High Pressure Structural Investigation of Benzoic Acid: Raman

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High Pressure Structural Investigation of Benzoic Acid: Raman Spectroscopy and X-ray Diffraction Lei Kang, Kai Wang, Xiaodong Li, and Bo Zou J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b05001 • Publication Date (Web): 28 Jun 2016 Downloaded from http://pubs.acs.org on June 29, 2016

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The Journal of Physical Chemistry

High Pressure Structural Investigation of Benzoic Acid: Raman Spectroscopy and X-ray Diffraction Lei Kang,† Kai Wang, *,† Xiaodong Li,‡ and Bo Zou,*,† †State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, China. ‡Beijing Synchrotron Radiation Laboratory, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100039, China.

Corresponding author. E-mail: [email protected], [email protected], Tel: 86-431-85168882

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Abstract The structural stability of benzoic acid (C6H5COOH, BA), a hydrogen-bonded molecular crystal, has been investigated by Raman spectroscopy and angle-dispersive X-ray diffraction (ADXRD) up to ~ 18 GPa at room temperature. Under ambient conditions, benzoic acid molecules are arranged in two set of parallel planes and held together by hydrogen bonding and van der Waals interactions. Small changes (e.g., emergence of new peaks, splitting of original peaks) can be observed in the Raman spectra at high pressures. However, no obvious changes can be observed in the X-ray diffraction measurements, which rules out any symmetry/structure changes within this pressure range. The pressure dependence of lattice parameters are presented, which show monotonously decrease without any anomalies. The experimental isothermal pressure-volume data are well fitted by the third-order Birch-Murnaghan equation of state, yielding bulk modulus B0 = 41.7(6) GPa and a first pressure derivative B0 ' = 4.5(4). Axial compressibility shows obvious anisotropy, the a-axis is more compressible than b-axis and c-axis. Moreover, the near symmetrization limit of hydrogen bonds at high pressures is proposed from the first-principles calculations. Based on the Raman, XRD, and the first-principles calculations analysis, we propose that the high pressure structural stability of benzoic acid is associated with the special hydrogen-bonded dimer structure.


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Introduction Molecular crystals have drawn considerable attention in the past decades because of their important roles in materials, chemistry, and life science.1-3 These crystals are composed of molecules held together by intermolecular interactions. Compared with covalent and ionic bonds, these interactions among molecules (hydrogen bonding, and van der Waals force, etc.) are relatively weak, so molecular crystals are usually more compressible and have relatively low melting temperature. In particular, hydrogen bond is the most important and extensively investigated intermolecular interaction, because of its key role in structural stability of many fundamental constituents of life, such as organic acids and water at varying thermodynamic conditions.4-6 Moreover, the physical and chemical properties of molecular crystals are mainly determined by the cooperation of two or more kinds of intermolecular interactions. Thus, a deep understanding of the nature of intermolecular interactions and their cooperation is necessary, which is a prerequisite of searching and designing new molecular crystal materials with excellent properties. As one of the well-known basic thermodynamic parameters, pressure is extensively employed to explore the properties of matter and design new materials, through its effective tuning of interatomic distances, potentials, and thus the crystal structures.7-10 Compared with covalent bond (150 - 400 KJ/mol), hydrogen bond (8 50 KJ/mol) is far weak, so geometric parameters of hydrogen bond (e.g., bond strength, bond length, bond angle) can be altered more easily by external forces, which is expected to result in dramatic modifications in molecular rearrangements. A



