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Enhanced Intermolecular Hydrogen Bonds Facilitating the Highly Dense Packing of Energetic Hydroxylammonium Salts Liya Meng, Zhipeng Lu, Yu Ma, Xianggui Xue, Fude Nie, and Chaoyang Zhang Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b01409 • Publication Date (Web): 10 Nov 2016 Downloaded from http://pubs.acs.org on November 11, 2016

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Crystal Growth & Design

Enhanced Intermolecular Hydrogen Bonds Facilitating the Highly Dense Packing of Energetic Hydroxylammonium Salts Liya Meng,†,‡ Zhipeng Lu,‡,§ Yu Ma, ‡ Xianggui Xue,‡ Fude Nie,‡ and Chaoyang Zhang*†,‡ †

College of Material Science and Engineering, Southwest University of Science & Technology, Mianyang, Sichuan 621900, China. ‡ Institute of Chemical Materials, China Academy of Engineering Physics (CAEP), P. O. Box 919-311, Mianyang, Sichuan 621900, China. § Department of Mathematics and Physics, Officers College of CAPF, Chengdu,610213,China.

Abstract: The energy and performance of energetic materials can be improved by increasing their crystal packing density. Thus, we propose a strategy involving salification with hydroxylammonium cations (HA+) to increase the packing coefficients (PCs) and packing densities of energetic ionic salts (EISs). Structural analyses and theoretical calculations of the observed EISs indicate that the strong intermolecular hydrogen bonds (HBs) between HA+ and anions are primarily responsible for the increase in EIS density. Such strong HBs usually exist in HA+-based energetic salts and rarely in other EISs but are absent in energetic crystals with neutral molecules. Such HBs induce high PCs and relatively high crystal packing densities by compensating the relatively lower molecular density of HA+ compared with other cations. Moreover, in combination with HBs in common explosives, we find a simple dependence showing that the shorter the strongest HB corresponds to the higher PC, suggesting that the strongest HB can be regarded as a simple indicator of PC. This study proposes that enhancing intermolecular HBs is the main strategy to increase compactness because H atoms usually exist in currently available energetic materials.

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1. INTRODUCTION Energetic ionic salts (EISs) have attracted increasing attention as alternatives to high-performance energetic materials.1–9 EISs possess advantages over traditional energetic materials composed of neutral molecules such as TNT, RDX, HMX, CL-20, and TATB. For instance, the combination of differently signed ions in pairs (Schemes 1 and 2) yields a series of EISs once a new ion is obtained. Such a combination7,10,11 reduces preparation cost and contributes to scientific findings of new rules and regularities. In addition, ionization favors molecular stability.12 For example, numerous highly energetic compounds are N-rich and electron-lacking, which cause the instability and thus limited practical applications of these compounds. Nevertheless, the stability of these compounds can be improved through negatively charging. Therefore, ionization is a feasible strategy to stabilize unstable energetic molecules.12,13 Scheme 1. Molecular structures of interested cations.

Scheme 2. Molecular structures of interested anions. O

N N N

N

-

N

N

N

N

N

N

-

N

N -

N

N

O

NO2

N

N

NO2

N

NO2

N

O

DBO2-

2-

BT2O

O

NO2

-

-

N

N

BT2N

O-

N

-

N

O

O2N

NNO2-

N

NO2

N

N

-

N

N

N

N N

NH2

N

O2NN N N

N

N

N

O-

O

-

N

N

-

N

O2NN

N

NO2 N

O -

NTX

DNAAF

N

N O2N

N

N-

N-

N

NH

O 2-

AFTA-

NO2

N

N

O

-

O

O2NN O2N

N

N

-

NNO2-

N

N

N

BTO2-

O

N

N

O-

O

DNBTO2-

DNABF2-

N

N

O-

N

HN

DPNA-

2


BNT2-

NH NNO2

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Table 1. Comparison in properties [crystal density (d, g/cm3) 25 velocity (D, m/s)] of interested EISs. Anions-based EISs 2-15

