Exploration of the Hydrogen-Bonded Energetic Material

Sep 18, 2014 - Feng Bai , Binsong Li , Kaifu Bian , Raid Haddad , Huimeng Wu , Zhongwu Wang , Hongyou Fan. Advanced Materials 2016 28 (10), 1989-1993 ...
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Exploration of the Hydrogen-Bonded Energetic Material Carbohydrazide at High Pressures Tingting Yan,† Kai Wang,† Xiao Tan,† Jing Liu,‡ Bingbing Liu,† and Bo Zou*,† †

State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, China Beijing Synchrotron Radiation Laboratory, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100039, China

‡

S Supporting Information *

ABSTRACT: We have reported the high-pressure behavior of hydrogen-bonded energetic material carbohydrazide (CON4H6, CHZ) via in situ Raman spectroscopy and angle-dispersive X-ray diffraction (ADXRD) in a diamond anvil cell with ∼15 GPa at room temperature. Significant changes in Raman spectra provide evidence for a pressure-induced structural phase transition in the range of ∼8 to 10.5 GPa. ADXRD experiments confirm this phase transition by symmetry transformation from P21/n to a possible space group P1,̅ which exhibits ∼23.1% higher density at 10.1 GPa compared to phase P21/n at ambient pressure. Moreover, the observed transition is completely reversible when the pressure is totally released. On the basis of the decreased number of hydrogen bonds, the shortened hydrogen bond lengths, and the variations in the NH and NH2 stretching Raman peaks in the high-pressure phase, we propose that this phase transition is caused by the rearrangement of the hydrogen-bonded networks.

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INTRODUCTION Energetic materials belong to an important class of materials that contain large amounts of stored chemical energy within their molecular structures. They can undergo rapid decomposition to release energy in the presence of an external stimulus.1 Generally, energetic materials can be classified as pyrotechnics, propellants, explosives, and gas generators. These high-energy-density materials have significant applications in the field of fireworks, mining, and armaments.2−5 Therefore, energetic materials have crucial importance in industrialized civilization. Furthermore, obtaining their detailed structural information at different high-pressure and/or high-temperature conditions is important because the performance of energetic materials is governed by their solid-state structures.6 In addition, nearly all molecular crystals including the most energetic materials exhibit polymorphism under strong external stimulus as they undergo major modifications in intermolecular interactions.7−22 As a pervasive intermolecular interaction, hydrogen bonds can be profoundly changed through the presence of an external pressure.23−25 Consequently, applying a high pressure can be a very efficient tool for studying hydrogenbonded systems. Moreover, the compression of materials can facilitate close packing, which changes the balance of intermolecular interactions that leads to new molecular reorientations.26−28 In this regard, high-pressure structural studies are an excellent way of understanding and modeling the performance characteristics of energetic materials. These studies also provide the exact nature of chemical bonds, intermolecular interactions, and pressure-induced changes that result in chemical reactions, such as detonation. © XXXX American Chemical Society

Over the past few decades, extreme effort has been given to study the high-pressure properties of hydrogen-bonded energetic materials.29−32 For example, Davidson and his coworkers obtained detailed structural information for energetic salt ammonium perchlorate (AP) at pressures exceeding ∼8 GPa through a combination of X-ray and neutron diffraction.33 Subsequently, they reported that another energetic material ammonium nitrate (AN-IV) underwent an isostructural phase transition to a “metastable” phase in a nonhydrostatic compression experiment. The metastable phase has a noticeably shorter N1···O1 hydrogen bond distance, which proves to be the most compressible until the cell axis over the whole pressure range.34 As a widely used commercial explosive, 1,3,5triamino-2,4,6-trinitrobenzene (TATB) has been verified by computational modeling that the intramolecular and intermolecular hydrogen bonds become nearly equivalent at a pressure of 67 GPa.35 McWilliams et al. carried out highpressure studies on 1-methyl-5-nitriminotetrazolate (TAGMNT) and identified a displacive phase transition that occurred near 13 GPa. This transition involves conformational changes in the molecules and modifications in the hydrogen-bonded networks. This process was followed by a continuous reconstructive transition at pressures up to ∼20 GPa, which ultimately resulted in the formation of a polymeric state.36 Recently, high-pressure experiments on urea nitrate (UN), a powerful improvised explosive with a representative layered Received: July 31, 2014 Revised: September 18, 2014

