Effects of Noncovalent Interactions on the Impact Sensitivity of HNS

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Effects of noncovalent interactions on the impact sensitivity of HNS-based cocrystals: A DFT study Xiao Zhao, Shiliang Huang, Yu Liu, Jinshan Li, and Weihua Zhu Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b01334 • Publication Date (Web): 18 Dec 2018 Downloaded from http://pubs.acs.org on December 18, 2018

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Crystal Growth & Design

Effects of noncovalent interactions on the impact sensitivity of HNSbased cocrystals: A DFT study Xiao Zhao,a Shiliang Huang,b Yu Liu,b Jinshan Li,b Weihua Zhua,* aInstitute

for Computation in Molecular and Materials Science, School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China bInstitute of Chemical Materials, China Academy of Engineering Physics, P. O. Box 919-327, Mianyang, Sichuan 621900, China ABSTRACT: We report crystal packing, electronic structure, Hirshfeld surface, Bader's atoms in molecules (AIM), and independent gradient model (IGM) for 2,2',4,4',6,6'-hexanitrostilbene

(HNS)

and

HNS-based

cocrystals

(HNS/4,4’-

Bipyridine (BP), HNS/trans-1,2-Bis(4-pyridyl)ethylene (BPE), and HNS/1,2-Bis(4pyridyl)ethane (BPA)) to understand how noncovalent interactions affect the impact sensitivity of the cocrystals. The results indicate that there are strong C-H···N interactions and π-stacking in the three cocrystals. Among the three cocrystals, HNS/BP has most strong intralayer hydrogen bonds and π-π interactions, which can be responsible for its lowest impact sensitivity. Among the emerging π-π interactions, the face-to-face interaction lying between components is the most important one. Therefore, the strong intralayer hydrogen bonds and face-to-face π-π interactions determines the cocrystal explosives to be low impact sensitivity. Our work may provide useful information for understanding the safe performance of the cocrystal explosives in atomic detail and broaden the application of supramolecular chemistry in the fields of the explosives. KEYWORDS: HNS; DFT; noncovalent interaction; HNS-based cocrystals, impact sensitivity.

*

Corresponding author. E-mail: [email protected] 1

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1. INTRODUCTION High energy density materials (HEDMs) are a class of compounds stored high chemical energy in their molecular structures and have been widely applied in military, mining, and space exploration.1 However, further development of HEDMs still faces the challenge that how to satisfy a balance between high detonation performance and low impact sensitivity.2 Cocrystallization 3-5 is known as an effective method to alter physicochemical properties of crystals without changing its molecular structure, which leads to new crystals with improving performance. In the field of HEDMs, cocrystallization can provide an effective way to modify the key properties (sensitivity, density, oxygen balance, thermal behavior etc.) of traditional explosives and thus achieves the purpose of decreasing sensitivity meanwhile without losing much more energy density.6-9 It is well documented that noncovalent interaction (NCI),10-14 such as hydrogen bonding, π stacking, van de Walls force, steric and electrostatic interaction, can serve as both main driven force of cocrystallization process and important factor that affect the structure and properties of formed explosive cocrystals. The cocrystal explosive is composed of different explosives through intermolecular interaction at molecular level. Preparation and characterization of energetic cocrystals composed of a 1:1 molar ratio of 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20) and 2,4,6-trinitrotoluene (TNT) suggested that the energetic cocrystal can form readily through a number of C-H hydrogen bonds despite of lacking predictable interaction (π-π stacking);15 and moreover, the cocrystal explosive CL-20/TNT possess high energy density and excellent insensitivity. The cocrystal of CL-20 and benzotrifuroxan (BTF) (1:1) can be accumulated by the same C-H···N hydrogen bonds and π-π interaction, which leads to more excellent detonation properties than

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pure components and provide another useful way to modify the properties of existing nitroamine-furazan explosives.16 The investigations on the cocrystal of CL-20 and 1,3,5,7-tetranitro-1,3,5,7-tetrazacyclooctane (HMX) (2:1) showed that high degree of widespread hydrogen bonding in the cocrystal makes CL-20/HMX possess good mechanical sensitivity.17 Landenberger et al.18 reported three attractive cocrystals composed of diacetone diperoxide (DADP) and 1,3,5-trichloro-2,4,6-trinitrobenzene (TCTNB), 1,3,5-tribromo-2,4,6-trinitrobenzene (TBTNB), or 1,3,5-triiodo-2,4,6trinitrobenzene (TITNB) and pointed out that unique halgen-hydrogen weak interaction in the cocrystal leads to the DADP-based explosives with excellent insensitivity. Afterwards, theoretical studies19 revealed the relationship between crystal packing and impact sensitivity of the three DADP cocrystals in detail and demonstrated that the improved intermolecular interaction and its anisotropy could be responsible for the difference in the sensitivity among these cocrystals. In addition, NCI plays an important role in the formation of energetic slats. Single crystal X-ray diffraction and theoretical calculations proved that the N/O-H bonds and π-π interaction can adjust the crystal structure of the cocrystal explosives and so determine the process that converts mechanical stimuli into the sliding of layers.20 Nowadays, supramolecular structures composed of both traditional energetic molecules and organic components have drawn much attention due to broadening the range of energetic cocrystal candidates.21 Limited by narrow range of functional groups (mostly nitro group) in typical energetic molecules, plenty of supramolecular synthons can not be applied to corystallization strategy of multiple energetic materials. Thus, many studies on the explosive cocrystals concentrate on altering the structure features and crystalline properties of the cocrystals containing nonenergetic components.22-26 Supramolecular chemistry of 17 new TNT-based cocrystals with