BT

2-16

BT2O

2-17

DBO

2-18

DNABF

2-19

DNBTO

BTO

2-20,21

-22

AFTA

-23

NTX

2-24

DNAAF

-7

DPNA

BNT

2-25

7,15-25

, and detonation pressure (P, kbar) and

properties

NH3OH+

NH4+

N2H5+

G+

AG+

DAG+

TAG+

d

1.742

1.590

1.531

1.586

1.568

1.520

1.535

P

317

189

236

176

193

205

238

D

8854

7417

8265

7199

7504

7711

8181

d

1.822

1.664

1.633

1.637

1.600

P

372

258

221

247

251

D

9264

8212

7752

8137

8256

d

1.986

1.951

1.75

1.742

1.696

1.769

P

394

350

270

266

263

298

D

8935

8618

8038

8078

8108

8513

d

1.963

1.834

1.812

1.769

1.680

1.811

P

425

334

354

271

265

335

D

9363

8748

9058

8225

8228

8836

d

1.952

1.696

1.840

1.788

1.764

1.730

P

390

297

342

263

272

328

D

9087

8388

8915

8102

8268

8919

d

1.915

1.8

1.725

1.639

1.596

1.749

P

424

316

340

233

243

246

D

9698

8817

9159

7917

8111

8028

d

1.822

1.688

1.706

1.642

1.608

1.710

P

33.4

26.6

29.0

23.6

26.7

29.0

D

9100

8436

8759

8173

8580

8859

d

1.85

1.73

1.698

1.697

1.687

1.639

P

41

32.8

27.4

29

29.9

29.2

D

9381

8767

8270

8503

8639

8617

ρ

1.90

1.83

1.84

1.74

1.84

1.76

P

42.2

39.6

39.3

29.2

36.5

33.6

D

9511

9474

9459

8585

9453

9227

d

1.86

1.83

1.86

1.72

1.79

P

385

356

387

283

322

D

9195

8880

9176

8319

8745

d

1.78

1.83

1.63

1.70

1.71

1.68

1.64

P

32.4

34.9

26.2

26.6

26.3

27

27

D

8856

9407

8455

8634

8603

8701

8705

The detonation performance of an energetic material can be improved by increasing its density; detonation equations revealed that detonation velocity and pressure are proportional to density and square of density, respectively.14 Therefore, determining which ion favors a high density

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is important in EISs. Careful inspection of the EISs synthesized by combining ions in pairs (Schemes 1 and 2) revealed that the salification with hydroxylammonium cations (NH3OH+, HA+) produces the highest densities relative to other cations combined with any same anion (Table 1). The table shows that HA+ salts (HASs) possess the highest densities in 10 out of the 11 groups of EISs; in the only one exception in which the anion is BNT2-, a HAS still possesses the second highest density7,15-25. In addition, the high densities of HASs contribute to their high detonation properties. Based on these observations, we can develop a strategy involving salification with HA+ to prepare highly dense EISs. Elucidating the underlying mechanism for the high densities of HASs is necessary. Taking a group of HA-based EISs and another group of BT2--based EISs as examples, we find that the strong intermolecular hydrogen bonds (HBs) in these EISs are responsible for their highly compact packing. Aside from increasing energy, high compactness also facilitates the safety of energetic materials.26 Considering that H atoms exist in most currently available energetic materials, the present work proposes to strengthen intermolecular HBs to obtain new high-quality energetic materials. 2. METHODOLOGIES This study focuses on elucidating the mechanism behind the high crystal densities (d) of HASs. Hence, the crystal packing of these HASs will be discussed on the basis of the experimentally determined crystal packing structures and in terms of molecular density (dM), packing coefficient (PC), and intermolecular interactions. First, d is determined by dM and PC. In the present work, the dM of ions was calculated by their mole weights and van de Waals volumes, with van de Waals radius assignments of C, H, O, and N atoms of 1.7, 1.2, 1.52, and 1.55 Å, respectively. Thereby, PC was obtained using the equation PC=d/dM.