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supramolecular structure, have been conducted. 37 The experimental results reveal that UN undergoes an irreversible phase transition. When the pressure is completely released, the density increases by ∼11.8%. These studies indicate that high pressure plays a crucial role for investigating the performance of energetic materials and synthesizing new phases with high density. As a component of rocket propellants, pyrotechnics, and explosive munitions, carbohydrazide (CON4H6, CHZ) has extremely good combustible characteristics. These important properties of CHZ have aroused great interest especially in recent years. Furthermore, it has always been used to design a variety of energetic coordination compounds with explosive properties because it is an azotic ligand with lone electron pairs and possesses relatively strong reduction ability.38−40 Singlecrystal X-ray diffraction analysis shows that CHZ crystal exhibits monoclinic symmetry with space group P21/n at ambient conditions.41 The lattice parameters are a = 3.72(5) Å, b = 8.83(4) Å, c = 11.96(3) Å, β = 91.97(1)°, V = 392.23(2) Å3, and Z = 4. As shown in Figure 1, the N−H···O and N−H···N

transition and the cooperativity of noncovalent interactions has been presented. This study is expected to contribute to crystal engineering for designing new materials with specific physical and chemical properties.

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EXPERIMENTAL SECTION CHZ was purchased from Alfa Aesar Co. and used without further purification. The symmetric diamond anvil cell (DAC) with 400 μm diamond culets was applied to generate a high pressure. A T301 steel gasket, placed between parallel diamond culets, was preindented to a thickness of 40 μm. A center hole with a diameter of 130 μm was then drilled as a sample chamber. Subsequently, CHZ was placed in the gasket hole together with one or two small ruby balls to determine the pressure via the R1 ruby fluorescent method. Experiments were carried out using liquid nitrogen as the pressure-transmitting medium (PTM). The ruby lines were sharp and well separated to the highest pressure. All of the experiments were performed at room temperature. High-pressure Raman measurements were obtained using an Acton SpectraPro 2500i spectrometer with a diode laser at 532 nm as the excitation source. The laser output power on the sample was maintained at 10 mW. All Raman spectra were collected using a backscattering configuration, and the acquisition time for each spectrum was 60 s. The Raman signals were recorded using a liquid nitrogen-cooled CCD camera (Pylon, 100B) with a spectral resolution set at 1 cm−1. The Gaussian and Lorentzian functions were combined for the Raman profiles. ADXRD experiments were carried out on 4W2 beamline at the High Pressure Station of the Beijing Synchrotron Radiation Facility (BSRF). A monochromatic 0.6199 Å beam was adopted for pattern collection. The beam size on the sample was ∼20 × 30 μm2. CeO2 was used as a standard sample to calibrate the geometric parameters before measurements. Bragg diffraction rings were recorded using an image plate area detector (Mar345) with an acquisition time of 300 s. Fit2D software was used to convert the diffraction rings to plots of intensity versus 2θ. Further analysis on structural evolution was performed via the Rietveld method by using the Materials Studio 5.5 Reflex program suite.