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number of investigations on the structural behaviors of hydrogen-bonded molecular crystals under high pressure have been reported.11-20 For example, urea experienced four structural changes at 0.48, 0.6, 2.8, and 7.2 GPa, respectively.17-18 During the phase transitions, large variations were observed in hydrogen bonds, including breakage, formation, and distortion.18-20 For formamide, the formation of CH···O hydrogen bonds occurred at high pressures.20 Besides, pressure-induced hydrogen bond symmetrization is also of significant importance, which helps to understand proton dynamics in the complex biological processes in nature.5,15,21-23 Overall, pressure is a powerful tool for investigating hydrogen-bonded molecular crystals, which is of fundamental importance not only for scientific research but also for potential practical applications. As the simplest aromatic carboxylic acid, benzoic acid (BA) can represent a model compound of many active pharmaceutical ingredients (e.g., aspirin, salicylic acid, flufenamic acid, diflunisal). Furthermore, the crystal structure of BA comprises common C22 (8) synthon. At ambient conditions, BA crystallizes in the monoclinic system P21/c with four molecules (two dimers) in the unit cell. Each two monomers are linked by two intermolecular O-H···O hydrogen bonds. All of the other intermolecular distances are over 3.0 Å, and correspond to van der Waals interaction.24-26 The crystal structure and hydrogen-bonded networks are shown in Figure 1. Consequently, the crystal structural stability of BA is mainly supported by the balance between van der Waals and hydrogen bonding interactions. The first high-pressure study of BA was conducted using NMR, which revealed two phase


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transitions at 0.1 and 0.4 GPa, respectively, the second transition was associated with the reduced asymmetry of the H-atom potential.27 Moreover, the relation between H-dynamics and O···O distances up to 0.32 GPa at 5 K in C6D5COOH was also investigated.28 Z. P. Wang et al.29 performed Raman and photoluminescence investigations on BA under high pressures, three phase transformations were deduced from

changes

of

lattice

modes,

the

third

one

was

explained

as

a

crystalline-to-amorphous transition. Subsequently, single crystal X-ray diffraction study up to 2.21 GPa did not show any clear evidence for structural transitions.30 Recently, FTIR and Raman spectroscopy were used to investigate changes of hydrogen-bonded dimers in benzoic acid crystal, modification of the dimer structure and a phase transition were deduced from several changes in Raman spectra in the pressure range of 6 - 8 GPa.31 In summary, although the structural stability of BA has received considerable attention, previous investigations provide contradicting information on the high-pressure behaviors. In particular, there is still no structural information about BA at higher pressures available due to lack of X-ray diffraction (XRD) data, which is important for understanding the mechanism of phase stability. Thus, a thorough study of BA combining vibrational spectroscopy and synchrotron X-ray diffraction is necessary. In situ synchrotron X-ray diffraction measurement is powerful to probe structural changes under extreme conditions. Raman scattering is sensitive to variations of hydrogen bonds as well as lattice vibrations. In this study, we report comprehensive investigations of BA using Raman spectroscopy and angle-dispersive X-ray



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diffraction (ADXRD) techniques up to ~ 18 GPa. The structural stability and behaviors of hydrogen bonds are presented. This work can be helpful for better understanding of hydrogen bond as well as the stability of hydrogen-bonded crystals.


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Experimental section Benzoic acid crystals were purchased from Alfa Aesar Co. (purity 99%) and used as received. Symmetric diamond anvil cells (DACs) with 400 µm culet-size diamonds were used in in situ high-pressure Raman spectroscopy and synchrotron X-ray diffraction measurements. T301 steel was used as gasket, a hole with diameter of 200 µm and thickness of 30 µm served as sample chamber. Considering BA crystal is soluble in most common pressure transmitting media, the XRD and Raman experiments were conducted without pressure medium. The quasi-hydrostatic pressure condition was confirmed by the sharp and well separated ruby R1 and R2 peaks. The standard ruby fluorescence method was employed to calculate pressures in the sample chamber.32 All of the experiments were performed at room temperature. High-pressure Raman experiments were carried out using Renishaw inVia Raman microscope with standard backscattering configuration. The visible 633 nm line laser with output power of 10 mW was applied as excitation source. The spectral resolution of the Raman system is around 1 cm−1. High-pressure angle-dispersive X-ray diffraction (ADXRD) measurements were conducted at 4W2 High Pressure Station of the Beijing Synchrotron Radiation Facility (BSRF) (wavelength 0.6199 Å, beam size 30 × 20 µm2). Part of ADXRD experiments were performed at BL15U1 beamline of Shanghai Synchrotron Radiation Facility (SSRF) (wavelength 0.6199 Å, beam size 4 × 7 µm2). Average acquisition time was set to 400 s. CeO2 powder was employed to calibrate geometric parameters before measurements. The diffraction rings were recorded with a MAR345 imaging plate detector. The collected 2-D images