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Intermolecular interactions account for PC. In the present work, we employed Bader’s QTAIM27 method and ab initio calculations to clarify the intermolecular interactions in all interested EISs. QTAIM analysis was conducted with Critic2, a code for analyzing the real-space quantum-mechanical interactions in periodic solids on the basis of electron densities and other related scalar fields.28, 29 The core and valence electron densities of a crystal were obtained from ab initio calculations implemented by VASP. The calculations and experimental confirmation of structures were carried out using the conjugate gradient method with 6×3×5 Monkhorst-Pack k-point sampling in reciprocal space and a plane-wave basis set with an energy cutoff of 550 eV. The DFT-D2 method of Grimme30 was employed to correct long-range dispersion interactions because the correction of these weak interactions is crucial to describe the crystal structures. In the DFT-D2 method, dispersion corrections are calculated not only for forces acting on the atoms but also for the stresses on the unit cell. Thus, a simultaneous optimization of all degrees of freedom is permitted. The self-consistent convergence criteria of energy were set to 1×10−7 and 1×10−6 eV for electronic and ionic relaxation, respectively. In our most recent work, the same ab initio method has been performed to optimize the crystal structure of TKX-50, showing good agreement with the experimental confirmation.31 This result also suggests the reliability of the applied method to EISs. The HB energy or the bond dissociation energy of HB (EHB) was predicted using the empirical equation EHB=-(1/2)v proposed by Espinosa et al.32 The prediction results were then used to assess HB strength in terms of electron densities (ρ) at the bond critical points, after which the potential energy densities (v) were obtained. Hirshfeld surface analysis, which is a straightforward tool to understand intermolecular interactions,33–35 was applied in this work. Hirshfeld surfaces in a crystal were constructed in terms of electron distributions, calculated as the sum of spherical atom electron densities. The normalized contact distance (dnorm) was determined by di and de, the distances from the surface to the nearest 5



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atom interior and exterior to the surface, respectively. dnorm enables the identification of regions of particular importance to intermolecular interactions. That is, a Hirshfeld surface is composed of many points, and each point parametrized as (di, de) can provide information about related contact distances from it. Smaller di + de suggests closer atom–atom contact. Both di and de were constrained in the range of 0 to 2.6 Å. Mapping these (di, de) points and considering their relative frequencies produces a 2D fingerprint plot. The fingerprint of any symmetrically dependent molecule in any crystal is unique. This unique feature is the basis for identifying the crystal environment of a given molecule. The color mapping distinguishes the intensity of points, and red and blue represent high and low intensities, respectively. Therefore, the locations of (di, de) points and their relative frequencies discernible on the surface and the 2D fingerprint plot can be used to ascertain the distances and intensities of these contacts. All the surfaces and fingerprint plots were created using CrystalExplorer3.0.57, and the surfaces were mapped over the dnorm range of −0.2 Å to 1.2 Å.36 3. RESULTS AND DISCUSSION 3.1 High PCs of HASs. Table 2. PCs (in %) of interested EISs and some common explosives. Explosives

NH3OH+

NH4+

N2H5+

G+

AG+

DAG+

TAG+

BT2--based

81.8

77.9

75.2

75.6

76.8

63.9

72.5

82.2

77.3

75.3

74.9

2-

BT2O -based 2-

82.8

82.0

2-

84.1

79.8

80.1

77.8

74.3

2-

DNBTO -based

83.3

77.3

80.7

78.3

77.7

BTO-based

84.9

80.2

80.0

75.3

72.8

DBO -based DNABF -based

-

80.3

-

NTX -based

79.9

TATB

79.4

FOX-7

80.0

LLM-105

78.4

β-HMX

79.0

ε-CL-20

77.1

AFTA -based

74.5

76.6 75.2

77.0 73.1

76.3 76.7

80.6 72.4

74.8

74.6

6

74.0

72.7


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Table 3. dM of various cations in BT2--based EISs and their d. density dM, g/cm3 3

