Packing Structures of CL-20-Based Cocrystals - ACS Publications

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Packing Structures of the CL-20-based Cocrystals Guangrui Liu, Hongzhen Li, Rui-jun Gou, and Chaoyang Zhang Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b01228 • Publication Date (Web): 02 Oct 2018 Downloaded from http://pubs.acs.org on October 6, 2018

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Crystal Growth & Design

Packing Structures of the CL-20-based Cocrystals Guangrui Liu, †,‡ Hongzhen Li, ‡ Ruijun Gou,*† and Chaoyang Zhang*‡,$ †

College of Environment and Safety Engineering, North University of China, Taiyuan 030051, China 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: The CL-20-based cocrystals (CCCs) are now the most active in the field of energetic cocrystals, due to an advantage of high energy density while a disadvantage of low stability of CL-20, which deserve to be tuned with desired structures and properties by cocrystallization. This work presents a comprehensive insight into the packing structures of 27 CCCs observed till 2017. Firstly, it shows a high multiplicity of the coformer molecules of CL-20, with various shapes and sizes. Regarding the conformers, the β-, γ-, η-, ε-, and ζ-forms appear in the CCCs, with a total above that observed in the CL-20 polymorphs; that two forms can exist in a same CCC highlights a difference in conformer between single component crystals and cocrystals; and the γ- and β-forms govern the CCCs with a total population of 87 %. This high conformational diversity serves as a reason for the abundance of CCCs. Meanwhile, various stoichiometric ratios from 1:1 to 1:6 except from 1:5 are observed, and the lower ones predominate the CCCs, with populations of 48 and 40 % for 1:1 and 1:2, respectively. Moreover, it exhibits wavelike, sandwich, channel and caged molecular stacking in the 27 CCCs. Among these stacking, O…H, O…N and O…O contacts dominate the weak intermolecular interactions, which feature the hydrogen bonding between the H atoms of CL-20 and the acyl/ether O atoms of the coformer molecules, and the p (of O atoms on the NO2 of CL-20)–π (of the big π-bonds of coformer molecules) interactions. The weak intermolecular interactions contribute to the small molecular volume variations of CL-20 after cocrystallization, with a maximum relative error of ~3 %. Besides, each CCC mediates the packing density between those of the two related pure components; and no CCC outperforms ε-CL-20 in packing density. Finally, we find that the high contents of O and N facilitate to increase packing coefficients and packing densities. All these findings are expected to richen the knowledge of both energetic materials and cocrystals, and enhance the rationalization of crystal design. 1 / 35Environment ACS Paragon Plus

Crystal Growth & Design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1. INTRODUCTION 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20)1 is currently the most powerful energetic material (EM) that has already been commercially available. The high energy density of CL-20 is attributed to its high packing density (d, for ε-CL-20, the most compact polymorph of CL-20 at ambient conditions, d=2.044 g/cm3), excellent oxygen balance (OB, -10.95 %) and high heat release (6.2 kJ/g). Nevertheless, the application of CL-20 remains still limited, due to some intractable difficulties like high sensitivity to external stimuli, ready polymorphic transformations and high cost. For example, the drop energy of ε-CL-20 is 13 J, lower than those of the moderately insensitive TNT (49 J) and the very insensitive TATB (120 J), suggesting its higher impact sensitivity in contrast to many common EMs.2,3 To overcome the difficulty of the high sensitivity, people have prepared a series of CL-20-based cocrystals (CCCs) by combining CL-20 molecules and other heterogeneous molecules together to form new crystals with new structures and improved properties and performances, such as reduced sensitivity and enhanced energy density. Cocrystallization has already been thought to be a strategy for creating new materials with tunable properties and performances.4 For CCCs, CL-20 serves at least as an energy source and sets a base for performance tuning due to its energetic superiority.

Figure 1. Evolution of the CL-20-based cocrystals (CCCs).

As a matter of fact, CCCs occupy the largest population of all energetic cocrystals (ECCs). This is largely attributed to two reasons: one is that CL-20 possesses an energetic superiority as pointed out above, and therefore more molecules can be adopted as coformers when keeping a certain level 2 / 35Environment ACS Paragon Plus

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of energy; and the other is that CL-20 itself tends to form a large quantity of cocrystals due to its high flexibility by the ready rotation of its NO2 groups.5 Regarding the term of a cocrystal, it becomes more and more extensively accepted that it refers to a two or multiple component molecular crystal. Therefore, solvate is a subset of cocrystal.6,7 We collect all CCCs ascertained experimentally till 2017 in Figure 1, with a total of 27 (there are two ones of CL-20/H2O2). By the way, I will apologize for anything overlooked. Interestingly, together with the three polymorphs of CL-20 at ambient conditions (β-, γ-, and ε-CL-20), the solvate of CL-20 (α-CL-20) was reported in a same paper.1 Among these cocrystals, CL-20/TNT manufactured by Matzger’s group is a milestone in the evolution of CCCs, or even the entire ECCs, as they firstly proposed a strategy of energetic cocrystallization to tune properties and performances of EMs and prepared the first energetic-energetic cocrystals (EECCs).8 This will make some forgotten EMs back.9 Afterwards, the amount of CCCs increases gradually. In particular, five and six CCCs came into being in 2012 and 2014, respectively. It should be noted that, in 2012, the CL-20/HMX cocrystal was manufactured by the same group and thought to possess a potential as a secondary explosive,10 because it is more energetic than HMX while is the same impact safe as HMX, which possesses the most excellent comprehensive performances among all existing EMs, and is applied extensively and called as the king of explosives. Unfortunately, the CL-20/HMX cocrystal has not been applied yet. Besides, Matzger’s group prepared two ECCs of CL-20/H2O2. Among them, the orthorhombic one was expected to possess a higher denotation velocity than ε-CL-20 that is currently the most powerful single component EM, by improving oxygen balance and maintaining a high density close to that of ε-CL-20, i.e., 2.03 vs. 2.04 g/cm3.11 It verified the energy enhancement as an advantage of energetic cocrystallization. After all, with respect to some special properties and performances, that a cocrystal outperforms its pure single components is

