English

• 1. INTRODUCTION

  Recently, a recent trend in the development of energetic materials field is replacing the traditional C–C frameworks energetic materials of nitrogen-rich heterocycles^[1-3]. Indeed, heterocycles show more efficient than traditional energetic materials, and they tend to possess important energetic properties, such as good oxygen balance (OB), positive heat of formation (HOF), high density and rational sensitivity towards external forces like impact or friction^{[4, 5]}. Furthermore, owing to the high nitrogen content, the decomposition product of this kind of compounds is mainly eco-friendly nitrogen gas, which makes them promising candidates as green energetic materials^{[6, 7]}. Among those heterocycles, furazan^[8-10], triazole^{[11, 12]}, tetrazole^{[13, 14]}, tetrazine^[15] and so on are normally used in modern energetic materials.

  Bis-triazole is one of the most promising nitrogen-rich heterocyclic backbones in energetic fields because its properties can be controlled by the selection of various substituents on two positions of each ring^{[16, 17]}. Better detonation properties can be achieved by introducing energy-rich functional groups such as nitro (-NO₂)^{[18, 19]} and nitramine (-NHNO₂)^{[20, 21]} in heterocyclic bis-triazoles based energetic materials. In addition, the nitrogen-rich heterocyclic linkages can also improve the properties of this kind of compound. Thus, the combination of bistriazole backbone with other energetic heterocyclic fragments as linkage is a promising strategy to acquire materials with excellent detonation performance^{[22, 23]}.

  Inspired by this, we attempted to design novel energetic materials by combining varying nitro-rich heterocyclic linkages with bis-triazoles rings (Fig. 1) and their optimized geometries in the potential energy surface were obtained using density functional theory. Then, the HOFs, density, detonation properties as well as the bond dissociation energies (BDE) were calculated. The relationship between the characteristics and the structure was also discussed so as to supply some useful data for the molecular design and synthesis of new energetic materials.

  Figure 1

  

  Figure 1.  Molecular structures of bridged bistriazole derivatives

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  2. COMPUTATIONAL METHODS

  All calculations were performed by using the Gaussian 09 quantum program^[24]. The DFT B3LYP/6-311+G* calculations^[25] were carried out all through the geometric optimization and frequency analysis, in which all optimized structures are energy minima on the potential energy surface (no imaginary frequencies). The gas phase HOFs were obtained via designed isodesmic reactions^[26], and the condensed phase HOFs were calculated through Hess' law^{[27, 28]}. The calculations involve HOMO-LUMO orbitals, electrostatic potential (ESP), thermodynamic parameters and BDE were implemented at the same level of theory based on the optimized structures. The empirical Kamlet-Jacob equations were famous to estimate the values of detonation velocity and detonation pressure for CHON energetic materials^[29].

  3. RESULTS AND DISCUSSION

  A series of new nitrogen-rich energetic molecules was designed by combining bis(1-nitramino-3-nitro-triazoles) derivatives with varying linkage groups (tetrazine M1, tetrazole-5-one M2, furazan M3, tetrazole M4, 3, 6-diamino-tetrazine M5, 1, 4-diamino-tetrazole-5-one M6, 4, 5-diamino-furazan M7, 1, 5-diamino-tetrazole M8). The optimized geometries of these new bridged bis(1-nitramino-3-nitro-triazoles) are shown in Fig. 2. The C–N and N–N bond lengths of trizole ring skeleton range from 1.23 to 1.38 Å and 1.34 to 1.39 Å, indicating these triazole rings are aromatic. The bond angles (C(1)–N(4)–C(2)) of triazole rings are between 101.7 and 107.1^o, which implies a certain tension in the triazole ring. Most nitro groups on the C atom are coplanar with the triazole rings, while the nitramine groups on the N atom twist out of the triazole rings. A systematic structure-property relationship has been established in the present study. The predicted energetic properties of bridged bis(1-nitramino-3-nitro-triazoles) derivatives are compared with the benchmark explosives like TNT (2, 4, 6-trinitrotoluene) and RDX (hexahydro-1, 3, 5-trinitro-1, 3, 5-triazine).

  Figure 2

  

  Figure 2.  Optimized geometries at the B3LYP/6-311+G* level

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  3.1 Electronic structure

  The electrostatic potential (ESP), the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the title compounds were calculated in order to obtain their substantial electronic structures and properties.