d, g/cm

NH3OH+

NH4+

N2H5+

G+

AG+

DAG+

TAG+

1.68

1.24

1.50

1.80

1.87

1.93

1.94

1.742

1.590

1.531

1.586

1.568

1.520

1.535

As pointed out above, HASs usually possess the highest d among cations (in Table 1). To ascertain the factor responsible for the highest d of HASs among cations, we calculated the PCs of all interested EISs and some common explosives composed of neutral molecules, as well as the dM of cations in BT2--based EISs, because d = PCd M . As demonstrated in Table 2, HASs possess the highest PC in each group. In general, HASs even possess higher PCs than common explosives with high packing densities, such as TATB,37 LLM-105,38 FOX-7,39 β-HMX,40 and ε-CL-20.41 In the group of BT2--based EISs listed in Table 3, the dM of HA+ is lower than those of G+, AG+, DAG+, and TAG+. Thus, the high PCs instead of the high dM of HA+ are responsible for the high d of HASs. High PCs also suggest strong intermolecular interactions. In other words, the strong intermolecular interactions cause the high d of HASs and are therefore crucial in understanding the strategy involving the deposition with HA to increase the d of EISs. 3.2 Intermolecular interactions in HASs

Figure 1. ESP of interested ions mapped onto the molecular surfaces of electronic density of 0.001 au. Grey, white, blue and red represent C, H, N and O atoms, respectively. 7



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We selected eight HASs from Table 1 and analyzed their crystal packing and intermolecular interactions to reveal the reason behind the high PCs, with consideration of availability of crystal structure and nonhydrates. We examined the molecular structures of the eight anions of BT2-, BT2O2-, DBO2-, DNABF2-, DNBTO2-, BTO2-, AFTA- and NTX-, and HA+. All anions possess planar conjugated structures in crystals. Such conjugated structures facilitate the enhancement of molecular stability, but some including BT2-, BT2O2-, DNBTO2-, BTO2-, AFTA- and NTX- are N-rich, which usually leads to weak molecular stability. Considering that molecular electrostatic potentials (ESPs) generally play a key role in intermolecular interactions, we provide the ESPs of the eight anions in Figure 1. Except for the positively charged moiety of the amino group of AFTA-, the remaining moieties of AFTA- and other anions are negatively charged; by contrast, HA+ is positively charged. The ESP difference in sign facilitates the electrostatic attraction between HA+ and each anion. To show the molecular electrostatic property quantitatively, we calculated the atomic ESP charges of each ion. Figure s1 of Supporting Information (SI) displays the atomic ESP charges of external moieties of the ions and supplies information similar to Figure 1. The electrostatic characteristic of HA+ and anions favors the electrostatic attraction between the three H atoms of HA+ and the atoms on the external moieties of anions. It also implies HBs. Rationally, such electrostatic attraction makes the HBs in EISs stronger than those in neutral energetic molecular crystals. The details of HBs will be discussed later. We then analyzed the molecular stacking in HASs. All anions are π-conjugated. Considering the smaller size of HA+ than these anions, we can deduce that the HASs are π-stacked with HA+ lying around the π-conjugated anions. Except for AFTA- with H atoms, the remaining anions are H-free, suggesting the good complementarity of H-bonding with HA+. This case is similar to the BTF-based co-crystals because BTF is H-free.42 As illustrated in Figure 2, all eight HASs are

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HB-aided π-stacked. These π-stacked anions in the figure, with larger sizes relative to HA+, can be regarded as the frames constructing the crystal. Meanwhile, HA+ cations lie among such anions and bridge them by strong electrostatic attraction and van de Waals attraction, mostly of HBs. This phenomenon is not observed in energetic crystals built by homogeneous neutral molecules because the intermolecular interaction governing the crystals is van de Waals attraction.43,44

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π-stacked layers becomes smoother.43,45 However, it is not the case. HA+ cations hinder the sliding relatively, that is, the sliding among these HAS crystals is more or less spatially unallowed, even though they aid the π-stacking. Considering the lower molecular stability of these HASs, we can deduce that they are more sensitive to external mechanical stimuli than TATB, consistent with experimental confirmations: the 4–50 J impact energy of these HASs versus 50 J of TATB15,17,18,43. Meanwhile, the anions of the remaining HASs are wave-like or crossing π-stacked, similar to the energetic crystals of FOX-7, LLM-105, DATB, etc.43 Obviously, the sliding might be more spatially prohibited.