3 / 35Environment ACS Paragon Plus


excited and desired. Regarding the status of energetic cocrystallization, it is in fact at its primary stage, therein it requires to solve many elementary and important questions or difficulties, such as a simple criterion for assessing cocrystal formation from two components or not, simple quantitative structure-property relationships, mechanisms for nucleation and growth, large scale production and application, and so forth. Overall, the energetic cocrystallization falls much behind the pharmaceutic one.12,13 And we might have a long way to make ECCs applicable. Nevertheless, undoubtedly, learning from the existing ECCs will facilitate us to enrich knowledge, overcome the difficulties and accelerate the creation of new EMs. Particularly, the energetic cocrystallization possesses a remarkable sense of crystal engineering. In the past decades, thousands of energetic molecules came into being and subsequently were put on the shelf, as they weren’t found to possess any application potential. Are these molecules completely useless? Cocrystallizing them might make a change. We are convinced that it is also a way to new EMs by cocrystallizing the existing molecules two ways, besides synthesizing new molecules. 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.14,15 Obviously, clarifying the relationships in the existing CCCs will facilitate the application of crystal engineering to EMs. In the present work, we focus upon the packing structures of existing CCCs. We find that five molecular conformers of CL-20 exist in the observed CCCs, including three in the polymorphs under common condition (β-, γ-, and ε-CL-20), one in a high-pressure polymorph (ξ-CL-20), and a new one that has not been observed as in a polymorph (η-CL-20). The high variability of the molecular conformers of CL-20 is attributed to the low barrier of the NO2 rotation, which is the origin of the high flexibility of the CL-20 molecule. Moreover, the CL-20 molecule can be 4 / 35Environment ACS Paragon Plus

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cocrystallized with other variously shaped and sized molecules and various stoichiometric ratios from 1:1 to 1:6 except from 1:5; and the lower ratios dominate the CCCs. The molecular stacking in CCCs appears in wavelike, sandwich, channel and caged shapes. Besides, we find a small difference in molecular density (dm), with the packing density (dc) governed by packing coefficient (PC) in this case. Interestingly, the PC of the observed CCCs increases with the increasing of the populations of close intermolecular interatomic contacts of O-O and N-O, while it reduces with that of O-H; also, the PC increases with the increasing of the contents of N and O, while with the reduction of H. In addition, the O…H, O…N and O…O contacts dominate the weak intermolecular interactions, which feature the hydrogen bonding (HB) between the acyl/ether O atoms of coformer molecules and the H atoms of CL-20, and the p (of O atoms on the NO2 of CL-20)–π (of the big π-bonds of coformer molecules) interactions. The weak intermolecular interactions contribute to the small molecular volume variations of CL-20 after cocrystallization. These new findings from the observed CCCs will help us to deepen the insight into ECCs and promote the rationalization degree of manufacturing new ECCs with desired components, structures and properties by crystal design in advance. Hopefully, it will set a base for creating new EMs along the concept of crystal engineering. 2. METHODOLOGIES 2.1 Objects As mentioned above, we focus upon the existing CCCs each with a crystallographic information file available. Thus, the partial crystallographic information and some properties of these CCCs, including molecular conformers of CL-20, stoichiometric ratios, melting point (Tm), DSC peak of thermal decomposition (Td) and measurement conditions, are collected in Table 1, with lattice parameters collected in Table S1 of Supporting Information (SI). These information sets a base for following analyses and calculations to deepen the insight into the CCCs. Figure 2 shows



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the former molecular structures of all the CCCs.1,8,10,11,16-33 As illustrated in the figure, the CL-20 molecules can be cocrystallized with variously shaped molecules, including chains, rings, condensed rings and poly-rings. Regarding the size of the coformer molecules, they can be as small as H2O and CO2, and as large as HMPA and TPPO. The various sizes and shapes of the coformer molecules suggest their high structural diversity and a high ability of the CL-20 molecule to be cocrystallized with them. Table 1. Partial Crystallographic Information and Some Properties of the Existing CCCs. Molecular conformer

Co-crystals

Tm , ℃

Td , ℃

H50, cm

Space Group

Density, g/cm3

Measurement conditions, K

2CL-20/1H2O 1

γ

Pbca

1.981

283-303

1CL-20/2DMF 16

γ

16

/

/

P-1

1.654

283-303

γ

160-200

210

/

Pbca

2.031

153(>16MP)

17

2CL-20/1CO2

18

1CL-20/1GTA

ζ

/

/

/

C2/c

1.650

283-303

1CL-20/1TNT 8

β

136

/

99(47) a

Pbca

1.910

95

1CL-20/4DO 19

γ

/

/

/

P-1

1.603

150

γ

/

/

/

Cc

1.858

100

β

/

/

/

P21/c

1.436

100

P21/c

2.001

95

1CL-20/1BL

19

1CL-20/3HMPA 19 10

2CL-20/1HMX 1CL-20/1BTF

20

1β/1γ

/

235

55(29)

b

β

198

235

1CL-20/1DNB 21

β

91

252

55(14) c

1CL-20/1AZ1 22

γ

136.6

216.8/242.8

/

1CL-20/1BQ

23

1CL-20/1NAQ 23 24

1CL-20/6CPL

1CL-20/1Xylene

25

P212121

1.926

283-303

Pbca

1.880

283-303

P21

1.971

100

c

β

/

/

>112(14)

