Crystal Packing of Low-Sensitivity and High-Energy Explosives

Article pubs.acs.org/crystal

Crystal Packing of Low-Sensitivity and High-Energy Explosives Yu Ma,† Anbang Zhang,†,‡ Chenghua Zhang,† 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: Low-sensitivity and high-energy explosives (LSHEs) are highly desired for their comprehensive superiority of safety and energy. Crystal packing is crucial to both the safety and energy, and therefore becomes of interest in energetic crystal engineering. This work carries out systemic analyses on the crystal packing of 11 existing LSHEs with both energy and safety close or superior to TNT. As a result, we find that the LSHE crystals wholly feature π−π stacking with the aid of intermolecular hydrogen bonding. Each LSHE molecule is πbonded with a big conjugated structure composed of all nonhydrogen atoms in the entire molecule. Intramolecular hydrogen bonding exists in most LSHE molecules with strongly active hydrogen bond (HB) donors of amino and hydroxyl groups, and various strength. These big π-conjugated structures and intramolecular HBs lead to planar molecules with high stability, settling a base of π−π stacking in crystals. With the help of intermolecular HBs, the π−π stacking holding the LSHE crystals appears in four modes. Among them, the face-to-face stacking (always offset) gives rationally the smallest steric hindrance when interlayer slide occurs in crystal, which is the reason for very low impact sensitivity. This work suggests that the planar conjugated molecular structure and intermolecular hydrogen bonding supporting the π−π stacking are necessary to the crystal engineering of LSHEs.

1. INTRODUCTION Crystal packing is crucial to the two most concernful properties of explosives, energy and safety. On the one hand, for a group of polymorphic explosives with a same chemical component (e.g., HMX or CL-20), the most compacted form (e.g., β-HMX or ε-CL-20) is usually desired in practical applications, in that the higher compactness is advantageous to both the energy and safety.1 For one thing, because the energy is usually represented by velocity of detonation (VOD), which is directly proportional to packing density, the most compacted form implies the highest energy among all polymorphs.2 For another thing, the higher compactness suggests less free volume in the crystal, which is more disadvantageousness to molecular degradation and hot spot formation, and consequently contribute to the higher safety.3 On the other hand, for whole explosive crystals, it deems that the layered crystal packing can be useful as a buffer against external stimuli, which leads to high safety. In other words, the layered crystal packing can effectively dissipate the external mechanical energy into intermolecular slide, avoiding too big intra/intermolecular potential increases to decay molecules, form hot spots, and subsequently cause final detonation.4−7 Surprisingly, it seems that the insights into the influences of crystal packing on the energy and safety are much richer than those into crystal packing itself, according to our best knowledge.8 There is no systemic analysis on crystal packing of explosives, a class of substantial materials. Owing to this © 2014 American Chemical Society

deficiency, it will make difficult for us to distinguish crystalpacking features of various explosives, or understand in-depth their crystal structure−property relationships. This motivates us in this work to provide insights into the explosive crystal packing and build a base for energetic crystal engineering hopefully, which is becoming more and more feasible by energetic cocrystallization.9−15 Relative to pure former crystals, it was found that molecular stacking in energetic cocrystals varies, resulting in tunable properties. For instance, the molecular stacking in the CL-20/TNT cocrystal is much different from that in the related two pure former crystals of CL-20 and TNT, causing varied detonation properties and impact sensitivity.11,15 It is worthwhile to classify explosives according to their properties to understand crystal packing-property relationships. Aforementioned properties energy and safety are viable to demarcate them as high-energy explosives and low-sensitivity explosives, respectively. However, up to the present, there is in fact no clear definition of them. Considering that TNT is the representative of the first generation of modern explosives, we regard it as a reference. That is to say, we will regard an explosive as a high-energy explosive or a low-sensitivity explosive once its energy or safety is close or superior to that of TNT. Because VOD is indicative of the energy and impact Received: June 3, 2014 Published: July 25, 2014 4703

dx.doi.org/10.1021/cg501048v | Cryst. Growth Des. 2014, 14, 4703−4713

Crystal Growth & Design

Article

Table 1. VOD and Edr of LSHEs and TNT explosives VODa (m/s) VODb (m/s) Edr (J)

TNT

TATB

NQ

DAAzF

DAAF

DATB

DNDP

NTO

TNA

FOX-7

LLM-105

TNB

7049 7363 24.5c16 36.3d16 39.239 28.9e1 49.0f1

8177 8640 >78.4cd1,16 120.217

8699 8960 >78.4cd1 43.521

8576 8752 >62.735 >68.636

8563 8625 >62.736 54.936 >78.437

7855 8167 >78.4cd >53.940

6965 7399 >78.4

8840 8747 57.438 71.621

7718 7864 43.439

8986 9000 24.740

8878 8944 28.671

7536 7494 24.539 24.621

a

Calculated using VLW method33 and bCalculated using EXPLO5/6.0234. cType 12B. dType 12. eMilitary standard sample, tested by 5 kg drop hammer. fAlcohol-refined samples, tested by 5 kg drop hammer.

hydrogen bonding aiding to support the π−π stacking is analyzed. Combining the stacking modes and molecular stability, we provide an understanding of the impact insensitivity of these LSHEs.

