Pressure-Induced Phase Transition in Hydrogen-Bonded

Oct 15, 2009 - diamond anvil cell up to pressures of 15 GPa. The abrupt changes in Raman spectra around 4.4 GPa have provided convincing evidence for ...
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J. Phys. Chem. B 2009, 113, 14719–14724

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Pressure-Induced Phase Transition in Hydrogen-Bonded Supramolecular Adduct Formed by Cyanuric Acid and Melamine Kai Wang,† Defang Duan,† Run Wang,† Dan Liu,† Lingyun Tang,‡ Tian Cui,† Bingbing Liu,† Qiliang Cui,† Jing Liu,‡ Bo Zou,*,† and Guangtian Zou† State Key Laboratory of Superhard Materials, Jilin UniVersity, Changchun 130012, P. R. China, and Beijing Synchrotron Radiation Laboratory, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100039, P. R. China ReceiVed: July 16, 2009; ReVised Manuscript ReceiVed: September 16, 2009

Single-crystal samples of the 1:1 adduct between cyanuric acid and melamine (CA · M), an outstanding case of noncovalent synthesis, have been studied by Raman spectroscopy and synchrotron X-ray diffraction in a diamond anvil cell up to pressures of 15 GPa. The abrupt changes in Raman spectra around 4.4 GPa have provided convincing evidence for pressure-induced structural phase transition. This phase transition was confirmed by angle dispersive X-ray diffraction (ADXRD) experiments to be a space group change from C2/m to its subgroup P21/m. On release of pressure, the observed transition was irreversible, and the new high-pressure phase was fully preserved at ambient conditions. We propose that this phase transition was due to supramolecular rearrangements brought about by changes in the hydrogen bonding networks. Introduction Significant effort has been devoted in the past years to the study, design, and understanding of hydrogen-bonded supramolecular architectures.1-5 These architectures have strong covalent bonds within each molecule and relatively weak intermolecular interactions between neighboring molecules, such as hydrogen bonds or van der Waals interactions. Their crystal structures and properties are determined mainly by the structure of molecules and intermolecular hydrogen bonds, which are stabilized by lone pair electrons.6-9 Pressure is ideally suited to the study of these crystals, as the application of high pressure can cause considerable variations in hydrogen bonding.10 Besides, the balance between hydrogen bonds and van der Waals interactions can be altered by pressure, which may potentially lead to structural transitions.11,12 Studies of hydrogen-bonded supramolecular architectures under high pressure can not only shed light on the nature of hydrogen bonds but also assist in the investigation of phase transitions, polymorphism, and crystal engineering. Therefore, a high-pressure study which provides an experimental foundation for the understanding of the structural stability of hydrogen-bonded supramolecular architectures is of fundamental importance. In the last decades, the number of high-pressure experiments performed on hydrogen-bonded molecular crystals has considerably increased.13-17 As we expected, changes in the hydrogen bonding interactions play a critical role in structural conformation in these systems. Nevertheless, there have been limited reports on structural changes of hydrogen-bonded supramolecular materials under high pressure.18,19 Recently, we have carried out high-pressure studies on melamine-boric acid adduct (M · 2B) and have shown that M · 2B undergoes a reversible pressure-induced amorphization.20 Morrison studied temperatureand pressure-induced proton transfers in the hydrogen-bonded adduct formed between squaric acid and bipridine.21 The high* Corresponding author. E-mail: [email protected]. † Jilin University. ‡ Chinese Academy of Sciences.

