ZHANG Hong-li, LIU Yu, ZHANG Hao-bin, LI Shi-chun, ZHU Chun-hua, XU Jin-jiang, YANG Shi-yuan,LI Jin-shan

(1. Institute of Chemical Materials, China Academy of Engineering Physics (CAEP), Mianyang 621999, China; 2. Southwest University of Science and Technology, Mianyang 621010, China)

1 Introduction

In the field of high explosives, many efforts have been focused on exploring more powerful, safer and environmentally friendly explosives. 1,1-diamino-2,2-dinitroethylene (C2H4N4O4), commonly referred to FOX-7, is a novel high explosive[1]. Compared with the most widely used and powerful explosive 1,3,5-trinitro-1,2,4-triazacyclohexane(RDX), FOX-7 combines two important aspects, i.e., high performance and low sensitivity[2]. However, FOX-7 possesses several polymorphs[3], which suppresses its application. It is well known that phase transitions of energetic materials commonly observed under high temperature directly affect the stability and performance[4]. Indeed, the most stable polymorph at ambient conditions is always sought owning to its highest detonation velocity[5]. Moreover, phase transitions induced by high temperature frequently result in crystal defects which can sensitize the explosive by forming hot spots and super-rate burning[5]. Therefore, developing a deeper understanding of phase behaviors can lead to valuable insight into the complex interplay of intra- and inter-molecular interactions which are responsible for shock-initiated chemical reactions.

Crystalline FOX-7 has three polymorphs, i.e.α,βandγ. The most stable phase at room temperature isα-FOX-7, and it can be converted to other polymorphs at higher temperatures, and upon cooling the explosive directly but incompletely restores to theαphase[6]. The phase transition behaviors of FOX-7 have been explored by X-ray diffraction methods, Raman, and thermal techniques[6-10]. However, the phase transition mechanism is still confusion. The effects of H-bonding and molecular structure might be the key for understanding the polymorphism. But the information on the behavior of the related H-bonding vibrations is not observed in FOX-7[11]. Indeed, the stability of FOX-7 mainly depends on the strong H-bonds between nitro oxygen atoms and amino hydrogen atoms. Furthermore, detailed analysis of the changes of H-bonding properties of the FOX-7 crystals under extreme conditions could provide important insight for understanding the detonation behavior of FOX-7 at molecular level[12]. Evers[7]and Crawford[8]have investigated the molecular structure and the effects of H-bonding of FOX-7 by X-ray single crystal methods. But few tools and techniques including X-ray single crystal diffraction could visually present the changes of H-bonding and molecular structure with increasing temperature in a single figure.

Recently, temperature-dependent Fourier transform infrared (FTIR) spectroscopy is recognized as a powerful technique forin-situcharacterization of phase transitions of energetic material. Temperature-dependent FTIR spectroscopy could give information about intermolecular vibrational modes mediated by the H-bonding and access knowledge about the structure and vibrational dynamics of solids[13-15]. Pressure-induced changes of FOX-7 were observed by using FTIR spectroscopy[9], and the effects of H-bonding and molecular structure with pressures were discussed. Temperature-induced changes were investigated by Bishop[16]using IR. The H-bonding effects of FOX-7 with temperatures were investigated but the changes of molecular structure were ignored. Thus, we sought to conduct a FTIR study of FOX-7 to observe and compare the effects of H-bonding and molecular structure on molecular vibrations with temperatures and to investigate the phase transitions of FOX-7. This study would also offer the valuable insight into the interesting secondary explosive behaviors under high temperature conditions.

2 Experimental

The FOX-7 samples were provided by Institute of Chemical Materials, Chinese Academy of Engineering Physics. FTIR spectra were taken with KBr pellets in Model Nicolet 6700 spectrograph. The resolution was 1 cm-1, and the scan range was 400-4000 cm-1. Spectra were recorded during heating from 55 ℃ to 190 ℃ at a constant heating rate of 5 ℃·min-1. Then, the spectra were obtained during the cooling process.

Powder X-ray diffraction(XRD) patterns were recorded on a Bruker D8 Advance with a CuKαradiation (λ=1.5406 Å), the voltage and current applied are 40 kV and 40 mA, respectively. Samples of FOX-7 were heated from 55 ℃ to 190 ℃, and data were collected from 5° to 60° (2θ) with an increment of 0.02° and a continuing time of 0.1 s for each increment.