sensitivity is a common index representing the safety, we adopt them to classify existing explosives. For a given explosive, the VOD is mainly determined by loading density, and thereby the measured VOD is usually fluctuated within a small range; whereas for its impact sensitivity, it was found it could be affected by instrumentation, techniques, and nature of sample.1,16,17 For example, on the level of crystal, crystal shape, size, imperfection, purity, and face can all influence impact sensitivity.18,19 Also, different measurements can usually give various results.1 To avoid confusion, in practice, the experimental evaluation of impact sensitivity of a new explosive is usually carried out with a representative reference like TNT, RDX, or HMX, for which impact sensitivity is evaluated under as similar conditions as possible. Because experimental impact sensitivities display a wide variation, we should make a comprehensive evaluation on them, according to which the explosives can be classified. In this work, all authorized data were unified by energy of drop (Edr). In practical applications, the lower-sensitivity and higherenergy explosives (LSHEs) are highly desired because of their dual superiorities of both energy and safety. In this work, when an explosive with VOD and Edr values close or superior to TNT, it is seen as a LSHE. It shows that, still, there is no completely strict definition of LSHE, because of the difficulty in making direct quantitative comparisons of impact sensitivities. This may be the complexity of explosive safety. In fact, to overcome the complexity, US DOE has built 10 standards of DOE IHE (Insensitive High Explosives) Qualification Tests to guarantee the safety before application.20 After a careful examination on most reported explosives,1,16,17,21 we collected 11 LSHE as listed in Table 1, including TATB,22 NQ,23 DAAzF,24 DAAF,25 DATB,26 DNDP,27 NTO,28 TNA,29 FOX-7,30 LLM105,31 and TNB.32 We do not think it is exhaustive, but is enough for the crystal packing analyses, as LSHEs are in fact very few because of the intrinsic energy-safety contradiction, i.e., high energy usually goes with low safety.41 It suggests that the pursuit of explosives with higher energy and higher safety is really ideal in most cases. Crystallographic information on these LSHEs is listed in Table S1 in the Supporting Information. With help of quantum chemical calculations and Hirshfeld surface analyses, we find that the whole LSHE crystals feature π−π stacking with aid of intermolecular hydrogen bonds (HBs). The LSHE molecules are planar each with a big π-bonded conjugated structure composed of all non-hydrogen atoms in the entire molecule. In general, strong intramolecular HBs exist in the LSHE molecules and enhance the molecular stability. And with respect to single HB, the intramolecular HB is usually stronger than the intermolecular one. Four modes of molecular stacking of these 11 LSHE crystals are discriminated, and the intermolecular

2. RESULTS AND DISCUSSION 2.1. Big π-Bonded Molecular Structures and Intramolecular Hydrogen Bonding. As illustrated in Figure 1, all

Figure 1. Molecular structures of the discussed LSHEs, intramolecular HBs represented by green dash and big π-bonds involved. Gray, green, blue, and red balls denote carbon, hydrogen, nitrogen, and oxygen atoms, respectively. Similar representations of HBs and atoms are considered in the following figures.

the LSHE molecules contain hydrogen atoms and are stable each with a big conjugated structure. Interestingly, all the nonhydrogen atoms in each LSHE molecule contribute to the conjugated structure involved. In the benzene derivatives TATB, DATB, DNDP, TNA and TNB, all nitrogen and oxygen atoms on nitro, amino and hydroxyl groups take part in conjugation and make the conjugation extended from the benzene rings to the bigger conjugated structures. For example, TATB possesses a big π24 18 bond with all 18 non-hydrogen atoms in the entire molecule and 24 π-electrons. Similarly, DATB, DNDP, TNA, and 18 20 18 TNB are π22 17, π14, π16, and π15 bonded, respectively, and each π-bond is with all non-hydrogen atoms in the entire molecule too. DAAzF and DAAF are conjugated molecules through two five-membered furazan rings bridged by an azo group and an azoxy group, respectively. At the same time, the nitrogen atoms of amino groups linked with the furazan ring join in the conju20 gation too. That is to say, they are π18 14 and π15 bonded, respectively. 4704



Article

Table 2. Geometry and AIM Analyses of the Intramolecular HBs in Related Molecules molecule TATB

NQ DAAzF DAAF DATB

DNDP TNA LLM-105 FOX-7

D−H (Å)

H···A (Å)

A···D (Å)

A−H···D (deg)

ρ (e/Å3)

v (a.u.)

EHB,(kJ/mol)

ΣEHB (kJ/mol)

0.865 0.849 0.757 1.054 0.868 0.954 0.960 0.880 0.880 0.798 1.110 1.110 1.110 1.110 0.812 0.889 0.924 0.884 0.918 0.928 0.943 0.849

1.797 1.838 1.948 1.692 1.816 1.770 1.930 2.366 2.366 2.238 1.815 1.684 1.744 1.848 1.920 1.809 1.873 1.992 2.122 2.081 1.883 2.039

2.439 2.484 2.553 2.480 2.502 2.492 2.587 2.897 2.897 2.804 2.585 2.433 2.518 2.602 2.616 2.593 2.616 2.626 2.711 2.698 2.608 2.619

136.0 131.6 130.6 127.7 134.4 129.9 123.5 119.0 119.0 128.4 122.4 127.7 122.3 121.1 143.3 145.7 135.8 127.5 121.0 122.7 131.7 124.9

0.0388 0.0360 0.0292 0.0494 0.0373 0.0414 0.0297 0.0136 0.0136 0.0156 0.0369 0.0504 0.0442 0.0351 0.0362 0.0285 0.0317 0.0250 0.0199 0.0214 0.0319 0.0233

