Design of New Bridge-Ring Energetic Compounds Obtained by Diels

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A: Molecular Structure, Quantum Chemistry, and General Theory

Design of New Bridge-Ring Energetic Compounds Obtained by Diels-Alder Reactions of Tetranitroethylene Dienophile Piao He, Haozheng Mei, Le Wu, Junqing Yang, Jian-Guo Zhang, Adva Cohen, and Michael Gozin J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b01555 • Publication Date (Web): 08 Mar 2018 Downloaded from http://pubs.acs.org on March 9, 2018

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The Journal of Physical Chemistry

Design of New Bridge-Ring Energetic Compounds Obtained by Diels-Alder Reactions of Tetranitroethylene Dienophile Piao Hea, Hao-Zheng Meia, Le Wua, Jun-Qing Yanga, Jian-Guo Zhang*a, Adva Cohenb and Michael Gozin*b a

State Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology, Beijing 100081 P. R. China. b School of Chemistry, Faculty of Exact Science, Tel Aviv University, Tel Aviv, 69978, Israel.

ABSTRACT The density functional theory method was employed to calculate three-dimensional structures for a series of novel explosophores. The design of new molecules (DA1-DA12) was based on the bridge-ring structures that could be formed via Diels-Alder (DA) reaction of selected nitrogen-rich dienes and tetranitroethylene dienophile. The feasibility of the proposed DA reactions was predicted on a basis of the molecular orbital theory. That strong interactions between HOMO of dienes, with electron-donating groups (Diene2, Diene6 and Diene8) and LUMO of tetranitroethylene dienophile, suggested thermodynamically favorable formation of the desired DA reaction products. In addition to molecular structures of the explored DA compounds, their physicochemical and energetic properties were also calculated in detail. Due to compact bridge-ring structures, new energetic molecules have highly-positive heats of formation (up to 1124.90 kJ·mol-1) and high densities (up to 2.04 g·cm-3). Also, as a result of all-right ratios of nitrogen and oxygen, most of the new compounds possess high detonation velocities (8.28-10.02 km·s-1) and high detonation pressures (30.87-47.83 GPa). Energetic compounds DA1, DA4 and DA12 exhibit a superior detonation performance over widely-used HMX explosive, and DA5, DA7 and DA10 could be comparable to the state-of-the-art CL-20 and ONC explosives. Our proposed designs and synthetic methodology should provide a platform for the development of novel energetic materials with superior performance.

* Corresponding authors: Jian-Guo Zhang, Tel & Fax: +86 10 68918091; E-mail: [email protected]; Michael Gozin, Tel: +972-3-640-5878; Email: [email protected] 1

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1. INTRODUCTION Energetic materials (EMs) are defined as a class of compounds storing a significant amount of chemical energy in their structures and capable of quick release of this energy upon some stimuli1-3, and have been widely studied and applied in civilian and military field.4-8 Desirable characteristics for new energetics include high density, positive heat of formation, high detonation performance, good thermal stability, low sensitivity and eco-friendly.9 In the quest for the development of novel high-energy materials, achieving a higher density of explosive compound plays a very important role in its performance. This is can be concluded in the Kamlet-Jacobs equation10, where the detonation velocity (VD) correlates linearly with the density and the detonation pressure (PD) correlates with the square of density. Oxygen balance (OB) is also one of critical parameters related to EM characteristics. Explosives that have values of OB close to zero were found to exhibit a very good performance.11 In addition, the nitro group is characterized as a typical energetic substituent with high oxygen content but simultaneously high sensitivity. While the amino group is an effective structural unit for enhancing the stability of energetic molecules.12 The cage-structures are prominent high-energy density materials (HEDM) due to high density and good oxygen balance which greatly improve the energetic performance. Three typical examples were presented (Figure 1). 2,4,6,8,10,12-Hexanitrohexaazaisowurtzitane (CL-20)13 is known to be the most-dense secondary CHNO-explosive (ε-CL-20 density of 2.044 g·cm-3), due to its isowurzitane cage-based structure. However, the high sensitivity14 and a high production cost of CL-20,

currently

impede

a

large

scale

use

of

this

material.

