Stability of Hydrogen-Bonded Supramolecular Architecture under High

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Stability of Hydrogen-Bonded Supramolecular Architecture under High Pressure Conditions: Pressure-Induced Amorphization in Melamine-Boric Acid Adduct )

Kai Wang,† Defang Duan,† Run Wang,† Aolei Lin,† Qiliang Cui,† Bingbing Liu,† Tian Cui,† Bo Zou,*,† Xi Zhang,‡ Jingzhu Hu,§ Guangtian Zou,† and Ho-kwang Mao

)

† State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, P. R. China, ‡Key Laboratory of Organic Optoelectronics and Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing 100084, P. R. China, §X17C of NSLS, CARS, University of Chicago, Upton, New York 11793 and Geophysical Laboratory, Carnegie Institution of Washington, Washington, D.C. 20015

Received December 7, 2008. Revised Manuscript Received January 15, 2009 The effects of high pressure on the structural stability of the melamine-boric acid adduct (C3N6H6 3 2H3BO3, M 3 2B), a three-dimensional hydrogen-bonded supramolecular architecture, were studied by in situ synchrotron X-ray diffraction (XRD) and Raman spectroscopy. M 3 2B exhibited a high compressibility and a strong anisotropic compression, which can be explained by the layerlike crystal packing. Furthermore, evolution of XRD patterns and Raman spectra indicated that the M 3 2B crystal undergoes a reversible pressure-induced amorphization (PIA) at 18 GPa. The mechanism for the PIA was attributed to the competition between close packing and long-range order. Ab initio calculations were also performed to account for the behavior of hydrogen bonding under high pressure.

Introduction In recent years, a number of supramolecular architectures with versatile functions have been successfully designed and synthesized via intermolecular interactions.1-3 In particular, hydrogen bonding is probably the most widely used interaction due to the reversibility, specificity, directionality, and cooperative strength of this class of interaction.4-8 Studies of intermolecular interactions can give valuable information to design new supramolecular materials. Moreover, the weak intermolecular interactions in solid state can be influenced easily by external actions,9 such as increasing pressure or decreasing temperature. The variation of the hydrogen bond as a function of temperature has been extensively studied.10,11 The effects produced by increasing pressure can be much larger than that of decreasing temperature, and they are easier to measure and study.12,13 The strength of the hydrogen bonds is relatively weak as compared to ordinary covalent bonds, *Corresponding author. E-mail: [email protected]. (1) Lehn, J. M. Supramolecular Chemistry: Concepts and Perspectives; VCH: Weinheim, 1995. (2) Wuest, J. D. Chem. Commun. 2005, 5830–5837. (3) Bohne, C. Langmuir 2006, 22, 9100–9111. (4) Prins, L. J.; Reinhoudt, D. N.; Timmerman, P. Angew. Chem., Int. Ed. 2001, 40, 2383–2426. (5) Steiner, T. Angew. Chem., Int. Ed. 2002, 41, 49–76. (6) Aakeroy, C. B.; Beatty, A. M. Aust. J. Chem. 2001, 54, 409–421. (7) Abe, Y.; Harata, K.; Fujiwara, M.; Ohbu, K. Langmuir 1996, 12, 636– 640. (8) Melendez, R. E.; Hamilton, A. D. In Topics in Current Chemistry 198, Design of Organic Solids; Weber, E., Ed.; Springer: Berlin: 1998; p; 97-129. (9) Espallargas, G. M.; Brammer, L.; Allan, D. R.; Pulham, C. R.; Robertson, N.; Warren, J. E. J. Am. Chem. Soc. 2008, 130, 9058–9071. (10) Jeffrey, G. A. An Introduction to Hydrogen Bonding; Oxford University Press: New York, 1997. (11) Parkin, A.; Adam, M.; Cooper, R. I.; Middlemiss, D. S.; Wilson, C. C. Acta Crystallogr. 2007, B63, 303–308. (12) Hemley, R. J. Annu. Rev. Phys. Chem. 2000, 51, 763–800. (13) Allan, D. R.; Clark, S. J. Phys. Rev. B 1999, 60, 6328–6334.

