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Compression and Probing C-H · · · I Hydrogen Bonds of Iodoform under High Pressure by X-ray Diffraction and Raman Scattering Dan Liu, Weiwei Lei, Kai Wang, Gang Bao, Fangfei Li, Jian Hao, Bingbing Liu, Tian Cui, Qiliang Cui,* and Guangtian Zou National Laboratory of Superhard Materials, Jilin UniVersity, Changchun 130012, People’s Republic of China ReceiVed: NoVember 11, 2008; ReVised Manuscript ReceiVed: March 10, 2009
High-pressure methods were applied to investigate the structural stability and hydrogen bonding of polar molecules of iodoform by synchrotron radiation X-ray diffraction and Raman spectra measurements, respectively. Up to a pressure of 40 GPa, no phase transitions were observed. The discontinuous frequency shift of the C-H stretching band is believed to be related to the enhancement of the C-H · · · I weak hydrogen bonds under high pressures. Ab initio calculations were performed, and the results predict the frequency shift of the C-H stretching vibration as C-H · · · I interacts via hydrogen bonding. The bulk modulus is 17.3 ( 0.8 GPa, with a pressure derivative of 5.2. 1. Introduction In recent years, there has been significant interest in polar molecules of organic matter in chemistry, physics, and materials sciences.1 The presence of polar molecules in crystals is an essential requirement for materials to have the necessary chemical and physical properties for important technological and industrial applications in areas such as optoelectronic transducers, pyroelectric materials, actuators, and photoconductivity.2 Iodoform (CHI3) is a good example of simple polar molecular crystals with permanent dipole moments and rigid molecular arrangement.3 Haloforms are made of molecular units of the type CHX3, where X ) F, Cl, Br, or I. As one of the holaforms, iodoform (CHI3) has unique electronic properties in photodissociation and photogeneration. It is a nonlinear optical material and has wide applications in the synthesis of novel chemical production and polymers.4-7 Recently, iodoform has been reported as a self-processing system for holographic recording with near-infrared sensitivity.8 Samoc´ et al. described the steady-state photoconductivity iodoform single crystals, which exhibit high photogeneration yields and interesting photoconductive characteristics.9 Iodoform has large thermal expansion coefficients and high compressibility. Thus, studying the high-pressure behavior of these polar molecular crystals not only is useful for science reasons, but also provides information on hydrogenous structure of polar molecules, which may have broad chemical applications. The early X-ray diffraction studies10-12 of iodoform at ambient pressure and room temperature suggested that it belongs to the space group P63, although a few reports showed the structure of iodoform should be P63/m with two molecules per unit cell.13,14 Almost all spectral studies of CHI3 have been interpreted in terms of the polar P63 structure.15-18 Previous studies of iodoform include Raman, infrared, and electrical measurements at high pressure and low temperature.15-20 There were reports showing the anomalous behavior of lattice mode intensities, suggesting a phase transition below 270 K.21-24 They were carried out over a very limited pressure range, up to about 10 GPa, and no evidence of structural phase transition was found by spectra methods. * Corresponding author. E-mail:
[email protected].