  For studies of the intermolecular interaction and sensitivities, the electrostatic potentials (ESP) of the designed compounds were obtained based on the optimized structures and visualized in Fig. 3. Positive regions (blue part) are observed energetic compounds, which mean weak bond strength that can be related to the increased impact sensitivities. As expected, the major negative regions occur in the nitramino and nitro moieties due to the lone pair electrons of oxygen atoms. The introduction of bridged ring that contents N–O bond into the bis(1-nitramino-3-nitro-triazoles) makes the ring a little more negative. However, such a difference is not obvious, which shows the difference between various bridged bis(1-nitramino-3-nitro-triazoles) towards impact stimulus is subtle. The major concentration of positive potential appears in those regions occupied by triazoles. As shown in Fig. 3, the positive values over the triazole moieties in compounds M5 and M8 are higher than the other ones, thus reflecting these two compounds may be sensitive to impact.

  Figure 3

  

  Figure 3.  Electrostatic potentials of studied compounds, ranging from –0.111 a.u. (red) to 0.111 a.u. (blue)

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  The HOMO-LUMO orbitals of the title molecules are illustrated in Fig. 4, while the red and blue colors denote the positive and negative phases, respectively. Both the HOMO and LUMO orbitals of M1 are contributed by the tetrazine bridged group. Conversely, the HOMOs of molecules M5~M8 occupy the whole molecule except the nitro groups, while the LUMO of M5 is mainly contributed by the linkage group, that of M6 occupies one 1-nitramino-3-nitro-triazole and that of M7 comprises two 1-nitramino-3-nitro-triazoles. The HOMO of M8 is made up of bridged group and triazole which in the right part of the molecule and LUMO comprises bridged group and another triazole. The values of energy gap between HOMOs and LUMOs of the title compounds range from 3.66 to 4.59 eV, which is relative to a comparably stable range.

  Figure 4

  

  Figure 4.  HOMO-LUMO orbitals of the studied compounds

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  3.2 Detonation performance

  In order to quantitatively evaluate the detonation performance of studied structures, the predicted density (d), solid phase enthalpy of formation (Δ_fH_{298K (s)}), oxygen balance (OB) and detonation detonations (Q, D, P) were calculated systematically, with the results listed in Table1.

  Table 1

  

  Table 1.  Properties of M1~M6 Compared with TNT and RDX

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  Salts	d (g/cm3)	OB (%)	ΔfH298K (s) (kcal/mol)	Q (kcal/mol)	D (km/s)	P (GPa)
  M1	1.84	–18.8	932.6	1431.0	8.56	32.9
  M2	1.88	–7.4	654.4	1372.5	8.76	35.0
  M3	1.86	–15.5	687.5	1487.7	8.84	35.4
  M4	1.84	–11.6	800.0	1396.0	8.63	33.5
  M5	1.87	–21.0	812.0	1297.2	8.53	33.0
  M6	1.87	–10.4	754.0	1358.1	8.77	34.9
  M7	1.87	–18.0	595.4	1321.6	8.57	33.3
  M8	1.83	–14.4	753.6	1300.9	8.52	32.5
  RDXa	1.82	–21.6	83.8	1591	8.75	34.7
  TNTa	1.64	–63.1	–63.12	1295	6.95	19.00
  a The experimental values of TATB and TNT taken from Ref. [27].

  Table 1 shows that the densities of the studied compounds are ranging from 1.83 to 1.87 g/cm³, higher than that of RDX. This may be attributed to the incorporation of nitro groups into the designed molecule species. Due to the high nitrogen content in the molecule, the series of the title compounds exhibits strongly positive solid phase enthalpy of formation (743.6~978.6 kJ/mol) that is a typical feature for most high-energy-density compounds.

  These new compounds possess high heats of detonation, most of which are above 1400 cal/g, and compound M5 has the highest value of 1567.5 cal/g, which indicates that the diamino-tetrazole bridged group can effectively increase the heats of detonation of the system. All the studied compounds possess calculated detonation velocities and detonation pressures which are superior to those of TNT. For M3 and M4, the detonation parameters are comparable to the RDX explosive. Taking into account the density and impact sensitivity, compounds M3 and M5 illustrate their high value of application.