pressure behavior of these systems also depended on the nature and the number of hydrogen bonds formed between constituent molecules. As in most supramolecular systems, the structures of the high-pressure phases of cyanuric acid-melamine adduct (CA · M) and the transition pressures are not known. However, pressure-induced decomposition reactions of cyanuric acid22 and structure phase transitions of melamine23,24 have recently been addressed. Note that both cyanuric acid and melamine have hydrogen-bonded networks in their molecular crystal structures. Hence it is of particular interest to investigate the influence of high pressure on the hydrogen-bonded supramolecular adduct formed by cyanuric acid and melamine. At ambient conditions, CA · M crystallizes into a monoclinic structure with the C2/m space group.25 In this structure, as illustrated in Figure 1, cyanuric acid and melamine molecules are held together by complementary hydrogen bonds yielding a rosette form which is stabilized by the collective strength of 18 hydrogen bonds.26 The rosettes are arranged in two dimensions to form planar sheets. While the cooperative hydrogen bonding constitutes the predominant intralayer interaction, the coupling between different layers is relatively weak van der Waals interaction. Owing to its outstanding structure, CA · M can be considered as a model system for studying the structure and phase stability of hydrogen-bonded supramolecular architectures. Interestingly, recent work has shown that the pet food contaminant responsible for the deaths of dogs and cats is caused by the intratubular precipitation of CA · M in the animals’ kidney.27 In the present work, the high-pressure Raman spectroscopy and synchrotron X-ray diffraction studies on CA · M crystal up to 15.8 GPa are presented, and we provide information about the high-pressure phase transition. The structural information at high pressure can be obtained from in situ angle dispersive X-ray diffraction (ADXRD) patterns with high intensity synchrotron radiation. High-pressure Raman spectroscopy can be a powerful tool to examine and analyze the modifications in molecule arrangements and hydrogen bonding interactions. The primary goal of this study is to provide a better understanding

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Figure 2. Raman spectra of CA · M single-crystal and powder samples at atmospheric pressure. The asterisks indicate the band observed only from the powder samples.

Figure 1. Crystal structure of CA · M at ambient pressure. Dashed lines represent intermolecular hydrogen bonds. Color legend: carbon (gray), nitrogen (blue), oxygen (red), hydrogen (white).

of the nature of the hydrogen bonding and the structural stability of hydrogen-bonded supramolecular architectures under high pressure.

by using the reflex module combined in the Materials Studio program (Accelrys Inc.). All experiments were carried out at room temperature. Ab initio constant pressure molecular dynamics were carried out with use of the Car-Parrinello scheme implemented in the CPMD code.30,31 Our simulations were carried out at 300 K with a supercell containing 108 atoms. The generalized-gradient approximation with Perdew-BurkeErnzerhof (PBE) exchange-correlation functional was used in the calculations.32 The norm-conserving pseudopotentials33 were employed with a cutoff energy of 100 Ry. Brillouin zone sampling was restricted to the supercell Γ point. Results and Discussion

Experimental Section Transparent CA · M single crystals used for this study were prepared by the hydrothermal synthesis method. The details of the synthesis conditions were described previously.25 The crystal structure of the samples was confirmed by the X-ray diffraction powder pattern. Powder samples used for the ambient measurements (Raman, XRD) were prepared by crushing single crystals using a mortar. High-pressure experiments were carried out using a MaoBell diamond anvil cell (DAC) with 0.5 mm diamond culets. A T301 stainless steel gasket was preindented by the diamonds and then drilled to produce a 0.15 mm diameter cavity for the sample. Then a CA · M single crystal was placed in the gasket hole together with a small ruby chip to determine the pressure using the standard ruby fluorescent technique.28 A 16:3:1 mixture of methanol-ethanol-water was used as a pressure-transmitting medium. By monitoring the separation and widths of both R1 and R2 lines, we confirmed that hydrostatic conditions were maintained throughout the experiment. Raman spectra were recorded using two Raman spectrometers, a Renishaw inVia Raman system and a Jobin Yvon HR800 Raman system. Results from two spectrometers were similar. The excitation source used for all Raman measurements was the 514.5 nm line of an argon ion laser in the backscattering geometry. The spectral resolution of both systems was around 1 cm-1. In situ ADXRD measurements were performed at 4W2 High-Pressure Station of Beijing Synchrotron Radiation Facility (BSRF). Monochromatic radiation at a wavelength of 0.6199 Å was used for pattern collection. Portions of ADXRD measurements were performed at 16IDB, of the HPCAT (highpressure collaborative access team) at the Advanced Photon Source in Argonne National Laboratory. The Bragg diffraction rings were recorded with an imaging plate detector, and the XRD patterns were integrated from the images with FIT2D software.29 The XRD patterns were then indexed and refined

