High-Pressure-Induced Reversible Phase Transition in Sulfamide

Article pubs.acs.org/JPCC

High-Pressure-Induced Reversible Phase Transition in Sulfamide Kai Wang,† Jing Liu,‡ Ke Yang,§ Bingbing Liu,† and Bo Zou*,† †

State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, People’s Republic of China Beijing Synchrotron Radiation Laboratory, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100039, People’s Republic of China § Shanghai Synchrotron Radiation Facilities, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201204, China ‡

ABSTRACT: Diamond anvil cells combined with Raman spectroscopy and synchrotron X-ray diffraction were used to analyze the compression behavior of sulfamide, a hydrogen-bonded crystal. The marked changes in the Raman spectra at approximately 5 GPa strongly suggest a structural phase transition associated with the rearrangement of hydrogen bonds. Results from angle-dispersive X-ray diffraction confirmed this pressureinduced phase transition, and the high-pressure phase was indexed and refined to a low-symmetry monoclinic structure with space group I2/m. Further phase transitions were not detected up to the maximum pressure of 10 GPa. The observed transition was completely reversible when the pressure was released to ambient conditions. The results from ab initio calculations reveal that the phase transition was mainly caused by changes in the hydrogen-bond networks in sulfamide.

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INTRODUCTION Organic molecular crystals have attracted much attention because of their impacts in scientific research and vast technical applications.1,2 The properties of these crystals depend on the nature of component molecules and molecular arrangement.3 Compared with the covalent bonds within each molecule, weak intermolecular interactions (e.g., electrostatic interaction, hydrogen bond, π−π stacking, and van der Waals interaction) essentially determine the attributes of molecular crystals.4 The hydrogen bond, an important and intensively studied interaction, efficiently stabilizes the crystal structures of organic molecules because of its directionality, specificity, and cooperativity.5 Considering the geometry of the hydrogen bond and the cooperativity of multiple interactions that can be markedly altered by external forces, high-pressure techniques can be used to analyze efficiently hydrogen-bonded molecular crystals and lead to structural transitions.6,7 High-pressure crystal analyses can clarify the nature of hydrogen bonds and intermolecular interactions as well as probe the polymorphism and pressure-induced phase transitions. Studies of the high-pressure behavior of hydrogen-bonded molecular crystals have increased in recent years. 8−13 Researchers have analyzed the high-pressure polymorphism and changes in hydrogen-bond networks in amino acids.14,15 The high-pressure characterization of organic energetic materials is closely related to the hydrogen bonds.16−18 Piezochromic materials have also recently attracted scientific interest.19,20 The corresponding high-pressure results indicate that the changes of molecular aggregation state and the consequent hydrogen-bond networks significantly influenced the fluorescence emission properties. The authors have previously analyzed the high-pressure effects on hydrogenbonded molecular systems21−26 and shown that the crystal © 2014 American Chemical Society

structure and properties can be modified, as well as completely changed, by adjusting the intermolecular hydrogen bonds under high pressures. However, in-depth analyses are necessary to establish the mechanisms of pressure-induced phase transitions and changes in hydrogen-bonded networks. Sulfamide (NH2SO2NH2) is an essential component of proton-vacancy conducting polymers that can function under high pH.27 Sulfamides comprise a large family of synthetic drugs with bacteriostatic activities that are used to diagnose infectious diseases.28 Single-crystal X-ray diffraction (XRD) analysis shows that sulfamide crystals at ambient pressures exhibit orthorhombic symmetry with space group Fdd2, and each unit cell contains eight tetrahedral molecules with an approximate mm2 symmetry (Figure 1).29 The sulfamide molecules in the crystals are hexagonally arranged and form infinite layers parallel to (010). In this molecular layer, one molecule is connected to four neighbors through equivalent weak N−H···O hydrogen bonds (bond length, 3.02 Å). The adjacent layers are linked by very weak N−H···N hydrogen bonds (bond length, 3.18 Å). Neutron diffraction measurements suggest that the amide groups adopt trigonal-pyramidal configurations in sulfamide crystals.30 One hydrogen atom in the amide group participates in the relative strong intralayer N−H···O hydrogen bonds, while the other hydrogen atom participates in the weak interlayer N−H···N hydrogen bonds. A previous study has employed Raman spectroscopy to explore hydrogen bonds in crystalline sulfamide.31 Considering the practical applications and unique hydrogen-bond networks of sulfamide, high-pressure techniques are necessary to investigate Received: May 12, 2014 Revised: July 18, 2014 Published: July 28, 2014 18640

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In this study, high-pressure Raman scattering and synchrotron angle-dispersive X-ray diffraction (ADXRD) were performed to determine the pressure-induced phase transition of sulfamide. Raman spectroscopy is highly sensitive to changes in the geometry of hydrogen bonds and high-pressure modifications in the local environment of sulfamide molecules.32 Synchrotron radiation-based ADXRD provides information on the high-pressure crystal structures of sulfamide.33 Ab initio calculations were also employed to determine the mechanisms of the experimentally observed results. The main purpose of this work is to further understand the nature of hydrogen bonds, evolution of hydrogen-bonded networks, and high-pressure stability of organic molecular crystals.