3 Results and Discussion

3.1 α→β Phase Transition3.1.1 Bands Assignments for Vibrational Modes

The temperature-dependent FTIR spectra of FOX-7 heating from 55 ℃ to 122 ℃ in the region of 3500-3150, 1670-970 cm-1and 900-400 cm-1are displayed in Fig.1. As shown in Fig.1a,ν1,ν2andν3have the tendency to shift toward higher wavenumber when the temperature increases. As shown in Fig. 1b,ν5(1632 cm-1) shifts toward lower wavenumber with increasing temperature. Subsequently,ν6overlaps or incorporates withν5. The frequencies of bands appearing at 1526 (ν7), 1470 (ν8) and 1394 (ν9), 1350 (ν10) cm-1remain nearly constant during heating (see in Fig.1b). Generally speaking, the frequencies of FOX-7 in most vibrational bands show little changes during heating in the range of 55 ℃ to 105 ℃. The FOX-7 molecules show strong intermolecular H-bonds within the wave-shaped layers. When the H-bonding is reduced or grows weaker, the stretching vibrational band usually shifts to higher frequency, whereas the deformation vibrational band usually moves to lower frequency[17]. According to the frequency shifts of H-bonding interaction, vibrational modes are clearly assigned[18-24], listed in Table 1. Besides,ν4can be assigned to Fermi resonance between the NH2(νas) and the overtone mode of the NH2scissor (2ν5), and the vibrational mode at ～1620 cm-1can be identified as a newly formed peak, which is assigned to Fermi resonance between NH2and NO2scissoring vibrations[25]. In contrast to other H-bonded energetic materials, such as 1,3,5-triamine-2,4,6-trinitrobenzene (TATB)[17], the weakening of H-bonding effect in most vibrational bands is inconspicuous with increasing temperature below 105 ℃.

a. 3500-3150 cm-1b. 1670-970 cm-1c. 900-400 cm-1

Fig.1 FTIR spectra of FOX-7 while the temperature increases from 55 ℃ to 122 ℃

Table 1 Assignments of vibrational bands in spectra of FOX-7

band/cm-1vibrationmodeswavenumber/cm-1band/cm-1vibrationmodeswavenumber/cm-1ν1'NH2(νas)3406ν11ν12ν13C—NH2(ν)1226,1170,1137ν2ν3NH2(νs)3329,3298ν14NH2(ρ)1023ν4Fermiresonance3223ν15NO2(γ)855ν5NH2(γ)1632ν16ν17ν18C—NO2(ν)788,750,737ν6CC(ν)1610ν19NH2(ω)621ν7ν8NO2(νas)1526,1470ν20NO2(ω)518ν9ν10NO2(νs)1394,1350ν21NO2(ρ)458

Note:ν=stretching,νas=asymmetric stretching,νs=symmetric stretching,γ=scissoring,ρ=rocking,ω=wagging.

3.1.2 Effects of Intermolecular Hydrogen Bonding Interaction and Molecular Structure

Above 105 ℃, the NH2(νas) peak near 3443 cm-1abruptly shifts to higher wavenumber, as shown in Fig.1a. The intensity of NO2(νs) ν10sharply decreases and shifts toward lower wavenumber (Fig.1b). As shown in Fig.1c,ν18disappears above 105 ℃. Spectral bands in the lower spectral range (650-467 cm-1) undergo significant changes in the range of 100 ℃ to 120 ℃. The smoothingν20is replaced by a broad medium strong peak above 105 ℃. The band at 458 cm-1vanishes, and a peak at 447 cm-1grows at about 111 ℃ and reaches the same intensity of the band near 458 cm-1. FTIR spectra of FOX-7 experience some abrupt changes on heating at 105 ℃, indicating that theα→βtransition occurs.