Figure 3. HBs in the sliding layers as encompassed by celeste rectangles.

10


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As pointed out above, the molecular structure and ESPs favor the strengthened HBs, which differ considerably from the common energetic crystals with homogeneous neutral molecules. This difference is possibly responsible for the high PCs of HASs. To confirm this possibility, we checked the HBs in HASs and obtained some interesting findings. First, HBs distribute in terms of their strengths. As illustrated in Figure 2, the relatively strong HBs (purple dash) are distributed along the sliding layer, whereas the weak HBs (green dash) are almost vertical to the layer. Figure 3 exhibits the intralayer HBs formed by HA+ and anions, as well as the anions themselves for HA-AFTA. The HB between two neighboring HA+ has not be observed in this case. Similar to TATB, these stronger HBs support the layers to extend infinitely.43

Figure 4. HBs around an anion.

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Figure 5. HBs around a HA+.

As shown in Figure 4, the strongest HBs in seven HASs excluding HA-AFTA are all of the same kind of (HA+) O-H…A (anion). This finding could be attributed to the higher electronegativity of the O atom than the N atom in HA+, favoring the formation of strong HBs; meanwhile, the strongest HB in HA-AFTA is (HA+) N-H…A (anion) because of additional promoters (i.e., additional anion–anion interaction) of HB formation. Even though the strongest HB always exists between HA+ and anions, the additional type of HBs in the crystal influences molecular stacking and therefore results in a different type of the strongest HB. The HBs around the HA+ in Figure 5 may be formed by HA+…anion and HA+…HA+. HA+ can act as both the donor and acceptor of HBs; hence, it can be encompassed by both HA+ and anions. This case is evidently different from the cases of HASs with H-free anions as shown in Figure 4, where the anions are surrounded by only HA+. In addition, the HBs between double HA+ are always interlayered and denoted by green dash,

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indicating that the HBs are relatively weak.

Figure 6. Two-dimensional fingerprint plots of HAS crystals.

The HASs featuring strong HBs can be confirmed by 2D fingerprint plots of HA+ in HAS crystals in Figure 6. At least two acute spikes appear at the left bottom of each plot, with a distance of the acuter spike to the original point of 1.65–1.84 Å (the sum values of horizontal and vertical coordinates). Such two spikes are of (HA+) H…O (anion) (the acuter ones) and (HA+) H…N (anion) (the others). Thus, these short distances suggest the strong HBs. According to the clarification of HB strength proposed by Jeffery,46 these spikes denote moderate and close strong HBs. In addition, excluding HA-DNBTO in Figure 6, other plots exhibit another eroded spike at the middle bottom, denoting the HBs between double HA+. These spikes are relatively far from the original point, representing the weakness of the HBs,46 which is in agreement with the above HB analysis of Figure 5. Despite the O…H contact between double HA+ for HA-NTX, QTAIM analysis confirms that it does not belong to a HB. A comparison of the 2D fingerprint plots of common energetic molecules in the crystal in Figure s2 of SI confirms that the intermolecular HBs in HASs are remarkably stronger than those in common explosive crystals. 13



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Figure 7. Populations of the close interatomic contacts of HA+ in HAS crystals and of neutral energetic molecules in their crystals.

Figure 7 also exhibits higher close interatomic contact populations of HBs in HAS crystals than in common explosive crystals. As illustrated in the figure, the sum contact populations of HBs of O…H and N…H in HAS crystals are mostly above 80%, even close to 90%, whereas those in common explosive crystals are below 70%, even below 40% for ε-CL-20. In contrast to the common explosive crystals and energetic co-crystals,42,47 the contribution of N…H contacts is more evident in HAS crystals than in common explosive crystals because of the high N-contents of the anions in EISs. Table 4. Geometry parameters, positions of bond critical points (BCPs), electronic density (ρBCP, in e/bohr3) and its Laplacian (∇ ∇2ρBCP, in e/bohr5) at BCPs, dissociation energy (EHB, in kJ/mol) of the strongest intermolecular HBs in HASs and some common explosives. ΣEHB/2 is the total dissociation energy of intermolecular HBs per ionic pair or molecular in the crystal. SM is the abbreviation of symmetry multiplicity. Explosives HA-BT