−0.0401 −0.0362 −0.0275 −0.0515 −0.0377 −0.0422 −0.0254 −0.0090 −0.0090 −0.0116 −0.0337 −0.0509 −0.0429 −0.0307 −0.0352 −0.0264 −0.0290 −0.0212 −0.0153 −0.0167 −0.0283 −0.0191

52.6 47.6 36.2 67.7 49.5 55.4 33.3 11.8 11.8 15.2 44.2 66.8 56.3 40.3 46.3 34.6 38.1 27.8 20.1 21.9 37.2 25.1

309.0

33.3 23.6 15.2 207.6

80.9 65.9 42.0 62.3

O−H···O interactions. Even more, EHB of two HBs in TATB and DATB respectively is more than 60 kJ/mol, suggesting the rather strong N−H···O interactions. In addition, the contribution of HBs to molecular stability can be quantitatively assessed by the sum of all EHB of a molecule (ΣEHB). As listed in Table 2, TATB and DATB possess the most ΣEHB among all LSHE molecules, 309 and 207.6 kJ/mol respectively, showing the large contributions to stabilize molecules, in consistent with their high decomposition temperatures.1 Also, this high molecular stability leads to low impact sensitivity of TATB and

And all carbon, nitrogen, and oxygen atoms in NQ, NTO, FOX-7, 12 14 20 and LLM-105 are included in big π-bonds of π10 6 , π9 , π10, and π15, respectively. From the above discussion, we can know that these LSHE molecules feature a strong conjugation effect each with a big π-bond involving all non-hydrogen atoms, which provides a base for π−π stacking in crystals. Even though there are some torsions of substituted groups from the molecular planes by examining the optimized single molecular structures of gaseous state using chemical quantum calculations and the molecular structures extracted directly from crystals (see Figures S1 and S2 in the Supporting Information), these molecules are, at least, approximately planar. Among all 11 LSHE molecules, the biggest torsion angle of O−N−C-C with 35.6° appears in solid FOX-7. Nevertheless, the length of two C-NO2 bonds and two C-NH2 bonds of FOX-7 is 1.388, 1.417, 1.309, and 1.315 Å, which are between the single and double C−N bonds, respectively, suggesting an obvious conjugation effect there. For other LSHE molecules, most torsion angles are within several degrees. This characteristic of planar π-conjugated structures of LSHE molecules is very important to their safety, including not only molecular stability but also crystal sliding properties. This will be discussed later. Besides the planar conjugation, intramolecular hydrogen bonding is another characteristic of LSHE molecules, which also enhances molecular stability. By AIM analyses, the HBs are verified in most LSHE molecules excluding NTO and TNB, and represented by green dash in Figure 1. Table 2 gives the details of their geometry and AIM analysis results. According to the clarification of HB strength proposed by Jeffrey,42 they are mostly mediate of N−H···O and O−H···O interactions, except from the weak HBs in DAAzF and DAAF due to the long donor−acceptor distances. With respect to the quantitative description of HB strength, the decomposition energy of HB, EHB,43 was applied. Apart from EHB of DAAzF and DAAF no more than 15 kJ/mol showing the weak HBs there, most EHB in Table 1 is above 30 kJ/mol, representing mediate N−H···O and

Figure 2. Four types of π−π stacking in the LSHE crystals.

Figure 3. Three LSHE crystals with face-to-face stacking: (a) TATB, (b) DAAzF, and (c) DAAF. 4705



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Table 3. Geometry and AIM Analyses of the Intermolecular HBs in Crystals explosive TATB

DAAF

DAAzF

NTO

FOX-7

LLM-105

NQ

DATB

DNDP

TNA

TNB

D

H

A

D−H (Å)

H···A (Å)

A···D (Å)

N4 N6 N2 N6 N2 N4 N2

H4 H6 H2 H5 H1 H3 H1

O1 O3 O5 O4 O6 O2 N4

1.054 0.954 0.849 0.868 0.965 0.757 0.868

2.239 2.348 2.381 2.371 2.392 2.396 2.316

2.929 2.933 2.951 2.991 2.991 2.99 3.162

121.3 119.1 125.0 128.7 126.7 136.3 164.6

N2

H2

N3

0.798

2.345

3.087

155.2

N2 N4

H1 H2

O2 N3

0.868 0.880

2.438 2.290

3.073 2.987

130.5 136.2

N4

H1

N2

0.880

2.261

3.130

169.1

N2 N14 N10 N6 N7 N3 N11 N15 N10 N14 N2 N6 N3 N4 N3 N3 N4 N4 N2 N6 N2 N6 N2 N1 N1 N2 N1

H1 H7 H5 H3 H4 H2 H6 H8 H5 H7 H1 H3 H1 H3 H2 H1 H4 H3 H1 H3 H2 H4 H3 H2 H1 H4 H1

O4 O7 O10 O1 N5 N1 N9 N13 O7 O10 O1 O4 O1 O4 O4 O3 O2 O1 O1 O5 O2 O3 O2 N3 O1 O1 O3

0.86 0.86 0.86 0.859 0.86 0.86 0.86 0.86 0.86 0.86 0.86 0.859 0.945 0.904 0.943 0.945 0.849 0.904 0.904 0.918 0.928 0.902 1.001 0.981 0.96 0.957 1.110

2.011 2.013 2.015 2.036 2.187 2.195 2.221 2.226 2.515 2.521 2.514 2.517 2.145 2.143 2.291 2.446 2.341 2.39 2.046 2.093 2.363 2.509 2.005 2.191 2.252 2.372 2.146