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aromatics27 indicated that the edge-to-face/face-to-face π stacking and amine-nitro interaction play a very key role in the formation of the cocrystals. Structure analysis of four CL-20 solvates demonstrated that the C-H···O and P=O···H weak intermolecular interaction can still served as the driving force of cocrystallization in the absence of typical π-π interaction.28 Although this kind of supramolecular structures can dramatically reduce the sensitivity of the cocrystals compared to pure energetic crystals, the cocrystals must pay slight loss of energy density and meanwhile some of them become to decompose easily. Supramolecular structures with nonenergetic molecules will provide us another valuable way to design excellent HEDMs and to solve the conflict between detonation performance and sensitivity. Very recently, three 2,2',4,4',6,6'-hexanitrostilbene (HNS) based cocrystals composed of HNS and 4,4’-Bipyridine (BP), trans-1,2-Bis(4-pyridyl)ethylene (BPE), or 1,2-Bis(4-pyridyl)ethane (BPA) were reported.29 The HNS/BP, HNS/BPE, and HNS/BPA cocrystals have much more lower impact sensitivity than pure HNS crystal due to ubiquitous NCI (hydrogen bonding, π stacking and van de Walls interaction) in the crystals. In this work, we investigated the crystal packing, electronic structures, Hirshfeld Surfce, Bader's atoms in molecules (AIM), and independent gradient model (IGM) of the HNS crystal and its cocrystals (HNS/BP, HNS/BPE, and HNS/BPA) using DFT. The differences in the noncovalent interaction between pure HNS and HNS-based cocrystals in both crystalline and cluster states were discussed. Our aim is to uncover the essential connection between variety of typical NCI and impact sensitivity for the cocrystals. That is, how the noncovalent interaction affect the impact sensitivity of the HNS-based cocrystals. Our studies may be a valuable fundamental work to guide the design of new functional energetic cocrystals.

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2. COMPUTIONAL METHODS The calculations in this study were performed by periodic density functional theory as implemented in the CASTEP code30 with norm-conserving pseudopotentials31 and a plane-wave expansion of the wave functions. The generalized gradient approximation (GGA) Perdew-Burke-Ernzerhof (PBE) exchange-correlation function32 with Grimme scheme for van de Walls correction was employed.33 The electronic wave functions of systems were defined by Pulay density-mixing scheme and its total energy was minimized by conjugate gradient technique. Crystal structures of these cocrystals were optimized by the Broyden, Fletcher, Goldfarb, and Shannon (BFGS) method.34 The cutoff energy of plane waves was set to 750 eV. Brillouin zone sampling was implemented by the Monkhost-Pack scheme. The k-point grid of 1×3×1 was used for pure HNS crystal and HNS/BP cocrystal and 1×2×1 for HNS/BPE and HNS/BPA cocrystals. Hirshfeld surfaces were constructed based on the sum of spherical atom electron densities, which were considered as an effective tool to investigate NCI in crystal structures.35-37 The normalized contact distance (dnorm) was defined by di, de (the distances from the surface to the nearest atom interior and exterior to the surface, respectively), and the vdW radii of atom according to Eq(1). The intensity of dnorm point can provide information about particular important regions of intermolecular interaction through color mapping (red and blue represent high and low intensities, respectively). 2D fingerprint point formed by combination of di and de could summarize not only different type and contribution of interactions, but also its corresponding area of the surface. All the Hirshefeld surfaces analysis in this work were generated using CrystalExplorer 3.0,38 and the surfaces were mapped over the dnorm range from -0.2 Å to 1.2 Å.