D-H…A

D-H, Å

H…A, Å

∠D-H…A, °

SM

ρBCP

∇2ρBCP

EHB

ΣEHB/2

O1-H1…N1

0.940

1.795

171.868

2

0.0444

0.0930

52.1

158.4

…

HA-BT2O

O2-H2 O1

0.960

1.670

176.544

4

0.0531

0.1063

68.3

317.5

HA-DBO

O12-H12…O3

0.920

1.786

173.645

2

0.0385

0.1100

45.1

236.5

HA-DNABF

O4-H4…O2

0.860

1.835

178.422

4

0.0345

0.1099

39.6

264.9

HA-DNBTO

O4-H4…O3

0.920

1.652

175.949

4

0.0533

0.1206

70.2

279.5

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HA-BTO

O2-H2…O1

0.907

1.714

168.900

4

0.047

0.123

59.1

273.7

HA-AFTA

N1S-H3S…O12

HA-NTX

1.000

1.724

173.770

4

0.0466

0.1155

58.1

352.6

…

0.940

1.691

174.599

4

0.0501

0.1208

64.5

262.5

…

1.054

2.239

121.3

2

0.0143

0.0548

12.3

57.1

…

0.904

2.143

143.7

4

0.0149

0.0645

13.9

60.5

…

0.904

2.046

149.4

4

0.0170

0.0786

17.1

50.4

…

1.094

2.360

152.6

4

0.0111

0.0389

8.4

67.1

…

0.937

2.439

135.7

4

0.0084

0.0366

6.6

31.5

O4-H4 O1 N4-H4 O1

TATB

N4-H3 O4

FOX-7 LLM-105 β-HMX ε-CL-20

N2-H1 O1 C2-H4 O2 C1-H1 O6

To describe quantitatively the HBs, QTAIM analyses were performed. The detailed geometry, electronic, and energy data of all HBs in related explosive crystals are given in Table s1 of SI, and those of only the strongest HBs are listed in Table 4. Regarding the eight HASs in the table, the H…A distances, ∠D-H…A, and HB dissociation energy range from 1.652 Å to 1.835 Å, from 168.9° to 178.4°, and from 39.6 kJ/mol to 70.2 kJ/mol, which are close to the criterion of strong HB, i.e., 1.2 Å to 1.5 Å, 175° to 180°, and 14 kcal/mol to 40 kcal/mol, respectively.46 In insensitive explosives TATB, LLM-105, and FOX-7 as well as sensitive ones β-HMX and ε-CL-20, the related geometric and energy parameters are far from the criterion. This finding indicates that the nature of the HBs in the HASs is a mixture of covalent and electrostatic attractions. The strongest HBs possess small proportions in intermolecular interactions and high degrees of covalent attraction; meanwhile, the remaining HBs in Table s1 of SI possess large proportions and high degrees of electrostatic attraction. From the above discussion, we can deduce that the intermolecular interactions in HASs are dominated by HBs. We related the length of the strongest HBs (in Table 4) with PCs (in Table 2) in Figure 8 because the distance of the shortest HB can largely reflect the intermolecular distance and thus the compactness of the crystal. As expected, PCs increase with enhancing HB strength. That is, PC can simply be assessed by the length of the shortest HBs. In Figure 8, HASs generally possess

the shorter lengths of the strongest HBs than common explosives.

15



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Figure 8. Relationship between PC and length of the strongest HB.

