ARTICLE pubs.acs.org/JPCB
Structural Properties and Halogen Bonds of Cyanuric Chloride under High Pressure Kai Wang,† Defang Duan,† Mi Zhou,†,‡ Shourui Li,†,§ Tian Cui,† Bingbing Liu,† Jing Liu,|| Bo Zou,*,† and Guangtian Zou† †
State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, China College of Physics, Jilin University, Changchun 130012, China § Institute of Atomic and Molecular Physics, Jilin University, Changchun 130012, China Beijing Synchrotron Radiation Laboratory, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100039, China
)
‡
bS Supporting Information ABSTRACT: The effects of high pressure on cyanuric chloride (C3N3Cl3), a remarkable crystal structure dominated by halogen bonds, have been studied by synchrotron X-ray diffraction and Raman spectroscopy in a diamond anvil cell. The results of high pressure experiments revealed that there was no obvious phase transition up to 30 GPa, indicating that halogen bonding is an effective noncovalent interaction to stabilize the crystal structure. Moreover, cyanuric chloride exhibited a high compressibility and a strong anisotropic compression, which can be explained by the layered crystal packing. Ab initio calculations were also performed to account for the high pressure Raman spectra and the high pressure behavior of halogen bonding.
’ INTRODUCTION Noncovalent interactions have attracted considerable interest in various fields of chemistry.1 Hydrogen bonding, the most common and important type of noncovalent interactions, has been extensively studied for many years.2 Recently, weak dimer interactions between a halogen atom in one molecule and a negative site in another were under active investigation.3 They are now referred to as halogen bonding to emphasize their striking resemblance with classical hydrogen bonding.4 By virtue of their strength, selectivity, and directivity, halogen bonding (HB) has been implicated as an important type of interaction in many fields of science.5,6 Recent studies indicated that halogen bonding was often competitive against hydrogen bonding in crystal structures of organic compounds.7 Exploration of the compressibility of noncovalent interactions under external action,8 such as increasing pressure or decreasing temperature, can enhance our understanding of such interactions and assist in investigation of phase transitions and polymorphism in molecular crystals. In the last decades, the variation of the hydrogen bonds as a function of temperature and pressure has been extensively studied.915 Recently, we have carried out high pressure studies on hydrogen-bonded supramolecular structures of melamineboric acid adduct and cyanuric acidmelamine adduct.1618 As we expected, changes in the hydrogen bonds played a critical role in the structural conformation in these systems. However, to the best of our knowledge, only a few previous studies have been performed to establish the effect of r 2011 American Chemical Society
external action on halogen bonds. Metrangolo et al. have studied the temperature dependence of halogen bond distance.19 High pressure has also been applied to freeze a series of halogencontaining compounds and to measure the compressibility of intermolecular contacts of halogen atoms.20,21 Therefore, a high pressure study which provides an experimental and theoretical foundation for the understanding of halogen-bonded crystal is of fundamental importance. A particularly interesting example of halogen bonding was provided by the structure of solid cyanuric chloride (2,4,6trichloro-1,3,5-triazine, C3N3Cl3). Figure 1 shows the molecular structure of cyanuric chloride and the electrostatic potential on the molecular surface (where red indicates negative and blue indicates positive). The molecule of cyanuric chloride contains only nitrogen (negative) and chlorine (positive) atoms on its periphery, and thus, halogen bonds are the controlling interactions in its crystals.22 Under ambient conditions, cyanuric chloride crystallizes into a monoclinic structure with the C2/c space group. In this structure, as illustrated in Figure 2, the molecules of C3N3Cl3 are forming planar layers parallel to the abplane. Within the layers, each molecule is linked to six neighbor molecules by halogen bonds.2325 Owing to its outstanding structure, cyanuric chloride can be considered as a model system Received: January 29, 2011 Revised: March 22, 2011 Published: March 31, 2011 4639
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Figure 1. Calculated electrostatic potential on the molecular surface of cyanuric chloride. The electrostatic potential scale ranges from 0.062 au (red) to 0.062 au (blue).
for studying the structural properties of halogen-bonded crystals under high pressure. In the present work, we report a combined experimental and computational study of cyanuric chloride as a function of pressure up to 30 GPa. 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 halogen bonding interactions. Besides, ab initio calculations were employed to gain insight into the changes of halogen bonding at high pressure. Our present study is an attempt to provide a better understanding of the nature of the halogen bonding and structural properties of the halogenbonded crystal under high pressure.