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(1) In order to obtain more accurate information about hydrogen bonding interaction, we performed a topological analysis on a variety of molecular clusters extracted from the unit cells of the cocrystals on the basis of QTAIM.39 The electron wave functions of molecules in the clusters were calculated at the level of B3LYP/6-31G(d,p)40 in the Gaussian 09W package.41 The electron densities (ρ) and Laplacians (▽ρ) at the critical points (CPs) were derived from the wave function of particular cluster structures and can be used to characterize the types and strength of bonding. Afterwards, the kinetic energy density (Gb), the potential energy density (Vr), and the hydrogen bonding energy (Eb)42 can be determined form the following three equations. Gb = (3/10)(3π2)2/3ρ5/3+(1/6) ▽2ρ

(2)

Vb = (ħ2/4m) ▽2ρ-2Gb

(3)

Eb = 1/2Vb

(4)

Besides, IGM model can provide a 3D real-space visualization of the interaction among several fragments of target structures.43 On the basis of reduced density gradient (RDG) method,44 the NCI plots of the differences between the sum of density gradient of atoms (δg), defined by Eq (5), and the electron density, the sign of the second Hessian eigenvalue in Laplacians (sign(λ2)ρ, ▽ρ = λ1 + λ2 + λ3, λ1 ≤ λ2 ≤ λ3), were calculated in real space based on pro-molecular density. The gradient isosurfaces of the NCI plots were able to identify a variety of interaction referring to key structural property of the cocrystals directly. Johnson et al44 have proved that the RDG method is an effective tool to exhibit the interactions (such as hydrogen bond, van de Walls force, and steric effect) among the molecules. Recently, the RDG method was successfully used to explain the interactions in the explosives reasonably.20 The IGM model based on RDG could isolate the interactions between 6


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molecular or user-defined fragments, which was a more convenient tool that help us analysis the interactions between intra- or inter-layer molecules compared to RDG. All the QTAIM and IGM analysis in this work were generated by Multiwfn 3.0.45

(5) 3. RESULTS AND DISCUSSION 3.1 Crystal Packing The initial structures of the HNS crystal and its cocrystals were taken from CCDC (Cambridge Crystallographic Data Centre)29,46 and fully relaxed without any constraints. Fig. 1 displays the unit cell of the HNS crystal and HNS/BP, HNS/BPE, HNS/BPA cocrystals. Pure HNS in a monoclinic lattice with point group P21/c have turned into C2/c through cocrystallization. The three cocrystals with different components maintain the same point group C2/c. The calculated lattice parameters are given in Table S1 together with their experimental values. Molecular stability is one of the most important effect factors on the impact sensitivity of the explosives, which is closely related to the strength of triggering bond. Since molecular components in the three cocrystals are nonenergetic, we selected three types of potential weakest bonds in the HNS molecules (C-C bond in six member rings, central C=C bond, and C-NO2 bond) to analyze their bond strength. As given in Table 1, the C-C (ring) bond length varies within a narrow range from 1.473 to 1.479 Å and the C-NO2 bond length changes slightly between 1.486 and 1.492 Å. Especially, the central C=C bond length hardly changes from 1.346 Å. The central C=C bond is demonstrated to be the most likely triggering bond in the HNS molecule 7



by the high pressure Raman spectra of HNS. Also, the torsion angle information on these crystals is listed in Table 1. It is found that the three nitro groups among all selected functional groups in the HNS molecule rotate much and their ranges are from -1.451 to 3.073, -11.820 to 18.427, and -47.529 to -117.248, respectively. The geometrical configuration of molecular components in the cocrystals are displayed in Fig. S1. Although there are small changes in molecular structures, it is obvious that the typical NCI in the cocrystals may be strongly related to their structural variation. Accordingly, the large changes in the geometric structures of the HNS molecules may further influence the physicochemical behaviors of the cocrystal explosives. It is well known that crystal packing is closely related to the impact sensitivity of the explosives through the inter/intra-layer molecular interactions. Generally, the strong intralayer molecular interactions coupling with weak interlayer molecular interactions are helpful for decreasing the impact sensitivity of the explosives and vice versa.47 Fig. 2 illustrates the crystal packing of the pure HNS crystal and three HNSbased cocrystals, where the molecular layers are marked by red dotted line box. As shown in Fig. 2, the molecular arrangements in these crystals is similar to “crossing stacking” configurations. But the orientations of adjacent molecular sets are different (approximately crossed). The HNS molecules in pure crystal are linked by hydrogen bonding network mostly; and moreover, the HNS molecules in the same orientation are overlap slightly (nitro group face to edge of benzene ring), which may lead to weaker vdW interaction rather than π-π interaction. Due to the lack of typical πstacking, the pure HNS crystal may be unable to buffer external mechanical stimuli through interlayer sliding.48-49 Thus, HNS is considered as a sensitive explosive with its experimental impact sensitivity value of 22 cm.50