A

newcomer

10-dinitro-2,6,8,12-tetraoxa-4,10-diaza- tetra-cyclo[5.5.0.05,9.03,11]-dodecane (TEX)15 shows a high ambient temperature density of 1.985 g·cm-3. A lower number of nitramine groups (in comparison to CL-20) reduces its sensitivity and makes it a potential insensitive energetic material. The third example of cage-type high performance explosives is octanitrocubane (ONC),16-17 which has a density of 1.98 g·cm-3 and exhibits 20-25% better detonation performance than HMX. Although OCN was predicted to have a VD of about 9.50 km·s-1 and PD of 46.00 GPa, synthetic difficulties for this compound inhibits its practical use.

2


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Figure 1. Three typical examples of cage-structure energetic compounds.

The Diels-Alder (DA) reaction is one of the most important cycloaddition reactions in organic chemistry, and it has become a powerful tool in preparation of molecules for biomedical applications, as well as for synthesis of several energetic compounds (Figure 2).18-19 These DA reactions can utilize a range of dienophiles and aza-dienes as starting materials, where a common reaction mechanism includes a concerted addition of starting materials, via a six-centered transition state, with simultaneous formation of two new bonds.20 In the terminology of orbital symmetry classification, the DA reaction is a [4πs + 2πs] cycloaddition and a thermally-allowed process.21 Recent scientific interests are connected with the utilization of aromatic heterocycles as dienes to design highly functionalized energetic derivatives with excellent performance.22

Figure 2. General features of Diels-Alder reaction.

Computational modeling and simulation techniques have been proven to be powerful and practical tools in the terms of the development of new energetic materials.11, 23-24 In this work, we designed versatile ploynitro energetic compounds with bridge-ring structures by employing nitrogen-containing heterocycles as dienes and tetranitroethylene as dienophile in the Diels-Alder reaction. (Figure 3) We described the density functional study on these novel high-energy compounds including molecular structures and predicted properties, as well as interactions between dienophile and diene based on the molecular orbital theory. 3



Figure 3. Design of molecular structures and synthesis pathways via Diels-Alder reaction.

2. COMPUTATIONAL METHOD The density functional theory (DFT)25 method has been widely used as an important and economical tool to deal with complex systems. All calculations of this work were performed via Gaussian 09 programs26. The geometric optimization and frequency analyses were carried out using the DFT B3LYP functional with the 6-311++G** basis set. All optimized structures were determined to be the local energy minima on the potential energy surface without imaginary frequencies. And the frontier molecular orbitals (FMOs) and infrared spectrum were calculated at the same level of theory based on optimized gas-phase structures. The density was predicted by using the equation proposed by Politzer27 referring to the interactions within the crystal. The heat of formation (HOF) was determined through deducting the enthalpy of sublimation

28-29

according to the Hess’ law30. Thereinto, the heat of formation of

gas-phase was calculated via atomization energies method31. The detonation velocity and detonation pressure are critical parameters of an energetic material and were predicted by the Empirical Kamlet-Jacobs equations10. Detailed calculation descriptions are represented in Supporting Information.

3. RESULTS AND DISCUSSION 3.1 Molecular Structures 4


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Our approach to design novel energetic molecules capitalizes on years of experience, coupled with interdisciplinary theoretical computations and experimental synthesis in this field. Therefore, we formulated three specific criteria that a newly-designed energetic molecule should: (i) have structural and electronic features that would allow sufficient stabilization of five- or six-member heterocyclic rings, preventing undesired dissociation/ decomposition of these rings, (ii) contain a relatively-high number of nitrogen and oxygen atoms, to ensure smallest possible OB and highest energy content, and (iii) contain a combination of amino and nitro groups to improve stability to mechanical impact and thermal stability of the resulted EM. On a basis of these criteria, our proposed designs of nitrogen-rich bridge-ring structures were oriented towards most thermodynamically stable molecules that would contain tetranitroethane bridge, fused (by DA reactions) into carbocyclic or heterocyclic rings (Figure 4). Peripheral nitro and amino groups in resulted compounds DA1-DA12 are non-coplanar with any of the formed rings, due to the electronic and steric repulsion effects. Additional information regarding optimized geometries of all DA products in this study is summarized in the Supporting Information.