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and thus, the hydrogen bond can be easily affected by compression.14 High pressure is ideally suited to the study of hydrogenbonded supramolecular architectures,15-17 as the application of high pressure can cause considerable variations in hydrogen bonding18,19 and significant reorganizations of crystal packing.20,21 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 is a powerful tool to investigate the modifications in molecular arrangements and the changes in hydrogen bonding interactions. Furthermore, ab initio calculations can provide fundamental insight into the features of hydrogen bonding interactions and give a reasonable interpretation of the experimental results. The pressureinduced amorphization (PIA) in hydrogen-bonded crystals was first reported in ice22 and subsequently was reported in other hydrogen-bonded molecular systems.23,24 Hydrogenbonded supramolecular architectures are also ideal candidates to understanding the basic process of PIA because of their hydrogen-bonded networks. (14) Boldyreva, E. V. J. Mol. Struct. 2004, 700, 151–155. (15) Boldyreva, E. V. Russ. Chem. Bull., Int. Ed. 2004, 53, 1369–1378. (16) Olejniczak, A.; Katrusiak, A. J. Phys. Chem. B 2008, 112, 7183– 7190. (17) Lin, A. L.; Wang, K.; Zhao, Y.; Zou, B. Chem. J. Chin. Univ. 2008, 29, 1181–1184. (18) Katrusiak, A. Crystallogr. Rev. 1996, 5, 133 – 175. (19) Sikka, S. K.; Sharma, S. M. Phase Transitions 2008, 81, 907–934. (20) Allan, D. R.; Clark, S. J. Phys. Rev. Lett. 1999, 82, 3464–3467. (21) 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. (22) Mishima, O.; Calvert, L. D.; Whalley, E. Nature 1984, 310, 393–395. (23) Rao, R.; Sakuntala, T.; Arora, A. K.; Deb, S. K. J. Chem. Phys. 2004, 121, 7320–7325. (24) Ciabini, L.; Santoro, M.; Gorelli, F. A.; Bini, R.; Schettino, V.; Raugei, S. Nat. Mater. 2007, 6, 39–43.

Published on Web 2/25/2009

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possible transformations, we have performed in situ XRD with complementary Raman measurements. Besides, ab initio calculations were employed to gain insight into the mechanism of hydrogen bonding at high pressure. Our present study is an attempt to provide a better understanding of the nature of the hydrogen bonding and structural stability of the supramolecular architectures under high pressure.

Experimental Section

Figure 1. (Top) Asymmetric unit of M 3 2B; dashed lines represent intermolecular hydrogen bonds. (Bottom) Arrangements of melamine and boric acid molecules in the crystal of M 3 2B; view along crystallographic a axis.

Melamine-boric acid adduct (M 3 2B), well-known as raw material of the boron nitride nanotubes,25,26 crystallizes in monoclinic space group P21/c at ambient conditions.27 In Figure 1, we show the asymmetric unit of M 3 2B and the stacking arrangement of hydrogen-bonded supramolecular networks. When viewed along the a-axis, melamine and boric acid molecules are connected together by hydrogen bonds to form a layerlike structure in the bc-plane.28 Because of its interesting features, M 3 2B can be considered as a good example for studying structure stability of hydrogen-bonded supramolecular architectures. A comprehensive study of the vibrational spectra of M 3 2B has been reported to provide information about hydrogen bonds in the supramolecular networks.29,30 Pressure-induced chemical decomposition of boric acid and structure phase transitions of melamine have recently been addressed.31,32 Hence, it is of particular interest to investigate the influence of high pressure on the hydrogen-bonded supramolecular structures formed by melamine and boric acid. In the present work, we report a combined experimental and computational study of M 3 2B as a function of pressure. To elucidate the influence of pressure on the structure and stability of M 3 2B and the role of hydrogen bonds in the (25) Ma, R.; Bando, Y.; Sato, T.; Kurashima, K. Chem. Mater. 2001, 13, 2965–2971. (26) Ma, R.; Bando, Y.; Sato, T. Chem. Phys. Lett. 2001, 337, 61–64. (27) Kawasaki, T.; Kuroda, Y.; Nishikawa, H. J. Ceram. Soc. Jpn. 1996, 104, 935–938. (28) Roy, A.; Choudhury, A.; Rao, C. N. R. J. Mol. Struct. 2002, 613, 61– 66. : (29) Atalay, Y.; Avcl, D.; Bas-oglu, A.; Okur, I. J. Mol. Struct. (THEOCHEM) 2005, 713, 21–26. (30) Panicker, C. Y.; Varghese, H. T.; John, A.; Philip, D.; Nogueira, H. I. S. Spectrochim. Acta 2002, 58A, 1545–1551. (31) Kuznetsov, A. Y.; Pereira, A. S.; Shiryaev, A. A.; Haines, J.; Dubrovinsky, L.; Dmitriev, V.; Pattison, P.; Guignot, N. J. Phys. Chem. B 2006, 110, 13858–13865. (32) Ma, H. A.; Jia, X.; Cui, Q. L.; Pan, Y. W.; Zhu, P. W.; Liu, B. B.; Liu, H. J.; Wang, X. C.; Liu, J.; Zou, G. T. Chem. Phys. Lett. 2003, 368, 668–672.