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were integrated and converted into one dimensional plots of intensity versus 2θ using FIT2D software.33 XRD patterns were indexed and refined using Reflex module combined in the commercial Material Studio 5.5 program (Accelrys Inc.). The structural optimizations and phonon properties were performed using norm-conserving pseudopotential with CASTEP code,34 based on the first-principles density functional theory (DFT) method.35 The local density approximation (LDA) with the Ceperley-Alder-Perdew-Zunger (CA-PZ) exchange-correlation functional was used in the high-pressure calculations. Structural optimizations, including atomic positions

and

lattice

constants

were

performed

Broyden-Fletcher-Goldfarb-Shanno algorithm (BFGS).36


using

the

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Results and discussion The Experimental and calculated Raman spectra of BA collected at ambient conditions is shown in Figure 2. The spectrum of BA was calculated using DFT method and it shows very similar features to our experimental result. The unit cell contains four molecules (two dimers) with space group P21/c and the point group is C2h. According to the group theory, 156 internal vibrational modes are predicted, including 78 Raman active (39 Ag and 39 Bg) and 78 IR active (39 Au and 39 Bu). There are also 21 intermolecular vibrations (lattice modes), out of which 12 modes are Raman active (6 Ag and 6 Bg), while the remaining 9 modes are IR active (5 Au and 4 Bu). However, some of the predicted Raman modes can not be detected because of their weak intensities. Two regions can be divided within the observed Raman spectra. Lattice modes can be observed in the low frequency region, which are involved in relative movements among molecules. Internal modes arising from molecular deformation are detected in the high frequency region. The assignments of Raman active modes under ambient conditions are based on literatures.37-40 Among these Raman modes, five are associated with hydrogen-bonding vibrations, which are labeled with νi (i=1-5), as is shown in Figure 2. The intermolecular O-H···O vibrations, which can be described by the relative motion of two monomers, were identified at about 94, 118, 196 and 422 cm-1 (ν1-ν4), while mode located at about 1445 cm-1 is related to O-H vibrations (ν5). The behaviors of these hydrogen-bond related modes as well as other vibrations are investigated at high pressure up to ~ 17.6 GPa.



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The evolutions of Raman spectra of BA crystal in the frequency regions 50 - 400 cm-1 at selected pressures up to ~ 17.6 GPa are presented in Figures 3(a). The Raman modes in this region contain lattice and hydrogen-bonding vibrations. These modes are related to movements of all atoms in a unit cell, thus, they are very sensitive to pressure because of the weak intermolecular interactions.41 The modes related to O-H···O vibrations are denoted by ν1, ν2, and ν3. When pressure is increasing, lattice and hydrogen-bonding modes shift monotonously towards higher frequencies without abnormal changes (blue shifts), though with different shift rates. The observed blue shifts indicate the reduction of intermolecular distances, which is due to the enhancements of interactions among adjacent molecules.42 Due to the different shift rates of the lattice modes, a peak marked with an asterisk appears at about 138 cm-1 at 0.3 GPa. This peak is not observed at ambient conditions because of its low intensity and overlapped by nearby strong peak. When pressure is raised from 0.3 to 1.0 GPa, the relative intensities among the lattice modes show great changes. Besides, the peak positions and bandwidth show smooth and expected changes which result from the mode anharmonicity at high pressures. There is no abnormal changes can be observed in the pressure range of 0 - 1.0 GPa, which is not agree with the reported phase transitions from NMR and Raman experiments.27,29 The single-crystal X-ray diffraction experiment also rules out the possibility of phase transition below 2.21 GPa.30 Upon further compression to 3.6 GPa, all the peaks become broadened and show gradual decrease in intensities. A peak marked with an asterisk appears in the low frequency region at around 55 cm-1, which could be due to the great blue shift of


