1 Introduction

Over the past decade, the synthesis and development of new high‑energy density materials(HEDMs) have attracted increasing attention in the world^[1,2] . New HEDMs not only focus on high density, high detonation velocity and pressure but also aim at the high positive heat of formation, high thermal stability, low sensitivity towards external forces such as impact and friction and environmental compatibility. However, among these characteristics, high performance and low sensitivity tend to be contradicting aspects, making the design and synthesis of energetic materials an interesting and challenging work^[3,4] . Heterocyclic energetic salts with high nitrogen content is one of the favorite topics in the search for high‑performance energetic materials recently^[5,6,7,8]. The energetic salts possess advantages over nonionic molecules since these salts tend to exhibit lower vapor pressures and higher thermal stability than their atomically similar nonionic analogues. This provides an efficient methodology for the design and synthesis of HEDMs^[9,10].

Oxadiazole is a favorite example of a nitrogen‑and oxygen‑rich ring that has been wildly used in the syntheses of drugs, ionic liquids and scintillators. The energetic materials based on 1,2,5‑oxadiazole (furazan) have been exhaustively investigated^{[11,12,13,14]}. Compared to furazan, 1,3,4‑oxadiazoles have been studied very rarely as energetic materials. The energetic materials based on 1,3,4‑oxadiazole have become more and more attractive due to their higher thermal stability and lower sensitivity^[15,16]. For example, di(4‑amino‑1,2,4‑triazolium) 5,5′‑dinitromethyl‑2,2′‑bis(1,3,4‑oxadiazolate) (A)^[17] shows a detonation velocity of 8601 m·s^-1 and a detonation pressure of 31.8 GPa; hydrazinium 2,5‑bis(trinitromethyl)‑1,3,4‑oxadiazolate (B)^[16], has a high density of 1.90 g·cm^-3 and excellent detonation performance (D=8900 m·s^‑1, p=36.3 GPa). And both of them have good thermal stabilities (A: 205.8 ℃; B: 190 ℃) and acceptable sensitivity towards impact and friction (A: IS: 16 J, FS: 200 N; B: IS: 19 J, FS: 80 N).

In our effort to seek more powerful, eco‑friendly and less sensitive explosives, we were interested in 5,5′‑dinitroamino‑2,2′‑bi(1,3,4‑oxadiazole), because of its high thermal stability (decomposition temperature 210 ℃), high density (1.99 g·cm^-3) and excellent explosive properties (D=9481 m·s^-1, p=41.9 GPa)^[18,19,20]. However, energetic salts based on 5,5′‑dinitroamino‑2,2′‑bi(1,3,4‑oxadiazole) have not been fully investigated. Herein, we report the synthesis and characterization of energetic salts based on 5,5′‑dinitroamino‑2,2′‑bi(1,3,4‑oxadiazole). All the salts are well characterized by IR and multinuclear NMR spectroscopy, differential scanning calorimetry (DSC) and elemental analysis, and some are further measured by single crystal X‑ray diffraction. Their key detonation performance and sensitivity towards impact and friction were determined by experimental and theoretical methods.

2 Experiments

2.1 General Methods

¹H and ¹³C spectra were recorded on a 500 MHz (Bruker AVANCE 500) nuclear magnetic resonance spectrometer operating at 500 and 125 MHz. The chemical shifts were given relative to external tetramethylsilane. IR spectra were recorded by using the attenuated total reflection mode for solids on a Thermo Scientific Nicolet is 10 spectrometer. The decomposition temperatures were determined on a differential scanning calorimeter (DSC823e instrument) at a heating rate of 5 ℃·min^-1. Elemental analysis was performed with a Vario EL III instrument. Sensitivity towards impact and friction was determined using an HGZ‑1 drop hammer and a BAM friction tester. The densities of the compounds were determined at room temperature by employing a gas pycnometer.

2.2 Synthetic Route

Synthesis of energetic salts based on 5,5′‑dinitroamino‑2,2'‑bi(1,3,4‑oxadiazole) are shown in Scheme 1.

Scheme 1 Synthesis of energetic salts based on 5,5′‑dinitroamino‑2,2'‑bi(1,3,4‑oxadiazole)

The starting material 5,5′‑diamino‑2,2′‑bi(1,3,4‑oxadiazole)(1) was prepared by treating oxalyl dihydrazide with cyanogen bromide in anhydrous ethanol for 24 h. Nitramino substituted compound 2 was obtained by direct nitration of 5,5′‑diamino‑2,2'‑bi(1,3,4‑oxadiazole) using 100% nitric acid at 0 ℃. The energetic salt derivatives of compound 2 were readily synthesized by the acid‑base neutralization reaction in which compound 2 reacted with such as ammonia, hydroxylamine, hydrazine, guanidine, aminoguanidine, 1,2,4‑triazole, 3‑amino‑1,2,4‑triazole, 4‑amino‑1,2,4‑triazole and carbohydrazide.