The molecular conformations and packing arrangements ofαandβpolymorphs can provide some insight. Theαandβ-FOX-7 have similar structures of two-dimensional wave-shaped layers (CCDC, SEDTUQ03 and SEDTUQ06), as shown in Fig.2. However, heating of the FOX-7 molecules to 120 ℃ flattens the wave-shaped layers. Within the layers, there are strong H-bonds. Indeed, the spectral changes of NH2(νas) at 3443 cm-1are attributed to the restructing of the H-bonding network in theα→βphase transition[4]. The CC and amino nitrogen atoms form a molecular plane, and the nitro oxygen atoms deviate strongly from the molecular plane. The results have been previously reported[8]that the NO2twist angle does not vary significantly on heating up to 100 ℃, but it increases abruptly above 100 ℃. The unique spectra signature ofν10is likely associated with changes of the NO2twist angle when the temperature increases. The analytical results of the IR spectra indicate that intermolecular hydrogen bonding interactions change in the phase transitions of the FOX-7, which cause the changes of the molecular structure. The results confirm that theα→βis a displacive transition with minor structural distortions[8].

a.α-FOX-7(25 ℃)

b. β-FOX-7(120 ℃)

Fig.2 View along a axis ofα-FOX-7 at 25 ℃ andβ-FOX-7 at 120 ℃

3.1.3 Transition State

Most materials experience relatively modest expansions with increasing temperature, resulting from the increasing anharmonic vibration amplitudes of the ingredient atoms or molecules[26]. The degree of linear thermal expansion has been quantified using a coefficient defined asα=(lT-l0)/(l0(T-T0))[27], wherelTis the axis length at the final temperature, andl0is the axis length at the initial temperature. The average linear thermal expansion coefficients ofα-FOX-7 areαa=2.18×10-5℃-1,αb=12.71×10-5℃-1andαc=4.29×10-5℃-1. The average linear thermal expansion coefficients ofβ-FOX-7 areαa=0.84×10-5℃-1,αb=16.89×10-5℃-1andαc=3.85×10-5℃-1 [28]. Similarly, the H-bonding length is formulated aslT=l0+αΔT[27], wherelTis the H-bonding length at final temperature,l0is the H-bonding length at initial temperature,αis the coefficient of thermal expansion, and ΔTis the change of temperature. Here, the value ofαforα- andβ-FOX is almost equal to 1×10-5℃-1. The intermolecular H-bonds of FOX-7 at 25 ℃ and 120 ℃ are presented in Fig.3, and their length are displayed in Table 2. The average H-bonds length between FOX-7 molecules is 2.58 Å at 25 ℃ and 2.76 Å at 120 ℃. We find thatl0+αΔT=(2.58+1×10-5×95) Å≈2.58Å=l0≠lT=2.76 Å. As described in Section 3.1.1, the length of H-bonding remains constant during heating from 55 to 100 ℃. We conclude that a intermediateltransoccurs in theα→βtransition, as is followed:lT=l0+αΔT+ltrans, wherelTis the H-bonding length at final temperature,l0is the H-bonding length at initial temperature,αis the coefficient of thermal expansion, ΔTis the change of temperature,ltransis the H-bonding length increamental resulting from the phase transition and increase gradually when the temperature increases from 100 ℃ to 110 ℃. Besides, the evolution of selected peaks frequencies with increasing temperature is presented. As shown in Fig.4, the frequencies of FOX-7 in all vibrational bands show abrupt changes during heating in the range of 100 ℃ to 110 ℃. Three or more points are observed in the range of 100 ℃ to 110 ℃, and each one of them is neitherα-FOX-7 norβ-FOX-7, which giving more evidence of the occurrence of a transition state in the phase transition. It is the transition state that makes the change of H-bonding length with temperatures, not consistent with the formula (lT=l0+αΔT). The intermediate should be a necessary step in the transition fromα-FOX-7 toβ-FOX-7. Traditionally, phase transition is considered to be one-step reaction.

a. α-FOX (25 ℃)

b. β-FOX (120 ℃)