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M 3 2B used for this study was prepared by hydrothermal synthesis.28 The details of the synthesis and the structure analysis have been reported. The crystal structure of the sample was confirmed by the X-ray diffraction powder pattern, and the Raman spectrum agreed well with that reported in the literature.28,30 High pressure experiments were performed using Mao-Belltype DAC at room temperature. A T301 stainless steel gasket was preindented by the diamonds and then drilled to produce a 0.15 mm diameter cavity for the sample. M 3 2B was then placed in the gasket hole together with a few small ruby chips for in situ measurements of the sample pressure using the standard ruby fluorescent technique.33 In the case of the layerlike structure of M 3 2B, which makes it a very soft material, experiments were carried out with no pressure transmitting medium. By observing the separation and widths of both R1 and R2 lines of ruby, we confirmed quasi-hydrostatic conditions over the whole pressure range of the present 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. The laser power on the sample was kept at 10 mW, and the Raman spectra were recorded in backscattering geometry. Prior to each measurement, the spectrometer was calibrated using the Si line. The resolution of the system was about 1 cm-1. The Raman spectra were collected in the range of 100-1200 cm-1 and 2800-3600 cm-1 as a function of pressure. Angle dispersive X-ray diffraction (ADXRD) measurements were performed at beamline X17-C of the National Synchrotron Light Source, Brookhaven National Laboratory. Monochromatic radiation at a wavelength of 0.4021 A˚ was used for pattern collection. The X-ray diffraction experiment after release of pressure was performed at 4W2 High Pressure Station of Beijing Synchrotron Radiation Facility (BSRF) with a wavelength of 0.6199 A˚. The Bragg diffraction rings were recorded with an imaging plate detector, and the XRD patterns were integrated from the images with FIT2D software.34 The XRD patterns were then indexed and refined by using the reflex module combined in the Materials Studio program. Ab initio calculations were performed with the pseudopotential plane-wave method based on density functional theory implemented in the CASTEP code.35 The local density approximation exchange-correlation functional was used in the calculations. Vanderbilt-type ultrasoft pseudopotentials were employed with a plane-wave cutoff energy of 300 eV.

Results and Discussion Representative X-ray diffraction patterns of M 3 2B are shown in Figure 2. It is evident that all the diffraction peaks shifted to higher angles with the increase of pressure, indicating a decrease in unit cell volume. As pressure increased, the peaks became broader, less intense, and some merged together (33) Mao, H. K.; Xu, J.; Bell, P. M. J. Geophys. Res. 1986, 91(B5), 4673– 4676. (34) Hammersley, A. P.; Svensson, S. O.; Hanfland, M.; Fitch, A. N.; Hausermann, D. High Pressure Res. 1996, 14, 235 – 248. (35) Segall, M. D.; Lindan, P. J. D.; Probert, M. J.; Pickard, C. J.; Hasnip, P. J.; Clark, S. J.; Payne, M. C. J. Phys.: Condens. Matter 2002, 14, 2717– 2744.