In Figure 2, we show the typical Raman spectra of singlecrystal and powdered samples measured at ambient conditions. Generally, the Raman spectra of the present work agreed well with those reported previously.34,35 In comparison with the single-crystal spectra, the Raman spectra obtained from powdered samples had the same pattern (except for the broad band at 118 cm-1), but the intensity was too low to be detected under high pressure. For a single crystal, it was difficult to control the direction of the crystal in the sample chamber of a DAC. Fortunately, no frequency deviations of the Raman bands were observed under repetitious measurements. So we studied the high-pressure Raman spectra of the CA · M single-crystal samples. To the best of our knowledge, no papers have been published for the assignments of Raman bands of CA · M. Therefore, we proposed a tentative assignment of the Raman bands at ambient pressure based on the comparison with reported spectra of cyanuric acid and melamine molecules.36-40 The wavenumbers of some Raman bands which can be examined under high pressure and their tentative assignments are provided in Table 1. High-pressure Raman spectra of CA · M single crystal in the spectral regions 100-300, 350-650, 650-1900, and 3000-3500 cm-1 are presented in Figures 3(a)-(d), respectively. The effect of pressure on Raman spectra of CA · M has been measured up to 15.8 GPa. As shown in Figure 3, the observed large variations in several Raman modes indicate that the sample has undergone a structural phase transition (phase I f phase II) around 4.4 GPa. The Raman spectra of the new phase were found to be stable up to the highest pressure applied in this work with no further discontinuous changes. On release of pressure to 1 atm, it is worth noting that the released Raman spectrum showed the characteristics of a high-pressure phase. Although it was difficult to determine the crystal structure of the high-pressure phase from the Raman data, the observed Raman spectra may provide some insight into the phase transition.

Hydrogen-Bonded Supramolecular Adduct TABLE 1: Tentative Assignment of the Major Raman Bands of CA · M frequencies (cm-1) cyanuric acid melamine CA · M 3208 3070

3471 3420 3330 3127

1727 1560 992 702

988 680 586

525 387

tentative assignment

3381 3338 3251

N-H stretching

1738 1689 1543 997 693 678 592 526

CdO stretching NH2 bending side-chain asym C-N stretching ring breathing ring out-of-plane bending symmetric ring breathing ring bending CdO in-plane bending + side-chain in-plane C-N bending ring: quadrant out-of-plane lattice vibration

406 152 102 97 75

Figure 3(a) shows representative Raman spectra of the external modes at several pressures. The spectrum observed at ambient conditions consists of four external modes (75, 97, 102, and 152 cm-1). With increasing pressure, all external modes showed substantial blue shift, nevertheless, with different shift rates. The blue shift observed for external modes is to be expected since the strength of the interactions between the adjacent molecules should increase with the reduction of intermolecular distances.41-44 At the same time, the intensity of the mode at 152 cm-1 reduced significantly with the increase of pressure. Furthermore, an abrupt change in the spectrum shape, which can be visualized as evidence for the pressureinduced phase transition, was observed at about 4.4 GPa. Apart from a general increase in frequency, this new pattern of peaks was preserved up to the highest pressure applied in this work. The pressure dependence of the Raman spectra in the frequency range 350-1950 cm-1 is shown in Figure 3(b) and (c). We assigned the observed Raman peaks to internal vibrational modes of the cyanuric acid and melamine molecules. With increasing pressure, all modes shifted gradually toward higher frequencies. As the crystal was compressed, the increases in frequencies could be explained by the decrease of interatomic distances and the increase in the effective force constants.45 However, the spectrum dramatically changed at about 4.4 GPa, exhibiting several new features. The most striking changes were the splitting of the mode in the frequency range 350-650 cm-1 (Figure 3(b)), which suggested that the phase transition resulted in lowering of crystalline symmetry. Furthermore, the splitting of the mode at 693 cm-1 could only be resolved above 8.6 GPa, due to the close proximity of the neighboring peaks. In addition to the splitting, the changes at this pressure also include the discontinuity in shifts of existing modes and two new peaks emerging at 1005 and 1175 cm-1. In Figure 3(d) we present typical Raman spectra of the N-H stretching modes under various pressures. In contrast to the other modes, the stretching mode frequencies of all N-H bonds decreased with increasing pressure. The red shifts were in accordance with general rules that an increase of pressure decreases the D-H stretching frequencies of weak and medium strength D-H · · · A bonds.46-50 This pressure effect could be attributed to an increase in the N-H · · · N hydrogen bond strength under pressure. At the same time, one can notice that the two peaks (3381 and 3338 cm-1) gradually merged together with increasing pressure. At about 4.4 GPa, the application of pressure resulted in large changes in peak positions. This is