Pna21

1.737

283-303

γ

132

235

>112(14) c

P21/n

1.774

283-303

γ

176

245

/

P-1

1.340

283-303

β

169

/

/

Pbca

1.823

153

1CL-20/2NMP·1H2O 26

γ

105

240.8

/

P21/c

1.602

283-303

1CL-20/2TPPO 27

1ε/1β

123.57

/

112(14) c

P-1

1.527

100

28

β

141

242

/

P21

1.750

200

2CL-20/1DNP 28

γ

170

237

/

P21/c

1.928

150

P-1

1.753

283-303

1CL-20/1DNG

1CL-20/2DNT

29

2CL-20/1MNO

30

β

120.8

216.4

44(20)

d

β

184

242

/

P21

1.947

150

2CL-20/1H2O2(1) 11

γ

165

250

24(29) e

Pbca

2.071

85

2CL-20/1H2O2(2) 11

γ

190

250

28(29) e

C2/c

2.022

85

1CL-20/2MAM

31

ζ

99.4

239.5

/

Pbcn

1.721

200

1CL-20/1MTNP 32

γ

215

222

6J(4J) f

P1211

1.932

293

1CL-20/1AZ2 33

η

175

217

/

P21

1.939

150.01

Note, numbers prior to the coformer molecules represent the stoichiometric ratio of each case. 2CL-20/1H2O2 (1) and 2CL-20/1H2O2 (2) represent the orthorhombic and monoclinic crystals, respectively. H50 represents impact sensitivity and is a drop hammer height, from where a given weight falling upon the compound gives a 50% probability of initiating an explosion. The impact sensitivity of CL-20/MTNP is denoted by impact energy with unit in J. The bracketed H50 are of ε-CL-20 under the same testing conditions for comparison. Because attribute, 6 / 35Environment ACS Paragon Plus

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sample status and testing condition are all responsible for H50, it is necessary to describe them for assessing the measurement values as follows: a a 2.94 kg and stainless-steel impactor, an anvil, and samples of 1 mg each sealed inside an aluminum DSC pan to consolidate the sample and prevent sample ejection during testing;8 b an apparatus designed to accommodate small amounts of material, and 0.5 mg samples each struck with a freefalling 2.27 kg weight dropped from variable heights;10 c a WL-1 instrument, a drop weight of 2 kg, and each sample of 30 mg;21,23,27 d a drop weight of 5 kg and each sample of 50 mg;29 e a drop weight of 5 lb and each sample of 0.5 mg;11 f the international standard BAM method.32

Figure 2. Molecular structures involved in observed CCCs.

2.2 Analysis and calculation methods. We are concerned ourselves about the packing structures of the observed CCCs. Thus, molecular conformers, molecular electrostatic potentials (ESP), packing styles, dm, dc, PC, and 7 / 35Environment ACS Paragon Plus


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intermolecular interactions are paid attention to. Molecular conformer analysis. A CL-20 molecule is composed of a cage of hexaazaisowurtzitane and six NO2 groups each linked with a N atom of the cage. Possibly, 24 conformers of CL-20 exist according to the orientation symmetry of NO2 in respect to the cage. Because the molecular conformers of CL-20 in its various polymorphs are distinguished by the polymorphs themselves, we can distinguish the conformer types by the polymorph ones. Up to the present, four polymorphs of CL-20 were observed, including three (β-, γ- and ε-forms) under common condition 1 and one (ζ-form) at high pressures 34. In the CL-20/AZ2 cocrystal 33, the CL-20 molecule appears in another one except from above four types, as named by η-CL-20, by respecting the original authors. In terms of the orientation symmetry of NO2 in respect to the hexaazaisowurtzitane cage, we analyzed the conformer type for each CL-20 molecule involved. Molecular stacking analyses. The molecular stacking analysis was performed based on crystallographic information determined experimentally. For each CCC, the centroid of each molecule were highlighted and distinguished by the molecular kind, and molecular stacking style was ascertained by the spatial distributions and the kinds of the centroids. PC calculations. PC was calculated according to equation PC =

∑V

m

/ V c , where Vc and Vc

are the molecular and crystal volumes, respectively. In the present work, the Vm was obtained by three means. Firstly, a volume enclosed through a surface with an assigned electronic density was considered as Vm. In this work, the electronic density was calculated at the theory level of B3LYP/6-311+G(d,p) 35, and the density of 0.003 a.u. was adopted for Vm calculations, as in our previous work (MI) 36. Next, the Vm was thought to be a volume enclosed through a van der Waals surface with van der Waals radii of 1.70, 1.20, 1.52 and 1.55 Å for C, H, O and N atoms, respectively, as Bondi radii (MII);37 also, those of 1.75, 1.09, 1.56 and 1.61 Å were considered for C,


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H, O and N atoms, respectively, as normalized Bondi radii (MIII).38 By means of Vm, dm was calculated through dm =M/NAVm, where M and NA are the molecular quantity and the Avogadro's number, respectively. Hirshfeld surface analyses. The principle of Hirshfeld surface analyses can be referred elsewhere. 39-41 Briefly, 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 nearest atom interior and exterior to the surface respectively, and the van der Waals radii of the atoms. dnorm enables the identification of the regions of particular significance 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 implies 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, we can get a two-dimensional 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 fingerprint plots were created using CrystalExplorer 3.0.57 in this work, and the surfaces were mapped over a dnorm range of -0.2 to 1.2 Å.42 ESP calculations. The density function theory at the level of B3LYP/6-311+G (d,p) was also adopted to calculate ESP of isolated former molecules and some molecular pairs in the asymmetric units. The ESP on the isosurface of electron density (ρ) of 0.001 au was shown using GaussView 5.0.8.



3. RESULTS AND DISCUSSION As mentioned above, we will pay attention to the packing structures of the CCCs in this work. Thus, the molecular conformers and stoichiometric ratios, molecular stacking styles, intermolecular interactions, packing coefficients and densities are individually discussed. 3.1 Molecular conformers and stoichiometric ratios.

Figure 3. Five types of molecular conformers of CL-20 in some typical CCCs.

Possibly, 24 conformers of CL-20 exist according to the orientation symmetry of the NO2 groups in respect to the cage. A previous calculation

5

confirmed eight stable conformers at the

B3LYP/6-31+G (d,p) level, the largest difference in the total energy among these conformers of 6.15 kcal/mol, and the barriers for the transformation among these conformers by NO2 rotation within several kcal/mol too. The ready NO2 rotation is the root for the high flexibility of CL-20 to be cocrystallized with other heterogeneous molecules. As illustrated in Figure 2, 26 molecules with various sizes and shapes have been found to form 27 CCCs (two of CL-20/H2O2) collected in Table 1. On the other hand, regarding the CL-20 molecules in the CCCs, five types of conformers are / 35Environment ACS Paragon 10 Plus

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found, including β-, γ-, η-, ε-, and ζ-forms, as demonstrated by some typical CCCs in Figure 3. According to previous results, the relative total energies of the β-, γ-, η-, ε-, and ζ-forms are in turn 0, 1.13, 1.27, 1.69 and 2.30 kcal/mol, showing a small difference in total energy among these conformers.5

Figure 4. Overlays of the molecular conformers of CL-20 in observed CCCs. Six NO2 groups are numbered by black Arabian numbers 1-6, respectively.