General procedures for compounds 3-11: NH₃·H₂O(2.0 mmol), hydroxylamine solution (2.0 mmol), hydrazine hydrate (2.0 mmol), guanidine carbonate(1.0 mmol), aminoguanidine bicarbonate(2.0 mmol), 1,2,4‑triazole(2.0 mmol), 3‑amino‑1,2,4‑triazole(2.0 mmol) and 4‑amino‑1,2,4‑triazole(2.0 mmol), carbohydrazide(2.0 mmol) were, respectively, added to a solution of compound 2 (0.258 g, 1.0 mmol) in 30 mL of acetonitrile and the mixture was stirred at room temperature. After 2 h the precipitate was filtered, washed with ether and air‑dried to yield.

2.3 Characterization

Ammonium 5,5′‑dinitroamino‑2,2′‑bi(1,3,4‑oxadiazolate)(3)

NH₃·H₂O(2.0 mmol) was added to a solution of compound 2 (0.258 g, 1.0 mmol) in 30 mL of acetonitrile and the mixture was stirred at room temperature. After 2 h the precipitate was filtered, washed with ether and air‑dried to yield. 0.237 g of compound 3 was obtained as a yellow solid in a yield of 81.1%. ¹H NMR(500 MHz, DMSO‑d ₆)δ:7.17 (s, 8H). ¹³C NMR(125 MHz, DMSO‑d ₆)δ: 166.15 (s), 148.32 (s). IR (KBr, ν/cm^-1): 3448, 3372, 3216, 3050, 1524, 1488, 1447, 1424, 1430, 1311, 1279, 1150, 1069, 1022, 963, 728. Anal. calcd for (%) for C₄H₈N₁₀O₆(292.17): C 16.44, H 2.76, N 47.94; found: C 16.52, H 2.53, N 47.52.

Hydroxylammonium 5,5′‑dinitroamino‑2,2′‑bi(1,3,4‑ oxadiazolate)(4)

Hydroxylamine solution (2.0 mmol) was added to a solution of compound 2 (0.258 g, 1.0 mmol) in 30 mL of acetonitrile and the mixture was stirred at room temperature. After 2 h the precipitate was filtered, washed with ether and air‑dried to yield. 0.279 g of compound 4 was obtained as a yellow solid in a yield of 86.1%. ¹H NMR (500 MHz, DMSO‑d ₆)δ: 7.09. ¹³C NMR (125 MHz, D₂O)δ: 167.41 (s), 149.97 (s). IR (KBr, ν/cm^-1): 3448, 3138, 2718, 1582, 1523, 1489, 1417, 1339, 1289, 1159, 1078, 1017, 987, 775, 728. Anal. calcd for (%) for C₄H₈N₁₀O₈(324.17): C 14.82, H 2.49, N 43.21; found C 14.71, H 2.38, N 43.33.

Hydrazinium 5,5′‑dinitroamino‑2,2′‑bi(1,3,4‑ oxadiazolate)(5)

Hydrazine hydrate (2.0 mmol) was added to a solution of compound 2 (0.258 g, 1.0 mmol) in 30 mL of acetonitrile and the mixture was stirred at room temperature. After 2 h the precipitate was filtered, washed with ether and air‑dried to yield. 0.254 g of compound 5 was obtained as a yellow solid in a yield of 78.9%. ¹H NMR(500 MHz, DMSO‑d ₆)δ: 6.16(br.). ¹³C NMR(125 MHz, DMSO‑d ₆)δ: 166.11 (s), 148.34 (s). IR(KBr, ν/cm^-1): 3330, 3303, 3222, 3161, 1516, 1490, 1435, 1301, 1280, 1158, 1110, 1071, 1014, 954, 786, 731. Anal. calcd for (%) for C₄H₁₀N₁₂O₆ (322.20): C 14.91, H 3.13, N 52.17; found C 14.78, H 3.02, N 52.32.