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Figure 3. Unit cell volume as a function of pressure. (Inset) anisotropic pressure response of the crystal lattices; solid lines are drawn as a guide to the eye. Figure 2. Representative X-ray diffraction patterns of M 3 2B at different pressures.

which indicated that the system was progressively evolving into a disordered state. The largest reduction in crystallinity was observed at 12 GPa and the X-ray diffraction peaks entirely vanished at 18 GPa, indicating the sample was completely amorphous in this condition. Moreover, the X-ray diffraction pattern indicated that M 3 2B returned to its ambient structure on complete release of pressure. The pressure dependences of the unit cell volume and lattice parameters at room temperature are illustrated in Figure 3. As was observed for many other organic crystals,36-38 the compressibility of M 3 2B was high and the compressional behavior of M 3 2B was anisotropic, with the a-axis being more compressible than the b- and c-axes. These compressional behaviors could be explained by taking into account the layerlike crystal packing and the hydrogen bond networks. The planar boric acid and melamine molecules were connected together by multiple hydrogen bonds to form a layerlike structure in the bc-plane, while the main interaction between the different layers was of van der Waals type. Therefore, the small compressibility of the b- and c- axis was explained by the strength of the multiple intermolecular hydrogen bonds between boric acid and melamine molecules. Moreover, the same arrangement of the molecules along the band c-axes was expected to behave in the same way. Because of weak van der Waals interactions between layers, the a-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,29,30 so we followed the assignments they presented. Figure 4 illustrates the Raman spectra of M 3 2B in the region of 100-1100 cm-1 under different pressures. Figure 5 gives the observed pressure-induced shifts of these modes. It can be seen that the external modes exhibited substantial blue shift (36) Boldyreva, E. V. J. Mol. Struct. 2003, 647, 159–179. (37) Dreger, Z. A.; Gupta, Y. M.; Yoo, C. S.; Cynn, H. J. Phys. Chem. B 2005, 109, 22581–22587. (38) Orgzall, I.; Emmerling, F.; Schulz, B.; Franco, O. J. Phys.: Condens. Matter 2008, 20, 295206.

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Figure 4. Selected Raman spectra of M 3 2B as function of increasing pressure in the wavenumber range 100-1100 cm-1. Spectra are vertically displaced for a clear presentation.

under pressure, as could be expected for organic crystals.39-41 This blue shift was due to a reduction of intermolecular distances, and the intermolecular coupling became stronger with pressure.42 Raman bands in this 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.43 Apart from a continuous shift to higher frequencies, the Raman bands also showed progressive broadening accompanied by a decrease in intensity. These features continued up to about 18 GPa, at which all the external lattice modes disappeared and excessive broadening of internal modes was also observed. The vanishing of well-defined lattice modes indicated a loss of a (39) Rao, R.; Sakuntala, T.; Godwal, B. K. Phys. Rev. B 2002, 65, 054108. (40) Park, T. R.; Dreger, Z. A.; Gupta, Y. M. J. Phys. Chem. B 2004, 108, 3174–3184. (41) Ciezak, J. A.; Jenkins, T. A.; Liu, Z.; Hemley, R. J. J. Phys. Chem. A 2007, 111, 59–63. (42) Goncharov, A. F.; Manaa, M. R.; Zaug, J. M.; Gee, R. H.; Fried, L. E.; Montgomery, W. B. Phys. Rev. Lett. 2005, 94, 065505. (43) Orgzall, I.; Franco, O.; Schulz, B. J. Phys.: Condens. Matter 2006, 18, 5269–5278.

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Figure 5. Pressure-induced Raman shifts of various modes of M 3 2B in the spectral region 100-1100 cm-1. Linear fits are shown for the frequency shifts. 23,42

long-range order. Additionally, the broadening of internal modes with a loss of intensity resulted from a random arrangement and distortion, both within the molecules and between them. Thus, the high pressure Raman investigation confirmed the conclusion from X-ray study that with increasing pressure the crystal lattice of M 3 2B became disordered and eventually amorphous at about 18 GPa. On release of pressure, there was progressive sharpening of all the observed Raman bands and increasing of the bands intensity, but large pressure hysteresis could be observed. After the pressure was released, the Raman spectrum was found to be the same as it was before pressure was applied, indicating that the compression is reversible. The PIA could be understood from the hydrogen-bonded supramolecular structure of M 3 2B and the cooperation between the van der Waals and hydrogen bonding interactions.44 The boric acid and melamine molecules formed the hydrogen bond networks through multiple hydrogen bonds. So the formation of such a long-range ordered crystal structure led to a large proportion of empty space. However, molecules in the crystals have a tendency to achieve closer packing by the application of high pressure.45 With increasing pressure, the increased energy of intermolecular interactions could rotate molecules or molecular fragments and distort the hydrogen bond networks. At sufficiently high pressure, the PIA took place to achieve closer packing with an effort to reduce the free energy. Thus, we attributed the PIA in M 3 2B to the competition between close packing and long-range order.46 The distorted hydrogen bonds could also be restored after the compression was removed, which accounts for the reversible crystalline-amorphous phase transition. Furthermore, it is significant to compare the high pressure behavior of M 3 2B with that of individual melamine and boric acid molecular crystals. Note that both of them consisted of molecules held together by intermolecular hydrogen bonds. Under high pressures, melamine32 and boric acid31 undergo phase transition or chemical decomposition at relative low pressure range. Nevertheless, the supramolecular adduct of M 3 2B undergoes a reversible PIA.