J. Phys. Chem. B, Vol. 113, No. 44, 2009 14721 primarily due to a significant modification of hydrogen bonds across the transition. With further increase of pressure, the mode at 3147 cm-1 showed a considerable red shift, whereas the mode at 3360 cm-1 was split into two peaks and no apparent frequency shift was observed. The pressure-induced frequency shifts of the Raman modes, indicating the presence of phase transition, are illustrated in Figure 4. At about 4.4 GPa, most vibrational modes showed a discontinuous shift, and some modes showed a sudden change in slope. This is also accompanied by the appearance of several new modes and the splitting of existing modes, indicating a lowering of the crystal symmetry in the high-pressure phase. Because all the N-H bonds participated in hydrogen bonds, their stretching modes are of particular importance for understanding the changes in intermolecular interactions. The observed abrupt changes of the N-H modes at about 4.4 GPa suggested that there was a considerable rearrangement of hydrogen bonds. Therefore, we can conclude that this highpressure phase transition significantly modified the hydrogen bonding networks in CA · M. The pressure-induced phase transition was also clearly observed by an optical microscope. Representative optical microscopy images of CA · M single crystal in the diamond anvil cell are shown in Figure 5. It can be seen that the crystal, which initially was transparent, was found to be opaque immediately after the phase transition. In other words, this phase transition was accompanied by cracking of the crystal. It is worth noting that the destroyed single crystal cannot become intact again upon full release of pressure. To understand the structural variation, it is necessary to perform X-ray diffraction studies to draw a convincing conclusion. Representative X-ray diffraction patterns of CA · M at various pressures are shown in Figure 6. The powder diffraction pattern of CA · M at ambient pressure was simulated using the structural data established by Rao.25 There were four peaks (110, 200, 020, 310) within the two-theta range covered in Figure 6. In our X-ray synchrotron experiments, the diffraction patterns changed significantly at 4.9 GPa, indicating a pressure-induced phase transition. Although the single crystal was destroyed as a result of phase transition, the crystal grain size was relatively large when compared with the X-ray beam diameter, and the diffraction patterns showed the existence of preferred orientation. On further increase of pressure, it is evident that all the diffraction peaks shifted to higher angles, exhibiting a decrease in unit cell volume. Moreover, no significant variations of the diffraction patterns of phase II were observed, demonstrating the new high-pressure phase remained stable up to 15.3 GPa, the highest pressure of the diffraction experiments. It is interesting to note that the intensity of the 001 peak decreased rapidly with increasing pressure. Upon release of pressure, the diffraction patterns of phase II were fully preserved to ambient conditions, which suggested that the high-pressure phase remained stable upon total release of pressure. Therefore, the high-pressure X-ray investigation confirmed the conclusion from the Raman study that CA · M undergoes a pressure-induced irreversible phase transition. To obtain high-quality XRD patterns and reliable structure information for phase II, the retrieved sample was measured out of the diamond anvil cell. Note that the high-pressure phase was preserved to ambient conditions. The diffraction pattern was most easily indexed by using monoclinic symmetry with space group P21/m. The indexed lattice constants were a ) 9.83(9) Å, b ) 3.81(4) Å, c ) 8.13(7) Å, and β ) 98.3(4)°, with unit cell volume V ) 302 Å3. Despite preferred orientation

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Figure 3. Raman spectra of CA · M single crystal at various pressures in the spectral region (a) 100-300 cm-1; (b) 350-650 cm-1; (c) 650-1900 cm-1; and (d) 3100-3500 cm-1. The curves are vertically displaced for clarity.

Figure 5. Optical microscopy images of the CA · M crystal in the DAC before (a) and after (b) phase transition.