2.769 2.776 2.778 2.791 2.912 2.911 2.923 2.928 3.073 3.08 3.089 3.1 3.016 2.921 3.013 3.008 3.027 3.193 2.861 160.49 3.224 3.115 2.997 3.145 3.022 3.291 2.971

146.5 147.3 147.4 146.1 141.9 140.7 138.7 138.8 123.4 123.5 125.0 125.8 152.5 143.7 132.9 118.1 138.1 148.0 149.4 3.0 154.3 125.0 170.6 163.9 136.6 160.8 128.8

N3 N1

H3 H2

O4 O4

1.110 1.110

2.201 2.319

3.098 2.990

136.1 116.9

N3

H4

O5

1.110

2.450

3.284

130.8

O3 O4 C4 N1 N1 C5 C5 C3 C4 C7

H3 H4 H2 H2 H1 H4 H4 H3 H6 H2

O2 O6 O3 O4 O6 O1 O3 O2 O1 O11

0.889 0.812 0.911 0.884 0.924 0.984 0.984 1.01 1.165 1.064

2.392 2.411 2.592 2.359 2.379 2.546 2.593 2.546 2.281 2.246

2.897 2.990 3.431 3.033 3.153 3.245 3.189 3.523 3.369 3.294

116.1 129.1 153.3 133.2 141.1 127.9 119.1 162.8 154.3 168.3

symmetry operation

ρ (e/Å3)

EHB (kJ/mol)

ΣEHB/2 (kJ/mo)l

−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, 1 + y, z; 1 + x, y, 1 + z; 1 + x, 1 + y, 1 + z; −1 + x, −1 + y, −1 + z; −1 + x, y, −1 + z; 1 + x, −1 + y, z; 1 + x, y, z −1 + x, y, z; −1 + x, 1 + y, z; 1 + x, y, 1 +z ; 1 + x, 1 + y, 1 + z; −1 + x, −1 + y, −1 + z; −1 + x, y, −1 + z; 1 + x, −1 + y, z; 1 + x, y, z −1 + x, y, z; 1 + x, y, 1 + z; −1 + x, y, −1 + z; 1 + x, y, z −x, −1/2+ y, 1/2 − z; −x, 1/2 + y, 1/2 − z; 1 + x, −1/2 − y, 1/2+ z; 1 + x, 1/2 − y, 1/2 + z −x, −1/2 + y, 1/2 − z; −1 + x, 1/2 − y, −1/2 + z; 2 − x, −1/2 + y, 1.5 − z; 1 + x, 1/2 −y, 1/2 + z 1 − x, −y, 2 − z 1 − x, −y, 1 − z −x, −y, 1 − z 2 − x, −y, 2 − 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 + x, 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 + x, y, z −1 + x, y, z; 1 + x, y, z 1/2 + x, 1/2 − y, 1/2 + z; −1/2 + x, 1/2 − y, −1/2 + z 1/2 + x, 1/2 − y, 1/2 + z; −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, 1/2 − y, 1/2 + z; 1/2 + x, 1/2 − y, −1/2 + z 1 + x, y, z; −1 + x, y, z; 1/2 + x, 1/2 − y, 1/2 + z; −1/2 + x, 1/2 − y, −1/2 + z −1 − x, 2 − y, 1 − z; −1/2 + x, 1.5 − y, −1/2 + z; 1/2 + x, 1.5 − y, 1/2 + z −x, 2 − y, 2 − z −1 + x, y, −1 + z; 1 + x, y, 1 + z 1/4 + x, 1/4 − y, −3/4 + z; −1/4 + x, 1/4 − y, 3/4 + z 1/4 + x, 1/4 − y, −3/4 + z; −1/4 + x, 1/4 − y, 3/4 + z 1/2 − x, −y, −1/2 + z; 1/2 − x, −y, 1/2 + z 1/4 − x, 1/4 + y, 1/4 + z; 1/4 − x, −1/4 + y, −1/4 + z −1 + x, 1 + y, z; 1 + x, −1 + y, z; −1 + x, −1 + y, z; 1 + x, 1 + y, z x, 1 + y, z; x, 1 + y, 1 + z; x, −1 + y, − 1+ z; x, −1 + y, z −1 + x, 1 + y, z; 1 + x, −1 + y, z; −1 + x, −1 + y, z; 1 + x, 1 + y, z −1 + x, 1 + y, z; 1 + x, 1 + y, 1 + z; −1 + x, −1 + y, −1 + z; 1 + x, −1 + y, z 1 − x, −1/2 + y, −1/2 − z; 1 − x, 1/2 + y, −1/2 − z 2 − x, −1/2 + −y, 1/2 − z; 2 − x, 1/2 + y, 1/2 − z 1 − x, 1 − y, −z 1 + x, 1/2 − y, 1/2 + z; −1 + x, 1/2 − y, −1/2 + z 2 − x, 1 − y, 1 − z; x, 1/2− y, −1/2 + z; x, 1/2 − y, 1/2 + z −x, 1/2 + y, 1/2−z; −x, −1/2+ y, 1/2 − z −x, −y, 1 − z x, 1/2 − y, 1/2 + z; x, 1/2 − y, −1/2 + z 1/2 − x, −1/2 + y, z; 1/2 − x, 1/2 + y, z