Figure 1. Ambient unit cell and hydrogen-bonded networks of CHZ. The dashed lines stand for N−H···O and N−H···N hydrogen bonds.

hydrogen bonds in its structure are linked with neighbor molecules to form hydrogen-bonded sheets. Each molecule participates in the formation of eight hydrogen bonds. Numerous hydrogen bonds strengthen the stability of the compound. The crystal is mainly constructed by these hydrogen bonds. Moreover, a relatively weak van der Waals interaction exists among molecules. Consequently, the structural stability of CHZ at high pressures is determined predominantly by the cooperativity between the two interactions. Understanding the high-pressure behavior of CHZ is expected to throw light on the nature of hydrogen bonding and the function of noncovalent interactions in the structural stability. The performance of energetic materials at high pressures will also be better understood through this behavior. In this paper, a joint in situ high-pressure Raman scattering and angle-dispersive X-ray diffraction (ADXRD) study of CHZ at pressures up to ∼15 GPa and at room temperature have been performed. Overall, CHZ undergoes a reversible phase transition from P21/n to a possible space group P1.̅ The detailed structural evolution can be obtained through the Raman analysis of external (intermolecular) and internal (intramolecular) vibration modes. In addition, the NH and NH2 stretching vibrations can provide valuable information about hydrogen bonds. Data of structural conformations in the high-pressure phase are provided by the ADXRD patterns. Moreover, analysis of the possible reason for the phase

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RESULTS AND DISCUSSION The point group symmetry of the CHZ crystal (Z = 4) is C2h(2/m). The irreducible representation of this symmetry is M = 36A g + 36A u + 36Bg + 36Bu

of which there are 3 acoustic modes Γacoustic = A u + 2Bu

and 141 optic modes Γoptic = 36A g + 35A u + 36Bg + 34Bu

Group theoretical classification of the 141 optical modes shows that the Raman-active modes belong to the 36Ag + 36Bg symmetry. The rest of them are infrared-active modes belonging to the 35Au + 34Bu symmetry. Some Raman modes could not be observed in our experiments probably because of very weak intensities. The Raman modes of CHZ are assigned based on reported studies.42,43 Figure 2 shows the summary of the Raman spectra ranging from 40 to 310 cm−1 at different pressures. In this region, B

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Figure 3. Corresponding peak positions of the external modes as a function of pressure. The shadow region represents the boundary of ambient phase I and high-pressure phase II.

Figure 2. Selected Raman spectra of CHZ in the external mode region in the range of 40−310 cm−1 at various pressures. The up arrows show the appearance of new peaks, and the down arrows denote the disappearance of peaks. Peak fitting and decomposition are obtained using a combination of Gaussian and Lorentzian functions.

the new band progressively gains intensity with ascending pressure. This increase is accompanied by the disappearance of two original modes in the plot of 10.4 GPa. When the pressure is increased to 9.7 GPa, two of the CNN deformation modes (662 and 695 cm−1) completely lose their intensities. Meanwhile, the CN stretching mode at a higher frequency vanishes, and a new peak emerges at 1084 cm−1, as shown in Figure 4b. Moreover, two modes marked with down arrows disappear in the NH2 scissoring modes; inversely, two modes with up arrows appear at 9.7 GPa. On further compression to 10.4 GPa, the NCO deformation mode shows asymmetric and splits into a doublet. Figure 5 reveals the corresponding peak positions versus pressure of the internal modes. In the range of 8.1−10.4 GPa, the slopes of some frequency−pressure curves show a clear discontinuity, which is attributed to the proposed phase transition. Below 8.1 GPa, most of the modes display ordinary blue shifts because of the decrease in interatomic distances at high pressures.45,46 However, the NH2 scissoring mode at 1618 cm−1 exhibits a red shift, and the other two modes at 1639 and 1655 cm−1 display blue shifts. These results indicate that CHZ has both strong and weak hydrogen bonds, respectively.47−50 Overall, from the internal modes, we can draw the preliminary conclusion that the phase transition involves a large change in the chemical environment around the molecules. The disappearance and appearance of peaks for the NH2 twisting and scissoring modes, as well as the change in shift rates of the NH2 scissoring mode, suggest that increasing the pressure causes NH2 to be distorted. Furthermore, changes in NCO deformation, CNN deformation, and CN stretching modes demonstrate the variation in the molecular skeleton during the phase transition. The typical Raman spectra in the range of 3000−3600 cm−1 and the pressure dependence of the corresponding modes are shown in Figures 6 and 7, respectively. Four modes can be detected at 0.1 GPa. The two bands at 3204 and 3327 cm−1 are assigned to NH2 stretching vibrations, labeled as ν1(NH2) and ν2(NH2). The two bands at 3305 and 3365 cm−1 are identified as N−H stretching vibrations, denoted as ν1(NH) and ν2(NH). At 8.1 GPa, two bands can be resolved out in the ν1(NH2) mode. Another new peak marked by the up arrow is observed