Diaminomethaniminium 5,5′‑dinitroamino‑2,2′‑bi(1,3,4‑oxadiazolate) (6)

Guanidine carbonate (1.0 mmol) was added to a solution of compound 2 (0.258 g, 1.0 mmol) in 30 mL of acetonitrile and the mixture was stirred at room temperature. After 2 h the precipitate was filtered, washed with ether and air‑dried to yield. 0.312 g of compound 6 was obtained as a yellow solid in a yield of 82.9%. ¹H NMR (500 MHz, DMSO‑d ₆)δ: 6.93 (s, 12H). ¹³C NMR (125 MHz, DMSO‑d ₆)δ: 166.05 (s), 157.91 (s), 148.36 (s). IR (KBr, ν/cm^-1): 3386, 3270, 3205, 1681, 1657, 1525, 1494, 1407, 1297, 1155, 1074, 635. Anal. calcd for (%) for C₆H₁₂N₁₄O₆ (376.25): C 19.15, H 3.21, N 52.12; found: C 19.03, H 3.42, N 52.24.

Amino(hydrazinyl)methaniminium 5,5′‑dinitroamino‑2,2′‑bi(1,3,4‑oxadiazolate) (7)

Aminoguanidine bicarbonate (2.0 mmol) was added to a solution of compound 2 (0.258 g, 1.0 mmol) in 30 mL of acetonitrile and the mixture was stirred at room temperature. After 2 h the precipitate was filtered, washed with ether and air‑dried to yield. 0.352 g of compound 7 was obtained as a yellow solid in a yield of 86.6%. ¹H NMR (500 MHz, DMSO‑d ₆)δ: 8.56 (s, 2H)δ: 7.02 (s, 8H) δ: 5.36 (s, 4H). ¹³C NMR (125 MHz, DMSO‑d ₆)δ: 164.65(s), 158.76(s), 146.85(s). IR(KBr, ν/cm^-1): 3385, 3310, 3164, 1664, 1590, 1522, 1488, 1403, 1277, 1153, 1073, 1016, 956, 776, 660. Anal. calcd for (%) for C₆H₁₄N₁₆O₆(406.28): C 17.74, H 3.47, N 55.16; found: C 17.68, H 3.52, N 55.22.

Di(1,2,4‑triazolium) 5,5′‑dinitroamino‑2,2′‑bi(1,3,4‑oxadiazolate) (8)

1,2,4‑ Triazole (2.0 mmol) was added to a solution of compound 2 (0.258 g, 1.0 mmol) in 30 mL of acetonitrile and the mixture was stirred at room temperature. After 2 h the precipitate was filtered, washed with ether and air‑dried to yield. 0.347 g of compound 8 was obtained as a white solid in a yield of 87.6%. ¹H NMR (500 MHz, DMSO‑d ₆)δ:10.06 (br.). 8.97(s, 4H). ¹³C NMR (125 MHz, DMSO‑d ₆)δ: 164.33(s), 146.48(s), 144.53(s). IR(KBr, ν/cm^-1): 3105, 1568, 1526, 1491, 1451, 1322, 1295, 1264, 1159, 1077, 1018, 945, 735, 625. Anal.calcd for(%) for C₈H₈N₁₄O₆(396.24): C 24.25, H 2.04, N 49.49; found: C 24.20, H 2.11, N 49.53.

Di(3‑amino‑1,2,4‑triazolium) 5,5′‑dinitroamino‑2,2′‑bi(1,3,4‑oxadiazolate) (9)

3‑Amino‑1,2,4‑triazole (2.0 mmol) was added to a solution of compound 2 (0.258 g, 1.0 mmol) in 30 mL of acetonitrile and the mixture was stirred at room temperature. After 2 h the precipitate was filtered, washed with ether and air‑dried to yield. 0.362 g of compound 9 was obtained as a white solid in a yield of 84.9%. ¹H NMR (500 MHz, DMSO‑d ₆)δ: 8.27(s, 2H)δ: 8.05 (br.). ¹³C NMR (125 MHz, DMSO‑d ₆)δ: 165.94 (s), 151.11 (s), 148.11 (s), 139.64 (s). IR (KBr, ν/cm^‑1): 3405, 3149, 2738, 1691, 1572, 1548, 1503, 1456, 1323, 1281, 1161, 1135, 1084, 1024, 946, 914, 887, 774,728. Anal. calcd for (%) for C₈H₁₀N₁₆O₆(426.27): C 22.54, H 2.36, N 52.57; found: C 22.21, H 2.39, N 52.49.