Conventional hydrogen bonds have been studied extensively, which play a vital role in supramolecular chemistry, materials science, biology, as well as in most chemical processes. During the past decade, much scientific interest has been dedicated to the weak intermolecular interactions involving C-H · · · X hydrogen bonds. They have been demonstrated to be of crucial importance in the formation of crystal structures, crystal packing, and protein folding.25-27 A weak hydrogen bond is an interaction C-H · · · X wherein a hydrogen atom forms a bond between two structural moieties C and X, of which one or both are of moderate to low electronegativity.28 However, the results of crystal structures are highly informative on the relative geometry; crystallography is silent on the disputes of whether weak hydrogen-bond interactions are attractive or appear merely as a result of steric constraints.29,30 Thus, the spectroscopic properties of C-H · · · X hydrogen bonds deserve further attention. There were theoretical discussions on the nature of weak hydrogen bonding. Experimental evidence of such interaction is notoriously difficult to obtain, mainly because weak hydrogen bonds usually coexist with many other types of strong interactions, making the contribution from C-H · · · X interactions difficult to distinguish.31-33 Several studies have shown the significant effect that pressure has on enhancing molecular aggregates containing charges and controlling the strength of C-H · · · X interactions.34 Although the C-H · · · O has attracted many research efforts, recent studies have shown that X is the more polarizable halogen (F, Cl, Br, I).35-39 In particular, recent achievement in this field is the recognition of hydrogen-bond synthons C-H · · · X-M (X ) F, Cl, Br, I and M ) metal), which have been exploited in the preparation of families of organic-inorganic hybrid materials.40-42 To obtain further information on weak C-H · · · X (X ) halogens) interactions, we study the polyhalomethane iodoform of polar molecules using their Raman spectra and results of ab initio calculations at various pressures. The purpose of this work is to report on the compressibility of iodoform and to explore its stability at high pressures by in situ synchrotron angle-dispersive XRD up to 40 GPa. It is well-known that the X-ray diffraction is a well direct and powerful tool to study structural characteristics of materials. The structure of iodoform is stable with no phase transitions
10.1021/jp900467z CCC: $40.75 2009 American Chemical Society Published on Web 04/03/2009
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in this pressure range, which confirmed the results of previous reports. In addition, we experimentally and theoretically studied the behavior of the hydrogen bond of iodoform under high pressure at room temperature using Raman spectra measurements in a diamond anvil cell up to pressure of 22 GPa and ab initio calculation, respectively. We further presented and discussed the structural and lattice dynamical behavior of iodoform. 2. Experimental Section Commercially available iodoform powder (99.5%) was loaded into a gasketed high-pressure Mao-Bell-type diamond anvil cell (DAC) with a culet face of 400 µm in diameter. The T301 stainless steel gasket was preindented by diamonds anvils to an initial thickness of about 70 µm, and then a center hole of 100 µm diameter was drilled as the sample chamber. Pressure was determined from the frequency shift of the ruby R1 fluorescence line.43 By monitoring the separation and widths of both R1 and R2 lines, we confirmed that hydrostatic condition was maintained throughout the experiments. The precision of our pressure measurements was estimated to be around 0.05 GPa. In situ high-pressure XRD measurement was performed at room temperature with an angular dispersive synchrotron X-ray source (λ ) 0.437119 Å) of the Advanced Photon Source (APS), Argonne National Laboratory (ANL). The diffraction data were collected using an MAR165 CCD detector. The Bragg diffraction rings were recorded with an imaging plate detector, and the two-dimensional XRD images were analyzed using FIT2D software, yielding one-dimensional intensity versus diffraction angle 2θ patterns.44 The simulation and analysis XRD patterns at different pressures were performed by Rietveld method using the Materials Studio Reflex program suite.45 During each refinement cycle, scale factor, background parameter, and cell parameter were optimized. In situ high-pressure Raman spectra were measured on a Renishaw inVia Raman microscope in the backscattering geometry. This was equipped with a high-power semiconductor diode laser (wavelength 514 nm, excited line 15 mW). Iodofrom powder and ruby particles were loaded into the sample chamber (120 µm in diameter and 70 µm thick) of a 500 µm culet MaoBell-type DAC. No pressure medium was used in these studies, and all measurements were conducted at room temperature. Pressure-induced shifts of overlapping Raman bands were analyzed by fitting the spectra to Lorentzian functions to determine the line shape parameters. Experimental details regarding the Raman and ruby fluorescence systems are presented below. Ab initio calculations were performed with the pseudopotential plane wave density functional method implemented in the CASTEP code treatment within the Materials Studio package.46 The local density approximation (GGA) exchangecorrelation functional was used in the calculations. Vanderbilt-type ultrasoft pseudopotentials were employed with a planewave cutoff energy of 280 eV and sampled over reduced k points. 3. Results and Discussion 3.1. Crystal Structure. The in situ XRD experiments of iodoform in DAC were performed at various pressures up to 40 GPa. The result is shown in Figure 1. Rietvelt refinements were carried out at various pressures using Materials Studio software to obtain the cell parameters. The diffraction pattern of CHI3 at ambient conditions can be easily indexed to orientation disorder hexagonal structure R-P63 iodoform crystal
Figure 1. X-ray diffraction patterns collected at various pressures for iodoform with incident wavelength λ ) 0.437119 Å.