Figure 5. Populations of the differently typed molecular conformers of CL-20 in CCCs.

Overlaying these five conformers of CL-20 in a same orientation facilitates to compare their geometry structures. Figure 4 exhibits almost complete overlays of the hexaazaisowurtzitane cages of the five conformers, with a difference in the orientation of the NO2 groups in respect to the cage. For the five types of molecular conformers of CL-20, their six NO2 groups 1-6 appear in e-v-e-v-v-v / 35Environment ACS Paragon 11 Plus


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(β), e-v-v-v-v-v (γ), e-v-v-v-e-v (η), e-e-v-v-v-v (ε) and v-v-v-v-v-v (ζ), respectively. e and v represent the NO2 groups equatorial and vertical to the cage, respectively. As mentioned above, the β-, γ- and ε-formed CL-20 exist in the polymorphs under common condition, the ζ-CL-20 is a high-pressure phase of >3.3 GPa, and the η-form has not been observed as a polymorph of CL-20. While, interestingly, the molecular conformer that exists in a high-pressure phase or does not exist as a polymorph can also be observed at ambient conditions. It shows that, by cocrystallization, the CL-20 molecule can exist in a manner of more molecular conformers relative to polymorphs under common condition. This is one difference between single component crystals and cocrystals. We also pay attention to the populations of the differently typed molecular conformers of CL-20 in the 27 CCCs, as shown in Figure 5. Firstly, it is interesting to find two types of conformers in a same CCC like CL-20/HMX (β/γ) 10 and CL-20/TPPO (ε/β)

27

. While, such case

has rarely been found in any polymorph of a single component crystal. This is another difference between single component crystals and cocrystals. Next, as illustrated in Figure 5, the ε-formed conformer only occupies a population of 2 % and doesn’t govern the CCCs. In fact, the ε-formed conformer only appears in CL-20/TPPO with a half occupancy and possesses the least population of all conformers. For the polymorphs of CL-20, ε-CL-20 is the most stable at ambient conditions, suggesting that the ε-formed conformer is dominant when CL-20 exists as a single component crystal. It exhibits a remarkable difference in conformer type between single component crystals and cocrystals. The difference should be reasonable, due to the difference in crystal field. That is, the CL-20 in any polymorph is surrounded by the homogenous molecules; while, those in a cocrystals is encompassed by both the homogenous and heterogeneous molecules. As a matter of fact, recent work showed that the most stable polymorph of CL-20, ε-CL-20, is dominated by its lattice energy (LE), instead of its molecular conformer energy (MCE), and the lowest total energy,

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i.e., the sum of LE and MCE, is the root for the most stable polymorph. 43 By the way, to confirm whether different conformers can also exist in CCCs, we varied the conformer types and relaxed the unit cells. As a result, unexceptionally, the varied conform types returned as original ones. It ascertained that the existing forms of these checked CCCs should be both kinetically and thermodynamically stable. Wholly, as demonstrated in Figure 5, the conformer with lower energy tends to appear in the cocrystal more frequently. That is, the β- and γ-formed conformers are more energetically favored than the ε-, ζ- and η-ones, and the former two possess a total population of 87 %, much higher than the latter three, 13 % totally. Nevertheless, there is no strict total energy-population relationship. From above conformer analysis, we can find three differences between the single component crystals and cocrystals of CL-20. The first is that the conformer types in the cocrystals are more abundant than in polymorphs; secondly, two types of conformers of a same molecule can appear in a same cocrystal, while allowed rarely in a polymorph; and finally, the populations of conformers distributed in cocrystals is much different from those in polymorphs at ambient conditions.

Figure 6. Populations of the stoichiometric ratios of CL-20 to coformer molecules in observed CCCs.

Besides, we pay attention to the stoichiometric ratios of the components in CCCs. Figure 6 shows the ratio of 1:1 governs the populations of all observed CCCs. Following the ratio of 1:1, the ratio of 1:2 (including 0.5:1) occupies 40 %. From the figure, we can draw a conclusion that a high / 35Environment ACS Paragon 13 Plus


stoichiometric ratio corresponds to a low population. 3.2 Molecular stacking styles. Previous studies have proven that the molecular stacking styles of energetic crystals are strongly responsible for the impact sensitivity of EMs.3,44-46 In particular, the face-to-face π-π stacking favors most the low impact sensitivity, with a typical representative of TATB, which is very impact insensitive and called the wood explosive, and the only candidate for insensitive high explosives by the Department of Energy of US. Currently, we extended this concept and thought that if crystal packing is remarkably distinguished by the strong intralayered and weak interlayered intermolecular interactions, it will favor low impact sensitivity, and the face-to-face π-π stacking is a typical case while not necessary.46 That is to say, in contrast to the pure CL-20, the impact sensitivity of the CCCs may be more or less improved by varying molecular stacking in crystal lattice, abiding by the concept of crystal engineering.

Figure 7. Four molecular stacking styles of observed CCCs. / 35Environment ACS Paragon 14 Plus

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Interestingly, despite no big π-structure for the CL-20 molecule, the molecular stacking in the CCCs appears regularly, and can be divided into four stacking styles: wavelike, sandwich, channel and caged stacking. That is, if the homogenous molecules in a CCC are two-dimensionally (2D) stacked, they will be thought to be stacked in a wavelike (Figure 7a) or sandwich (Figure 7b) manner; if one kind of molecules are one-dimensionally (1D) stacked, removing these molecules will cause numerous channels remained, and therefore the CCC will be seen to be packed like channels (Figure 7c); and if one kind of molecules are stacked in a zero-dimensional (0D) manner, the CCC is caged stacked (Figure 7d). We will separately describe these four stacking styles of observed CCCs as follows. In combination with the molecular stability of coformers, these stacking styles can influence the impact sensitivity of the CCCs.