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After the cocrystallization of HNS with nonenergetic components, the crystal stacking of the HNS-based cocrystals remain crossing. Except for the interactions between the HNS molecules, the HNS-component interactions result in much more overlaps between interlayer molecules. Compared to solely weak vdW interactions between the HNS molecules, the formed cocrystals exhibit lower impact sensitivity (106.9 cm for HNS/BP, 100.9 cm for HNS/BPE and 44.2 cm for HNS/BPA respectively). This may be since there are new π-stacking interactions formed in the cocrystals. Detailed inter/intra-layer molecular arrangements by the H-bonding interactions are displayed in Fig. 3. Stacked layers are composed of HNS and organic molecules in 1:1 molar ratio connected by intermolecular hydrogen bonding (two CH···O and one H-C···N). Intralayer interactions in the HNS/BP and HNS/BPE cocrystals are nearly the same except for a few differences in the strength and construction of the H-bonding nets. The steric effect of the central methylene in BPA results in quite strong N···H bonding between molecular layers and weak intralayer H bonding. However, HNS/BPA possesses the most highest impact sensitivity among the three cocrystals due to different noncovalent interactions and nonenergetic property of BPA. 3.2 Electronic structure To further illustrate the interactions among molecular fragments in the crystals, density of states and spatial electron density difference between molecular layers were investigated. Fig. 4a displays the electron delocalization between different HNS layers, where the blue and yellow denotes a gain or loss of electron density, respectively. Generally, the electron density of the O atoms in nitro group was delocalized along with the reduction of electron density in the H atom of the C ring and the polarization

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of C-H bond, which indicates that there exists the O···H hydrogen bonding in the pure HNS crystal. Meanwhile, the variation of electron density between three types of nitro groups (planar < nearly vertical < slightly torsional) leads to the formation of different H-bonding, namely, different functional groups have different contributions on the Hbonding net. According to crystal packing structure, there are a few vdW interactions between planar nitro and edge of C ring, but it is not found in the HNS electron density difference maps due to its relatively weak strength. Except for ubiquitous O-H bonding in the HNS molecules, there are new N···H bonds in the systems after the addition of organic molecules. It is seen in Fig. 4b-d that the electrons of the N atoms in the organic molecules delocalize towards adjacent H atoms and result in more tight connection between two molecules in the same layer. Also, the electron density of the C and N atoms in the organic molecules have accumulated much more, suggesting that the face to face HNS-coformer, coformercoformer π-stacking interactions between the layers are stronger than the HNS-HNS interactions in pure HNS. Besides, the delocalization of electron density in methylene indicates the existence of N-H bonding between different HNS/BPA layers, which agrees with the prediction of crystal packing. Figure 5 displays density of states (DOS) of the N, O, and H states in the HNS crystal and its cocrystals. There is an overlap between the DOS peaks of the O pstates and H s-states in the pure HNS crystal, indicating a strong interaction between two close atoms. The DOS of the H s-states in the cocrystals is extensive and weak, meaning that the reactivity of the H atoms becomes more active after cocrystallization. Thus, the hydrogen bonding nets in the cocrystals become more strong and intensive due to more resonance between the p orbitals of O and the s orbital of H (shown in

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Fig. 5a). Compared to the pure HNS crystal, the DOS peaks of O p-states present a slight blue shift in the valence bands of the cocrystals, suggesting that the p orbitals of the O atoms participate in wide hybridization between adjacent atoms. Similarly, It is found in Fig. 5b that the DOS peaks of the N p-states in the cocrystals possess larger bandwidth in the valence bands than those in the pure HNS crystal. Different from little overlap between the DOS peaks of the N p-states and H s-states in the pure HNS crystal, which represents intramolecuar vdW interactions, there is an intensive resonance range from -7.5 to -2.5 eV in the cocrystals. This suggests that there are more strong N-H bonding (including inter- and intralayers) in the cocrystals. It is found that the π-stacking structures formed in the cocrystals have important effects on their impact sensitivity. Here we investigated partial DOS of C p-state and N p-state to undercover the character of π-π interaction. In the low energy region from -15 to -5 eV, the overlap between two type DOS peaks is supposed to the contribution from the covalent C-N bonds in original molecular structure. In the rest of the valence bands, especially valence band maximum from -5 eV to Fermi level, there emerge very strong resonances between p orbitals in the cocrystals, indicting the existence of π-π interactions between molecular layers (both HNS-coformer and coformercoformer). An analysis of the electron density difference and DOS of all the crystals indicates that the H-bonding net and π stacking play an very important role in affecting the property of the HNS-based cocrystal explosives. 3.3 Hirshfeld Surface As discussed above, a variety of NCI have emerged after the HNS combined with nonenergy molecules to form the cocrystals. Next we studied Hirshfeld surface and 2D fingerprint to identify the location and population of all kinds of particular