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EXPERIMENTAL SECTION Sulfamide (purity, 99%) was obtained from Alfa Aesar and used without further purification. The sample was then ground to a fine powder using a mortar and pestle. A symmetric diamond anvil cell (DAC; culet diameter, 0.4 mm) was employed to generate high pressure. A preindented type 301 stainless steel gasket (thickness, 0.04 mm; hole diameter, 0.14 mm) was used as the sample chamber. The powder sample and a ruby ball, which was used to measure pressure,34 were packed into the sample chamber using argon as the pressure-transmitting medium. All experiments were performed at room temperature. A spectrometer (focal length, 500 mm) combined with a liquid nitrogen-cooled CCD (Acton SP-2500 and PyLoN:100B, Princeton Instruments) was utilized to acquire high-pressure

Figure 1. Ambient pressure crystal structure and hydrogen-bond networks of sulfamide. Red and blue dot lines represent intralayer N− H···O hydrogen bonds and interlayer N−H···N hydrogen bonds, respectively.

their hydrogen bonds, molecular arrangements, and crystal structures.

Figure 2. Pressure evolution of Raman spectra in the spectral regions of (a) 50−1280, (b) 1420−1780, and (c) 2800−3500 cm−1. The arrows denote the appearance of new peaks, and the stars highlight the splitting of peaks. 18641

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Raman spectra. A 532 nm single-mode DPSS laser (power output, 50 mW) was used as the light source for Raman excitation. High-pressure synchrotron ADXRD experiments were conducted at the 4W2 high-pressure station of the Beijing Synchrotron Radiation Facility and repeated at the 15U1 beamline of the Shanghai Synchrotron Radiation Facility. XRD data were recorded using an imaging plate detector and transformed into one-dimensional XRD patterns with the FIT2D program.35 The high-pressure XRD results were then analyzed using the powder reflex module in the Materials Studio Software. Ab initio calculations of the high-pressure crystal structure and hydrogen bonds were performed using a pseudopotential plane wave based on the density functional theory; these calculations were implemented in the CASTEP code.36

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RESULTS AND DISCUSSION The sulfamide Raman spectrum that was obtained at ambient conditions agrees very well with that previously reported by Lucazeau et al.31 The assignments of the Raman bands are shown in Figure 2. The evolutions of sulfamide Raman spectra in the vibrational ranges of 50−1300, 1420−1780, and 2850− 3500 cm−1 with increasing pressure are illustrated in Figure 2a− c, respectively. The Raman spectra at approximately 5 GPa underwent multiple simultaneous changes, which demonstrated that the sulfamide exhibits a pressure-induced phase transition (phase I → phase II). The Raman spectra of the high-pressure phase (phase II) failed to discontinuously change with increasing pressure, indicating the stability of this phase up to the maximum pressure (10 GPa). The high-pressure phase reverted to the original phase at approximately 2.5 GPa, as indicated by the Raman spectra on decompression. The pressure-induced phase transition of sulfamide was completely reversible with a hysteretic pressure of 2.5 GPa. The hysteretic decompression indicates a first-order structural transition, and the energy barrier between phases I and II was relatively large. The pressure-induced frequency shifts for all the Raman peaks (Figure 3) implied the occurrence of the phase transition. Most Raman peaks exhibited a discontinuous shift, and the slope of several peaks abruptly changed at 5 GPa. Furthermore, several new peaks were observed, and some existing modes markedly split at the transition pressure. Although the detailed phase II crystal structure was not clearly observed, high-pressure Raman spectral analysis provided valuable information on the pressureinduced phase transition. The external modes of molecular crystals are highly sensitive to changes in the crystal structure. At ambient conditions, the sulfamide Raman spectrum possesses six external modes, but only three relative strong peaks (48, 72, and 88 cm−1) can be clearly resolved under high pressure. Given that intermolecular hydrogen bonds (e.g., N−H···O and N−H···N) play a key role in crystalline sulfamide, most of the external modes are involved in the hydrogen-bond vibrations.31 The pressureinduced evolution of the external modes is shown in Figure 2a. All the three low-frequency peaks exhibited a substantial blue shift with increasing pressure, while the shift rates were different. High pressure reduced the distances of adjacent molecules and increased the intermolecular interactions that resulted in the observed frequency blue shift of the external modes.37,38 The spectral shape of the external modes noticeably changed at approximately 5 GPa, thereby implying the phase transition. Three new high-intensity Raman peaks were observed with high intensities at 99, 142, and 155 cm−1

Figure 3. Frequency shift of the Raman modes as a function of pressure. The vertical dashed line denotes the occurrence of phase transition.