Figure 4. Frequency shift of the Raman modes as a function of pressure. The vertical dashed line marks the onset of discontinuity.

effects that precluded any accurate Rietveld refinement to obtain atomic positions, a Pawley refinement provided an excellent description of the observed diffraction patterns (as shown in

Figure 7). It is worth noting that the high-pressure space group P21/m, which is a subgroup of C2/m, has lower symmetry than C2/m. The symmetry lowering of high-pressure crystal structure was consistent with the splitting of Raman modes in the Raman spectra of phase II. The present Raman and X-ray results provided strong evidence of a pressure-induced phase transition in this hydrogenbonded adduct. An important question is what changes were happening in the hydrogen bonding networks under high pressure? However, we do not have enough experimental data to provide precise information about the structure of hydrogen bonds. So we performed ab initio molecular dynamics calculations to investigate the effect of pressure on the hydrogen bonding networks and to gain insight into the mechanism of

Hydrogen-Bonded Supramolecular Adduct

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Figure 6. Representative angle-dispersive X-ray diffraction patterns of CA · M at different pressures and after pressure release. Figure 8. (a) Rosette formed between CA · M pairs through interpair hydrogen bonds (HBs). (b) Two-dimensional hydrogen-bonded planar sheet at ambient pressure. (c) Two-dimensional hydrogen-bonded zigzag-shaped network at high pressure. The arrows indicate the formation of zigzag structures.

Figure 7. Pawley refinements of the diffraction patterns collected on release of pressure. The P21/m phase fit is good for the diffraction pattern shown (Rp ) 1.22% and Rwp ) 2.52%).

this phase transition. The calculated results revealed that the hydrogen-bonded networks were modified from planar sheets (Figure 8(b)) to two-dimensional zigzag-shaped networks (Figure 8(c)) at high pressure. Most interestingly, this particular zigzag structure was generated by distortion of the interpair hydrogen bonds (marked in dashed blue lines in Figure 8), while the CA · M pairs (Figure 8(a)) and the intrapair hydrogen bonds (marked in dashed black lines in Figure 8) still maintained planar structure. We reasoned that this intriguing behavior was due to the difference in stability between inter- and intrapair hydrogen bonds. In other words, the intrapair hydrogen bonds were more stable than interpair hydrogen bonds under high pressure. With the knowledge of this hydrogen bonding model, we proposed the following mechanism for the pressure-induced phase transition. In the low-pressure range, the application of pressure resulted mainly in the reduction of interlayer distance and the increase of van der Waals interactions. The hydrogen bonding interactions within each layer were also strengthened, but the interlayer distance reduced more than the distance between the molecules in the layers. On further compression, the layered structure gradually became unstable because the balance of the van der Waals and hydrogen bonding interactions was disturbed. At sufficiently high pressures (4.4 GPa), the planar hydrogen bonding networks can no longer support the increased energy of intermolecular interactions. Thus, the phase transition and rearrangement of hydrogen bonding networks occurred to reduce the free energy. The features observed in the Raman spectra of N-H stretching modes under high pressure were also consistent with

the hydrogen bonding model presented in Figure 8. Before phase transition, the red shift of all modes could be ascribed to the strengthening of both interpair and intrapair hydrogen bonds. At 4.4 GPa, the large discontinuities in the N-H stretching modes implied a substantial modification of the hydrogen bonding networks. After phase transition, the continual red shift peak can be assigned to the planar intrapair hydrogen bonds which are further strengthened by pressure. Furthermore, the other two peaks which show no apparent frequency shift at higher pressures can be assigned to the distorted interpair hydrogen bonds. On release of pressure, the interpair hydrogen bonds were still distorted to a certain extent, while the intrapair hydrogen bonds could be restored, which explained why some peaks returned to their original positions. These results strongly suggest that hydrogen bonding played a decisive role in the observed pressure-induced phase transition. However, further high-pressure neutron diffraction studies are required to provide reliable information on the hydrogen atomic positions. Conclusion In summary, we have performed high-pressure studies of CA · M single crystal using Raman spectroscopy and synchrotron X-ray diffraction. The observed abrupt changes in the Raman spectra indicated strongly the occurrence of a phase transition at about 4.4 GPa, and the high-pressure phase was found to be stable up to 15.8 GPa. The X-ray diffraction experiment has confirmed the phase transition in CA · M and identified it as a transition from C2/m to P21/m symmetry. Moreover, the observed phase transition was irreversible, and the new highpressure phase was fully preserved after release of pressure. In addition, we proposed that the phase transition was accompanied by rearrangement of the hydrogen bonding networks according to the experimental and calculation results. This paper will be helpful for achieving more insight into the nature of hydrogen bonds and the structural stability of supramolecular systems at high pressure. Acknowledgment. The authors are grateful to Dr. Ho-kwang Mao and Dr. Yue Meng for help on experiments. This work is