Figure 2. Rietveld full-profile refinements of the diffraction patterns collected on compression at ambient pressure. The P63 phase fit is good for the diffraction pattern shown, with an Rwp ) 3.06%. Red, blue, and black solid lines represent experimental, calculated, and residual patterns, respectively. The inset report is the structure of P63.
with polar space group no. 173. All dipole moments of the molecules should be parallel to the crystallographic c-axis. In the spectra collected at ambient, the Rietvelt refinements converge to the structure model with residuals Rwp ) 3.06%, as shown in Figure 2. The lattice parameters refined within this space group were a ) 6.813 Å, b ) 7.559 Å, and the unit-cell volume V0 ) 151.9(3) Å3 (Z ) 2) (inset in Figure 2), which are in good agreement with those previously reported in the literature.47 With increase of pressure, the diffraction lines shift toward higher 2θ angles accompanied by a change of relative intensities. The CHI3 sample was compressed normally, showing no sign of changes in the diffraction patterns up to 40 GPa. As the pressure increased, the Bragg peaks broadened, and some merged due to the differences of compressibility along the a and c axes. However, the structure of iodoform appeared to be quite stable up to 40 GPa. Rietvelt refinements were carried out at various pressures using Materials Studio software to obtain the cell parameters. The effects of pressure on the lattice parameters, relative change, and unit-cell volumes V changes are shown in Table 1. The pressure-volume results for the CHI3 phases are presented in
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TABLE 1: Unit-Cell Parameters and Volume of Iodoform at Various Pressures P (GPa)
symmetry
a (Å) d ( 0.001
c (Å) ( 0.001
V (Å3) ( 0.1
0 0.7 2.2 3.5 5.5 7.9 9.7 12 15 17.4 20.6 22.6 26.6 32.4 34.5 37 40
P63 P63 P63 P63 P63 P63 P63 P63 P63 P63 P63 P63 P63 P63 P63 P63 P63
6.813 6.691 6.605 6.525 6.439 6.353 6.291 6.219 6.153 6.108 6.052 6.049 5.942 5.892 5.858 5.861 5.835
7.559 7.162 6.862 6.694 6.525 6.395 6.323 6.171 6.063 5.977 5.911 5.883 5.753 5.693 5.672 5.667 5.630
151.9 138.9 129.6 122.7 117.1 111.8 108.0 101.0 99.1 96.5 93.7 93.2 87.6 85.2 83.8 82.9 81.6
Figure 3. Pressure-volume data of iodoform at 300 K. Solid blue symbols represent P63 phase experiment data, and solid black lines are third-order Birch-Murnaghan equation of state fits to P63 phase data.
Figure 3. All changes in the relative unit-cell volume with pressure are fitted to a third-order Birch-Murnaghan equation of state:
P ) 3/2B0[(V0/V)7/3-(V0 /V)5/3] × {1 + 3/4(B0′-4) × [(V0 /V)2/3-1]}
(1)
where B0 is the bulk modulus and B0′ is pressure derivative. On the basis of these results, B0 and B0′ were calculated to be 17.3 ( 0.8 GPa and 5.2, respectively. Fitting to the X-ray diffraction patterns at each pressure led to the determination of the cell length as a function of pressure. The present data show a smooth and monotonic behavior with pressures, and the c-axis length is approximately 2 times more compressible than the a-axis length. The plots in Figure 4 show the pressure-induced changes in the reduced unit-cell parameters and the c/a ratio (inset). The measured c/a ratio always remained close to unity, although it increased slightly as a function of pressure, showing that the R-iodoform has large crystallographic anisotropy in compressibility. Below 12 GPa, c is slightly longer than a, and above 12 GPa, c becomes the shortest axis. 3.2. Raman Spectra. In our experiments, six Raman active phonon modes are discernible at ambient conditions, and all of the observed modes correspond well to the band positions reported in the literature for iodoform.20,48,49 Figure 5a shows four representatives Raman bands of iodoform in the frequency range of 100-700 cm-1 at different pressures. A weak and broad band at 109 cm-1 is the ν6 mode, corresponding to the degenerate C-I3 deforming bands. The rather narrow band at 154 cm-1 comes from the symmetric C-I3 deforming bands (ν3). Two distinct bands at 437 and 538 cm-1 are results of the symmetric C-I3 stretching (ν2) and the degenerate C-I3 stretching (ν5), respectively. Two more bands are observed in the range 1000-3000 cm-1: the one at 1067 cm-1 is attributed to the degenerate C-H bending vibration mode (ν4), while the other near 3013 cm-1 is assigned to symmetric C-H stretching (ν1), as shown in Figure 5b. In Figure 6, we notice that the peak frequencies of the C-I3 vibration modes and C-H bending vibration modes of iodoform have smooth monotonic pressure dependence, which have blue shifts in frequency with increasing pressure. Nevertheless, the C-H stretching band exhibits anomalous nonmonotonic pressure-induced frequency shifts. The C-H stretching band moves in the opposite direction as the pressure increases; it is red-
Figure 4. The lattice parameters of iodoform at different pressure. The inset shows the pressure dependencies of c/a ratio for iodoform.