’ EXPERIMENTAL SECTION Cyanuric chloride used for this study was purchased from the Sigma-Aldrich Chemical Co. and was used without further purification. High pressure experiments were carried out using a symmetric diamond anvil cell (DAC) with 0.4 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, cyanuric chloride was placed in the gasket hole together with a small ruby chip to determine the pressure using the standard ruby fluorescent technique.26 By monitoring the separation and widths of both R1 and R2 lines, we confirmed that quasi-hydrostatic conditions were maintained throughout the experiment. The Raman scattering measurements were carried out using the Renishaw system (inVia Raman microscope) with a 514.5 nm argon ion laser as the excitation source. In situ ADXRD measurements were performed at the 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 16BMD, of the HPCAT (high-pressure 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.27 The XRD patterns were then indexed and refined by using the reflex module combined in the Materials Studio program (Accelrys Inc.). All experiments were carried out at room temperature. Ab initio calculations were performed with the pseudopotential plane-wave method based on density functional theory
Figure 2. Planar sheets and crystal structure of cyanuric chloride at ambient pressure. Dashed lines represent intermolecular halogen bonds. HB1 and HB2 denote the two crystallographic independent halogen bonds.
implemented in the CASTEP code.28 The generalized gradient approximation (GGA) with the PerdewBurkeErnzerhof (PBE) exchange-correlation functional was used in the high pressure calculations. The norm-conserving pseudopotentials were employed with a plane-wave cutoff energy of 770 eV. The program Gaussian 0329 was used for calculations of the electrostatic potential on the molecular surface.
’ RESULTS AND DISCUSSION Representative X-ray diffraction patterns of cyanuric chloride at various pressures are shown in Figure 3. The highest pressure reached in this study was about 30 GPa. With the increase of pressure, it is evident that all the diffraction peaks shifted to higher angles, indicating a decrease in unit cell volume. We note that the shift of the (002) diffraction peak has the largest value, due to the high compressibility along the c-axis. As pressure increased, the peaks became broader and less intense and some merged together. Moreover, it is evident that the intensity of some peaks (022, 51-1, 420) increased with increasing pressure while some peaks (110, 200, 11-1) gradually decreased to disappear. We speculate there are two reasons for this behavior. First, by applying high pressure, the lattice constants and molecular geometry can be changed appreciably which leads to the changes of relative diffraction peak intensities. Figure S1 of the Supporting Information shows the comparison of the experimental results and the simulated XRD patterns based on ab initio calculated results. In this figure, we display the relative changes in some peak intensities (110, 200, 11-1, 022, 51-1, 420) as compared to the 020 peak intensity. As is obvious from Figure S1 of the Supporting 4640
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Figure 4. The unit cell volume as a function of pressure. (inset) Anisotropic pressure response of the crystal lattices. The solid lines are drawn as a guide to the eye.
Figure 3. Representative X-ray diffraction patterns of cyanuric chloride at elevated pressures.
Information, the changes of experimental and simulated XRD patterns showed the same tendency. That is, some peak intensities (022, 51-1, 420) increased with increasing pressure while some peak intensities (110, 200, 11-1) gradually decreased. Second, as a general feature in high pressure experiments, the whole signal strength becomes weaker with increasing pressure. Thus, it is difficult to detect the three weak peaks (110, 200, 11-1) in the high-pressure range. From the above observations and analysis, we can conclude that there is no obvious phase transition that occurred up to 30 GPa. Therefore, the high-pressure diffraction patterns can be indexed according to the monoclinic unit cell. The pressure dependences of the unit cell volume and lattice parameters at room temperature are illustrated in Figure 4. All changes in the relative unit-cell volume with pressure are fitted to the usual two-parameter Murnaghan equation of state: 2 3 0 B0 4 V0 B0 15 P ¼ 0 V B0 where B0 and B00 are the ambient bulk modulus and its pressure derivative, respectively. The best fits were found to be B0 = 8.4 ( 0.5 GPa and B00 = 10.7 ( 0.4 GPa.30 As was observed for many other molecular crystals,3133 the compressibility was high and the compressional behavior of cyanuric chloride was anisotropic, with the c-axis being more compressible than the a- and b-axes. These compressional behaviors could be explained by taking into account the layered crystal packing. The planar cyanuric chloride molecules were connected together by multiple halogen bonds to form a layered structure in the ab-plane, while the main interaction between the different layers was ππ stacking interaction. Therefore, the