vibrations associated with the external modes are expected to be seen. The evolution of external modes can serve as an indication of structural change. The spectrum observed at 0.1 GPa consists of eight external modes (62, 69, 97, 113, 128, 136, 150, and 173 cm−1). At 8.1 GPa, a new external mode marked with an up arrow appears at 88 cm−1. This mode implies the onset of the phase transition from phase I to phase II. At 8.9 GPa, another three new modes emerge, which are also marked by the up arrows. The five modes marked by the down arrows in the curve of 9.7 GPa disappear when the pressure reaches 10.4 GPa, suggesting the phase transition is completed. No discontinuity is detected in the external modes above 10.4 GPa, which indicates that phase II is stable and does not undergo further changes up to 15.4 GPa, the highest pressure employed in this experiment. Figure 3 illustrates the pressure dependence of these external modes. An obvious discontinuous range from 8.1 to 10.4 GPa is seen in the figure, which is consistent with the proposed phase transition. Furthermore, the mode at 69 cm−1 initially shifts to higher frequency and then shifts to lower frequency until the completion of the phase transition. Similarly, the new mode at 87 cm−1 displays a red shift in the phase transition region. After the phase transition, all external modes exhibit normal blue shifts when the pressure is increased up to 15.4 GPa. These blue shifts are caused by the decrease in intermolecular distances and the increase in the strength of intermolecular interactions.44 From the evolution of the external modes, it can be expected that CHZ undergoes a phase transition between 8.1 and 10.4 GPa. Figure 4 shows the high-pressure Raman spectra representing the internal modes of CHZ in ranges of 300−800 cm−1, 800− 1300 cm−1, and 1400−1800 cm−1. Internal modes are sensitive to high pressure and can efficiently provide fundamental information about the chemical environment around a specific group. Complex behaviors of these internal modes are observed between 8.1 and 10.4 GPa. The two NH2 twisting modes (372 and 409 cm−1) evolve into triplet bands at 8.1 GPa. In addition, C

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Figure 4. Representative high-pressure Raman spectra of the internal modes of CHZ in the ranges of (a) 300−800 cm−1, (b) 800−1300 cm−1, and (c) 1400−1800 cm−1.

Figure 5. Frequency shifts of the selected internal modes ranging from 300 to 1800 cm−1 as a function of pressure. Linear fits are performed for clarity. The shadow region denotes the boundary of phase I and phase II.

Figure 6. Evolution of the NH and NH2 stretching vibrations at different pressures in the region of 3000−3600 cm−1.

at 8.9 GPa. Moreover, a new peak emerges, and the two modes with the down arrows cannot be observed upon further compression to 10.4 GPa. The new set of these modes can be interpreted as the end of the phase transition. As seen in Figure 7, in phase I, the ν1(NH2) and ν2(NH) modes reveal red shifts, whereas the ν2(NH2) and ν1(NH) modes display blue shifts. The difference in shift directions can be interpreted as different strengths of hydrogen bonds. The distance between H and O/ N atoms in weak hydrogen bonds is shortened, which results in an enhanced electrostatic attraction. Thus, the N−H distance is elongated, leading to subsequent red shifts. The blue shifts in strong hydrogen bonds are caused by the sustaining enhancement of hydrogen bonds at high pressures. The red shift of ν1(NH2) and the blue shift of ν2(NH2) are consistent with the behavior of the NH2 scissoring modes. The discontinuity in the pressure range from 8.1 to 10.4 GPa indicates that the proposed phase transition is correlated with rearrangement of the N−H· ··O and N−H···N hydrogen-bonded sheets.