0.01399 0.01173 0.01054 0.01047 0.01019 0.00956 0.01159

12.8 10.7 9.6 9.4 9.1 8.6 9.0

60.2

0.01120

8.6

0.00766 0.01320

7.2 10.8

0.01343

10.4

0.01985 0.01982 0.01971 0.01882 0.01640 0.01624 0.01543 0.01529 0.00787 0.00777 0.00782 0.00775 0.0148 0.01418 0.01184 0.00997 0.00944 0.00908 0.01568 0.01639 0.00787 0.00714 0.01974 0.01679 0.01193 0.00927 0.01618

20.7 20.6 20.4 19.2 14.1 13.9 13.0 12.8 6.9 6.8 6.8 6.7 13.5 13.4 10.4 8.8 8.3 7.9 16.2 15.6 6.6 6.1 19.3 13.5 10.6 7.5 15.1

0.01279 0.01218

11.5 10.8

0.00857

7.1

0.00974 0.00884 0.00600 0.01068 0.00805 0.00762 0.00670 0.00589 0.01172 0.01169

9.5 8.4 4.7 9.3 7.2 6.1 5.6 4.6 9.4 9.4

A−H···D (deg)

42.4

42.4

40.48

62.3

44.5

50.9

44.5

22.6

32.8

18.8

contacts in the molecule of impact sensitive β-HMX, 2.19 and 2.37 Å, no critical points associated with these possible interactions were located.44 Therefore, it seems for energetic molecule

DATB, i.e., an Edr value greater than the 78.4 J shown in Table 1. Differently, an AIM analysis carried out by Zhurova et al. showed that despite the existence of two short O···H intramolecular 4706



Article

HBs, planar TATB, DAAzF and DAAF molecules extends to infinite two-dimensional layers to stack crystals. In Figure 4a, all the surrounding atoms, the oxygen atoms on the nitro groups and the hydrogen atom on the amino groups, take part in forming N−H···O interactions. In DAAzF of Figure 4(b), the N−H···N interactions take place between the hydrogen atoms on amino groups and the nitrogen atoms on furazan rings of their neighboring molecules. Azo groups do not contribute to HBs. The case of DAAF in Figure 4c is similar to that of DAAzF, with only a difference that the oxygen atoms on azoxy groups are active to accept hydrogen atoms to form HBs. Comparing EHB in Tables 2 and 3, we can find that these intermolecular HBs are much weaker than the intramolecular ones. However, this intralayer hydrogen bonding is still big enough to support the layer, because the summed EHB of one TATB molecule, for example, is up to 60 kJ/mol. Similarly, these weak but many HBs hold the DAAzF and DAAF layers in crystals. NTO, FOX-7, and LLM-105 exhibit the wavelike stacking of the second type shown in Figure 5. Interestingly, these three

that the intramolecular hydrogen bonding is preferred in the LSHE molecules. 2.2. π−π Stacking and Intermolecular Hydrogen Bonding. Examining crystal packing of all these LSHEs, we find it is wholly π−π stacking with the aids of intermolecular HBs. The π−π stacking can be divided into four types in terms of the orientations of molecular planes in crystal, as illustrated in Figure 2: (a) face-to-face type. All molecular planes in crystal are parallel to one another; (b) wavelike type and (c) crossing type. All the molecular planes in crystal are parallel to two planes, which are not parallel to each other. If we do not find an intermolecular crossing along these two planes, the crystal belongs to the wavelike type; otherwise, it belongs to the crossing type; and (d) mixing type. This type is beyond all the three types aforementioned. Figure 3 shows the face-to-face stacking of TATB, DAAzF, and DAAF. Strictly, only TATB belongs to this type. DAAzF and DAAF are classified to this type because of very small fluctuations on their crystal layers. Furthermore, as described later, classifying DAAzF and DAAF into the face-to-face type is helpful to understand their low impact sensitivity, in spite of their no high molecular stability. Careful examining the faceto-face stacking in the three crystals, we find that they are all offset. Previous study showed that the interlayer distance will influence both dispersion and electrostatic interactions, while the layer offset will make mainly electrostatic interactions changed.4 As a matter of fact, for example, the observed offset site of the TATB layer corresponds to the lowest potential site, suggesting the most stable stacking.4 After offset, same groups among neighboring layers are staggered to avoid electrostatic repulsion. Relative to full face-to-face stacking, this offset shortens the interlayer distances, increases the interlayer attractions and makes crystal packing more compact. The interlayer distances of TATB, DAAzF, and DAAF are about 3.1, 3.2, and 3.3 Å, respectively, shorter than those of graphite (3.4 Å)45 and the full face-to-face stacked benzene dimer (3.8 Å).46 This π−π stacking is supported by intermolecular HBs. That is to say, the intralayer intermolecular interactions are mainly HBs. As illustrated in Figure 4, by means of intermolecular

Figure 5. Three LSHE crystals with wavelike stacking: (a) NTO, (b) FOX-7, and (c) LLM-105.