Di(4‑amino‑1,2,4‑triazolium) 5,5′‑dinitroamino‑2,2′‑bi(1,3,4‑oxadiazolate) (10)

4‑Amino‑1,2,4‑triazole (2.0 mmol) was added to a solution of compound 2 (0.258 g, 1.0 mmol) in 30 mL of acetonitrile and the mixture was stirred at room temperature. After 2 h the precipitate was filtered, washed with ether and air‑dried to yield. 0.356 g of compound 10 was obtained as a white solid in a yield of 83.5%. ¹H NMR (500 MHz, DMSO‑d ₆)δ: 9.25 (s, 4H), 8.43 (br). ¹³C NMR (125 MHz, DMSO‑d ₆)δ: 164.86 (s), 147.03 (s), 144.07 (s). IR (KBr, ν/cm^-1): 3312, 3120, 1571, 1555, 1537, 1493, 1495, 1322, 1277, 1161, 1086, 1040, 1019, 984, 798, 784, 622. Anal. calcd for (%) for C₈H₁₀N₁₆O₆ (426.27): C 22.54, H 2.36, N 52.57; found: C 22.37, H 2.48, N 52.31.

Didiaminouronium 5,5′‑dinitroamino‑2,2′‑bi(1,3,4‑oxadiazolate) (11)

Carbohydrazide (2.0 mmol) was added to a solution of compound 2 (0.258 g, 1.0 mmol) in 30 mL of acetonitrile and the mixture was stirred at room temperature. After 2 h the precipitate was filtered, washed with ether and air‑dried to yield. 0.392 g of compound 11 was obtained as a white solid in a yield of 89.5%. ¹H NMR (500 MHz, DMSO‑d ₆)δ: 8.50. ¹³C NMR (125 MHz, DMSO‑d ₆)δ: 166.11 (s), 159.14 (s), 148.38 (s), IR (KBr, ν/cm^-1): 3320, 3275, 3213, 1697, 1623, 1602, 1555, 1529, 1492, 1429, 1350, 1316, 1281, 1220, 1168, 1120, 1083, 1019, 843, 776, 690. Anal. calcd for (%) for C₆H₁₄N₁₆O₈ (438.28): C 16.44, H 3.22, N 51.13; found: C 16.40, H 3.54, N 50.28.

3 Results and Discussion

3.1 Single Crystal Structure Analysis

A solvent evaporation method was applied to grow good quality crystals of 9·2H₂O and compound 10 for X‑ray diffraction. The crystals of 9·2H₂O and compound 10 were grown from a solution of water/CH₃OH. Their structures are shown in Fig.1 to 4 and the crystallographic and structural refinement data are listed in Table .1

Compound 9·2H₂O crystallizes in the triclinic space group Pī with one formula unit in the unit cell and a calculated density of 1.761 g·cm^-3 at 172(2) K. The asymmetric unit consists of one 3‑amino‑1,2,4‑triazole cation, one and a half 5,5′‑dinitroamino‑2,2′‑bi(1,3,4‑oxadiazolate) anion and a lattice water molecule. As shown in Fig.,1, the anion exhibits a central symmetric structure where an inversion centre lies. The packing diagram of 9·2H₂O is built up by hydrogen bonds(Fig.2). The cations connect to anions by the classical hydrogen bonds (N(7)—H(7)…O(2), 2.800(2) Å; N(7)—H(7)…N(3), 3.051(2) Å; N(7)—H(7)…N(4), 3.353(2) Å; N(8)—H(8)…N(2), 3.021(2) Å) and the extensive hydrogen‑bonding in the structure may also contribute to its good thermal stability.

Fig.1 Molecular structure of 9·2H2O

Fig.2 Ball and stick packing diagram of 9·2H2O

Table 1 Crystallographic data for 9·2H2O and 10

crystal	9·2H2O	10
CCDC	1544254	1543503
formula	C8H10N16O6·2H2O	C8H10N16O6
formula mass	462.35	426.32
temperature /K	172(2)	171(2)
crystal system	triclinic	monoclinic
space group	P‑1	P21/c
volume /Å3	435.87(6)	812.65(14)
a/Å	6.0458(5)	6.9450(6)
b/Å	6.6277(6)	6.3230(6)
c/Å	11.3541(9)	18.576(2)
α/(°)	97.206(3)	90
β/(°)	101.955(3)	94.983(4)
γ/(°)	97.438(3)	90
Z	1	2
ρ/g·cm-3	1.761	1.742
F(000)	238.0	436
F 2	1.050	1.031
R 1,wR 2 [all data]	0.0612, 0.0887	0.0851, 0.1087
R 1,wR 2[I>=2σ(I)]	0.0413, 0.0816	0.0511, 0.0973