shifted in frequency at pressures below 2 GPa, while blue-shifted beyond 2 GPa and up to 22 GPa. This discontinuity in frequency shift should be related to the mechanism of C-H · · · I hydrogen bonding. In the C-H · · · I hydrogen bonding, the C-H acts as the proton donor and is rather weak as compared to the conventional hydrogen bond. At pressures 2 GPa, the C-H unit’s hydrogen atom is pushed toward the carbon atom by the I atom as a consequence of the strength modification of the C-H · · · I hydrogen bond with increasing pressure. Therefore, blue shift of the frequency of C-H stretching mode is observed due to the decreased C-H bond length. Our observation of the discontinuity phenomenon is consistent with the rule of hydrogen bond by Hamann et al.50 They concluded that an increase in pressure should decrease
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Figure 5. Raman spectra of iodoform at different pressures. For clarity, the spectra have been divided into two parts: (a) 100-700 cm-1 and (b) 1000-3020 cm-1. The six Raman modes are labeled ν1, ν2, ν3, ν4, ν5, and ν6, respectively.
TABLE 2: Ab Initio Calculations of C-H Stretching Bond Lengths (Å) Involving the C-H · · · I Hydrogen Bonding at Different Pressures
Figure 6. Pressure dependence of Raman frequency shifts of iodoform at room temperature. Arrow shows anomalous nonmonotonic frequency shifts.
Figure 7. A perspective view of the bc-plane shows the 3D-network structure of iodoform with the intermolecular hydrogen-bonding interactions. The blue dashed lines represent C-H · · · I hydrogen bonds.
the C-H stretching frequencies of weak hydrogen bonds. If the H · · · I distance were small enough (as it could be in very strong hydrogen bonds or at very high pressures), it would make the C-H and H · · · I repulsive forces an important part of the total forces. Therefore, the frequencies of C-H stretching band
pressure
assignment
bond lengths (Å)
0 GPa 1 GPa 2 GPa 3 GPa 5 GPa 10 GPa 15 GPa
C-H stretching C-H stretching C-H stretching C-H stretching C-H stretching C-H stretching C-H stretching
1.0872(2) 1.0851(1) 1.0987(3) 1.0981(1) 1.0972(2) 1.0959(1) 1.0934(4)
display blue shift above 2 GPa. Moreover, the result of weak C-H · · · I interactions between the neighboring layers may be the key factors for the formation of the polar structure.51 In Figure 5, we did not observe any major changes in the Raman spectra except that all of the Raman peaks significantly broadened features on compression, which may be attributed to the inhomogeneous pressure stress field in the high-pressure chamber. Furthermore, with the increase of pressure, the weak C-H stretching ν1 Raman peak diminishes gradually and is indiscernible as the pressure increases to 13.3 GPa. The Raman spectrum of iodoform appears to remain unchanged upon compression up to 22 GPa, indicating that no structural or chemical transformation takes place over this pressure for that phase. A complementary insight into high-pressure behavior of the C-H · · · I hydrogen bonding is obtained by carrying out the ab initio calculations. The calculated pressure-induced changes in the C-H bond lengths involving the hydrogen bonding are shown in Table 2. At a pressure lower than 2 GPa, there is a smooth increase of the C-H bond length, implying pressureinduced red shifts in Raman spectra. The result of changes of the C-H bond has revealed that the bond strength was slightly reduced and the C-H · · · I bond gets stronger interaction due to charge overlap. At a pressure higher than 2 GPa, the C-H bond length becomes shorter, which is in good agreement with our Raman spectra data. The blue shifts may originate from the overlap repulsion effect enhanced by hydrostatic pressure. These indicate the discontinuity of the frequency shift of the associated C-H stretching frequency upon the formation of hydrogen bonding as C-H · · · I interactions. Conclusion In summary, we performed in situ high-pressure synchrotron XRD and Raman spectra studies of iodoform up to 40 and 22
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GPa, respectively. In this work, the investigations of the hydrogen-bonding interactions in polar molecule iodoform using Raman spectra were carried out, and a useful correlation of the C-H stretching frequencies with that of the C-H · · · I hydrogenbond parameters was obtained. We observed that the pressure dependence of C-H stretch bands yields red frequency shifts at pressures below 2 GPa, and changes to blue frequency shift beyond 2 GPa. This discontinuity phenomenon in Raman spectrum is consistent with the rule of hydrogen bond as previously reported. These Raman experimental results were also supported by the ab initio calculation. In addition, a bulk modulus B0 ) 17.3 ( 0.8 GPa at a fixed B0′ ) 5.2 for R-iodoform was estimated from the patterns obtained at high pressure. These results should be helpful for the understanding of physical properties of iodoform and future investigation into metal complexes. Acknowledgment. We are grateful to Ho-kwang Mao and Keh-Jim Dunn for many useful discussions and Yue Meng for his assistance with the experiments. X-ray diffraction measurements were carried out at the Advanced Photon Source, Argonne National Laboratory. This work was supported by the National Science Foundation of China (no. 50772043) and The Postgraduate Innovative Foundation Program of Jilin University (MS20080217) and National Basic Research Program of China (grant nos. 2005CB724400 and 2001CB711201). Note Added after ASAP Publication. This paper was published ASAP on April 3, 2009. Data in Table 2 were corrected. The updated paper was reposted on April 7, 2009. Supporting Information Available: Figure showing the d spacing of diffraction lines as a function of pressure of iodoform. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Curtin, D. Y.; Paul, I. C. Chem. ReV. 1981, 81, 525. (2) Zhang, H.; Wang, X. M.; Zhang, K. C.; Teo, B. K. J. Solid State Chem. 2000, 152, 191. (3) Samoc´, A.; Samoc´, M.; Gierman´ska, J.; Sworakowski, J.; Koiodziej, H.; Williams, D. F. J. Phys. D: Appl. Phys. 1985, 18, 2529. (4) Zheng, X. M.; Phillips, D. L. Chem. Phys. Lett. 2000, 324, 175. (5) Karna, S. P.; Dupuis, M.; Perrin, E.; Prasad, P. N. J. Chem. Phys. 1990, 12, 7418. (6) Munn, R. W.; Kelly, J. F.; Aicken, F. M. Chem. Phys. 1999, 245, 227. (7) Hoffmann, R. W.; Mu¨ller, M.; Menzel, K.; Gschwind, R.; Schwerdtfeger, P.; Malkina, O. L.; Malkin, V. G. Organometallics 2001, 20, 5310. (8) Noiret, N.; Meyer, C.; Lougnot, D. J. Pure Appl. Opt. 1994, 3, 55. (9) Samoc´, A.; Samoc´, M.; Sworakowski, J.; Koropecky, I.; Nespurek, S. Mol. Cryst. Liq. Cryst. 1981, 78, 1. (10) Khostsyanova, T. L.; Kitaigorodskii, A. I.; Struchkov, Y. T. Zh. Fiz. Khim. 1953, 27, 647. (11) Wykoff, R. W. G. Crystal Structures; Interscience: New York, 1964; Vol. 5.
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