explosives are most powerful (the highest VOD) among all 11 LSHEs as indicated in Table 1. This is related with their high packing densities 1.915, 1.893, and 1.919 g/cm3, and good oxygen balances. On the wavelike layers of NTO, FOX-7 and LLM-105, the wave periods involve 4, 2, and 4 molecules, respectively. Their interlayer distances are about 3.0, 3.2, and 2.9 Å, respectively. From the HBs shown in Figure 6, it can be found that the neighboring NTO molecules are not hydrogen bonded together at wave crests and wave hollows, distinct from FOX-7 and LLM-105 where the HBs support the wavelike layers successively. Therefore, it seems that the wavelike layers in NTO are composed of numerous plates, each with a width of two NTO molecules. Figure 6a exhibits the plates each consisting of two rows of NTO molecules, which are compactly hydrogen bonded with each other. N−H···N and N−H···O interactions exist in the hydrogen bonding with various EHB from several to 20 kJ/mol, as listed in Table 3. Some N−H···O interactions are the strongest intermolecular HBs among the all LSHEs. The nitro groups of NTO are located at wave crests or wave hollows, and do not contribute to the HBs, much different from other nitro-contained LSHE crystals. Similar to TATB, all surrounding atoms of FOX-7 are involved in the intermolecular HBs in Figure 6b. It is interesting that there are several types of

Figure 4. Intermolecular hydrogen bonds, represented by green dash, in three face-to-face stacked LSHE crystals: (a) TATB, (b) DAAzF, and (c) DAAF. 4707



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intermolecular HBs formed by a way of split-level. This is a distinct characteristic of the HBs involved in this crossing stacked crystals. Figure 8 exhibits the intermolecular HBs in these four LSHE crystals. Similar to TATB and FOX-7, compact intermolecular HBs are formed in the NQ crystal, as illustrated in Figure 8a. TNA and DATB have similar molecular

Figure 6. Intermolecular hydrogen bonds, represented by green dash, in three wavelike stacked LSHE crystals: (a) NTO, (b) FOX-7, and (c) LLM-105.

intermolecular HBs in FOX-7 crystal, including R12(6), R21(6), R23(10), R32(10), and R22(6), according to Bernstein et al.’s definition.47 The case of LLM-105 is between those of NTO and FOX-7. That is, as indicated in Figure 6c, half of the oxygen atoms sited around the wave crests and wave hollows do not take part in forming HBs. And HBs formed there are rather weak, with EHB of only about 6 kJ/mol in Table 3. As to the crossing stacking, it involves NQ, TNA, DATB, and DNDP. It seems that these crystals are composed of layers along two crossed directions in Figure 7. The most distortion appears in TNA, that is, one of nitro groups is distorted about 22° from the benzene plane in the molecule due to the rather big intermolecular nitro−nitro repulsion. The interlayer distances of NQ, TNA, DATB and DNDP are about 3.2, 3.5, 3.3, and 3.1 Å, respectively. As for the intermolecular HBs, as shown in Figure 7 and Figure S3 in the Supporting Information, they

Figure 8. Intermolecular hydrogen bonds, represented by green dash, in four crossing stacked LSHE crystals: (a) NQ, (b) TNA, (c) DATB, and (d) DNDP.

structures to TATB, but lack active hydrogen atoms for HB formation. Therefore, we can find from panels b and c in Figure 8 that their oxygen atoms do not contribute completely to the intermolecular HBs. The similar case occurs for DNDP in Figure 8d. Respectively, one, three, and two oxygen atoms of TNA, DATB, and DNDP do not take part in the HBs. In addition, EHB in Table 3 show the intermolecular HBs in NQ and DATB are stronger that those in TNA and DNDP. Regarding the final type of mixing stacking, it appears in TNB only. TNB is a crystal with Z = 16 and Z′ = 2, leading to much more molecular orientations relative to other LSHE crystals and mixing stacking. As shown in Figure 9, TNB has two kinds of

Figure 9. Mixing stacking of TNB crystal. Figure 7. Four LSHE crystals with crossing stacking: (a) NQ, (b) TNA, (c) DATB, and (d) DNDP.

interlayer distances of 3.1 and 3.7 Å. Similar to TNA and DNDP, hydrogen atoms linked with benzene ring is active to form HBs but with weak strength of about 9 kJ/mol of EHB in Table 3. Besides, comparing Figure 9 with Figures 4, 6, and 8, we find that HB in TNB is very sparse, relative to other LSHE crystals. In addition, we employ ΣEHB/2 to show the strength of the intermolecular hydrogen bonding. All listed in Table 3, the more

connect noncovalently the molecules to build three-dimensional net structures of these crystals, instead of the two-dimensional layered structures of the two former kinds of stacking. And Figure S3 in the Supporting Information shows the 4708



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intermolecular hydrogen bonding, from 60.2 to 44.5, 32.8, and 18.5 kJ/mol. 2.3. Hirshfeld Surface Analyses. Hirshfeld surface can effectively reflect the crystal packing. From the surfaces in Figure 10, we can summarize two features. One is that all the surfaces appear in plate shapes, representing their planar conjugated molecular structures. And the other is that all (TATB, DAAF and FOX-7) or most red dots (remaining 8 LSHEs) located on the edges of each plate-like surface show the predominant intermolecular interactions there, according to Spackman, et al’s definition mentioned above: the red and blue on the surfaces denote the high and low close contact populations, respectively.48−50 These two features of Hirshfeld surfaces of 11 LSHEs are in accordance with above discussion on the crystal packing. As a matter of fact, the red dots sited on the edges denote mainly the intermolecular H···O and H···N interactions, in agreement with above analyses of intralayer intermolecular HBs supporting the layers to form face-to-face π−π stacking, whereas those on the plate faces usually belong to π−π stacking, such as O···C, C···C, and C···N interactions. This can also be ascertained by the two-dimensional fingerprint plots in Figure 11, each with a pair of remarkable spikes on the bottom left and a pair of wings on the up right, denoting the HBs among intralayer neighboring molecules and