Compound 10 crystallizes in the monoclinic space group P2₁/c with Z=2 and a cell volume of 812.65(14) ų. A density of 1.742 g·cm^-3 was determined at 171 K from the X‑ray diffraction analysis. As presented in Fig.,3, the structure of anion is similar to that of compound 10 and there are little differences in bond lengths and angles. The anion within the asymmetric unit (—CNNOC—N—NO₂) is nearly coplanar with an root mean square deviation value of 0.0237 Å. As can be seen from Fig.,4, large amount of hydrogen bonds can be found between the anions and cations (N(1)—H(1)…O(3), 2.888(3) Å; N(1)—H(1)…N(5), 2.726(3) Å; N(4)—H(4)…N(7), 2.984(4) Å). The oxygen atoms (O(2), O(3)) in the anions are involved in the formation of non‑classical hydrogen bonds with the CH fragments (C(2)—H(2)…O(3), 2.950(3) Å; C(2)—H(2)…O(2), 3.024(4) Å).

Fig.3 Molecular structure of compound 10

Fig.4 Ball and stick packing diagram of compound 10

3.2 Computational Details

Computations were performed by using the Gaussian 09 suite of programs. The elementary geometric optimization and the frequency analysis were performed at the level of the Becke three parameter, Lee‑Yan‑Parr (B3LYP) functional with the 6‑311+G^** basis set^[21]. All the optimized structures were characterized to be local energy minima on the potential surface without imaginary frequencies. Atomization energies were calculated by the CBS‑4M. The lattice energy of the trinitroethyl derivatives were predicted by the formula suggested by Jenkins et al.

The predictions of heats of formation (HOF) used the hybrid DFT/B3LYP methods with the 6‑311+G^** basis set through designed isodesmic reactions. The isodesmic reaction processes, that is, the number of each kind of formal bond is conserved, were used with the application of the bond separation reaction (BSR) rules. The molecule was broken down into a set of two heavy‑atom molecules containing the same component bonds. The isodesmic reactions used to derive the HOF of compounds 3-11 are shown in Scheme 2.

Scheme 2 Isodemic reactions of N,N'‑([2,2'‑bi(1,3,4‑oxadiazole)]‑5,5'‑diyl)dinitramide anion

The change of enthalpy for the reactions at 298K can be expressed by Equ.(1):

ΔH _{298 K}=ΣΔ_f H _P–ΣΔ_f H _R (1)

Where ΣΔ_f H _P and ΣΔ_f H _R are the HOF of the reactants and products at 298 K, respectively, and ΔH _{298 K} can be calculated from the following expression in Equ.(2):

ΔH _{298 K}=ΔE _{298 K}+Δ(pV)=ΔE ₀+ΔZPE+ΔH _T+ΔnRT (2)

where ΔE ₀ is the change in total energy between the products and the reactants at 0 K; ΔZPE is the difference between the zero‑point energies (ZPE) of the products and the reactants at 0 K. ΔH _T is the thermal correction from 0 to 298 K. The Δ(pV) value in Equ.(2) is the pV work term. It equals ΔnRT for the reactions of an ideal gas. For the isodesmic reactions Δn=0, so Δ(pV)=0. On the left side of Equ.(2) apart from target compound all the others are called reference compounds. The HOF of reference compounds are available either from experiments or from the high level computing such as CBS‑4M.

Based on a Born‑Haber energy cycle (Scheme 3), the heat of formation of a salt can be simplified by Equ.(3):

ΔH _f ⁰(ionic salt, 298 K)=ΔH _f ⁰(cation, 298 K)+ ΔH _f ⁰ (anion, 298 K)-ΔH _L (3)

where ΔH _L is the lattice energy of the salt which could be predicted by the formula suggested by Jenkins et al^[22]. as given in Equ.(4):

ΔH_L =U_POT+[p(n _M/2‑2)+q(n _X/2‑2)]RT (4)

where n_M and n_X depend on the nature of the ions Mp⁺ and Xq^-, respectively, and are Equ.(3) for monatomic ions, Equ.(5) for linear polyatomic ions, and 6 for nonlinear polyatomic ions. The equation for lattice potential energy U _POT takes the form of Equ.(5):

U _POT=γ(ρ_m/M_m )1/3+δ (5)

where ρ_m is the density ,g·cm^-3. M _mis the chemical formula mass of the ionic material and the coefficients γ , kJ·mol^-1 ·cm .δ are 8375.6 and -178.8 kJ·mol^-1, respectively.