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interactions. Previous investigation pointed out that typical impact-sensitive explosives are usually non-planar molecular and are supposed to create rugged Hirshfeld surface in the system.51 Fig.7a displays the isosurface of pure HNS, where red and blue on the surfaces represent for high and low close contact interaction, respectively. Red dots standing for close contact of O and H are bestrewed the surfaces of pure HNS and HNS in the cocrystals (shown in Fig. 7b, d, and f ), which contributes to the formation of H-bonding net. Stronger N···H interaction locating on the edge of surface emerges after HNS combines with organic component along by the weakening of the O···H close contact. Meanwhile, the surface of the crystals become slightly planar after cocrystallization, which mainly due to extensive existence of C···C interactions on the right side of the surfaces. These interactions are supposed to be the π-π stacking between the HNS-component and componentcomponent discussed above. The strength of C···C close contact in the HNS of HNS/BPE is relatively weak although the block of the H surface remains planar. The surfaces of the three planar component molecules in the cocrystals are cubelike, where the red dots (Fig. 7a and d) on the edge of block donate mainly intralayer hydrogen bonding and its strength increase in the order of BP > BPE > BPA. Strong N···H and H···N interactions on the surface of BPA destroy the π stacking structure between molecular layers, which may be main cause that the HNS/BPA cocrystal has lower impact sensitivity than HNS. Similarly, the C···C interactions in the BPE molecule of HNS/BPE is also weak, which is in agreement with the situation in the HNS molecule of HNS/BP. This can explain the differences in impact sensitivity between HNS/BP and HNS/BPE.

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Fig. 8 displays the 2D fingerprint of all molecule in the crystals. It is obvious that the introduction of nonenergetic molecules leads to stronger N-H bonds. These are represented by the spikes in the left top of Fig. 8b, d, and f and in the left bottom in Fig. 8c, e, and g. Except the O···O and N···O close contact belongs to weak vdW interactions, the red region of about 1.6 -1.8 Å (almost central) donates the interlayer π interactions formed by C···C and C···O close contact. BPA has wider range of contact than other components. A pair of outboard spikes on the left bottom donates interlayer hydrogen bonding formed by relatively weak N···H interaction between neighboring BPA molecules. Fig. 9 presents the populations of close contacts of HNS and component molecule in the cocrystals. It is found that the populations of the O···H, N···O, and O···O close contacts are almost 70% of total populations in the pure HNS crystal. This indicates that these close contacts plays an important role in determining its main behaviors. As the populations of the O···H close contacts in the HNS of the cocrystals increase, those of the C···O, N···O, and O···O contacts decrease. This may due to the emergence of N···H, C···N, and C···C interactions. The C···N and C···C contacts in BP have higher populations than those in BPE and BPA, which may lead to HNS/BP possessing the most low impact sensitivity among the three cocrystals. In all, the strong N-H bonds and π stacking were formed among the three HNS-based cocrystal explosives during the cocrystal process. The differences in the H-bonding net and π-π interactions among the HNS-based supramolecular structures may explain the variation of impact sensitivity for the three cocrystals. 3.4 AIM and IGM

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As we know, the anisotropy of intra- and interlayer interactions is closely related to the impact sensitivity of the cocrystal explosives.19 To further understand the differences in the interactions among the three cocrystals, we selected six representative clusters derived from the unit cells of the HNS-based corystals, as shown in Figure S2, to calculate their AIM and RDG (based on IGM model), which can describe the H-bonding net and π-π interactions quantitatively. The pure HNS can not be considered due to lacking face-to-face π stacking, Table 2 lists the topological parameters of selected clusters from the pure HNS crystal and its cocrystals. It is found that the H-bonding net in the pure HNS crystal is consist of two different sets of hydrogen bonds, which are formed by two face-to-face HNS molecules and an isolated HNS molecule linked via a diagonal HNS one, respectively. The bond length of the hydrogen bond C-H···O ranges from 2.266 to 2.670 Å and the bond energy Eb is in the range from 4.01 to 12.97 kJ·mol-1. The intralayer interactions of the HNS-based cocrystals are also composed of two sets of hydrogen bonds. One is called “planar H-bonds”, which contains two C-H···O and one C-H···N binds. The other is called “crossing H-bonds”, which includes several CH···O hydrogen bonds formed by two crossing molecules. Among them, the planar H-bonds are main driving force between molecular layers, and moreover, the C-H···N bond is strongest one, whose bond length ranges from 2.051 to 2.208 Å and Eb is in the range of 15.70-22.45 kJ·mol-1. The crossing H-bond in the intralayer of BPA has only one weak C-H···O bond, which is supposed to be weakest one and its bond length is 2.484 Å and Eb is 8.10 kJ·mol-1. Comparably, the intra H-bonds of BP (83.65 kJ·mol-1) has higher total Eb than those of BPE (63.43 kJ·mol-1) in spite of relatively small amount, which can be also indicated by the plot shapes of NCI in Fig. S3

