2007, 75, 104111. (9) Zhang, C.; Xue, X.; Cao, Y.; Zhou, J.; Zhang, A.; Li, H.; Zhou, Y.; Xu, R.; Gao, T. CrystEngComm 2013, 15, 6837−6844. (10) Ma, Y.; Zhang, A.; Zhang, C.; Jiang, D.; Zhu, Y.; Zhang, C. Cryst. Growth Des. 2014, 14, 4703−4713. (11) Wang, X. Theoretical Study on RDX thermal decomposition in gas phase and effect of solvents on RDX decomposition. Master’s Degree Thesis, ICM, CAEP, 2005. (12) McCrone, W. C. In Physics and Chemistry of the Organic Solid State; Fox, D., Labes, M. M., Wessberger, A., Eds.; Wiley: New York, 1965; Vol. II, p 726. (13) Nielsen, A. T.; Chafin, A. P.; Christian, S. L.; Moore, D. W.; Nadler, M. P.; Nissan, R. A.; Vanderah, D. J.; Gilardi, R. C.; George, F.; Flippen-Anderson, J. L. Tetrahedron 1998, 54, 11793. (14) Ou, Y.; Wang, C.; Pan, Z.; Chen, B. Chin. J. Energetic Mater. 1999, 7, 100−102. (15) Landenberger, K. B.; Matzger, A. J. Cryst. Growth Des. 2010, 10, 5341−5347. (16) Landenberger, K. B.; Matzger, A. J. Cryst. Growth Des. 2012, 12, 3603−3609. (17) Bolton, O.; Matzger, A. J. Angew. Chem., Int. Ed. 2011, 50, 8960−8963. (18) Bolton, O.; Simke, L. R.; Pagoria, P. F.; Matzger, A. J. Cryst. Growth Des. 2012, 12, 4311−4314. (19) Yang, Z.; Li, H.; Zhou, X.; Zhang, C.; Huang, H.; Li, J.; Nie, F. Cryst. Growth Des. 2012, 12, 5155−5158. (20) Zhang, C.; Cao, Y.; Li, H.; Zhou, Y.; Gao, T.; Zhang, H.; Xu, J.; Yang, Z.; Jiang, G. CrystEngComm 2013, 15, 4003−4014. (21) Zhang, C.; Yang, Z.; Zhou, X.; Zhang, C.; Ma, Y.; Xu, J.; Zhang, Q.; Nie, F.; Li, H. Cryst. Growth Des. 2014, 14, 3923−3928. (22) Zhang, C.; Xue, X.; Cao, Y.; Zhou, J.; Zhang, A.; Li, H.; Zhou, Y.; Xu, R.; Gao, T. CrystEngComm 2014, 16, 5905−5916. (23) Oyumi, Y.; Brill, T. B.; Rheingold, A. L. J. Phys. Chem. 1985, 89, 4824. (24) Trotter, J. Acta Crystallogr. 1963, 16, 698. (25) Archibald, T. G.; Gilardi, R.; Baum, K.; George, C. J. Org. Chem. 1990, 55, 2920. (26) Choi, C. S.; Prince, E. Acta Crystallogr. 1972, B28, 2857. (27) Choi, C. S.; Boutin, H. P. Acta Crystallogr. 1970, B26, 1235. (28) Gilardi, R.; Flippen-Anderson, J. L.; Evans, R. Acta Crystallogr. 2002, E58, o972. (29) Cady, H. H.; Larson, A. C.; Cromer, D. T. Acta Crystallogr. 1966, 20, 336. (30) Akopyan, Z. A.; Struchkov, Yu. T.; Dashevii, V. G. Zh. Strukt. Khim. (Russ.) (J. Struct. Chem.) 1966, 7, 408. (31) Zhang, M. X.; Eaton, P. E.; Gilardi, R. Angew. Chem., Int. Ed. 2000, 39, 401. (32) Sućeska, M. EXPLO5 6.02 program; Zagreb: Kroatien, 2014. (33) Dobratz, B. M.; Crawford, P. C.LLNL Explosives Handbook: Properties of Chemical Explosives and Simulants, 1974. (34) Zhang, C.; Shu, Y.; Huang, Y.; Zhao, X.; Dong, H. J. Phys. Chem. B 2005, 109, 8978−8982 and references therein. (35) Lee, K. Y.; Coburn, M. D. 3-Nitro-1,2,4- Triazole-5-One, A Less Sensitive Explosive, United States Patent, 1988, Patent Number: 4733610. (36) Storm, C. B.; Stine, J. R.; Kramer, J. F. Chem. Phys. Energetic Mater. 1990, 309, 605−639. (37) Latypov, N. V.; Bergman, J.; Langlet, A.; Wellmar, U.; Bemm, U. Tetrahedron 1998, 54, 11525−11536. (38) Klasovity, D.; Zeman, S.; Ruzicka, A.; Jungova, M.; Rohac, M. J. Hazard. Mater. 2009, 164, 954−961.

interactions depend on the O···O distances to a certain extent of 2.7−3.3 Å. Comparing the intermolecular O···O interaction strength in SHEs with the intermolecular HB strength in LSHEs, we can conclude that the SHE crystals can be deconstructed more readily and then be more sensitive. It confirms that energetic cocrystallization is indeed a promising way to improve performances because it can improve the intermolecular interactions among existing explosive molecules. In combination with previous work,5−10 we can explain, to a large extent, explosive sensitivity from the perfect crystal packing of viewpoint. On the basis of this, we may try to design molecules and further crystals to engineer the crystal structure to achieve a desirable structure where an attribute, such as relative impact sensitivity, is minimized.

■

ASSOCIATED CONTENT

S Supporting Information *

Crystallographic information on 10 SHEs and intramolecular hydrogen bonds in ONDO. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are very grateful for the financial support from the Science and Technology Fund of CAEP (2011A0302014 and 2012A0302013), the Science and Technology Innovation Fund of ICM (KJCX-201305), and the National Natural Science Foundation of China (21173199). In addition, the comments and suggestions of the referees are acknowledged.

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ABBREVIATIONS BCHMX cis-2,4,6,8-tetranitro-1H,5H-2,4,6,8-tetraazabicyclo(3.3.0)octane BTF benzotrifuroxan CL-20 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (HNIW) DAAF trans-(d,d)-3,3′-diamino-4,4′-azofurazan DAAzF trans-(p,p)-3,3′-diamino-4,4′-azofurazan HMX 1,3,5,7-tetranitro-1,3,5,7-tetrazocane HNB hexanitrobenzene ONC octanitrocubane ONDO 1,1,1,3,6,8,8,8-octanitro-3,6-diazaoctane PETN pentaerythritol tetranitrate RDX 1,3,5-trinitro-1,3,5-triazinane TATB 1,3,5-triamino-2,4,6-trinitrobenzene TNAZ 1,3,3-trinitroazetidine TNB 1,3,5-trinitrobenzene

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REFERENCES

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