Figure 8. Wavelike stacking of observed CCCs.

The wavelike stacking is found in four CCCs. As illustrated in Figure 8, in the wavelike stacked CCCs, homogenous molecules in each layer are closely contacted one another to form wavelike arrangement. This stacking is similar to those of FOX-7 and LLM-105, which are impact insensitive EMs.3 Because both the CL-20 and the coformer molecules of each case don’t possess big π-bonded planar structures, the sliding hindrances of these CCCs are reasonably higher than / 35Environment ACS Paragon 15 Plus


those of TATB, FOX-7 and LLM-105 with such structures. Regarding the shapes of coformer molecules cocrystallized with CL-20 in this case, they feature nonplanar. As to the four coformer molecules, BL and NMP possess nonconjugated ring structures, and GTA and MAM are chains. It should be noted that the CL-20/NMP·H2O cocrystal

26

is the only ternary ECC, despite a hydrate.

Experimentally, it was found that CL-20 favors to be crystallized as various hydrates (α-CL-20) in case of the water contained in a solution, as the CL-20 hydrates are more energetically favored to the polymorphs of CL-20.47 So, it is deemed that other ternary hydrous CCCs can also be formed with water in solutions.

Figure 9. Sandwich stacking of observed CCCs.

The second stacking style is the sandwich stacking, as another 2D arrangement of the homogenous molecules in the CCCs. Different from above 2D molecular arrangement of wavelike staking, the homogenous molecules of any layer of the sandwich packed CCCs are planarly arranged. In the six sandwich stacked CCCs, three possess coformers of π-conjugated molecules, i.e., TNT, Xylene and DNB. For the CCC of CL-20/TNT (Figure 9a), it seems that the TNT molecules are homogenously face-to-face π-π stacked. A similar case is observed for the cocrystal of CL-20/DNB (Figure 9b) or CL-20/Xylene (Figure 9c). It suggests that such face-to-face π-π stacking of homogenous molecules of TNB, DNB or Xylene may be energetically or spatially / 35Environment ACS Paragon 16 Plus

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favored; otherwise, they will be fully separated by the CL-20 molecules. For the HMX molecule with a ring structure, and DNG and DMF with chains (Figure 2), they also exhibit a high ability to be self-assembled into layers. As illustrated in Figures 9d-9f, the close contacts of homogenous molecules suggest their strong interactions. Relative to the typical face-to-face π-π stacking of TATB, these sandwich stacking is not expected to favor much the low impact sensitivity, as there still exist much spatial hindrance for sliding shear.

Figure 10. Chanel stacking of observed CCCs.

The third style is the channel stacking. In this type of stacking, one kind of molecules are homogenously arranged as numerous channels, and the other kind of molecules are just inserted in the channels. Obviously, the inserted homogenous molecules in channels are fully isolated from one another by heterogeneous molecules. The channel stacking possesses the largest population among the CCCs, i.e., 12 out of 27. Regarding the objects isolated, the CCCs can partitioned into two cases: one is that the channels formed by the stacking of the CL-20 molecules, such as the cases of / 35Environment ACS Paragon 17 Plus


CL-20/DNP (Figure 10a), CL-20/NAQ (Figure 10b), CL-20/BTF (Figure 10c), CL-20/AZ1 (Figure 10f), CL-20/AZ2 (Figure 10g), CL-20/MTNP (Figure 10h), CL-20/BQ (Figure 10i) and CL-20/MNO (Figure 10j); and the other is that, for the remaining cases, the inserted molecules in the channels are of CL-20.

Figure 11. Caged stacking of observed CCCs.

The final case is that the CL-20 molecules or the coformer molecules are fully heterogeneously isolated, i.e., in the caged stacking, a coformer molecule is completely encompassed by heterogeneous molecules in crystal. Except from the CL-20/CPL cocrystal (Figure 11a), in which the two coformer molecules possess a small difference in molecular volume, a significant such difference is exhibited for the remaining cocrystals (Figures 11b to 11e). That is, in the cases of the cocrystallization of CL-20 with H2O, CO2 and H2O2, separately, it seems that the coformer molecules are filled in the cava among the CL-20 molecules. Thereby, we think that, if a CCC can be formed and the coformer molecule is much smaller than CL-20, the CCC can largely be caged stacked. As a matter of fact, H2O, CO2 and H2O2 are the three smallest molecules in Figure 2, the related CCCs are unexceptionally caged stacked. The above-mentioned four stacking styles are classified by means of the arrangement of / 35Environment ACS Paragon 18 Plus

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homogenous and heterogeneous molecules in the CCCs. As pointed out above, because the stacking style can be responsible for the properties and performances of crystals, and cocrystallization is an effective approach to tune the style to make the crystal with desired properties, the cocrystallization possesses a significance of energetic engineering. In general, a cocrystal mediates the properties and performances of the two pure components. For example, a current insight into the thermal stability of the CL-20/HMX cocrystal showed that the mediation of Td of the cocrystal between those of the two pure components is originated from the thermal exchange between them. 48 While, an extreme case that a cocrystal outperforms the pure components can also take place. 11,49 The outperforming is attributed to the optimization of the component and/or crystal packing.50 It is now still difficult to establish a stacking-property relationship of the CCCs, due to the poor integrity of the property data, as shown in Table 1. 3.3 Intermolecular interactions The apparent crystal packing is originated from the intermolecular interactions. Nevertheless, regarding energetic molecules, it is believed that they usually lack donors or acceptors for strong intermolecular interactions. For example, previous work confirmed that the CL-20 molecule possesses neither strong intermolecular hydrogen bond acceptor (HBA) nor strong donor (HBD);23 and the ECCs formation was thought to be attributed to an entropic effect, as a small energy variation occurs during the formation.51

Figure 12. ESPs of various molecular coformers of CL-20 mapped onto the molecular surfaces of electronic density of 0.001 au. The grey, white, blue and red represent C, H, N and O atoms, respectively. These / 35Environment ACS Paragon 19 Plus


representations are considered in the following figures. And the color chart is also applicable in following figures involving ESPs.