ADXRD experiments were carried out up to 15.5 GPa to confirm the phase transition and to provide information about the structural variations of CHZ at high pressures. Figure 8 depicts the representative high-pressure diffraction patterns of CHZ. At 0.4 GPa, the pattern in phase I can be indexed with space group P21/n, which is coincident with a previous report.41 Below 7.9 GPa, the peaks become broader and less intense, and some merge together with increasing pressure. All of the diffraction peaks shift to high angles due to the decrease in dspacings. In the pressure range of 7.9−10.1 GPa, several indications of the phase transition are observed. At 7.9 GPa, a new peak, marked with an up arrow, emerges and gradually gains intensity. The (031) and (120) peaks completely vanish when the pressure reaches 9.2 GPa. Meanwhile, a new peak appears and continues to persist up to 15.5 GPa, the highest pressure in the experiment. The (021) peak can no longer be detected above 10.1 GPa. These changes provide sufficient D

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Figure 7. Frequency shifts of the NH and NH2 stretching vibrations as a function of pressure. The shadow region represents the boundary of the two phases.

Figure 9. (a) Rietveld refinement of the ADXRD pattern of the CHZ at ∼11.2 GPa: blue line, experimental; red dotted line, simulated; black line, residual pattern. (b) Compression of unit cell volume of phase I and phase II with respect to pressure.

∼2.2% is observed over the phase transition. It is worth noting that the volume of phase P1̅ decreases by ∼18.8% at 10.1 GPa compared to phase P21/n at ambient pressure. That is to say, the density of phase P1̅ increases by ∼23.1%. This property is crucial to energetic materials. A rocket propellant may generate strong air pressure at the moment of combustion. This air pressure might transform unburned CHZ to phase P1̅, and phase P1̅ could be expected to release more chemical energy due to the higher density. Therefore, phase P1̅ may play a bigger role in the subsequent process of rocked propellant combustion. The results of Raman and ADXRD measurements strongly demonstrate the existence of a pressure-induced structural phase transition in CHZ. On the basis of the experimental results and the proposed crystal structure of phase II, we put forward the possible reason for the phase transition. Figure 10 depicts the comparison of the structures of the two phases. At ambient pressure, hydrogen bonding and van der Waals interactions are the dominant interactions within CHZ. The cooperativity of these two interactions is expected to dictate the behavior of CHZ at high pressures. With ascending pressure, the distances between the molecules are increasingly reduced (5.5 Å at 0 GPa, 4.2 Å at 7 GPa). Hydrogen bonding and van der Waals interactions are enhanced between the CO, NH, and NH2 groups, which results in the increase in the total intermolecular interactions. As a consequence, groups of CO, NH, and NH2 start to become distorted above ∼8 GPa to keep the structure balanced. The structure cannot afford the increased intermolecular interactions with further compression between ∼8 and 10.5 GPa. Therefore, hydrogen-bonded networks are rearranged, and CHZ transforms into the new

Figure 8. Representative ADXRD patterns of CHZ at high pressures with the liquid nitrogen as PTM. The wavelength for data collection is 0.6199 Å.

evidence for the phase transition of CHZ, in accordance with the conclusion drawn from the Raman analysis. To obtain more information on phase II, the Rietveld refinement of the diffraction pattern at 11.2 GPa was conducted based on rigid body approximation, as shown in Figure 9a. For polycrystalline samples of organic molecules inside a DAC, it is always challenging that there is much background and insufficient signal-to-noise in the diffraction pattern. Thus, we propose that the phase II might belong to the crystal system of Triclinic with space group P1̅ and with the following crystal parameters: a = 5.36(3) Å, b = 5.23(7) Å, c = 7.44(6) Å, α = 62.61(4)°, β = 108.81(2)°, γ = 120.73(4)°, V = 158.58(8) Å3, and Z = 2. Upon complete release of pressure, the diffraction pattern of phase II returns to the original state, indicative of the reversibility of the phase transition. Figure 9b gives the pressure dependence of the unit cell volume. A volume collapse by E