  3.3 Thermal stability

  The weakest bond in a molecule is widely regarded as the"trigger bond", and its breaking is the common primary step of energetic materials toward external heat, impact, and electric spark stimulus^{[30, 31]}. Generally, the smaller the energy needed for breaking a bond, the weaker the bond. The BDE of the trigger bond is obtained to evaluate the thermal stability of energetic compounds, which reflects their practicability. For the compounds studied, two possible bond dissociations were considered: (1) the C–NO₂ bond in the side chain; (2) the N–NHNO₂ bond in the side chain. Table 2 presents the BDEs of the relatively weaker bonds of the compounds studied. Based on our calculations, all of the bond dissociation energies are far larger than 120 kJ/mol, which indicated the molecules designed meet the stability requirement of a practical energetic material. In general, the stability of C–NO₂ bond is not affected by changing different linkages, whereas, the BDE value of N–NHNO₂ bond varied with the bridged groups. When the heterocyclic linkages with amino group, the nitramino groups are less stable, suggesting the whole molecules have lower stability. Thus, introduction of -NH- tends to decrease the stability of the derivatives that may be due to the longer bridged group.

  Table 2

  

  Table 2.  Calculated Bond Dissociation Energies (BDE, kJ/mol)

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  	M1	M2	M3	M4	M5	M6	M7	M8
  C–NO2	235.2	234.7	235.9	233.6	239.4	237.9	231.3	232.3
  N–NHNO2	247.9	231.3	250.8	257.0	208.0	206.6	201.4	217.0

  4. CONCLUSION

  In this work, eight novel bis(1-nitramino-3-nitro-triazoles) compounds were designed by introducing various heterocyclic linkages into the molecule. Then, their heats of formation, energetic properties, electronic structure, molecule stability and impact sensitivity were investigated using the DFT B3LYP/6-311+G*. Based on our calculations, the adding of functional groups and nitrogen-rich heterocyclic linkages show positive influence on HOF of the title molecules. All of the studied compounds possess positive HOFs which confirm their energetic nature. In addition, the heterocyclic linkages have more favorable effect on the detonation performance than the corresponding diamino-heterocyclic ones. By analyzing BDE of the weakest bond of the designed compounds, the BDE values are much higher than 120 kJ/mol, with relatively ideal thermal stability. However, the addition of amino groups is not conducive to improve the stability of derivatives. In terms of detonation performance and stability, compounds M3 and M4 yield promising results and illustrate their high value of application.

  
  
• 1. [1]

     Zhao, G.; Kumar, D.; Yin, P.; He, C.; Imler, G. H.; Parrish, D. A.; Shreeve, J. M. Construction of polynitro compounds as high-performance oxidizers via a two-step nitration of various functional groups. Org. Lett. 2019, 21, 1073-1077. doi: 10.1021/acs.orglett.8b04114

  2. [2]

     Domasevitch, K. V.; Gospodinov, I.; Krautscheid, H.; Klapötke, T. M.; Stierstorfer, J. Facile and selective polynitrations at the 4-pyrazolyl dual backbone: straightforward access to a series of high-density energetic materials. New J. Chem. 2019, 43, 1305-1312. doi: 10.1039/C8NJ05266B

  3. [3]

     Ma, J.; Tang, Y.; Cheng, G.; Imler, G. H.; Parrish, D. A.; Shreeve, J. M. Energetic derivatives of 8-nitropyrazolo[1, 5-a][1, 3, 5]triazine-2, 4, 7-triamine: achieving balanced explosives by fusing pyrazole with triazine. Org. Lett. 2020, 22, 1321-1325. doi: 10.1021/acs.orglett.9b04642

  4. [4]

     Qu, Y.; Babailov, S. P. Azo-linked high-nitrogen energetic materials. J. Mater. Chem. A 2018, 6, 1915-1940. doi: 10.1039/C7TA09593G

  5. [5]

     Gao, H.; Zhang, Q.; Shreeve, J. M. Fused heterocycle-based energetic materials (2012–2019). J. Mater. Chem. A 2020, 8, 4193-4216 doi: 10.1039/C9TA12704F

  6. [6]

     Klapötke, T. M. New nitrogen-rich high explosives. Struct. Bond 2007, 125, 85-121.