As a matter of fact, the relatively weak intermolecular electrostatic interactions usually direct the intermolecular interactions of CCCs. Figure 12 exhibits the negative ESP regions related to NO2 groups and positive ones related to CH2 groups of various molecular conformers of CL-20, without extremely red or blue region on the molecular surface of CL-20, representing no high ESP. It confirms that the CL-20 molecule is not a strong intermolecular interaction donor or acceptor, in according with previous analysis. 23 Carefully checking the molecular structures of coformers in Figure 2, we find that most of them are C=O/P=O/N=O contained molecules, including DMF, CO2, GTA, BL, HMPA, BTF, NMP, BQ, NAQ, CLP, TPPO, MNO and MAM (13 out of 26 totally); the big-π bond contained coformers, including TNT, BTF, DNB, AZ1, BQ, NAQ, Xylene, DNT, MTNP and AZ2 occupy a large population too (10 out of 26 totally); some coformer molecules contain ethereal O atoms, including GTA, DO, BL and MAM; except from two small molecules of H2O and H2O2, the remaining coformers are three nitroamines of HMX, DNG and DNP. Experimentally, it has been ascertained that the high solubility of CL-20 in carbonyl-contained solutions, such as acetone and acetic ether; the low solubility in alcohol, ether and nitroalkane; and the insolubility in hydrocarbon, halogenated hydrocarbon and water. That the largest coformer population of C=O/P=O/N=O contained molecules accords with the high solubility in carbonyl-contained solutions of CL-20 suggests that the solubility may be referred for coformer selection.


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Figure 13. ESP surfaces of coformer molecules with acyl or ether O atoms.

Figure 14. Lengths of HBs between the CL-20 molecules and coformer molecules with acyl or ether O atoms.

Figures 13 and 14 illustrate the ESP surfaces of acyl or ether O atoms contained molecules and the lengths of HBs between these molecules and CL-20, respectively. The remarkably red regions of the ESP surfaces of the O atoms in Figure 13 represent there is highly negative electricity, setting a base for attracting positive charges, like the positively charged H atoms of CL-20. The regions of the acyl and ether O atoms in Figure 13 are more remarkably red than those of NO2 in Figure 12,



implying a higher ability of the former to be bonded with the positively charged H atoms of CL-20. This could be a thermodynamic origin for forming the related CCCs, i.e., a heterogeneous assembly of the CL-20 and coformer molecules is energetically preferred to a homogenous one of the CL-20 molecules themselves. As a matter of fact, except from the CL-20/BTF cocrystal, the acyl and ether O atoms of the coformer molecules are non-covalently bonded with the H atoms of CL-20 (Figure 14). The HBs with the shortest bond length of 2.279 Å are weak according to the clarification of HB strength proposed by Jeffrey.52 Still, these HBs are stronger than other kinds of intermolecular interactions and responsible for the molecular stacking. Besides, it is interesting to find that most of acyl and ether O atoms each servers as a binary HBA, which compensates for the HB weakness to enhance intermolecular interactions. In fact, these binary HBs have not been observed in the polymorphs of CL-20.53 By the way, as illustrated by Figure 14g, the HB between the BTF and CL-20 molecules is formed through a H…N linkage, instead of a H…O one, due to a dominance of p-π interaction. It will be discussed later.

Figure 15. ESP surfaces of coformer molecules each with at least a big π-bond.


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Figure 16. Plot showing p (of O atoms on the NO2 of CL-20) –π (of coformer molecules) intermolecular interactions by interatomic distances between two neighboring molecules.

Besides the H atoms, the NO2 groups also serve as a part of external moieties of CL-20. Despite the possible HBs formed between these NO2 groups and the H atom of the coformer molecules, the p–π interactions are observed as one of dominant interactions in the CCCs. In the p– π interactions, p and π refer to the p-electrons of the NO2 groups and the electrons of the big π-bonds of the coformer molecules, respectively. In fact, the p–π interactions belong to the sum of electrostatic and van der Waals ones. Figure 12 exhibits the negative electricity of the NO2 groups by the ESP surfaces; while, the positive one of the big π-bonds appears in some coformer molecules of Figure 15, including BTF, AZ1, AZ2, MTNP, TNT, DNB, DNT and BQ. In Xylene (Figure 15e), contrarily, the benzene ring is electron-rich and therefore negatively charged. While, regarding NAQ (Figure 15j), its naphthalene ring is almost neutral. Figure 16 exhibits the interatomic distances between two neighboring molecules abstracted from the observed CCCs. Because the normalized Bondi radii are 1.75, 1.09, 1.56 and 1.61 Å for C, H, O and N atoms 38, that the interatomic distances in the figure each is close to the sum of radii of two related atoms implies typical van der Waals interactions. / 35Environment ACS Paragon 23 Plus


Page 24 of 35

Figure 17. Plot showing the ESP surfaces of single coformer molecules (top) and CL-20-coformer molecular pairs (bottom), and typical HBs in the related CCCs (middle).