Figure 4. Three dimensional molecular structures of compounds DA1-DA12. 5



3.2 Dienophile and Diene Interactions For better understanding of concerted cycloaddition process in the studied DA reactions, we also investigated molecular geometries and frontier orbitals of selected nitrogen-rich heterocycles dienes (Diene1-Diene8) and tetranitroethylene dienophile precursors. Optimized structures are shown in Figure 5. The structures of dienes contain furazan-fused carbocyclic ring (Diene1, Diene2), urea-fused carbocyclic ring (Diene 4, Diene5 and Diene6), furazan-fused N-heterocyclic ring (Diene3) and 2H-imidazole rings (Diene7 and Diene8), while nitro or amino substituents on these rings provide a diversity in electron density to these dienes. Notably, tetranitroethylene dienophile has two diagonally-placed nitro groups in trans positions that are oriented in a coplanar manner to a C=C bond plane, while the remaining two nitro groups are positioned perpendicularly the plane.

Figure 5. Optimized structures of tetranitroethylene and diene candidates (Diene1-Diene8).

Molecular orbital theory supported the concerted mechanism in Diels-Alder reaction. For these purposes, we calculated the frontier orbitals of reactants, to predict the hardness or softness of the explored DA reactions. The interactions between LUMO of tetranitroethylene and HOMOs of a series selected dienes are shown in Figures 6 and 7. Typically, in DA reactions, a diene and a dienophile approach each other in approximately parallel planes. The symmetry properties of the orbitals permit stabilizing interactions between C(1) and C(4) of a diene and a dienophile. In Figure 6, Diene1, Diene2, Diene4, Diene5, Diene6 and Diene8 show parallel p orbitals on diene atoms in HOMO matching with LUMO of tetranitroethylene dienophile for the [4πs + 2πs] cycloaddition in DA reaction. The reaction-relevant diene atoms of Diene3 and Diene7 show p orbitals in HOMO vertical to LUMO of dienophile, which may result in a hindering of their DA reactions. 6


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Figure 6. Shapes of LUMO of tetranitroethylene dienophile, and HOMOs of selected dienes.

-0.3 0.9 1.5

1.9 3.0

3.1

3.9

3.9

dienophile de-1

de-2

de-3

de-4

de-5

de-6

de-7

de-8

Figure 7. Energies of HOMOs-LUMOs of tetranitroethylene dienophile and selected dienes.

As shown in Figure 7, the strong interactions between LUMO of tetranitroethylene dienophile and the HOMOs of Diene2, Diene6 and Diene8 with low energy gap. This relationship permits description of potential cycloaddition reactions as “allowed”, strongly suggesting that these specific reactions are likely to be thermodynamically favorable. In contrast, HOMOs of Diene1, Diene3, Diene5 and Diene7 have relatively high energy gap with LUMO of tetranitroethylene, which indicates that potential cycloaddition reactions could be “forbidden”. Diene4 occupies the middle of

7



energy gap that may suggest a certain possibility of its successful reaction with tetranitroethylene dienophile and formation of compound DA11. These molecular orbitals calculation results could be attributed to the electronic substituent effect on the DA reaction probability. When a dienophile bears multiple electron-withdrawing nitro groups and dienes are substituted with electron-donating amino group, the strongest interactions between the LUMO of the dienophile and the HOMOs of the dienes are expected. The reactants are oriented so that the carbons having the highest coefficients of these two frontier orbitals can begin the bonding process, leading to the observed preference of this DA reaction.

3.3 Infrared Spectroscopy Calculated infrared IR spectra of compounds DA1-DA12 are shown in Figure 8. For all these compounds, the vibration modes of bridged rings are expected to appear in 800-900 cm-1 region, while symmetrical and asymmetrical stretching vibrations, characteristic to nitro groups, could be found in a range between 1300 and 1600 cm-1. For carbonyl group-containing compounds DA6, DA7, DA8 and DA9, the C=O bonds have strong stretching vibrations with peaks at 1831, 1899, 1816 and 1824 cm-1. Furazan-containing compounds DA1-DA5, showed bending vibration modes (corresponding to this moiety) at about 600 cm-1, while triazole-containing compound DA12 showed a characteristic vibration of N=N bond of the triazole ring with peaks at 1200 cm-1. Most of the DA compounds in this study exhibit peaks at frequencies over 3000 cm-1, characteristic to the vibrational modes of C-H and N-H bonds, except of the compound DA5 that has no hydrogen atoms in its structure. The peaks at 3500 cm-1 could be attributed to the stretching vibrations of N-H bonds of amino groups in DA2, DA4, DA8, DA11 and DA12 molecules. Compounds DA10, DA11 and DA12 have stretching vibrations of their CH2 group, with weak peaks in a range between 3100 and 3200 cm-1.