3.3 HBs in BT-based EISs Table 5. Geometry of the strongest HBs of BT-based EISs. Explosive

D-H…A

D-H, Å

HA-BT

O1-H1…N1

0.940

1.795

171.868

NH4-BT

N3-H2…N2

0.98

1.962

179.608

N2H5-BT

N5-H1…N6

1.02

1.857

168.680

…

H…A, Å

∠D-H…A, °

G-BT

N7-H7A N2

0.91

2.035

169.229

TAG-BT

N8-H8B…N4

0.91

2.100

168.428

The strategy of salification with HA+ for high-density EISs can be supported from another aspect, i.e., a group of EISs with the same anion. Thus, the nonhydrated BT-based EIS was selected to compare the HBs between BT2- and various cations. The geometric details of the HBs are listed in Table s2 of SI, and those of the strongest HBs are shown in Table 5. As exhibited in the table, the H…A distance of the strongest HB is evidently shorter in HA-BT than in other BT-based EISs, resulting in a PC of 81.8% of HA-BT, about 72.5%–77.9% of others. This result further confirms that the strengthened HBs are responsible for the enhanced PCs of HASs. 4. CONCLUSIONS We propose a strategy involving salification with HA+ to increase the crystal densities of EISs 16


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and reveal the principle behind the strategy through structural analyses and theoretical calculations. In contrast to other EISs, the enhanced crystal densities of HASs are essentially attributed to the strengthened intermolecular HBs. These strong HBs with rather high strength are absent in common explosive crystals and rarely present in other EISs. Such HBs increase PCs while compensate the shortcoming of the low dM of HA+, resulting in relatively high crystal densities, even above 1.95 g/cm3 17,19. Relative to heavy elements, CHON in energetic materials with a certain degree of specific pulse or gaseous release generally possesses a low dM. To overcome this shortcoming, intermolecular interactions such as HB, π-stacking, and halogen-bonding must be strengthened to elevate PC. The present work shows that electrostatic attraction favors the formation of strong HBs. From this aspect, differentiating polarities of component molecules could facilitate the compact stacking of energetic salt crystals, single component crystals, and co-crystals. ■ ASSOCIATED CONTENT Supporting Information Atomic ESP charges of external moieties of interested ions, two-dimensional fingerprint plots of neutral energetic molecules in crystal, full Information of intermolecular HBs in interested HASs, and full geometric information of intermolecular HBs in interested BT-based EIS. This material is available free of charge via the Internet at http://pubs.acs.org. ■ AUTHOR INFORMATION Corresponding Author C. Y. Zhang, email: [email protected]; Tel: 86-816-2493506. Notes The authors declare no competing financial interest. ■ ACKNOWLEDGMENT 17



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The authors gratefully acknowledge the support of Science Challenge Project, the National Natural Science Foundation of China (U1530262, 11602241 and 21673210). ■ ABBRIVIATIONS BT

5,5′-bistetrazolate

BT2O

5,5′-bis(tetrazole-2-oxide)

DBO

5,5′-dinitromethyl-3,3′-bis(1,2,4-oxadiazolate)

DNABF

3,3′-dinitramino-4,4′-bifurazane

DNBTO

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

AFTA

4-amino-furazan-3-yl-tetrazol-1-olate

NTX

5-nitrotetrazolate-2N-oxide

DNAAF

3,3′-dinitroamino-4,4′-azoxyfurazanate

DPNA

N-(3,4-dinitro-1H-pyrazol-5-yl)nitramidate

BNT

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

HA

hydroxylammonium

NH4

ammonium

N2H5

hydrazinium

G

guanidinium

AG

aminoguanidinium

DAG

diaminoguanidinium

TAG

triaminoguanidinium

BTF

benzotrifuroxan

TATB

1,3,5-triamino-2,4,6-trinitrobenzene

LLM-105

2,6-diamino-3,5-dinitro-1,4-pyrazine 1-oxide

FOX-7

1,1-diamino-2,2-dinitroethylene

β-HMX

β-1,3,5,7-tetranitro-1,3,5,7-tetrazocane

ε-CL-20

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

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For Table of Contents Use Only Enhanced Intermolecular Hydrogen Bonds Facilitating the Highly Dense Packing of Energetic Hydroxylammonium Salts Liya Meng, Zhipeng Lu, Yu Ma, Xianggui Xue, Fude Nie, and Chaoyang Zhang

Enhanced intermolecular hydrogen bonds facilitating for the highly dense packing of energetic hydroxylammonium salts

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Enhanced Intermolecular Hydrogen Bonds ... - ACS Publications

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