Scheme 3 Born‑Haber cycle for the formation of energetic salts

3.3 Physicochemical Properties

Thermal stability is one of the most important physicochemical properties of energetic materials. The thermal stabilities of compounds 3-11 were determined by differential scanning calorimetry (DSC) at a heating rate of 5 ℃·min^-1 using dry nitrogen in Al pans. For all compounds, the decomposition temperatures were determined by the decomposition onset temperatures. As show in Fig.,5, all the compounds decompose directly without melting, and except for compound 3, that melts at 73.5 ℃. In particular, the decomposition process of 4-6, and 9 possess two steps with the onset temperatures. The decomposition onset temperatures of the energetic compounds are observed in a range from 146.7(4) to 239.9 ℃(6). Compounds 6-10 are observed in a range from 181.7(9) to 239.9 ℃(6), As is known, thermal stability above 180 ℃ is an essential requirement for energetic compounds for adaptation for practical use^[22]. The guanidinium salt 6 is the most thermally stable one at 239.9 ℃. Both compound 6 and 9 have higher decomposition temperatures than RDX (204°C).

Fig.5 DSC curves of compounds 3-11

3.4 Energetic Performance

Impact and friction sensitivities are high priorities for secondary explosives. Low sensitivities of energetic materials can reduce the risk of serious and fatal accidents during the applications. Compounds 3-11 were tested for their sensitivity towards friction and impact using the BAM methods. The impact and friction sensitivities of compounds 3-11 fall in the range of 10-28 J or 120-280 N. The energetic salts are well stabilized and exhibit better impact and friction sensitivities (10-28 J for IS and 160-360 N for FS) than their parent compounds. All the compounds are less sensitive than RDX(7.5 J), the IS values of 3-4, 6-11 are higher than that of TNT (15 J). In the terms of the friction sensitivities, all of the energetic salts are much higher than RDX (120 N). By comparing with nitrogen‑rich cations, the salts have additional ionic and hydrogen bonding, and thus exhibit lower impact and friction sensitivities.

The enthalpy of formation (HOF) is essential for calculating the detonation performance. The heats of formation of the synthesized compounds 3-11 were calculated based on appropriate isodesmic reactions. Calculations were carried out using the Gaussian 09 program suite. The geometry optimization of the structures and frequency analyses were carried out using the B3LYP functional with the 6‑311+G^** basis set. All of the optimized structures were characterized by true local energy minima on the potential energy surface without imaginary frequencies. The results are summarized in Table 2, the enthalpy of formation ranging from -0.671(6) kJ·g^-1 to 1.143 kJ·g^-1 (10). With the exception of 3, 4 and 6, the energetic salts possess positive heats of formation, which are higher than TNT (-1.30 kJ·g^-1) and RDX (0.32 kJ·g^-1) due to a large number of C—N and N—N bonds, and compound 10 has the highest value of 1.143 kJ·g^-1.

By using the calculated heats of formation and the experimental densities (gas pycnometer) of the new energetic compounds, the detonation pressures (p) and detonation velocities (D) were calculated by using the EXPLO5 v6.02 program. As is shown in Table 2, the calculated detonation velocities fall in the range between 7693 m·s^-1(6) and 9136 m·s^-1(3), and detonation pressures range from 21.3 GPa(6) to 34.2 GPa(3). Compound 3-5 show excellent detonation properties which are much higher than those of RDX (8795 m·s^-1, 34.9 GPa), and to those of HMX (9320 m·s^-1, 39.6 GPa).

4 Conclusions

A new family of energetic salts featuring bi(1,3,4‑oxadiazole) were prepared and fully characterized. In addition, the structures of 9·2H₂O and 10 were confirmed by single‑crystal X‑ray diffraction analysis. According to the DSC results, most of the energetic salts showed acceptable thermal stabilities, with the decomposition temperatures ranging from 146.8 to 239.9 ℃. The calculated detonation velocities lie in the range between 7693(6) and 9136(3) m·s^-1, and the detonation pressures range from 21.3 to 34.2 GPa, which are larger than those of TNT. The detonation velocities and pressures of compounds 3-5 are comparable to those of RDX. Based on the impact and friction tests, all the salts are less sensitive than RDX. Ammonium salt which has excellent detonation properties (D=9136 m·s^-1, p=34.2 GPa) and acceptable sensitivities (IS=20 J, FS=180 N) is a promising potential candidate for energetic materials.

Reference