small compressibility of the a- and b-axes was explained by the strength of the multiple intermolecular halogen bonds between molecules. Because of weak ππ stacking interaction between layers, the c-axis was the most compressible one as expected. To understand the structural variation, it is necessary to combine X-ray diffraction and Raman spectroscopy to draw a convincing conclusion. The ambient pressure Raman spectrum observed by us agreed well with those reported in the literature.3436 In Table S1 of the Supporting Information, we gave the observed and calculated Raman bands along with their frequencies and assignments. The two low frequency bands (75 and 92 cm1) can be assigned to external modes corresponding to the intermolecular vibration of cyanuric chloride. The Raman bands observed at 176, 215, and 407 cm1 were assigned to the internal modes corresponding to intermolecular vibrations of the CCl bond. The Raman bands in the region 4501500 cm1 are related to the triazine ring vibrations. We note that the bands 650 and 793 cm1 were assigned to out-of-plane ring vibrations and the other bands were assigned to in-plane ring vibrations. Figure 5 shows experimental and calculated high pressure Raman spectra in the spectral region 30360 cm1. Only two strong external modes can be experimentally measured under high pressure. The two external modes exhibited substantial blue shift under pressure, as could be expected for molecular crystals.3740 This blue shift was due to a reduction of intermolecular distances, and the intermolecular coupling became stronger with pressure. At sufficient pressure, one external mode was merged with the internal mode at about 250 cm1. High pressure Raman spectra in the spectral region 3001700 cm1 are presented in Figure 6. Generally, most of the Raman bands in the internal mode region also 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.4145 The pressure-induced frequency shifts of the Raman modes are illustrated in Figure 7. It is evident that most of the Raman bands shifted gradually toward higher frequencies with no discontinuous changes. Therefore, our high pressure Raman investigation has demonstrated that no obvious phase transition occurred in the whole pressure range we studied. There are two significant points to note from Figure 6. First, the behavior of the pressure-induced frequency shift for the 4641
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Figure 5. Experimental and calculated high pressure Raman spectra in the region of 30360 cm1.
Figure 7. The frequency shift of the Raman modes as a function of pressure.
Figure 6. Experimental Raman spectra in the region 3001700 cm1. The frequency range containing the strong diamond Raman line (13101410 cm1) is excluded.
CCl stretching mode (407 cm1) became almost constant between 3 and 6 GPa. This is in agreement with earlier observations by Shimada et al.46 Such behavior can be understood as the result of the pressure induced enhancement of the intermolecular halogen bonding interactions. Second, in contrast to the other modes, the out-of-plane ring vibration (650 and 793 cm1) frequencies decreased with increasing pressure. It is also interesting to note that the band at 650 cm1 was gradually split into two peaks after the external pressure exceeded 10 GPa. These out-of-plane ring vibrations should show blue shifts because of the large compressibility between layers and the increasing repulsive force under high pressure. The mode Gr€uneisen parameters along with corresponding pressure coefficients are calculated and listed in Table S2 of the Supporting Information. As can be seen, there are three pressure regions of the CCl stretching mode and the Gr€uneisen parameters related to out-ofplane ring vibrations show negative values. However, the same results (red shift, splitting) obtained from our ab initio calculation can effectively explain this abnormal behavior. For visual
comparison, the experimental and calculated high pressure Raman spectra were presented in Figure S2 of the Supporting Information in the region 580840 cm1. As we observe from the figure, the calculated results were in good agreement with the experimental ones. With increasing pressure, the two Raman bands at 650 and 793 cm1 shifted to the low frequency side accompanied with the increase of intensity. Moreover, the Raman band at 650 cm1 was split into two peaks at about 10 GPa, which can be explained by the different pressure-induced Raman shift value of the two adjacent modes at ambient pressure (see Table S1 of the Supporting Information). From the above comparison, we can conclude that the ab initio calculations we performed are accurate enough to reveal the high pressure behavior of cyanuric chloride. An important question is what changes happened to the halogen bonds and the halogen-bonded networks under high pressure? At ambient pressure, the cyanuric chloride crystal has two crystallographic independent halogen bonds within layers (see Figure 2). One halogen bond (HB1) is exactly linear (180°), whereas the other (HB2) differs very substantially from linearity (172.9°). The two independent Cl 3 3 3 N distances are 3.100 and 3.113 Å, respectively. The calculated CCl 3 3 3 N bond angles and pressure-induced changes in the CCl and Cl 3 3 3 N distance were shown in Figure 8. From our calculations, there was a smooth decrease of all distances over the whole investigated pressure range. It is also interesting to notice that the pressureinduced changes in Cl 3 3 3 N distance of HB1 are larger than those of HB2, whereas the changes in CCl bond length are smaller than those of HB2. In other words, HB1 and HB2 were both strengthened under high pressure, but HB1 has stronger attractive Cl 3 3 3 N interaction. Moreover, the bond angle of HB1 was maintained at 180°, while the bond angle of HB2 decreased steadily with increasing pressure. That is, the cyanuric chloride molecules rotated around their HB1 direction (b-axis) under high pressure. Figure 9 shows the calculated high pressure crystal 4642
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compressibility and a strong anisotropic compression, which can be explained by the layered crystal packing. In addition, most Raman peaks exhibited a blue shift with increasing pressure, whereas the peaks corresponding to out-of-plane ring vibrations showed red shift. This red shift was believed to be related to the “fish-scale” arrangement of halogen-bonded networks under high pressures. Ab initio calculations were also performed to account for the high pressure Raman spectra and the high pressure behavior of halogen bonding. We hope these findings will contribute achieving more insight into the nature of halogen bonding and the structural properties of halogen-bonded systems at high pressure.