  7. [7]

     Singh, R. P.; Verma, R. D.; Meshri, D. T.; Shreeve, J. M. Energetic nitrogen-rich salts and ionic liquids. Angew. Chem. Int. Ed. 2006, 45, 3584-3601. doi: 10.1002/anie.200504236

  8. [8]

     Wang, B.; Zhang, G.; Huo, H.; Fan, Y.; Fan, X. Synthesis, characterization and thermal properties of energetic compounds derived from 3-amino-4-(tetrazol-5-yl)furazan. Chin. J. Chem. 2011, 29, 919-924. doi: 10.1002/cjoc.201190189

  9. [9]

     Shaposhnikov, S. D.; Korobov, N. V.; Sergievskii, A. V.; Pirogov, S. V.; Mel'nikova, S. F.; Tselinskii, I. V. New heterocycles with a 3-aminofurazanyl substituent. Russ. J. Org. Chem. 2002, 38, 1351-1355. doi: 10.1023/A:1021668216426

  10. [10]

      Wang, Q.; Shao, Y.; Lu, M. Salt formation: route to improve energetic performance and molecular stability simultaneously. Cryst. Growth Des. 2020, 20, 197-205. doi: 10.1021/acs.cgd.9b00997

  11. [11]

      Dippold, A. A.; Klapötke, T. M.; Martin, F. A.; Wiedbrauk, S. Nitraminoazoles based on ANTA – a comprehensive study of structural and energetic properties. Eur. J. Inorg. Chem. 2012, 14, 242-2443.

  12. [12]

      Bian, C.; Zhang, M.; Li, C.; Zhou, Z. 3-Nitro-1-(2H-tetrazol-5-yl)-1H-1, 2, 4-triazol-5-amine (HANTT) and its energetic salts: highly thermally stable energetic materials with low sensitivity. J. Mater. Chem. A 2015, 3, 163-169. doi: 10.1039/C4TA04107K

  13. [13]

      Klapötke, T. M.; Kurz, M. Q.; Schmid, P. C.; Stierstorfer, J. Energetic improvements by N-oxidation: insensitive amino-hydroximoyl-tetrazole-2N-oxides. J. Energ. Mater. 2015, 33, 191-201. doi: 10.1080/07370652.2014.963743

  14. [14]

      Benz, M.; Klapötke, T. M.; Stierstorfer, J. Combining performance with thermal stability: synthesis and characterization of 5-(3, 5-dinitro-1H-pyrazol-4-yl)-1H-tetrazole and its energetic derivatives. Z. Anorg. Allg. Chem. 2020, 646, 1380-1388. doi: 10.1002/zaac.202000123

  15. [15]

      Saikia, A.; Sivabalan, R.; Polke, B. G.; Gore, G. M.; Singh, A.; Rao, A. S.; Sikder, A. K. Synthesis and characterization of 3, 6-bis(1H-1, 2, 3, 4-tetrazol-5-ylamino)-1, 2, 4, 5-tetrazine (BTATz): novel high-nitrogen content insensitive high energy material. J. Hazard. Mater. 2009, 170, 306-313. doi: 10.1016/j.jhazmat.2009.04.095

  16. [16]

      Klapötke, T. M.; Schmid, P. C.; Schnell, S.; Stierstorfer, J. Thermal stabilization of energetic materials by the aromatic nitrogen-rich 4, 40, 5, 50-tetraamino-3, 3΄-bi-1, 2, 4-triazolium cation. J. Mater. Chem. A 2015, 3, 2658-2668. doi: 10.1039/C4TA05964F

  17. [17]

      Wu, J. T.; Zhang, J. G.; Xin, Y.; Wu, K. Energetic oxygen-containing tetrazole salts based on 3, 4-diaminotriazole. Chem. Asian J. 2015, 10, 1239-1244. doi: 10.1002/asia.201500075

  18. [18]

      Lang, Q.; Sun, Q.; Wang, Q.; Lin, Q.; Lu, M. Embellishing bis-1, 2, 4-triazole with four nitroamino groups: advanced high-energy-density materials with remarkable performance and good stability. J. Mater. Chem. A 2020, 8, 11752-11760. doi: 10.1039/D0TA03008B

  19. [19]

      Yu, Q.; Chinnam, A. K.; Yin, P.; Gregory, H. I.; Parrish, D. A.; Shreeve, J. M. Finding furoxan rings. J. Mater. Chem. A 2020, 8, 5859-5864. doi: 10.1039/D0TA01538E

  20. [20]

      Hu, L.; Gao, H.; Shreeve, J. M. Challenging the limits of nitrogen and oxygen content of fused rings. J. Mater. Chem. A 2019, 8, 17411-17414.