(44) Deb, S. K.; Rekha, M. A.; Roy, A. P.; Vijayakumar, V.; Meenakshi, S.; Godwal, B. K. Phys. Rev. B 1993, 47, 11491–11494. (45) Sharma, S. M.; Sikka, S. K. Prog. Mater. Sci. 1996, 40, 1–77. (46) Sikka, S. K.; Sharma, S. M. Curr. Sci. 1992, 63, 317–320.

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Figure 6. Selected Raman spectra of M 3 2B as function of increasing pressure in the N-H and O-H stretching frequency region. (a) Stretching modes of the hydrogen-bonded N-H and O-H bonds. (b) Stretching modes of the free N-H bonds. Gray arrows indicate red shifting of N-H and O-H stretching with a pressure increase.

Figure 7. Pressure-induced Raman shifts of the N-H and O-H stretching modes. Error bars are given where errors are larger than the size of the symbol.

Although the hydrogen atom positions could not be obtained from the X-ray diffraction data, Raman spectroscopy under high pressures provides detailed insights into the properties of hydrogen-bonded systems. Figure 6 shows the observed pressure-induced variations in the Raman spectra of the N-H and O-H stretching modes, while Figure 7 represents the pressure-induced Raman shifts of these modes. At ambient conditions, the bands observed at 3522, 3488, and 3416 cm-1 correspond to the free N-H bonds. The weak broad Raman bands centered around 3360 and 3305 cm-1 could be attributed to the stretching modes of the hydrogenbonded N-H bonds. The O-H stretching mode appeared as weak broad Raman bands around 3183 cm-1, and the other O-H stretching modes were too weak and broad to be identified as distinct peaks. In contrast to other modes, all the frequencies of N-H and O-H stretching vibrations underwent a moderate red shift with increasing pressure. Furthermore, the extent of the pressure-induced shifts differed among these modes. The hydrogen-bonded O-H and N-H stretching modes were found to be more affected by pressure Langmuir 2009, 25(8), 4787–4791

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Table 1. Ab Initio Calculated Changes in Bond Lengths (A˚) for Selected Hydrogen Bonds; Positive and Negative Values Indicate Elongation and Shortening, Respectively N(5)-H(4) 3 3 3 O(3)

O(3)-H(9) 3 3 3 N(3)

O(6)-H(12) 3 3 3 O(1)

N(6)-H(6)

pressure (GPa)

Δr(N-H)

Δr(H 3 3 3 O)

Δr(O-H)

Δr(H 3 3 3 N)

Δr(O-H)

Δr(H 3 3 3 O)

Δr(N-H)

2 4 6 8 10

0.006 0.010 0.013 0.017 0.019

-0.048 -0.076 -0.106 -0.127 -0.136

0.011 0.015 0.019 0.020 0.023

-0.038 -0.060 -0.081 -0.098 -0.098

0.012 0.022 0.032 0.042 0.072

-0.045 -0.080 -0.109 -0.136 -0.185

0.0028 0.0037 0.0044 0.0051 0.0057

Table 2. Ab Initio Calculated Bond Populations (e) for Selected Hydrogen Bonds under Different Pressures N(5)-H(4) 3 3 3 O(3) pressure (GPa) 0 2 4 6 8 10