Figure 10. Hirshfeld surfaces of 11 LSHE molecules in crystal stacking, each shows by two plots with a torsion of 180°. TNB has Z′ = 2, as illustrated by (k) and (l) separately. a−g denote contacts of O···H, N···H, O···N, O···O, O···C, C···C, and C···N, respectively.

and the more active HB acceptors and donors contained in the molecules suggest the stronger hydrogen bonding in general. Typically, TATB, DATB, TNA, and TNB, with fewer and fewer HB donor amino groups, possess the increasingly weakened

Figure 11. Two-dimensional fingerprint plots in crystal stacking. TNB has Z′ = 2, as illustrated by (k) and (l) separately. 4709



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Figure 12. Populations of close contacts of 11 LSHE molecules in crystal stacking. TNB has Z′ = 2, as illustrated by TNB(1) and TNB (2).

interlayer π−π stacking, respectively. That is, π−π stacking with aid of intermolecular HBs. In general, the shorter de + di of the spikes suggest the stronger HB. Comparing those de + di of the LSHEs, we deduce that NTO, FOX-7, LLM-105, and NQ possess stronger HBs in contrast to the remaining, consistent with the EHB using AIM analyses in Table 3. From the close contact populations in Figure 12, we can confirm that the hydrogen bonding involving O···H and N···H possesses the most percentage around 40%, and even above 50% in TATB, NQ, DAAF, DAAzF, FOX-7, and LLM-105, suggesting the hydrogen bonding is an important characteristic of LSHEs. O··· O interactions occupy the second place. The rather high populations of O···O interactions can be readily understood by the oxygen atoms surrounding each molecule and π−π stacking.8 The remaining interactions are mainly involved in π−π stacking. 2.4. Stacking Effect on Impact Sensitivity. An interesting concern is the stacking effect on impact sensitivity. Several teams have carried out quantum chemical and empirical potential calculations to uncover the stacking mode-sensitivity relationships.4−7,51,52 And steric hindrance caused by interlayer sliding is usually thought to be a crucial factor influencing impact sensitivity. Here, we can also understand the low impact sensitivity of these LSHEs by straightforward diagrams of crystal packing, instead of calculations.4−7,51,52 As illustrated in Figure 13,

Besides TATB with high molecular stability, DAAzF and DAAF, with BDE close to HMX,53 are very impact insensitive (their Edr > 54.9 J, much higher than that of HMX, 7.35 J21). That is to say, crystal packing is indeed an important factor determining safety, and the advantage of crystal packing will cause low sensitivity despite low molecular stability. As a whole, the face-to-face stacked explosive crystals are most insensitive to impact, with high values of Edr in Table 1. And TATB even possesses the highest Edr of 120.2 J among the all LSHEs. As to the wave-like stacking in Figure 13b, the sliding is limited to one line, with much less sliding orientations relative the face-toface type. The crossing stacking in Figures 13c involves two cases: the sliding is strongly unallowed (NQ, TNA, and DATB) and constrainedly allowed (DNDP). While, as shown in Figure 13d, for mixing stacking type of TNB, sliding along any orientation is strongly forbidden, implying very high impact sensitivity. In fact, TNB has the smallest Edr of 24.5 J among the all LSHEs in Table 1. It should be noted that NQ, DATB and DNDP belong to crossing stacking but still with very low impact sensitivity similar to the face-to-face stacked TATB, DAAzF and DAAF, suggesting that the sensitivity mechanism is very complicated and any factor considered alone will be insufficient. To be much clearer to show the crystal packing-impact sensitivity relationship, we adopt inter/intrapotentials increased during sliding in Figure 14 to predict the possibility of hot spot

Figure 14. Inter/intramolecular potential (p)−sliding distance (d) dependences of four kinds of stacking. a and b denote the sliding along right/left and front/back, respectively.

Figure 13. Plot showing interlayer sliding allowed (green in the indicator light) and unallowed (red in the indicator light) of four kinds stacking. On the indicator lights, right/left and up/down arrows represent the sliding along right/left and front/back, respectively. Red double arrows point to the sites of sliding unallowed.

formation. The interlayer sliding along a or b in the face-to-face stacked crystals will lead to so small inter/intramolecular potential increases as not to decay molecules (in Figure 14a), that is, it is difficult to form hot spots and cause final combustion or detonation. As shown in Figure 14b, the slide of the wavelike stacking along a will increase the potentials much, corresponding to the unallowed slide, whereas the slide along b causes a

the aforementioned four stacking types will lead to different steric hindrances when interlayer sliding happens. The face-to-face stacking in Figure 13a will cause the easiest slide, as it occurs along the molecular plane, resulting in very low impact sensitivity. 4710



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nearest atom interior and exterior to the surface respectively, and the van der Waals radii of the atoms, and represented by eq 1.

small increase, corresponding to the allowed slide. Figure 14c shows a big increase along a constrainedly allowed sliding orientation, for example, a sliding vertical to the front view of Figure 7d of DNDP will cause the intermolecular hydrogen bonding changes and increase the potentials much. It is also indicative of abrupt potential increases for the other cases, suggesting the strongly unallowed sliding. For the mixing stacking in Figure 14d, small sliding will resulting in a catastrophic potential increase to decompose molecules to form hot spots. Regarding the crystal packing influencing impact sensitivity, the cases of the wavelike and crossing stacking mediate the face-to-face and mixing stacking, as the Edr listed in Table 1.