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Figure 10. Comparison of crystal structures and hydrogen-bonded networks of CHZ: (a) ambient phase I and (b) possible high-pressure phase II. Hydrogen bonds N−H···O and N−H···N are marked as dashed lines.

behavior of the NH and NH2 stretching vibrations in Raman spectra demonstrates that the phase transition is likely attributed to rearrangement of the N−H···O and N−H···N hydrogen bonds. High-pressure investigations on CHZ can provide deeper insight into the nature of hydrogen bonding and the performance of energetic materials at high pressures.

conformation. The Raman results support the proposed reason. For example, modes related to N−H···O and N−H···N hydrogen bonds, including NH2 twisting, NCO deformation, and NH2 scissoring modes, all vary remarkably during phase transition. These variations indicate that donors and acceptors of hydrogen bonds adopt new orientations. Meanwhile, drastic changes in the NH and NH2 stretching modes significantly modify the hydrogen-bonded networks. Furthermore, the external modes are much more sensitive to intermolecular coordinates than internal modes and thus offer abundant information about pressure-induced changes in intermolecular interactions. Consequently, the disappearance and emergence of peaks in the external region suggest great changes in intermolecular interactions. As seen in Figure 10, molecules undergo distortions and rotations and move away from the original locations. Moreover, hydrogen bonds are broken, and new hydrogen bonds are formed. The number of hydrogen bonds has decreased after the phase transition. In addition, the hydrogen bonds in the high-pressure phase are shorter than those in the lower-pressure phase, as shown in Figure S1 (Supporting Information). The results suggest that hydrogen bonds exhibit an important function in the structural transition of CHZ. Although the high-pressure phase II is also a molecular crystal, the rearranged hydrogen bonds and deformed molecules can be restored by pressure release, which denotes reversible phase transition. This reversibility is attributed to the fact that only hydrogen bonds and van der Waals interactions involved along the transition can be related to small energy barriers that are easily overcome when pressure is released. Further single-crystal X-ray diffraction and neutron diffraction are necessary to determine reliable atomic positions, and computer simulations can help to interpret and predict structural changes of CHZ at high pressures.

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ASSOCIATED CONTENT

S Supporting Information *

The method of Rietveld refinement of high-pressure phase II; The molecular structure and atom label of CHZ (1). Hydrogen bond parameters of the ambient pressure phase (2) and the high-pressure phase (3), respectively (Figure S1); The C, N, and O atom positions of the proposed high-pressure phase II of CHZ from Rietveld refinement (Table S1). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86-0431-85168882. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by NSFC (Grant Nos. 91227202 and 11204101), RFDP (no. 20120061130006), National Basic Research Program of China (Grant No. 2011CB808200), China Postdoctoral Science Foundation (Grant No. 2012M511327), and Project 2014087 Supported by the Graduate Innovation Fund of Jilin University. ADXRD measurement was performed at the 4W2 beamline, Beijing Synchrotron Radiation Facility (BSRF), that is supported by Chinese Academy of Sciences (Grant Nos. KJCX2-SW-N03 and KJCX2-SW-N20).

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CONCLUSION In summary, Raman spectroscopy and ADXRD techniques have been utilized to study the changes in hydrogen-bonded energetic material CHZ at high pressures. Both Raman and ADXRD data show that compression causes a reversible structural phase transition in the pressure range of ∼8 to 10.5 GPa. The ADXRD data are consistent with a high-pressure structure change from P21/n to a possible space group P1̅. The density of phase P1̅ increases by ∼23.1% at 10.1 GPa compared to phase P21/n at ambient pressure. Phase P1̅ could be expected to release more chemical energy in a subsequent process of rocket propellant combustion. In addition, the

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