N-H 0.64 0.64 0.63 0.63 0.62 0.62

H3 3 3O 0.14 0.15 0.16 0.17 0.18 0.19

O(3)-H(9) 3 3 3 N(3) O-H 0.46 0.46 0.46 0.46 0.46 0.46

than those of free N-H bonds. Additionally, the rapid broadening of the Raman modes with a loss of intensity was also disorder-induced. 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 3 3 3 A bonds.47-49 The behavior of red shifts can be understood qualitatively in terms of the electrostatic attraction between the proton and the atom A. When the D 3 3 3 A distance was reduced by compression, the electrostatic attraction between H 3 3 3 A increased, thus lengthening the D-H bond and reducing its stretching frequency. It is worth noting a slight decrease in the Raman shifts of these modes corresponding to the free N-H bonds. These N-H stretching modes should show blue shifts because they did not participate in hydrogen bonding. However, a slight lengthening of the N-H distance at high pressure which was obtained from our ab initio calculation shown below can explain this abnormal behavior. To gain further insight into the high-pressure behavior of hydrogen bonding, we performed ab initio calculations. The calculated pressure-induced changes in the D-H and H 3 3 3 A distances are shown in Table 1. Positive and negative values indicate elongation and shortening, respectively. From our calculations, there was a smooth decrease of the H 3 3 3 A distance over the whole investigated pressure range. At the same time, the D-H bond length increased with increasing pressure, implying pressure-induced red shifts in Raman spectra.50 It is also interesting to note that the free N-H bonds exhibited the same tendency on compression. This effect may be due to the attractive interaction between the positive H and electron rich A (around H) forcing the N-H bond elongation. Additionally, a Mulliken bond population analysis of the systems was performed to give a qualitative description of the relative strengths of the hydrogen bonds.51 Table 2 shows the calculated bond populations of hydrogen bonds under different pressures. The pressure(47) Reynolds, J.; Sternstein, S. S. J. Chem. Phys. 1964, 41, 47–50. (48) Moon, S. H.; Drickamer, H. G. J. Chem. Phys. 1974, 61, 48–54. (49) Hamann, S. D.; Linton, M. Aust. J. Chem. 1976, 29, 1641–1647. (50) Joseph, J.; Jemmis, E. D. J. Am. Chem. Soc. 2007, 129, 4620– 4632. (51) Segall, M. D.; Shah, R.; Pickard, C. J.; Payne, M. C. Phys. Rev. B 1996, 54, 16317–16320.

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H3 3 3N 0.26 0.28 0.29 0.29 0.30 0.30

O(6)-H(12) 3 3 3 O(1) O-H 0.48 0.47 0.46 0.45 0.44 0.42

H3 3 3O 0.22 0.24 0.25 0.27 0.29 0.32

N(6)-H(6) 0.70 0.70 0.70 0.70 0.70 0.69

induced changes of the D-H (include the free N-H) bond populations have revealed that the bond strength was slightly reduced. With increasing charge overlap at higher pressures, the H 3 3 3 A bond gets stronger. The bond population analysis of hydrogen bonds confirmed the interpretation from the bond distance analysis, which was in accord with our Raman spectra measurements.

Conclusion In contrast to phase transition and chemical decomposition of individual melamine and boric acid molecular crystals, the hydrogen-bonded supramolecular adduct of M 3 2B undergoes a reversible PIA. This may be the first example of PIA in a hydrogen-bonded supramolecular adduct, and we attributed this amorphization to the competition between close packing and long-range order. Furthermore, M 3 2B exhibited a high compressibility and a strong anisotropic compression, which can be explained by the layerlike crystal packing. In addition, the Raman modes of N-H and O-H stretching vibrations underwent a moderate red shift with increasing pressures and ab initio calculations contributed to a good description of this behavior. These findings 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 Prof. Dr. Jing Liu and Dr. Lingyun Tang for help on experiments. This work is supported by NSFC (Nos. 20773043, 20673048, and 10674053), PCSIRT (IRT0625), NCET-06-0313, RFDP (No. 20060183073), the Key Research Program of Education Ministry of China (No. 03057), the National Basic Research Program of China (Nos. 2001CB711201, 2005CB724400, and 2007CB808000), the Cultivation Fund of the Key Scientific and Technical Innovation Project of MOE of China, and the Postgraduate Innovative Foundation Program of Jilin University (No. 20080108). This work is also supported by NSF COMPRES EAR01-35554 and by US-DOE contract DE-AC02-10886 to NSLS. Portions of this work were performed at 4W2 HP-Station, Beijing Synchrotron Radiation Facility (BSRF) which is supported by Chinese Academy of Sciences (No. KJCX2-SW-N20KJCX2-SW-N03). DOI: 10.1021/la804034y

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