New Assembly of Acetamidinium Nitrate Modulated by High Pressure

Article pubs.acs.org/JPCC

New Assembly of Acetamidinium Nitrate Modulated by High Pressure Shourui Li,† Qian Li,† Rui Li,‡ Jing Liu,‡ Ke Yang,§ 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 § Shanghai Synchrotron Radiation Facilities, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201204, China ‡

S Supporting Information *

ABSTRACT: High pressure is an essential thermodynamic parameter in exploring the performance of condensed energetic materials. Combination of high-pressure techniques and supramolecular chemistry opens a new avenue for synthesis of high energy density materials. Herein, we fabricate a new high-pressureassisted assembly of energetic material acetamidinium nitrate (C2N2H7+·NO3−, AN) with P-1 symmetry after a 0−12 GPa−0 treatment at room temperature, which exhibits a density that is 9.8% higher than that of the initial P21/m phase. Evolution of intermolecular lattice modes in Raman spectra and synchrotron Xray diffraction (XRD) patterns provide strong evidence for this transition in the 1.3−3.4 GPa range. The mechanism involves relative motions between ionic pairs in the hydrogen-bonded array and distortions of building blocks.

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INTRODUCTION The search for energetic materials with superior properties is an unfailing subject for scientists. Many factors should be considered once the specific material is applied for practical applications, including energy, safety, cost, etc. One such vital parameter is density, as the detonation velocity of products is proportional to density on the basis of the first approximation.1 Designing energetic materials with targeted functions has received intense interest. Particularly, the successful synthesis of 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (HNIW) has inspired scientists tremendously since 1987, the density of which is as high as 2.04 g/cm3.2 The current goal of high-energy material research is to develop more safe, insensitive, and powerful explosives, oxidizers, energetic plasticizers, and binders.3 Special attention has been focused on eco-friendly synthesis of heterocycles, nitramines, high nitrogen content materials, etc.4,5 Recently, the concept of supramolecular chemistry has been applied for designing new assemblies of energetic materials with optimized properties, including TNT and HNIW-based cocrystals, as well as the cocrystal of TNT and HNIW.6−8 As we know, high pressure can decrease the distance between neighboring atoms and thus generate high density, especially for condensed molecular crystals9−11 or molecular ion systems12−14 because of noncovalent interactions in the structure. In addition, the extreme conditions from ambient conditions to the high pressure of 50 GPa and temperature of 5500 K can be realized during the detonation process.1 Accordingly, energetic materials can experience many changes, such as polymorphic transition and © XXXX American Chemical Society

the ultimate decomposition reaction into gases at very high pressure and temperature. In situ high-pressure studies are vital for understanding the performance of energetic materials under these extreme conditions. High-pressure behaviors of ammonium nitrate,13 ammonium perchlorate,14 HNIW,15 1,1diamino-2,2-dinitroethylene (FOX-7),16 cyclotetramethylene tetranitramine (HMX), 17,18 pentaerythritol tetranitrate (PETN),19,20 cyclotrimethylene trinitramine (RDX),11,21 and 1,3,5-triamino-2,4,6-trinitrobenzene (TATB)22,23 have recently been monitored using diffraction and vibration methods. The thorough knowledge of fundamental information under thermodynamic conditions is crucial for understanding the performance of energetic materials during the detonation process and in turn can provide more insight into designing and synthesizing materials. Supramolecular chemistry is “chemistry beyond the molecule”, and has drawn considerable interest of chemical and material scientists.24,25 The cooperativity of noncovalent interactions is responsible for the supramolecular architecture formation and its characteristics, including hydrogen bonding, π-stacking, electrostatic, and van der Waals interactions. Pressure has proven to be a powerful tool for studying these noncovalent interactions within organic crystals, covering from amino acids,26 pharmaceutical27 and hydrogen-bonded28 compounds, to suprmolecular assemblies.29,30 Meanwhile, as a Received: April 9, 2014 Revised: September 2, 2014

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dx.doi.org/10.1021/jp503495k | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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Figure 1. Ambient structure of AN: (a) unit cell and (b) hydrogen-bonded array in ac plane. The dashed lines stand for N−H···O hydrogen bonds, and the shadow in panel b denotes ion pairs.