We also pay attention to the case of the coformer molecules without above-mentioned acyl/ ether O atom or big π-bond. These coformer molecules refer to H2O, H2O2, HMX, DNG and DNP. As demonstrated in Figure 17, these molecules exhibits evident polarity, with evident red and blue regions on the ESP surfaces, showing negative and positive electricity, respectively. Because these coformer molecules each possesses both O and H atoms, as HBAs and HBDs, respectively, the same as CL-20, the HBs are formed in the CL-20-coformer molecular pairs. Due to the weakness of the HBAs and HBDs, these HBs are always weak. N-O

O-O

O-H

Other

CL-20/H2O CL-20/MTNP CL-20/CO2 CL-20/BTF CL-20/H2O2-1 CL-20/HMX_1 CL-20/HMX_3 CL-20/HMX_4 CL-20/DNB CL-20/HMX_2 CL-20/H2O2-2 CL-20/MNO_2 CL-20/DNP_2 CL-20/AZ1 CL-20/TNT-2 CL-20/TNT-1 CL-20/DNP_1 CL-20/MNO_1 CL-20/BL_1 CL-20/BL_2 CL-20/AZ2 CL-20/p-Xylene CL-20/DNT CL-20/BL_3 CL-20/BL_4 CL-20/BQ CL-20/DNG CL-20/MAM CL-20/NAQ CL-20/GTA CL-20/DMF CL-20/DO CL-20/NMP·H2O CL-20/TPPO_2 CL-20/TPPO_1 CL-20/CPL CL-20/HMPA

0

10

20

30

40

50

60

70

80

90

100%

Figure 18. Populations of close interatomic contacts of the CL-20 molecules involved in the CCCs. / 35Environment ACS Paragon 24 Plus

Page 25 of 35

Hirshfeld surface is a straightforward tool to indicate the intermolecular interactions in crystal. Figures S1 to S4 of SI exhibit the two-dimensional fingerprint plots of the CL-20 molecules involved in all the observed CCCs, among which the O…H, O…N and O…O contacts dominate the intermolecular interactions. This can also be ascertained by the populations of various contacts in Figure 18. The dominance of the O…H, O…N and O…O contacts are very common in CHON contained EMs.2,3 From Figure 2, we can find H, O and N atoms predominate the external moieties of all former molecules. Because of high dominance of H atoms on the external of HMPA, CPL, TPPO, NMP, DO and DMF, the high dominance of the O…H contacts are observed in the related CCCs. 3.4 Packing coefficient and density. 0.90

0.85

MI MII MIII

0.80

PC 0.75

0.70

--

0.65 C C L-2 L- 0 2 /C C 0/H PL L C C -20 MP L- L / A 20 -2 DM / N 0/ F M GT C PH A L 2 C -2 O C L- 0/B L- 20 Q 2 C 0/T /DO L- P C 20/ PO L M C -20 AM L- /N C 20/ AQ L- D C 20/ NG L D C -2 N C L-2 0/B T L- 0 T F C 20/ /DN L- X B 20 yle / n C MT e C CL L-2 NP L- -2 0/ 20 0 BL / /D C H2 NP L- O 2 2 C 0/M (2) L C -20 NO L- /A C 20/ Z2 L T C -20 NT L / C -20 AZ1 L- /H 2 C CL 0/H 2O L- -2 M 20 0 X /H /C 2O O2 2( 1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 19. PCs of the observed CCCs from various calculations.

As one of the most concerned properties of EMs, dc is also focused upon because it has a strong relation on the detonation behaviors. In principle, dc is determined by PC and dm. In this work, we adopted three methods for PC calculations. As illustrated in Figure 19, the calculated PCs depend on the methods applied, while with a close change tendency. By comparison, we find that,



for a CCC, a largest PC is usually given by MI, a smallest on by MII, and a middle one by MIII. Therefore, the results from III are employed for following discussion. 2.1 2.0 1.9 1.8

dc,g/cm3

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1.7 1.6 1.5 1.4 1.3 65

70

75

80

85

PC, %

Figure 20. PC-dc dependence of observed CCCs. CL-20/CPL CL-20/H2O2_1 CL-20/HMPA CL-20/CO2

CL-20/DMF

CL-20/HMX

CL-20/GTA

CL-20/H2O

CL-20/NMP/H2O

CL-20/AZ1

CL-20/BQ

CL-20/TNT

CL-20/DO

CL-20/AZ2

CL-20/TPPO

CL-20/MNO

CL-20/MAM

CL-20/H2O2_2

CL-20/NAQ

CL-20/DNP

CL-20/DNG

CL-20/BL

CL-20/DNT CL-20/MTNP CL-20/BTF CL-20/Xylene CL-20/DNB

dCL-20

dmolecule_2

dco-molecule

Figure 21. dm of observed CCCs.

As pointed out above, dc is principally determined by PC and dm, i.e., dc is the product of PC and dm. While, it is interesting to find that there is an evident PC-dc tendency in Figure 20: the higher PC, the larger dc. Because dc is the product of PC and dm, and dc increases with PC / 35Environment ACS Paragon 26 Plus

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increasing, that dm maintains as a constant should be the reason for the tendency. To verify this, we calculated the dm of CL-20 (dCL-20), the coformer molecule (dmolecule_2), and the CL-20 and the coformer molecules as a whole (dco-molecule). As demonstrated in Figure 21 and Table S2 of SI, dco-molecule fluctuates around 2.43 g/cm3 with a maximum relative error 0.35 g/cm3 (14.4 %). That is, the close dco-molecule is the main reason for the PC-dc tendency shown in Figure 20. Moreover, from Table S2 of SI, dCL-20 is ranged from 2.567 to 2.723 g/cm3, with an average of 2.644 g/cm3 and a maximum relative error of ~3 %. It implies a small Vm difference of CL-20 in the CCCs.53 Besides,

we

can

know

from

Figure

21

that

there

is

always

an

order

of

dCL-20>dco-molecule >dmolecule_2, for each cocrystal. It shows a very high dm of CL-20, due to its highly compact caged molecular structure. It also indicates that we haven’t found out a coformer molecule with a higher dm above CL-20, even though a considerable quantity of coformer molecules possess dm close to that of CL-20, including H2O2, CO2, HMX, AZ1, AZ2, DNP, MTNP and BTF. Regarding the related CCCs, the reduced dc is attributed to the PC reduction relative to the densest form of CL-20, ε-CL-20. In fact, relative to the dc of ε-CL-20, the dc is reduced more or less by the cocrystallization of all the CCCs, as listed in Table S2 of SI. 85

85

85

(b) N-O

(a) O-O 80

80

75

75

75

70

70

70

65

65

65

60

60 0

10

20

30

40

50

(c) H-O

PC,%

PC,%

80

PC,%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0

5

p, %

10

15

20

p,%

Figure 22. p-PC dependence of observed CCCs.