8


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Figure 8. Calculated IR spectra for molecules DA1-DA12.

3.4 Physicochemical and Energetic Properties Table 1 presents the calculated physiochemical and energetic properties of our designed DA energetic compounds and common explosives. Most of DA compounds were generally designed to have a good oxygen balance, where compounds DA5, DA7 and DA10 were arranged to have OB values close to zero, making them especially attractive for the further exploration, synthesis and experimental evaluation. The densities for new DA compounds were calculated to be in range between 1.83 and 2.04 g·cm-3, while densities for DA5 and DA7 were predicted to be comparable to CL-20 (2.04 g·cm-3). All of the DA compounds in this study were found to exhibit positive heats of formation, ranging from 431.03 to 1124.90 kJ·mol-1, where compound DA5 reached the highest value. The heats of detonation (Q) of the DA compounds are in a range of 6426 J·g-1 (in case of DA8) and 8333 J·g-1 (in case of DA5), varying with the structures and functional groups. Remarkably, except of DA8, all other DA compounds in this study were found to have higher Q values than CL-20 explosive. Also, these DA compounds were predicted to have high VDs (8.28-10.02 km·s-1) and impressive PDs (30.87-47.83 GPa). More specifically, in terms of VD, compounds DA1, DA5, DA7, DA10 and DA12 may exhibit a superior performance over CL-20 9



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(9.38 km·s-1) and even over ONC (9.50 km·s-1) explosives, where in terms of PD, performance of DA5, DA7 and DA10 could be compared to CL-20 (44.10 GPa).

Table 1. Physicochemical and energetic properties of DA compounds and reference explosives.

Compounds DA1 DA2 DA3 DA4 DA5 DA6 DA7 DA8 DA9 DA10 DA11 DA12 RDX32 HMX32 CL-2032 ONC17

∆fH298K(s)[a] [kJ·mol-1] 843.68 722.92 991.98 896.48 1124.90 533.01 802.94 431.03 695.11 758.80 600.63 935.17 79.00 102.41 377.04 392.92

ρ[b] [g·cm-3] 1.97 1.83 1.91 1.93 2.04 1.93 2.01 1.84 1.90 1.98 1.83 1.92 1.80 1.90 2.04 1.98

Q[c] [J·g-1] 8274 7371 7699 7942 8333 7392 8064 6426 6758 8140 7577 8161 6274 6261 6550 --

VD[d] [km·s-1] 9.52 8.63 8.96 9.39 10.02 9.04 9.72 8.28 8.56 9.88 9.08 9.54 8.75 9.10 9.38 9.50

PD[e] [GPa] 42.40 33.42 36.94 40.74 47.83 37.82 44.71 30.87 33.62 45.87 36.92 41.98 34.70 39.30 44.10 46.00

OB[f] [%] -15.31 -44.68 -36.85 -17.85 3.80 -25.92 -6.13 -55.90 -48.04 4.37 -26.14 -17.66 -21.62 -21.62 -10.96 0

[a] Heat of formation, [b] density, [c] heat of detonation, [d] detonation velocity, [e] detonation pressure, [f] oxygen balance for CaHbOc: 1600(c-2a-b/2)/MW, (MW = molecular weight).

For selection of the best performing DA compounds in this study, their calculated detonation parameters were compared to RDX and HMX reference explosives. As could be seen in Figure 9, DA compounds in this work, except of DA2, DA8, and DA9, have better detonation properties than RDX. Moreover, compounds DA5, DA7 and DA10 have outstanding calculated performance, with VDs above 9.70 km·s-1 and PDs above 44.70 GPa, making these molecules promising candidates for new green HEDMs.

10


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Figure 9. Overall performance of DA compounds compared with RDX and HMX.