clean “catalyst”, pressure has already been extensively utilized in accelerating novel chemical reactions and synthesizing unique materials.31−34 The studied organic crystals usually comprise a single component (molecule); however, supramolecular chemistry deals with organized assemblies of two or more molecules or molecular ions. Here, we propose high-energy density materials can be synthesized through combining supramolecular chemistry and high-pressure techniques. On one hand, supramolecular architectures are composed of two or more components via noncovalent interactions. This softness endows them with great flexibility and offers more possibilities of transformations from low-density to high-density phases at high pressure. On the other hand, high pressure can modulate the cooperativity of various noncovalent interactions in supramolecular systems. For example, behaviors of hydrogen bonding and π-stacking interactions in ammonium squarate are “separated” by high pressure, as the first phase transition arises from rearrangements of hydrogen bonds, while the second one is dominated by π-stacking without much impact of hydrogen bonds.35 We believe the strategy of pressure-assisted assembly can be utilized for designing and preparing functionalized materials. Recently, we have examined responses of energetic materials guanidinium nitrate (GN) and urea nitrate (UN) to high pressure. Although rearrangements of hydrogen bonds in GN is evidenced at ∼1 GPa, the transformation is reversible.36,37 Replacing amino group in GN with hydroxyl generates the energetic material UN, which has been used in several attacks. UN experiences a sluggish phase transition in the 9−15 GPa range. Importantly, the new assembly can be reserved to ambient pressure and room temperature, which shows ∼11.8% increase in density.38 However, one problem is the critical range for this transition seems somewhat high in practical applications. In this work, we tailor the cation with methyl group replacing hydroxyl and prepare the energetic material acetamidinium nitrate (C2N2H7+·NO3−, AN).39 Under ambient conditions, AN crystallizes in space group P21/m with Z = 2 in the unit cell. The lattice parameters are a = 6.457(2) Å, b = 6.442(2), c = 6.884(1), and β = 97.56(2)°.40 As shown in Figure 1, arrangements of ion pairs construct the hydrogenbonded array in the ac plane, while adjacent arrays stack along the b-axis in alternating orientations with mirror symmetry. The motivation for this work is based on the consideration that the hydroxyl in UN acts as hydrogen bond donors, while the methyl group in AN does not participate in hydrogen bonds. Such tailoring can bring down the stability of hydrogen-bonded arrays and lower the critical pressure for corrugation of arrays. This can offer advantageous conditions for new assembly formation. The responses of AN to high pressure can provide new insight into the synthesis of high-energy density materials

following the strategy of high-pressure-assisted assembly in combination with supramolecular chemistry. In addition, we attempt to obtain more insight into the structure−activity relationships within supramolecular architectures by comparing high-pressure behaviors of GN, UN, and AN. Raman scattering and synchrotron X-ray diffraction (XRD) have proven to be routine methods in monitoring condensed molecular materials at high pressure. The small modification through phase transitions can give rise to enormous changes in lattice vibrations because of different symmetries and selection rules. High-resolution synchrotron powder XRD data can be used to determine the crystal structure of the obtained new assembly with the aid of Raman scattering.

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EXPERIMENTAL SECTION Synthesis of C2N2H7+·NO3−. In the typical procedure, 0.01 mmol of Co(NO3)2·6H2O and 0.04 mmol of ketoxime ((CH3)2CNOH) were dissolved in 20 mL of acetonitrile (CH3CN).41 The solution was stirred at 60 °C for 8 h. The obtained material was washed with acetone three times. Then AN crystals were dried in an oven at 40 °C. The identity and crystallinity of AN were ensured by laboratory XRD (Cu Kα, 1.5406 Å), synchrotron XRD patterns, and by comparing experimental and simulated Raman spectra under atmospheric pressure (Figure S1 of the Supporting Information). High-Pressure Experiments. Flawless crystals were selected and ground to powder of a few micrometers using a mortar. The obtained powder was used for in situ high-pressure experiments at room temperature. High pressure was generated by a symmetric diamond anvil cell (DAC) equipped with 400 μm culet diamonds. A T301 steel gasket was placed between parallel diamond culets, which had been preindented to 45 μm. A 130 μm diameter hole was drilled in the gasket center and used as the sample compartment. We used ruby R1 line for pressure calibration.42 Experiments were carried out using silicone oil (5 cSt viscosity) as the pressure-transmitting medium (PTM) and without any PTM because methanol and ethanol can dissolve AN quickly (∼2 s). Lattice modes and synchrotron X-ray diffraction (XRD) patterns were also collected when nitrogen was adopted as PTM (see Figures S2 and S3 of the Supporting Information). The structural evolution is the same in essence as that with silicone oil as PTM and without any PTM. The hydrostatic pressure condition was guaranteed on the basis of Pascal’s law when silicone oil was employed. When no PTM was loaded, the quasi-hydrostatic pressure could be gained in view of AN softness. All of the experiments were conducted at room temperature. Raman spectra were detected by an ARC-SP 2558 spectrograph (Princeton Instruments) furnished with a charge coupled detector (CCD) PyLon:100B. The excitation line was 532 nm B