(indicated with arrows). However, the intensities of the original peaks were significantly reduced and then eventually disappeared. The three high-pressure Raman bands remained until 10 GPa, except for the frequency blue shift. The changes in the external Raman modes directly revealed the appearance of phase II at 5 GPa. The internal Raman modes of sulfamide molecules also changed significantly at 5 GPa, which confirms the occurrence of the pressure-induced phase transition. The Raman peaks from 200 to 1800 cm−1 are assigned to the internal framework modes according to the previously reported literature. Most of the observed Raman peaks in this frequency range continuously shifted to high wavenumbers with increasing pressure (Figure 2b). The length of the covalent bonds in sulfamide molecules decreases upon the application of static pressure, and the corresponding effective force constants increase and yield blueshifted internal Raman peaks.39,40 At 5 GPa, the Raman spectrum considerably changed and new characteristics have emerged. The δSO2, νsSN 2, νs SO2, and δNH2 modes significantly split at 528, 908, 1150, and 1557 cm−1, respectively. The crystalline symmetry was reduced after phase transition, which indicates the splitting of the Raman peaks. Three new peaks at 314, 361, and 431 cm−1 (indicated with arrows) have emerged, and several shifts in the peaks (e.g., νsSN2 mode at 908 cm−1) are discontinuous. The intensities of some peaks significantly decreased (e.g., δSN2 mode at 360 cm−1), and some even completely disappear at 5 GPa (such as the tO2SN2 mode at 435 cm−1). The frequencies of the δNH2 mode at 1557 cm−1 gradually decreased above 5 GPa. This red shift is attributed to the strengthened hydrogen bonds at high pressures.41 The results from internal Raman mode analysis indicate that the molecular conformation, local environment of 18642

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and broadened at phase transition and some peaks (such as [040]) disappeared gradually above this pressure, suggesting that the crystallinity of the sulfamide sample was decreased after phase transition. The high-pressure phase (phase II) remained stable upon compression up to 10.8 GPa, the highest pressure in the XRD experiments. Although the poor quality of the highpressure XRD patterns prevented satisfactory Rietveld refinement to obtain reliable atomic positions, a Pawley refinement yielded a good fitting for the XRD patterns of phase II (Figure 5). The crystal structure of phase II could be indexed and

sulfamide molecules, and hydrogen bonds in the sulfamide crystal remarkably changed during phase transition. Considering that all amine groups in sulfamide participate in hydrogen bonds, the high-pressure behavior of N−H stretching modes should be specified. The N−H stretching Raman peaks (3099, 3222, 3256, and 3332 cm−1) with a shift to low frequencies with increasing pressure is shown in Figure 2c. These red-shifted peaks are in agreement with the rule that high pressures strengthen the electrostatic attraction in hydrogen bonds (D−H···A) and increase the covalent D−H bonds that reduce the D−H stretching frequencies.41−43 The strong intralayer N−H···O and weak interlayer N−H···N bonds in the sulfamide crystal were relatively strengthened under high pressures. The application of approximately 5 GPa of pressure resulted in substantial changes in the Raman bands, which are associated with the phase transition. The four original Raman peaks disappeared, and three new Raman peaks are observed at 3248, 3308, and 3357 cm−1. Considering all the amine groups involved in intermolecular hydrogen bonds and the abrupt changes in N−H stretching modes, the hydrogen-bond networks significantly rearranged after phase transition. The Raman peaks at 3248 cm−1 were continuously red-shifted, whereas those at 3308 and 3357 cm−1 exhibited unnoticeable shifts in frequency with increasing pressure. To analyze the pressure-induced phase transition, highpressure synchrotron ADXRD experiments for sulfamide were conducted. The pressure-dependent variations in the XRD patterns are illustrated in Figure 4. All the diffraction peaks of

Figure 5. Pawley refinements of the diffraction patterns collected at 6.2 GPa. Dots and solid lines represent observed and simulated profiles.