3.5 Sensitivity Correlations The sensitivities of EMs play a critical role in determining their safety and potential applications. The sensitivity to mechanical impact is usually measured by the h50%, which is the height from which 50% of the drops that would produce initiation of the examined material. Several good correlations were found connecting sensitivity to impact with various other molecular properties, particularly within families of chemical compounds.8, 17, 33-37 The connections between the positive ା ି തതതത തതതത ܸ ௌ and negative ܸௌ surface potentials (Model 1) or ν the degree of balance between the surface

potentials of positive and negative regions (Model 2) of a molecule and its impact sensitivity were explored. In addition, the heat of detonation (Q) is also correlated with the impact sensitivity (Model 3), suggesting that as values of Q are getting bigger, the sensitivity of the energetic molecule would increase. Using these three empirical models, proposed by Rice and Hare,36 the h50% values for designed DA compounds were estimated and corresponding results were summarized in Table 2.

11



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Table 2. Predicted and experimental h50% values (in cm) for DA compounds and reference explosives. Compound

Model 1

Model 2

Model 3

Exp.36

DA1 DA2 DA3 DA4 DA5 DA6 DA7 DA8 DA9 DA10 DA11 DA12 RDX HMX CL-20

33 21 15 19 68 16 25 28 15 29 30 23 49 21 16

29 29 29 29 29 29 29 38 30 29 30 29 31 31 29

28 28 28 28 28 28 28 28 28 28 28 28 39 41 29

------------28 32 24.214

As could be seen from Table 2, Models 2 and 3 show relatively consistent results. In contrast, the predicted results from Model 1 are significantly different, leading to overestimation or underestimation of the impact sensitivity of the new compounds, where the substituents show a strong influence on the impact sensitivity. The h50% values for most of the DA compounds in this study have close values to the CL-20 explosive, indicating that DA compounds with multiple germinal and vicinal nitro groups would probably be relatively sensitive that could be easily triggered by mechanical impact. In comparison, the compounds with vicinal NH2 groups show higher h50% values, clearly implying that introduction of this group can result in a significant stabilization and decrease in the impact sensitivity of such EM. The newly designed DA compounds were predicted to exhibit acceptable impact sensitivities, with h50% values ranging between 29 and 38 cm (Model 2), where compounds DA8, DA9 and DA11 have a comparable sensitivity to HMX explosive (with h50% of 31 cm). However, due to the complexity of the impact sensitivity modeling and calculations, the abovementioned estimates should be regarded as suggestive results. 4. CONCLUSION

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In the present work, new polynitro bridge-ring explosophores were designed on a basis of Diels-Alder reaction between nitrogen-rich heterocyclic dienes and tetranitroethylene dienophile. Molecular geometries and corresponding IR spectra were calculated for twelve prospective energetic molecules (DA1-DA12), using DFT method. The interactions of selected diene candidates (Diene1-Diene8) and tetranitroethylene dienophile were also studied by using the molecular orbital theory, in order to better understand the concerted cycloaddition mechanism in these Diels-Alder reactions. Our results clearly indicate that there is a strong substituent effect on the DA reaction. Electron-rich dienes (with amino) have high-energy HOMO and can interact strongly with the LUMO of electron-poor dienophile (with nitro). This can be used to predict whether specific reactions (Diene2, Diene6 or Diene8 with tetranitroethylene) are likely to be thermodynamically favorable, which would be very important for a future synthetic phase of this project. The physicochemical and energetic properties (density, heat of formation, detonation parameters and impact sensitivity) of newly designed compounds were investigated as well. These DA compounds could have high densities (1.83-2.04 g·cm-3) and highly-positive heats of formation (431.03-1124.90 kJ·mol-1) due to the strained-ring skeleton and energetic nitro groups in their structures. Several of these compounds (DA5, DA7 and DA10), with OB values close to zero, were predicted to exhibit very high VDs of above 9.70 km·s-1 and PDs of above 44.70 GPa, making them promising candidates of EMs with superior performance. The proposed straightforward synthetic methodology could play a positive role in exploring novel EMs.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: ××. Optimized geometries, Selected parameters, Dienophile and dienes, Calculations description.

AUTHOR INFORMATION Corresponding Author Jian-Guo Zhang, Tel & Fax: +86 10 68918091; E-mail: [email protected]; Michael Gozin, Tel: +972-3-640-5878; Email: [email protected]. Notes 13



The authors declare no competing financial interest.

ACKNOWLEDGMENTS The supports of the 111 project (B17004) in China and State Key Laboratory of Explosion Science and Technology (YBKT16-04 and YBKT16-15) are gratefully acknowledged.

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Design of New Bridge-Ring Energetic Compounds Obtained by Diels

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