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discussed below. In summary, the strength of intralayer H-bond ranks in the order of BP > BPE > BPA, which is in good agreement with experimental impact sensitivity. As for interlayer interactions, the interlayer H-bond of BP is similar to the intralayer one of BPE and includes two types of C-H···O. One is two sets of C-H···O between paralleled HNS molecules. The other is an isolated C-H···O between HNS and component from different layers. The bond length of the interlayer H-bond in BP ranges from 2.377 to 2.713 Å and Eb is in the range 5.07-10.12 kJ·mol-1, which is slightly higher than that of the interlayer H-bond in BPE (lies in the range of 3.376.85 kJ·mol-1). However, the interlayer interaction in BPA becomes stronger due to the emergence of two symmetrical C-H···N (Eb is 7.59 and 7.75 kJ·mol-1, respectively) linked by the N atom of ring and central methylene, which is in agreement with the aforementioned conclusions drawn from crystal structures. This is finally proved to be main reasons that the HNS/BPA cocrystal possesses the most highest impact sensitivity among the three HNS-based cocrystals.

The anisotropy of intermolecular interactions is highly related to the impact sensitivity of the cocrystal explosives. Based on the AIM analysis of H-bonding net, it is obvious that the central methylene in BPA is responsible for HNS/BPA having the highest impact sensitivity among the three cocrystals. But there are still small differences in impact sensitivity between HNS/BP and HNS/BPE. Fig. 10 and Fig. S3 displays the NCI plots for intermolecular interactions (interlayer and intralayer respectively) and the shape of isosurface that denotes the strength of interaction. As discussed above, the π-π interactions between molecular layers can be divided into three types: 1) Partial overlapping interactions between HNS molecules; 2) Edge-toedge π interactions between HNS and component; 3) Face to face π interactions

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between components. The shape and color of the NCI plots for first two interactions in the three interlayer clusters are similar. Therefore, the differences between the last one may determine the variation of the impact sensitivity of the three cocrystal explosives. The NCI plots between BPA present no face-to-face interactions here, which may due to non-planar molecular structure of BPA. Instead, the strong attractive interactions and steric hindrance of C-H···N are leading in this area, which is against the sliding between molecular layers and buffers to external mechanical energy. Moreover, the isosurface of interactions between BPE is smaller than that between BP. This may be due to relatively weak overlap between two face-to-face C ring of components, leading to the small accumulation of electron density. Thus, the strength of π-π interaction between components is as follow: HNS/BP > HNS/BPE > HNS/BPA, in good agreement with the order of experimental impact sensitivity. This suggests that the π-π interaction between components plays a key role in controlling the safety performance of the HNS-based cocrystals. 4. CONCLUSIONS We performed DFT calculations to study the crystal packing, electronic structures, Hirshfeld Surfce, Bader's AIM, and IGM of the HNS crystal and its cocrystals (HNS/BP, HNS/BPE, and HNS/BPA) in order to reveal the effects of noncovalent interactions on their impact sensitivity. When HNS combines with aromatic nonenergetic molecule to form a cocrystal, the strong C-H···N interactions and πstacking structures were formed. The strength of the intralayer hydrogen bonds in the HNS-based cocrystals increases in the following order: HNS/BP > HNS/BPE > HNS/BPA, but the situation for the interlayer hydrogen bonds is opposite. This indicates that the anisotropy of the H-bonding net is closely related to impact

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sensitivity. HNS/BPA has strong interlayer interaction between C-H···N linking by methylene, which destroys the π-stacking structure but leads to lower impact sensitivity than HNS. HNS/BPE is slightly higher impact sensitivity than HNS/BP. Main cause is that small electron density accumulates between two parallel edge-toedge C-rings in BPE and so leads to further assembling of the BPE molecules. Among the three cocrystals, HNS/BP has most strong intralayer hydrogen bonds and π-π interactions, which can be responsible for its lowest impact sensitivity (best safety performance). In all, the coupling of H-bond net and π-stacking structure plays a crucial role in decreasing the impact sensitivity of the HNS-based cocrystals. Among the emerging π-π interactions, the face-to-face interaction lying between components is the most important one. Therefore, the strong intralayer hydrogen bonds and face-to-face π-π interactions determines the cocrystal explosives to be low impact sensitivity. Our work may provide useful information for understanding the safe performance of the cocrystal explosives in atomic detail and broaden the application of supramolecular chemistry in the fields of the explosives.

ASSOCIATED CONTENT A Supporting Information file includes: Crystal parameters and impact sensitivity of pure HNS crystal and HNS/BP, HNS/BPE, HNS/BPA cocrystals. Geometrical configuration of components molecular. Detailed information for geometry structure, bond paths and (3,-1) critical point in particular clusters. NCI plots for gradient isosurfaces for intralayer clusters.