dnorm =

d i − d ivdW rivdW

+

de − revdW revdW

(1)

dnorm enables the identification of the regions of particular importance to intermolecular interactions. That is to say, a Hirshfeld surface is composed of lots of points, and each point parametrized as (di, de) can provide information about related contact distances from it. The smaller di + de suggests the closer atom−atom contact. Both di and de were constrained in a range of 0 to 2.6 Å. Mapping these (di, de) points and considering their relative frequencies, one can get a two-dimensional (2D) fingerprint plot. For any symmetrically dependent molecule in any crystal, the fingerprint is unique. This is the base for identifying a crystal environment of a given molecule. The color mapping distinguishes the intensity of points, and the red and the blue represent the high and low intensities, respectively. Therefore, through the locations of (di, de) points and their relative frequencies discernible on the surface and the 2D fingerprint plot, we can ascertain the distances and intensities of these contacts. All the surfaces and fingerprint plots are created using CrystalExplorer3.0.57 And in this work, the surfaces were mapped over a dnorm range of −0.2 to 1.2 Å. Bond Dissociation Energy (BDE) Comparison of DAAzF, DAAF, and HMX. After confirming the weakest bonds among the DAAzF, DAAF, and HMX by NBO methods, we calculated the energy increases after partitioning these bonds at the level of M06-2X/6-311+G (d,p) and regarded them as BDE for comparing molecular stability. Evaluation of Detonation Velocities (VOD). Because of the various values of VOD by tests and theoretical calculations, we uniformed them by calculating VOD using VLW method33 and the popular package of EXPLO5.0.34 As listed in Table 1, all LSHEs have a higher VOD than TNT.

3. CONCLUSIONS After analyzing crystal packing of 11 LSHEs whose both energy and safety close or superior to TNT, we find that, from molecule to crystal, they possess the characteristics as follows: (1) Each molecule is π-conjugated with all non-hydrogen atoms in the entire molecule. Intramolecular hydrogen bonding exists in most LSHE molecules and enhances their stability. (2) The crystal packing of LSHEs is the π−π stacking in principle with the aid of intermolecular hydrogen bonding supporting the layers. Four types of the stacking are classified to reveal the crystal packing and understand the stacking effect on impact sensitivity. The face-to-face stacked crystals like TATB are most insensitive to impact as it can readily lead to interlayer sliding with too small inter/intramolecular potential increases to analyze molecules and form hot spots, whereas the mixing stacked crystals like TNB are strongly forbidden to slide along any orientation, suggesting the easy molecular decomposition caused once slide occurs. This is the reason for the crystal packing as a factor influencing impact sensitivity. Besides, we find the hydrogen bonding occupies the most population in intermolecular interactions and the O···O interactions are sited on the second place of LSHEs. The intramolecular HBs are usually stronger than the intermolecular ones. From this study, we can confirm that in the crystal engineering of LSHEs at least, the π−π stacking with the aid of intermolecular hydrogen bonding is requisite. Methodologies. Full Optimization of Single Molecular Structures. All related LSHE molecular structures were fully optimized at the level of M06-2X/6-311+G (d,p)54 using the Gaussian 09 package.55 All the stationary structures were ascertained by no imaginary frequency. M06-2X is one of the most efficient functional describing noncovalent interactions, which extensively intramolecularly and intermolecularly exists in the discussed systems.54 AIM Analyses. In AIM analyzing, the required wave functions of the single molecules and the molecular pairs extracted from crystals were calculated at the level of M06-2X/6-311+G (d,p). Hydrogen bonding energy (EHB) was assessed through analyzing the electron density (ρ) at the bond critical point, by which the potential energy density (v) can be obtained.55 Then, EHB was predicted using an empirical equation EHB = −(1/2)v proposed by Espinosa et al.43 Hirshfeld Surface Analyses. We employ Hirshfeld surfaces48−50 to show the intermolecular interactions in the crystals. Hirshfeld surfaces in a crystal are constructed in terms of the electron distribution, calculated as the sum of spherical atom electron densities. The normalized contact distance (dnorm) is determined by di and de, the distances from the surface to the

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ASSOCIATED CONTENT

S Supporting Information *

Crystallographic information on 11 LSHEs, front views and side views, and maximum torsion angles from the molecular plane of 11 LSHE molecules, and split-level formation of hydrogen bonds in four crossing stacked LSHE crystals.This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for the financial support from the Science and Technology Fund of CAEP (2011A0302014 and 2012A0302013), the Science and Technology Innovation Fund of ICM (KJCX201305), and the National Natural Science Foundation of China (21173199).

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ABBREVIATIONS 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 4711


Crystal Growth & Design DAAz DATB DNDP FOX-7 HMX LLM-105 NQ NTO RDX TATB TNA TNB TNT

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Ftrans-(p,p)-3,3′-diamino-4,4′-azofurazan 1,3-diamino-2,4,6-trinitrobenzene 4,6-dinitro-1,3-diphenol 1,1-diamino-2,2-dinitroethylene 1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane 2,6-diamino-3,5-dinitro-1,4-pyrazine 1-oxide 2-nitroguanidine 5-nitro-1,2-dihydro-3H-1,2,4-triazol-3-one 1,3,5-trinitro-1,3,5-triazacyclohexane 1,3,5-triamino-2,4,6-trinitrobenzene 2,4,6-trinitroaniline 1,3,5-trinitrobenzene 2,4,6-trinitrotoluene

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