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from a diode pumped solid state (DPSS) laser. The silicon (520 cm−1), diamond (1332 cm−1), as well as indene (2892.2 cm−1, 3054.7 cm−1) lines were used for spectrometer calibration before measurements. All of the Raman spectra were collected with backscattering geometry, and the acquisition time for each spectrum was 60 s. The overlapped CH and NH vibrations, as well as lattice modes, are fitted using a combination of Lorentzian and Gaussian line shapes. Synchrotron XRD experiments were conducted on the 4W2 beamline at the High Pressure Station of the Beijing Synchrotron Radiation Facility (BSRF). The beam size was ∼20 × 30 μm2 on the sample. The wavelength was 0.6199 Å. We chose CeO2 as the standard for geometry calibration before measurements. Bragg diffraction rings were recorded using an image plate area detector (Mar345) with the acquisition time of 400 s and were converted to plots of intensity versus 2θ using software Fit2D.43 Further analysis of XRD data was performed with commercial software Materials Studio 5.5. Calculation Details. Raman spectra were calculated with pseudopotential plane wave method based on density functional theory in CASTEP code with Materials Studio 5.5.44 Local density approximation exchange-correlation functional and norm-conserving pseudopotential were used. BFGS algorithm was applied for structural optimization. Convergence tests gave the plane-wave cutoff energy of 830 eV. The Monkhorst−Pack grid for the electronic Brillouin zone integration was 2 × 2 × 2. The simulation was not finished until the forces on the atom were less than 0.01 eV/Å and the stress components less than 0.02 GPa. The self-consistent field (SCF) tolerance was set as 5 × 10−1 eV/atom.

Figure 2. Representative ADXRD patterns of AN at high pressures: (a) with silicone oil as PTM and (b) without any PTM. (c) Rietveld refinement of the retrieved pattern with respect to the new assembly with unit cell depicted as the inset. The pattern in panel c is obtained by subtracting the background of the released pattern in b.

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RESULTS AND DISCUSSION Angle-dispersive X-ray diffraction (ADXRD) measurements were performed to monitor structural properties of AN at high pressures (Figure 2). Below 1.3 GPa, all of the diffraction peaks shift to high angles because of d-spacing reduction upon compression as expected. When silicone oil is used (Figure 2a), there are many changes in the pressure range 1.3−3.4 GPa. For example, the new peak at 2θ = 6.09° marked by the asterisk emerges at 1.3 GPa. As illustrated in the inset in Figure 2a, another new peak with the up arrow intensifies gradually with increasing pressure, whereas the peak marked by the down arrow obviously weakens. These changes are continuous and indicate AN adopts a new assembly above 3.4 GPa (denoted as phase II). Additionally, examination of Figure 2a,b indicates that the transition process seems not to be influenced by pressure conditions, which benefits practical applications. This also demonstrates that layered supramolecular architectures may offer quasi-hydrostatic pressure conditions due to their softness. The only distinction is that some peaks are too weak to be observed when no PTM is applied (Figure 2b), which arises form preferred orientation of polycrystalline in two different experiments. Compared with that of urea nitrate,28 the critical pressure range for AN has indeed been lowered by simply changing substituent groups, as we expected. Importantly, the new assembly is fully recoverable to ambient conditions. Because diffraction patterns of the two assemblies do not have much obvious difference, we consider this transformation as a distortion type. In particular, the (020) peak, which reflects diffraction among hydrogen-bonded arrays in initial assembly motif, does not experience remarkable variation over the entire pressure range. This result implies that the new assembly