ACKNOWLEDGMENTS

17



This work was supported by the National Natural Science Foundation of China (Grant No. 21773119), the NSAF Foundation of National Natural Science Foundation of China and China Academy of Engineering Physics (Grant No. U1530104), and the Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions..

REFERENCES

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Figure 1. Unit cells for (a) pure HNS crystal, (b) HNS/BP, (c) HNS/BPE, and (d) HNS/BPA cocrystals. Figure 2. Crystal Packing of (a) HNS, (b) HNS/BP, (c) HNS/BPE, and (d) HNS/BPA. The green dash lines represent hydrogen bonds. Figure 3. Detailed hydrogen bonds in the intralayers of (a) BP, (b) BPE, and (c) BPA and in the interlayers of (d) BP, (e) BPE, and (f) BPA. The dash lines stand for related hydrogen bonds. Figure 4. Spatial electron density differences between different molecular layers in (a) HNS, (b) HNS/BPE, (c) HNS/BPE, and (d) HNS/BPA crystals. The blue and yellow colors display a gain or loss of electron density (isovalue is 0.01). Figure 5. Density of states (DOS) of the O-p states, N-p states, and H states in all the crystals. The Fermi energy is shown as a dashed vertical line. Figure 6. DOS of the C-p states and N-states in all the crystals. Figure 7. Important close contacts of (a) HNS, (b) HNS in HNS/BP, (c) BP in HNS/BP , (d) HNS in HNS/BPE, (e) BPE in HNS/BPE, (f) HNS in HNS/BPA, and (e) BPA in HNS/BPA on their Hirshfeld surfaces marked by red dots. The pairs of a-j stand for H···O, O···H, H···N, N···H, C···O, O···C, N···O, O···N, H···H, and C···C interactions, respectively. Figure 8. Two-dimensional fingerprint plots of close interatomic contacts of (a) HNS, (b) HNS in HNS/BP, (c) BP in HNS/BP, (d) HNS in HNS/BPE, (e) BPE in HNS/BPE, (f) HNS in HNS/BPA, and (e) BPA in HNS/BPA. Figure 9. Populations of close contacts of HNS and component molecules in all the crystals. Figure 10. NCI plots of gradient isosurfaces (0.006 a.u.) for particular interlayer clusters of (a) BP, (b) BPE, and (c) BPA. The blue, green, and red scale colors on the surfaces denote strong attractive, weak attractive, and nonbonded interaction, respectively.

Table 1. Lengths of selected weakest bonds and several torsion angles for the pure

HNS and component molecules in the HNS-based cocrystals.

weakest bond/ Å

C-C(ring)

Pure HNS

HNS/BP

HNS/BPE

HNS/BPA

1.479

1.474

1.473

1.473

21



torsion angle/°

Page 22 of 35

C-C(olefins)

1.346

1.345

1.349

1.348

C-N

1.492

1.491

1.486

1.49

C7-C2-C1-H1

125.612

124.501

-130.331

128.060

C4-C5-C6-H6

-177.562

-178.525

178.668

-179.469

C2-C3-N1-O1

0.818

3.073

3.424

-1.451

C4-C5-N3-O5

18.427

18.788

-11.82

9.339

C6-C7-N2-O4

-47.529

-48.428

-117.248

-57.448

Table 2: Topological parameters of selected clusters from the HNS crystal and HNSbased cocrystals at the (3, -1) critical point: calculated hydrogen bond length (Å), angle (°), density of all electrons (ρ, e·Å-3), Laplacian of electron density (▽2ρ, e·Å-5), potential energy density(Vb), and hydrogen bond energy(Eb, kJ·mol-1)