refined to a monoclinic structure with space group I2/m. The refined lattice parameters were as follows: a = 5.37(4) Å, b = 6.12(3) Å, c = 5.15(6) Å, and β = 88.12(1)°, and the cell volume was 169.5(5) Å3. The symmetry of monoclinic phase II was lower than that of the orthorhombic phase I, which is in accordance with the splitting results of Raman spectral analyses. The XRD patterns after decomposition to ambient pressure reveal that the high-pressure phase was completely recovered to its original crystal structure. Overall, the results for highpressure XRD analyses revealed that sulfamide undergoes reversible pressure-induced phase transitions at around 5 GPa, which are similar to those of Raman investigations. The results for Raman and synchrotron ADXRD experiments strongly indicate the existence of a pressure-induced phase transition in sulfamide. However, existing experimental data do not provide detailed information on the geometry of hydrogen bonds at high pressure. Therefore, ab initio calculations have been performed to explore the high-pressure behavior of hydrogen-bond networks and investigate the mechanism of phase transition. The present results show that high-pressure variations mainly occur at the intralayer N−H···O hydrogen bonds. The patterns of hydrogen bonds considerably evolve in the molecular layers parallel to (010) (Figure 6). The highpressure pattern was formed by the distortion of the original N−H···O hydrogen bonds (parallel to the a-axis) and formation of new N−H···O hydrogen bonds between sulfamide molecules along the c-axis. Results from calculations reveal that, below 5 GPa, the applied pressure continuously reduces the molecular distances and strengthens the hydrogen bonds. When the pressure reaches 5 GPa, the balance between hydrogen bonds and close packing effects is broken, and the crystal structure becomes unstable. Therefore, phase transition and rearrangement of hydrogen-bond networks occur to achieve close molecular packing and reduce the total free energy of the system.

Figure 4. Representative ADXRD patterns at elevated pressures. Indications for a phase transition are marked by a star and arrows.

the orthorhombic sulfamide crystals shift to high angles upon compression, which suggests the reduction in the volume and intermolecular distance. The abrupt changes in the XRD patterns at approximately 5 GPa indicate phase transition. One new diffraction peak was observed at 5.2 GPa, in which its intensity gradually increased with pressure (star and arrows, Figure 4). Most of the original XRD peaks suddenly decreased 18643

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molecular framework. The observed phase transition was completely reversible following the decompression to ambient pressure. This work can be helpful for further understanding the nature of hydrogen bonds and the high-pressure stability of organic molecular crystals.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: 86-431-85168882. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by NSFC (Nos. 91227202, and 11204101), RFDP (No. 20120061130006), the National Basic Research Program of China (No. 2011CB808200), and the China Postdoctoral Science Foundation (No. 2012M511327). Angle-dispersive XRD measurement was performed at 4W2 beamline, Beijing Synchrotron Radiation Facility (BSRF), which is supported by the Chinese Academy of Sciences (Nos. KJCX2-SW-N20, KJCX2-SW-N03). Portions of this work were performed at the 15U1 at the Shanghai Synchrotron Radiation Facility (SSRF).

Figure 6. Calculated intralayer hydrogen-bond networks (a) at 0 GPa and (b) at 10 GPa. Dashed lines represent hydrogen bonds.

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The proposed mechanism for the phase transition is in accordance with the characteristics of high-pressure Raman and XRD experiments. For example, the red shift of the N−H stretching Raman peaks is attributed to the strengthening of interlayer and intralayer hydrogen bonds. During phase transition, the remarkable changes in the N−H stretching Raman peaks significantly modify the hydrogen-bond networks. Given the knowledge of the calculated model, the Raman peaks of the high-pressure phases at 3357 and 3248 cm−1 can be assigned to the as-formed and originally distorted N−H···O hydrogen bonds, respectively. Considering the geometry of sulfamide molecules and all the tetrahedral atoms participating in the hydrogen bonds, the rearrangement of the corresponding networks causes a slight elastic deformation in the molecular framework. This is the reason that the significant changes of the framework Raman modes and the crystallinity reduced to a certain extent after phase transition. The rearranged hydrogen bonds and deformed molecules can be restored by pressure release, which accounts for the reversible pressure-induced phase transition. The results suggest that the hydrogen bonds exhibit a central function in the reversible structural transition of sulfamide. However, further single-crystal X-ray diffraction and neutron diffraction studies are necessary to determine precise atomic positions and reliable hydrogen-bond geometries at high pressure.

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CONCLUSION In conclusion, high-pressure Raman spectroscopy and synchrotron ADXRD experiments were conducted to analyze the pressure-induced phase transition in crystalline sulfamide. The abrupt changes in the Raman spectra at 5 GPa strongly suggested the emergence of a high-pressure phase, which was stable up to 10 GPa. The phase transition was confirmed by high-pressure XRD experiments. The high-pressure phase was indexed and refined to a monoclinic structure with space group I2/m. Results from ab initio calculations and Raman spectra clarified the transition mechanism in terms of the rearrangement of hydrogen-bond networks and the deformation of the 18644

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