25

60 20

30

40

50

p,%

60

70

80


85

85

85

80

80

75

75

75

PC,%

PC,%

80

70

70

70

65

65

65

60 5

10

15

20

25

30

35

40

(c) H

(b) O

(a) N

PC,%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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60 10

60 15

20

25

30

35

40

5

m,%

r,%

10

15

20

25

30

35

40

45

50

55

r,%

Figure 23. r-PC dependence of observed CCCs.

Next, we focus upon the influences of the populations of various types of interatomic contacts of CL-20 molecules with their neighboring molecules in lattice (p) and elementary molar ratio (r) on PC. As pointed out above, the O…H, O…N and O…O contacts dominate the intermolecular interactions of CL-20 and their neighboring molecules in the CCCs. As demonstrated in Figure 22, there exist some obvious tendencies: PC increases with the p(O…O) or p(O…N) increasing, while reduces with the increase of p(O…H). Surprisingly, these tendencies are contrary to our common knowledge, as the increase of p(O…H) represents the enhancement of intermolecular interactions and further the increase of PC. As a matter of fact, both di and de in respect to the Hirshfeld surfaces were constrained in a range of 0 to 2.6 Å for calculating p, suggesting that the largest distance of 5.2 Å was possibly accounted for statistic. In other words, the larger p(O…H) of CL-20 does not always suggest the stronger HB or the stronger intermolecular interactions. Due to no stronger HBA or HBD in the CCCs as in common energetic crystals, the HBs in the CCCs are weak or even very weak as claimed above. In the case of the weak intermolecular interactions, increasing the O…O and O…N contacts is more efficient to increase interactions or PCs than the O…H contacts. A similar case also appears in the crystal packing of cubane and its nitro-derivatives, as heptanitrocubane and pentanitrocubane with higher p(O…O) and lower p(O…H) possess the largest PC.36 Owing to the same reason, we can readily understand the tendencies shown in Figure 23: the higher elementary ratios of O and N lead to the higher PC; while, that of H does reversely.


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4. CONCLUSIONS Summarily, we carry out a comprehensive insight into the crystal packing structures of 27 CCCs observed till 2017, and focus upon the conformer types and their populations, the stoichiometric ratios of the components, molecular stacking style, intermolecular interactions, PCs, and packing densities, by means of theoretical calculations and analyses. Five conformers including the β-, γ-, η-, ε-, and ζ-forms of CL-20 and five stoichiometric ratios are observed in the CCCs. These conformers exist as in all the polymorphs at ambient conditions as well as high pressures, and even as not observed in the polymorphs. This is attributed to the high flexibility of CL-20 cocrystallized with coformer molecules, which is originated from the ready rotation of NO2 groups with low energy barriers of several kcal/mol. Meanwhile, a high ability of CL-20 to be cocrystallized can also be ascertained by the high multiplicity of the shapes and sizes of the coformer molecules. Besides, that two types of conformers of CL-20 can appear in a same CCC makes a cocrystal different from a single component crystal. It also shows wavelike, sandwich, channel and caged molecular stacking in the CCCs, and the O…H, O…N and O…O contacts dominating the weak intermolecular interactions. The weak intermolecular interactions feature HBs between the acyl/ether O atoms of coformer molecules and the H atoms of CL-20, and the p (of O atoms on the NO2 of CL-20)–π (of the big π-bonds of coformer molecules) interactions, and contribute to the small molecular volume variations of CL-20 after cocrystallization. Besides, the CCCs each mediates packing density between those of the two related pure components, and does not outperform ε-CL-20 in packing density. In addition, we find that the high contents of O and N facilitate to increase packing coefficients and packing densities. All these findings are expected to richen our knowledge in the fields of both energetic materials and cocrystals, and enhance the rationalization of crystal design in a crystal engineering way.54



Symbol AZ1 AZ2 BL BQ BTF CL-20 CPL DMF DNB DNG DNP DNT DO FOX-7 GTA HMPA HMX LLM-105 MAM MNO MTNP NAQ NMP·H2O TATB TNT TNT TPPO Xylene

Full name 7H-tris-1,2,5-oxadiazolo[3,4-b:3',4'-d:3",4"-f] azepine tris[1,2,5]oxadiazolo[3,4-b:3',4'-d:3",4"-f] azepine-7-amine γ-butyrolactone para-benzoquinone benzotrifuroxan 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane caprolactam N,N-dimethylformamide 1,3-dinitrobenzene 2,4-dinitro-2,4-diazaheptane 2,4-dinitro-2,4-diazapentane 2,5-dinitrotoluene 1,4-dioxane 2,2-dinitroethylene-1,1-diamine glyceryl triacetate hexamethylphosphoramide 1,3,5,7-tetranitro-1,3,5,7-tetrazacyclooctane 2,6-diamino-3,5-dinitro-1,4-pyrazine 1-oxide methoxy-NNO-azoxymethane N,N'-dimethyl-N,N'-dinitrooxamide 1-methyl-3,4,5-trinitropyrazole 1,4-naphthoquinone N-methyl-2-pyrrolidone·H2O 2,4,6-trinitro-1,3,5-triaminobenzene 2,4,6-trinitrotoluene 2,4,6-trinitro-toluene triphenylphosphine oxide p-xylene

Supporting Information (SI) Partial crystallographic information and some component, structure and property information of the existing CCCs, and two-dimensional fingerprint plots of the CL-20 molecules involved in the observed CCCs. These materials are available free of charge via the Internet at http://pubs.acs.org.

■ Information of Authors Corresponding Author R. J. G, email: [email protected]


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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 (U1530262) and the National Natural Science Foundation of China (TZ-2018004).

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For Table of Contents Use Only Packing Structures of the CL-20-based Cocrystals Guangrui Liu, Hongzhen Li, Ruijun Gou, and Chaoyang Zhang

This work comprehensively studies the molecular conformers of CL-20, molecular stacking, and packing densities of the CL-20-based cocrystals observed till 2017.


Packing Structures of CL-20-Based Cocrystals - ACS Publications

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