The demand for high-performance explosives and propellants has led to intensive investigations relative to improving their energetic, mechanical, and storage properties and safe handling. One of the key steps is the development of new energetic ingredients, such as energetic plasticizers. Energetic plasticizers are important constituents of rocket and gun propellants, which aid in improving the mechanical properties of the propellant grain^[1-5].The basic objectives of developing energetic plasticizers are: (1) increase of the thermal stability, (2) increase in energy content, (3) adjustment of the oxygen balance in a formulation, (4) improvement of the plasticizer functions in formulations. Furazanyl ether compounds have attracted a considerable amount of attention as an energetic plasticizers due to their high energy density, good thermal stability and positive heat of formation ^[6-14]. When a bridged oxygen atom is introduced into furazans, it could be significantly increased both the flexibility and plasticizer of the molecule. A good example is the previously synthesized compound 3, 3′-dinitrodifurazanyl ether (FOF-1), which shows a low melting point (T_m=63-64 ℃), high density (ρ=1.90 g·cm^-3) and good thermal stability (T_dec>200 ℃).

In order to raise the thermal stability and energy level of the energetic plasticizer, introducing furazanyl ether as backbone into fluorodinitromethyl group could be an effective method. Chemically, the substitution of one nitro group in the trinitromethyl moiety with a fluorine atom reduces the pseudohalide character^[15]. In view of the observations above, a detailed study of the synthesis and characterization of 3, 4-bis(3-fluorodinitromethylfurazan-4-oxy)furazan is presented in this work. In addition, thermal stability, sensitivity measurements and the detonation parameters were investigated.

Elemental analyses of C, H and N were performed on a VARI-El-3 elementary analysis instrument. Infrared spectra were obtained from KBr pellets on a Nicolet NEXUS870 Infrared spectrometer in the range of 4000-400 cm^-1. Differential scanning calorimetry(DSC) studies were carried out on a Q200 apparatus(TA, USA) with a heating rates of 10 K·min^-1, using dry oxygen-free nitrogen as atmosphere with a flowing rate of 50 mL·min^-1. The TG-DTG experiment was performed with a SDT-Q600 apparatus(TA, USA) operating at a heating rate of 10 K·min^-1 in a flow of dry oxygen-free nitrogen at 50 mL·min^-1. The density was measured using a pycnometer method at room temperature. The impact sensitivity was determined with a ZBL-B impact sensitivity instrument. The heat of formation for 3, 4-bis(3-fluorodinitromethylfurazan-4-oxy) furazan was theoretically computed using the Gaussian 09 (Revision A.02) program package^[16]. To obtain very accurate energies, the enthalpies (H) were calculated by using the CBS-4M method^[17-18].

Dipotassium of 3, 4-bis(3-dinitromethylfurazan-4-oxy) furazan (2) was prepared according to the published procedures^[19]. Other chemicals were obtained from commercial sources and used without further purification.

Scheme 1(Scheme1)

Scheme1

3, 4-Bis(3-fluorodinitromethylfurazan-4-oxy) furazan(1): To a suspension of dipotassium 3, 4-bis(3-dinitromethylfurazan-4-oxy)furazan (0.35g, 0.67mmol) in an hydrous acetonitrile (15 mL) was added XeF₂ (0.46 g, 2.7 mmol) with stirring and reaction mixture was stirred for 20 h at 30 ℃. The acetonitrile was evaporated, and the residue was treated with water. The white crystals was collected by filtration to afford 1 (0.18 g, 56.2%) as a colorless crystals. mp 50 ℃; ¹³C NMR (DMSO-d₆, 125 MHz) δ 159.85, 153.33, 138.03, 114.21; ¹⁹F NMR (DMSO-d₆, 470.5MHz) δ: -106.84; IR (KBr, ν/cm^-1) 1608, 1576, 1545, 1507, 1350, 1309, 1250, 1194, 1033, 982; Anal.Calcd. for C₈N₁₀O₁₃F₂ (%): C 19.93, N 29.05; Found C 20.08, N 29.13.

The physicochemical and energetic properties of 3, 4-bis(3-fluorodinitromethylfurazan-4-oxy) furazan were determined or calculated, and listed in Table 1. It was found that 3, 4-bis(3-fluorodinitromethylfurazan-4-oxy)furazan possesses a good thermal stability with decomposition temperature of 197.8 ℃, moderate impact sensitivity of 11 J. The density of 3, 4-bis(3-fluorodinitromethylfurazan-4-oxy)furazan is 1.88 g·cm^-3. The oxygen balance is calculated to be -6.6%, and this value is lower than that of NG (3.5%), but higher than that of FEFO(-10%).

表 1(Tab. 1)

Tab. 1 The physicochemical properties and detonation parameters of 3, 4-bis(3-fluorodinitromethylfurazan-4-oxy)furazan compared with the well-known plasticizers NG, FEFO and FOF-1 compound 1 NG FEFO FOF-1 formula C8F2N10O13 C3H5N3O9 C5H6F2N4O10 C4N6O7 molar mass 482.1 227.1 320.1 244 nitrogen content/% 28.1 18.5 17.5 34.4 density/g·cm-3 1.88 1.59 1.60 1.90 melting point/℃ 50 13 14.5 63 oxygen balance/% -6.6 3.5 -10 -6.5 impact sensitivity/J 11 0.2 13.5 - enthalpy of formation/kJ·mol-1 -128.5 -371 849.8 350.5 detonation velocity/m·s-1 8644.5 7813 7500 8930 detonation pressure/GPa 34.0 25.7 25 -

Tab.1 The physicochemical properties and detonation parameters of 3, 4-bis(3-fluorodinitromethylfurazan-4-oxy)furazan compared with the well-known plasticizers NG, FEFO and FOF-1

Based on the calculated heats of formation and the experimentally measured densities, the detonation pressures and detonation velocities were calculated based on Kamlet-Jacobs equations and shown in Table 1. Compound 1 has a detonation velocity of 8644.5 m·s^-1 and a detonation pressure of 34.0 GPa, which are significantly better than those of nitroglycerine(NG) (25.7 GPa, 7813 m·s^-1)^[20] and bis(2-fluoro-2, 2-diniroethyl)formal (FEFO)(25 GPa, 7500 m·s^-1)^[21], and comparable with detonation velocity of 3, 3′-dinitrodifurazanyl ether (FOF-1) (8930 m·s^-1)^[22], revealing a higher energy level as a new high-performance energetic plasticizer.

In this study we reported the synthesis and structural property, spectroscopic characterization and detonation performances of 3, 4-bis(3-fluorodinitromethylfurazan-4-oxy)furazan (1). Compound 1 exhibits good physicochemical and detonation properties, such as high density(1.88 g·cm^-3), good thermal stability (T_dec=197.8 ℃), moderate impact sensitivity(11 J), acceptable oxygen balance(-6.6%), and high detonation pressure(34.0 GPa) and detonation velocity(8644.5 m·s^-1). In many aspects, such as thermal stability, density, sensitivity, and detonation parameters, it is far superior to NG. This promising result makes the 3, 4-bis(3-fluorodinitromethylfurazan-4-oxy)furazan interesting for future application as a potential high-energy dense plasticizer.