’ ASSOCIATED CONTENT
bS
Supporting Information. Changes of relative peak intensities in the experimental and simulated high pressure XRD patterns, experimental and calculated high pressure Raman spectra in the region 580840 cm1, observed and calculated Raman frequencies, pressure coefficients, and the mode Gr€uneisen parameters. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
Figure 8. Ab initio calculated bond angles and changes in bond lengths for the two independent halogen bonds.
Figure 9. The calculated geometry of halogen bonds and the “fishscale” arrangement of halogen-bonded networks at 30 GPa. Note that the bond angle of HB1 was maintained at 180°.
and halogen bond structure of cyanuric chloride crystal at 30 GPa. The calculated results revealed that the halogen-bonded networks were modified to distinctive “fish-scale” arrangement24 at high pressure. This “fish-scale” can effectively explain the observed and calculated red shift of the ring vibrations at 650 and 793 cm1.
’ CONCLUSION In summary, we have performed high-pressure studies of cyanuric chloride using Raman spectroscopy and synchrotron X-ray diffraction. The experimental results indicated that halogen bonding is an effective noncovalent interaction to stabilize the crystal structure. Moreover, cyanuric chloride exhibited a high
’ ACKNOWLEDGMENT This work was supported by NSFC (Nos. 21073071, 20773043, 51025206, and 10979001) and the National Basic Research Program of China (Nos. 2011CB808200 and 2007CB808000). This work was performed at the 4W2 beamline, Beijing Synchrotron Radiation Facility (BSRF), which is supported by Chinese Academy of Sciences (Grant Nos. KJCX2SW-N20 and KJCX2-SW-N03). Portions of this work were performed at HPCAT (Sector 16), Advanced Photon Source (APS), Argonne National Laboratory. HPCAT is supported by CIW, CDAC, UNLV, and LLNL through funding from DOENNSA, DOE-BES, and NSF. APS is supported by DOE-BES, under Contract No. DE-AC02-06CH11357. ’ REFERENCES (1) Johnson, E. R.; Keinan, S.; Mori-Sanchez, P.; Contreras-García, J.; Cohen, A. J.; Yang, W. J. Am. Chem. Soc. 2010, 132, 6498–6506. (2) Steiner, T. Angew. Chem., Int. Ed. 2002, 41, 48–76. (3) Politzer, P.; Lane, P.; Concha, M. C.; Ma, Y.; Murray, J. S. J. Mol. Model. 2007, 13, 305–311. (4) Metrangolo, P.; Resnati, G. Halogen Bonding: Fundamentals and Applications; Springer: Berlin, 2008. (5) Saha, B. K.; Nangia, A.; Jask olski, M. CrystEngComm 2005, 7, 355–358. (6) Metrangolo, P.; Meyer, F.; Pilati, T.; Resnati, G.; Terraneo, G. Angew. Chem., Int. Ed. 2008, 47, 6114–6127. (7) Metrangolo, P.; Neukirch, H.; Pilati, T.; Resnati, G. Acc. Chem. Res. 2005, 38, 386–395. (8) Espallargas, G. M.; Brammer, L.; Allan, D. R.; Pulham, C. R.; Robertson, N.; Warren, J. E. J. Am. Chem. Soc. 2008, 130, 9058–9071. (9) Shimizu, H.; Nagata, K.; Sasaki, S. J. Chem. Phys. 1988, 89, 2743–2747. (10) Katrusiak, A. Crystallogr. Rev. 1996, 5, 133–175. (11) Allan, D. R.; Clark, S. J. Phys. Rev. B 1999, 60, 6328–6334. 4643
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