22


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Cluster

D-H

H···A A-H···D

ρ×100

▽ρ×10

Vb×100

Eb

Intramolecular BP

1.088

2.051

157.885

2.202

0.675

-1.71

22.45

1.087

2.436

158

1.067

0.3531

-0.7739 10.16

1.088

2.355

146.732

1.236

0.3945

-0.8973 11.78

1.09

2.433

131.576

0.898

0.3311

-0.6343

8.33

1.1

2.151

175.692

2.151

0.4924

-1.345

17.66

1.091

2.252

131.371

1.363

0.479

-1.011

13.27

1.09

2.524

167.25

0.8202

0.2661

-0.5416

7.11

1.092

2.705

157.739

0.6143

0.2188

-0.3771

4.95

1.092

2.627

148.151

0.7179

0.2452

-0.4538

5.96

1.091

2.561

145.219

0.7683

0.2603

-0.4988

6.55

1.091

2.403

133.694

0.9416

0.3476

-0.6739

8.85

1.103

2.139

173.819

2.21

0.5038

-1.383

18.16

1.089

2.304

133.736

1.242

0.4246

-0.903

11.85

Intramolecular BPA 1.091

2.484

159.812

0.9167

0.2902

-0.6173

8.10

1.092

2.442

134.095

0.8456

0.3205

-0.5999

7.88

1.101

2.208

173.246

1.935

0.4392

-1.196

15.70

1.09

2.318

132.571

1.211

0.4146

-0.8774 11.52

1.091

2.713

114.098

0.6301

0.2461

-0.3859

5.07

1.088

2.484

155.964

0.758

0.2751

-0.5211

6.84

1.088

2.377

120.788

1.0874

0.4075

-0.7708 10.12

1.09

2.84

118.974

0.4463

0.177

-0.2569

3.37

1.09

2.516

121.94

0.7589

0.3026

-0.5219

6.85

1.09

2.628

164.292

0.5753

0.2103

-0.364

4.78

Intermolecular BPA 1.099

2.51

148.93

1.023

0.3096

-0.5906

7.75

1.099

2.51

148.929

1.009

0.3078

-0.5785

7.59

1.091

2.688

114.773

0.6436

0.2487

-0.3943

5.18

1.09

2.696

162.286

0.4871

0.1861

-0.2979

3.91

1.09

2.443

124.919

0.8795

0.3393

-0.6187

8.12

1.09

2.266

150.224

1.352

0.4087

-0.9883 12.97

Intramolecular BPE

Intermolecular BP

Intermolecular BPE

Pure HNS

23



Page 24 of 35

1.088

2.67

150.404

0.4797

0.1934

-0.3051

1.088

2.351

130.483

1.128

0.3951

-0.8041 10.56

1.088

2.481

117.392

0.89

0.3447

-0.6089

7.99

1.088

2.404

157.364

0.9128

0.3173

-0.6444

8.46

24


4.01

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Figure 1. Unit cells for (a) pure HNS crystal, (b) HNS/BP, (c) HNS/BPE, and (d) HNS/BPA cocrystals. Gray, blue, red and white spheres stand for C, N, O and H atoms, respectively. .

25



Figure 2. Crystal Packing of (a) HNS, (b) HNS/BP, (c) HNS/BPE, and (d) HNS/BPA. The green dash lines represent hydrogen bonds.

26


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Figure 3. Detailed hydrogen bonds in the intralayers of (a) BP, (b) BPE, and (c) BPA and in the interlayers of (d) BP, (e) BPE, and (f) BPA. The dash lines stand for related hydrogen bonds.

27



Figure 4. Spatial electron density differences between different molecular layers in (a) HNS, (b) HNS/BPE, (c) HNS/BPE, and (d) HNS/BPA crystals. The blue and yellow colors display a gain or loss of electron density (isovalue is 0.01).

28


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Figure 5. Density of states (DOS) of the O-p states, N-p states, and H states in all the crystals. The Fermi energy is shown as a dashed vertical line.

29



Figure 6. DOS of the C-p states and N-p states in all the crystals.

30


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Figure 7. Important close contacts of (a) HNS, (b) HNS in HNS/BP, (c) BP in HNS/BP , (d) HNS in HNS/BPE, (e) BPE in HNS/BPE, (f) HNS in HNS/BPA, and (e) BPA in HNS/BPA on their Hirshfeld surfaces marked by red dots. The pairs of a-j stand for H···O, O···H, H···N, N···H, C···O, O···C, N···O, O···N, H···H, and C···C interactions, respectively.

31



Figure 8. Two-dimensional fingerprint plots of close interatomic contacts of (a) HNS, (b) HNS in HNS/BP, (c) BP in HNS/BP, (d) HNS in HNS/BPE, (e) BPE in HNS/BPE, (f) HNS in HNS/BPA, and (e) BPA in HNS/BPA.

32


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Figure 9. Populations of close contacts of HNS and component molecules in all the crystals.

33



Figure 10. NCI plots of gradient isosurfaces (0.006 a.u.) for particular interlayer clusters of (a) BP, (b) BPE, and (c) BPA. The blue, green, and red scale colors on the surfaces denote strong attractive, weak attractive, and nonbonded interaction, respectively.

34


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For Table of Contents USe Only Effects of noncovalent interactions on the impact sensitivity of HNS-based cocrystals: A DFT study Xiao Zhao, Shiliang Huang, Yu Liu, Jinshan Li, Weihua Zhu*

When HNS combines with aromatic nonenergetic molecule to form a cocrystal, the strong C-H···N interactions and π-stacking structures were formed. The strong intralayer hydrogen bonds and face-to-face π-π interactions determines the HNSbased cocrystal explosives to be low impact sensitivity.

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Effects of Noncovalent Interactions on the Impact Sensitivity of HNS

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