  21. [21]

      Joo, Y. H.; Shreeve, J. M. High-densityenergetic mono-or bis(oxy)-5-nitroiminotetrazoles. Angew. Chem. Int. Ed. 2010, 49, 7320-7323. doi: 10.1002/anie.201003866

  22. [22]

      Chinnam, A. K.; Yu, Q.; Imler, G. H.; Parrish, D. A.; Shreeve, J. M. Azo- and methylene-bridged mixed azoles for stable and insensitive energetic applications. Dalton Trans. 2020, 49, 11498-11503. ] doi: 10.1039/D0DT02223C

  23. [23]

      Yu, Q.; Yang, H.; Imler, G. H.; Parrish, D. A.; Cheng, G.; Shreeve, J. M. Derivatives of 3, 6-bis(3-aminofurazan-4-ylamino)-1, 2, 4, 5-tetrazine: excellent energetic properties with lower sensitivities. ACS Appl. Mater. Inter. 2020, 12, 31522-31531. doi: 10.1021/acsami.0c08526

  24. [24]

      Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery Jr., J. A.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, Revision B. 01, Gaussian, Inc., Pittsburgh PA 2003.

  25. [25]

      Perdew, J. P. Density-functional approximation for the correlation energy of the inhomogeneous electron gas. Phys. Rev. B: Condens. Matter Mater. Phys. 1986, 33, 8822-8824. doi: 10.1103/PhysRevB.33.8822

  26. [26]

      Wiberg, K. B.; Ochterski, J. W. A comparison of different theoretical models for calculating isodesmic reaction energies for small ring and related compound, J. Comput. Chem. 1997, 18, 108-114. doi: 10.1002/(SICI)1096-987X(19970115)18:1<108::AID-JCC10>3.0.CO;2-I

  27. [27]

      Rice, B. M.; Pai, S. V.; Hare, J. Predicting heats of formation of energetic materials using quantum chemical calculations. Combust. Flame 1999, 118, 445-458 doi: 10.1016/S0010-2180(99)00008-5

  28. [28]

      Byrd, E. F.; Rice, B. M. Improved prediction of heats of formation of energetic materials using quantum mechanical calculations. J. Phys. Chem. A 2006, 110, 1005-1013. doi: 10.1021/jp0536192

  29. [29]

      Kamlet, M. J.; Jacobs, S. J. Chemistry of detonations. I. A simple method for calculating detonation properties of C-H-N-O explosives. J. Chem. Phys. 1968, 48, 23-35. doi: 10.1063/1.1667908

  30. [30]

      Qiu, L.; Gong, X. D.; Zheng, J.; Xiao, H. M. Theoretical studies on polynitro-1, 3-bishomopentaprismanes as potential high energy density compounds. J. Hazard. Mater. 2009, 166, 931-938. doi: 10.1016/j.jhazmat.2008.11.099

  31. [31]

      Chung, G. S.; Schimidt, M. W.; Gordon, M. S. An ab initio study of potential energy surfaces for N8 isomers. J. Phys. Chem. A 2000, 104, 5647-5650. doi: 10.1021/jp0004361

• 

  Figure 1  Molecular structures of bridged bistriazole derivatives

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  Figure 2  Optimized geometries at the B3LYP/6-311+G* level

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  Figure 3  Electrostatic potentials of studied compounds, ranging from –0.111 a.u. (red) to 0.111 a.u. (blue)

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  Figure 4  HOMO-LUMO orbitals of the studied compounds

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  Table 1.  Properties of M1~M6 Compared with TNT and RDX

  Salts	d (g/cm3)	OB (%)	ΔfH298K (s) (kcal/mol)	Q (kcal/mol)	D (km/s)	P (GPa)
  M1	1.84	–18.8	932.6	1431.0	8.56	32.9
  M2	1.88	–7.4	654.4	1372.5	8.76	35.0
  M3	1.86	–15.5	687.5	1487.7	8.84	35.4
  M4	1.84	–11.6	800.0	1396.0	8.63	33.5
  M5	1.87	–21.0	812.0	1297.2	8.53	33.0
  M6	1.87	–10.4	754.0	1358.1	8.77	34.9
  M7	1.87	–18.0	595.4	1321.6	8.57	33.3
  M8	1.83	–14.4	753.6	1300.9	8.52	32.5
  RDXa	1.82	–21.6	83.8	1591	8.75	34.7
  TNTa	1.64	–63.1	–63.12	1295	6.95	19.00
  a The experimental values of TATB and TNT taken from Ref. [27].

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  Table 2.  Calculated Bond Dissociation Energies (BDE, kJ/mol)

  	M1	M2	M3	M4	M5	M6	M7	M8
  C–NO2	235.2	234.7	235.9	233.6	239.4	237.9	231.3	232.3
  N–NHNO2	247.9	231.3	250.8	257.0	208.0	206.6	201.4	217.0

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•