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supported by NSFC (Nos. 20773043, 20673048, and 10674053), PCSIRT (IRT0625), NCET-06-0313, RFDP (No. 20060183073), the National Basic Research Program of China (Nos. 2005CB724400 and 2007CB808000), and Project 20080108 Supported by Graduate Innovation Fund of Jilin University. This work was performed at 4W2 HP-Station, Beijing Synchrotron Radiation Facility (BSRF), which is supported by Chinese Academy of Sciences (No. KJCX2-SW-N20KJCX2-SW-N03). References and Notes (1) Lehn, J. M. Supramolecular Chemistry: Concepts and PerspectiVes; VCH: Weinheim, 1995. (2) Aakeroy, C. B.; Beatty, A. M. Aust. J. Chem. 2001, 54, 409–421. (3) Desiraju, G. R. Nature 2001, 412, 397–400. (4) Prins, L. J.; Reinhoudt, D. N.; Timmerman, P. Angew. Chem., Int. Ed. 2001, 40, 2383–2426. (5) Wuest, J. D. Chem. Commun. 2005, 5830–5837. (6) Jeffrey, G. A. An Introduction to Hydrogen Bonding; Oxford University Press: New York, 1997. (7) Steiner, T. Angew. Chem., Int. Ed. 2002, 41, 49–76. (8) Zheng, W. T.; Sun, C. Q. Prog. Solid State Chem. 2006, 34, 1–20. (9) Sun, C. Q.; Tay, B. K.; Lau, S. P.; Sun, X. W.; Zeng, X. T.; Li, S.; Bai, H. L.; Liu, H.; Liu, Z. H.; Jiang, E. Y. J. Appl. Phys. 2001, 90, 2615– 2617. (10) Sikka, S. K.; Sharma, S. M. Phase Transit. 2008, 81, 907–934. (11) Lee, K. M.; Chang, H. C.; Jiang, J. C.; Chen, J. C. C.; Kao, H. E.; Lin, S. H.; Lin, I. J. B. J. Am. Chem. Soc. 2003, 125, 12358–12364. (12) Allan, D. R.; Clark, S. J. Phys. ReV. Lett. 1999, 82, 3464–3467. (13) Shimizu, H.; Nagata, K.; Sasaki, S. J. Chem. Phys. 1988, 89, 2743– 2747. (14) Katrusiak, A. Crystallogr. ReV. 1996, 5, 133–175. (15) Boldyreva, E. V. J. Mol. Struct. 2004, 700, 151–155. (16) Allan, D. R.; Clark, S. J. Phys. ReV. B 1999, 60, 6328–6334. (17) Lamelas, F. J.; Dreger, Z. A.; Gupta, Y. M. J. Phys. Chem. B 2005, 109, 8206–8215. (18) Boldyreva, E. V. Russ. Chem. Bull., Int. Ed. 2004, 53, 1369–1378. (19) Olejniczak, A.; Katrusiak, A. J. Phys. Chem. B 2008, 112, 7183– 7190. (20) Wang, K.; Duan, D.; Wang, R.; Lin, A.; Cui, Q.; Liu, B.; Cui, T.; Zou, B.; Zhang, X.; Hu, J.; Zou, G.; Mao, H. K. Langmuir 2009, 25, 4787– 4791. (21) Martins, D. M. S.; Middlemiss, D. S.; Pulham, C. R.; Wilson, C. C.; Weller, M. T.; Henry, P. F.; Shankland, N.; Shankland, K.; Marshall, W. G.; Ibberson, R. M.; Knight, K.; Moggach, S.; Brunelli, M.; Morrison, C. A. J. Am. Chem. Soc. 2009, 131, 3884–3893. (22) Montgomery, W.; Crowhurst, J. C.; Zaug, J. M.; Jeanloz, R. J. Phys. Chem. B 2008, 112, 2644–2648. (23) Yu, D.; He, J.; Liu, Z.; Xu, B.; Li, D.; Tian, Y. J. Mater. Sci. 2008, 43, 689–695.

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