Fig.3 H-bonding interactions of FOX-7

Table 2 H-bonds length between FOX-7 molecules at 25 ℃ and 120 ℃

D—H…Aα-FOX(25℃)D—H/ÅH…A/ÅD…A/Åβ-FOX(120℃)D—H/ÅH…A/ÅD…A/ÅN(4)—H(4)…O(2)0.8492.3413.0270.8622.4832.945N(4)—H(3)…O(4)0.9042.1432.9210.7952.2092.979N(4)—H(3)…O(2)0.9043.1053.0270.7952.7822.945N(4)—H(3)…O(1)0.9042.3903.1930.7954.1573.925N(3)—H(1)…O(1)0.9452.1453.0160.7932.5322.988N(3)—H(1)…O(3)0.9452.4463.0080.7932.3963.156N(3)—H(2)…O(4)0.9432.2913.0130.8052.7033.414N(3)—H(2)…O(1)0.9563.7713.0160.8052.7952.988

3.2 β→γ Phase Transition

With further application of temperature to 190 ℃, spectra were acquired and displayed in Fig.5. Inβ-phase, the two NH2(νas) peaks are well separated and dramatically shifted to higher frequencies. As temperature increases, the two peaks are broad and strongly overlapped with each other in the range of 3500-3350 cm-1as well as being accompanied by the loss of NH2(νs) modes at ～3329 cm-1. NO2-related vibrations such asν12andν20exhibit significant changes, indicating that theγtransition was obtained. Compared to the changes observed in theα→βtransition, the frequencies of most vibrational bands remain nearly constant in theβ→γtransition, implying no further weakening of the H-bonding network. The changes in NO2-related vibrations are smaller during theβ→γtransition than theα→βtransition. The previous results[9]showed that the displacement of the nitro groups with respect to the molecular plane remains invariant inβ- andγ-FOX-7. FTIR spectra of FOX-7 exhibit minor changes in theβ→γtransition. A combination of stable H-bonding network and structural similarities is responsible for subtle differences in the FTIR spectra for two polymorphs over the whole temperature range from 122 ℃ to 190 ℃.

Standard FTIR spectra ofα,βandγpolymorphs of FOX-7 are identified and confirmed by X-ray diffraction patterns (see in Fig.6). The FTIR spectra ofα-,β- andγ-FOX-7 exhibit significant changes including NH2and NO2-related vibrations such asν12,ν18,ν20andν21. The former is due to the restructing of the H-bonding network, and the latter is due to the changes in the displacement of nitro groups with respect to the molecular plane. The characteristic FTIR absorption bands of each phase of FOX-7 are presented in Table 3.

a. ν1′

b. ν4

c. ν7

d. ν10

Fig.4 Peak positions of different vibrational modes as a function of temperature

Table 3 The characteristic FTIR absorption bands of each phase of FOX-7

3440cm-1NH2(νas)3329cm-1NH2(νs)1170cm-1C—NH2(ν)737cm-1C—NO2(ν)650-467cm-1(ν19,ν20)NH2(ω),NO2(ω)α ×√√√sharppeaksβ √√√×broadpeaksγ √×××anotherbroadpeaks

Note: √ represents the emergence of peak, × represents the disappearance of peak.

Fig.5 FTIR spectra of FOX-7 at five different temperatures (α-FOX-7 at 55 ℃;β-FOX-7 at 122 ℃;γ-FOX-7 at 190 ℃)

Fig.6 XRD patterns of FOX-7 at five different temperatures (α-FOX-7 at 55 ℃;β-FOX-7 at 122 ℃,γ-FOX-7 at 190 ℃)

4 Conclusions

Temperature-dependent FTIR spectroscopy has been employed to study the phase transitions of FOX-7 in real time. The main conclusions are as follows：

(1) In the process of phase transitions of the FOX-7, intermolecular hydrogen bonding interactions change, which caused the changes of the molecular structure.

(2)The transition state was confirmed by the special thermal expansion properties of H-bonding length and the special frequencies ofν1′,ν4[Fermi resonance between the NH2(νas) and the overtone mode of the NH2scissor(2ν5)],ν7andν10[NO2(νas)] in theα→βtransition with increasing temperature.α-FOX-7 does not directly transforms toβ-FOX-7, the transition state should be a necessary step.

(3)Standard FTIR spectra ofα,βandγ-FOX-7 were identified and confirmedin-situby powder X-ray diffraction (XRD). Several characteristic peaks ofν12[C—NH2(ν)],ν18[C—NO2(ν)] andν20[NO2(ω)] can be used to identify each phase of FOX-7.

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