consists of quasi two-dimensional (2D) hydrogen-bonded networks and that phase II possesses a distorted state of phase I. To obtain reliable structural information, we performed Rietveld refinement of the retrieved pattern based on rigid body approximation,45 presented in Figure 2c. The pattern is obtained by subtracting the background of the released pattern in Figure 2b. The results suggest the new assembly has P-1 symmetry. The indexed parameters are a = 5.930(1) Å, b = 6.645(3) Å, c = 6.882(1) Å, α = 80.845(3) °, β = 78.139(4) °, and γ = 79.066(1) ° with Z = 2. The validity of the resolved structure is also supported by Raman analysis in the latter discussion. The comparison between these two assemblies is displayed in Figure 3. The figure demonstrates that the quasi 2D hydrogen-bonded array is reserved in the new assembly and relative arrangements between adjacent arrays in two cases are similar to each other. However, there are two important distinctions indicative of pressure-assisted assembly. The uppermost assembly process is the building blocks deviating from ideal hydrogen-bonded arrays (Figure 3a,b). Another characteristic is adjacent ion pairs in the array experiencing small slippage during assembly formation (Figure 3c,d), accompanied by the formation of new hydrogen bonds. These features endow the new assembly with higher packing efficiency. Actually, the density of the new assembly is 1.556(3) g/cm3, ∼9.8% higher than that of the ambient structure. Because the detonation velocity is proportional to the density, C



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Figure 3. Comparison of structures with regard to two assemblies of AN. Panels a and b illustrate the relative orientations of hydrogen-bonded arrays in phases I and II, respectively. Panels c and d display the arrangements of ionic pairs in hydrogen-bonded arrays in phases I and II, respectively.

nearly consistent with that of ADXRD results. The profiles of lattice region remain unchanged above 3.6 GPa, indicating the new assembly is stable up to 12.5 GPa. The slopes in phase II are lower than those in phase I, suggesting the new assembly possesses a closer packing (Figure 4b). Overall, lattice modes provide obvious evidence for new assembly formation. This sluggish transition does not bring about abrupt changes in the lattice region, consistent with the distortion nature of the pressure-assisted assembly. The variations of internal modes can offer vital information on chemical environment around specific groups. Selected Raman spectra and peak positions in the 420− 1020 cm−1 and 1020−1220 cm−1 ranges are described in Figure 5. CN deformation exhibits red shift initially, followed by blue shift above 3.1 GPa (Figure 5a). This process reveals the acetamidinium cation undergoes a conformation change during new assembly formation, similar to deformation of uronium cation in UN under high pressure.28 In Figure 5c,d, the C−CH3 stretching mode has the highest shift rate among internal vibrations, implying the chemical surrounding can be easily disturbed by external pressure. This stems from the large void space around methyl group, accounting for the high rate of 3.3 cm−1/GPa in the initial phase. Interestingly, this vibration in the new assembly has a higher shift rate of 4.3 cm−1/GPa, suggesting the C−CH3 group evolves into more released surroundings. In Figure 6, significant modifications with CH stretching modes occur at 1.3 GPa. The first change is the splitting of the dominant vibration, and the second is the emergence of a new vibration marked by the up arrow. With increasing pressure, the intensity of the new peak is enhanced, whereas the peak marked by the asterisk weakens progressively. These features indicate obvious local modifications of methyl groups, in line with the methyl deviation from ideal hydrogenbonded arrays during new assembly formation. Now that NH radicals participate in hydrogen bonds acting as donors, their responses to high pressure play an important role in understanding arrangements of building blocks. Three modes are resolved in DAC (Figure 6b). The v1 mode mainly

this new assembly is expected to possess higher detonation power. The new assembly formation itself can also be corroborated by Raman scattering analysis; in particular, the distribution in the lattice region is sensitive to symmetry change. Evolution of lattice modes of AN and corresponding peak positions versus pressure are depicted in panels a and b of Figure 4, respectively. A new peak marked by the up arrow emerges in the plot of 1.4 GPa and intensifies to 3.6 GPa. The critical pressure range is

Figure 4. (a) Summary of selected Raman spectra in the lattice modes region in the 60−400 cm−1 range. (b) Corresponding peak positions as a function of pressure. D



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Figure 5. Selected Raman spectra at high pressures in the (a) 420−1020 cm−1 and (b) 1020−1220 cm−1 ranges. Panels c and d give peak positions at various corresponding pressures.

vibration exhibits red shift initially, followed by blue shift above 0.6 GPa (Figure 6c). The other two display blue shifts only. The difference in shift directions (red or blue shift) is interpreted as different strengths of hydrogen bonds.46 For weak or medium hydrogen bonds, electrostatic interaction between H and O is enhanced, accompanied by distance contraction between N and O on compression. This enhancement process can trigger elongation of NH distance, so as to compensate for compression effect, resulting in red shift correspondingly. Only blue shifts are expected with regard to strong hydrogen bonds because of sustaining enhancement. On the whole, no apparent discontinuity is detected for the v1 mode; however, the v3 mode weakens gradually and cannot be traced anymore above 4.1 GPa (Figure 6b). Meanwhile, a new shoulder vibration is distinguished (Figure 6b at 4.1 GPa). Hence, we infer that ion pairs do not undergo large changes during pressure-assisted assembly, whereas subtle slippage between adjacent ion pairs is evidenced. These variations are verified by the structural model of the new assembly derived from ADXRD analysis, and vice versa. Released Raman spectrum of AN in the lattice region (60− 290 cm−1) as well as in the CH vibrations region (2780−3100 cm−1) is summarized in panels a and b of Figure 7, respectively. As can be seen, the released spectrum exhibits features of highpressure phase, including the vibrations labeled by the rhombus (Figure 7a) and the asterisk (Figure 7b). In addition, the recovered sample was stable upon consecutive compression (see Figure S4 of the Supporting Information for more details about the evolution of external modes of the recovered sample with pressure). Routine ab initio calculations offer essential assistance in characterizing structural changes in molecular assemblies. Especially, CH and NH stretching vibrations give vital information on molecular connectivity. Figure 8 compares the observed and simulated CH and NH vibrations of the two

Figure 6. Evolution of CH stretching (a) and NH stretching vibrations (b) at high pressures. The plots of peak positions versus pressure are given in panel c.

involves NH stretching within ion pairs, while the other two are correlated with vibrations among neighboring ion pairs. The v2 E



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substituent replacement. These results demonstrate that through simple tailoring of building blocks we can control features of high-pressure-assisted assembly, such as from reversible to irreversible phase transition, as well as reduction of critical transition pressure with respect to energetic materials GN, UN, and AN.

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ASSOCIATED CONTENT

* Supporting Information S

Comparison of experimental and simulated Raman spectra of ambient phase of AN (Figure S1), evolution of AN lattice modes at high pressures with nitrogen as PTM (Figure S2), selected ADXRD patterns of AN at high pressures with nitrogen as PTM (Figure S3), and typical Raman lattice modes of the recovered AN under high pressure (Figure S4). This material is available free of charge via the Internet at http:// pubs.acs.org.

Figure 7. Raman spectrum of AN: (a) the lattice modes region and (b) CH vibrations upon release of pressure.

assemblies in Raman spectra. Our calculations can be conducive to our understanding of CH and NH vibrations. The new assembly exhibits a new intense CH vibration, originating from distortion of methyl groups, marked by the asterisk in Figure 8b. NH vibrations indicative of relative slippage between adjacent ion pairs are also reproduced computationally. As is well-known, LDA functional in DFT underestimates primitive cell volumes. The error mainly arises from the inadequacy of describing weak van der Waals interactions with DFT, which has important influence on vibrational properties of molecular/ molecular ion crystals. In addition, the Raman spectra calculations with CASTEP were performed at zero temperature, while the experiments were conducted at room temperature. These factors can account for the difference between the observed and simulated results. In brief, the general agreement in variation tendency of peak positions and intensities is satisfactory, suggesting the correctness of the assembly structure.

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AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by National Natural Science Foundation of China (91227202 and 21073071), RFDP (20120061130006), and the National Basic Research Program of China (2011CB808200) and Project 2014050 Supported by Graduate Innovation Fund of Jilin University. Synchrotron XRD experiments were performed at 4W2 HP-Station, Beijing Synchrotron Radiation Facility (BSRF), which is supported by Chinese Academy of Sciences (Grant KJCX2-SW-N20, KJCX2SW-N03). Portions of this work were performed at beamline 15U1 at the Shanghai Synchrotron Radiation Facility (SSRF).

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CONCLUSION A new assembly of AN has been fabricated after a 0−12 GPa−0 treatment. Both Raman scattering and ADXRD measurements offer strong evidence for the symmetry transformation from P21/m to P-1. We view this transition as a distortion type because the quasi 2D hydrogen-bonded networks are reserved in the new assembly. Meanwhile, the new assembly has a density that is 9.8% higher than that of the ambient assembly, leading to a higher detonation velocity. Importantly, the critical pressure range has dropped to 1.3−3.4 GPa through simple

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Figure 8. Observed (a) and simulated (b) CH and NH Raman modes of the new assembly; panels c and d denote the CH and NH vibrations of initial assembly of AN. F



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