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this mode from low frequency. Some peaks cannot be detected in the low-pressure range because of intensities and/or instrument restriction. When the pressure increases to 17.6 GPa, the spectral features still show similar features compared with that of 3.6 GPa, except a general increase of frequencies of the Raman peaks and the continuous decrease of intensities. The spectrum seems flat at 17.6 GPa, no peaks can be resolved. There are two common reasons for the flat features in Raman spectra under high pressure. One is amorphous state due to the loss of long-range order, and the other is that the luminescence background is so strong that the Raman signal is obscured. For this experiment, accompanied by the increasing luminescence background, the signal is too weak at high pressure, and eventually cannot be resolved. This phenomenon is also found in the previous Raman experiment.31 In the entire pressure range (0 - 17.6 GPa), we did not find any strong evidence for the phase transition proposed at 0.5, 3.6, and 6 - 8 GPa, respectively, in lattice region.27,29,31 Moreover, the Raman spectrum can be fully recovered when complete release of pressure, as is shown in Figure 3(a), which

is

contrast

with

previous

Raman

data

that

an

irreversible

crystalline-to-amorphous transitions is detected at pressure above 11.1 GPa.29 The pressure dependence of Raman modes in the frequency range of 50 - 300 cm-1 is depicted in Figure 3(b). All these peaks show monotonously towards higher frequencies, no discontinuities can be observed. It is also worth noting that the shifts of lattice modes show obvious slopes at high pressures, which is due to the weak intermolecular interactions in BA structure. Raman spectra as a function of pressure in the range of 400 - 1900 cm-1 are



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summarized in Figures 4(a). Analysis of Raman internal modes can be used to explore the local changes of chemical environment around specific groups.43 With increasing pressure, most of the internal modes shift gradually towards higher frequencies. Pressure decreases the distances between neighboring molecules and between groups and atoms within individual molecules, which results in the increases of forces of repulsion and steepening the curvatures of the potential wells that govern the vibrations.44 Two hydrogen bonding related modes can be observed in this spectral region. The combination of O-H···O stretching and (CC) ring bending vibrations is marked by ν4, and the O-H bending mode is denoted as ν5. With increasing pressure, the intensities of ν4 and ν5 show continuous decrease, and eventually disappear at higher pressures. With further compression to 3.6 GPa, the peak assigned to torsion of (CC) ring at about 620 cm-1 shows splitting, which is related to the different pressure dependence of the two modes that cannot be resolved at ambient conditions. There is also a reversal in intensity of the two peaks (marked with arrows) around 800 cm-1. Moreover, a new peak marked with an asterisk appears at about 1507 cm-1, which can be explained that the intensity of the mode is too weak (lower than background) at ambient conditions and pressure increases its intensity. Red shift is observed for the peak at about 1635 cm-1 marked with an arrow, which can be explained as the smaller chemical force constant and increased bond length induced by pressure.45,46 The peak is related to (CC) ring stretching vibration, and it merges into one peak with the nearby strong peaks at 5.1 GPa. No obvious changes for structural transition in this frequency range are observed, which indicate the stability of the crystal structure. The


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evolution of the internal modes as a function of pressure in the range of 400 - 1900 cm-1 is depicted in Figure 4(b). With the increasing pressure, most of the internal modes display increasing frequencies. However, the internal modes show obvious smaller pressure coefficients compared with that of lattice modes. This can be explained that the covalent bonds are stronger than noncovalent bonds, and therefore, show much lower compressibility. Figures 5(a) and 5(b) display the evolution of the typical Raman spectra and the corresponding pressure dependence of internal modes in the range of 2800 - 3400 cm-1. Peaks in this region are related to C-H and O-H stretching vibrations. However, the stretching bands of O-H (v(O-H)) are very weak in Raman spectra. They are largely hidden by the extremely intense stretching bands of C-H at about 3000 cm-1， and cannot be distinguished from overtones and combinations in the same spectra range.47 The phenomenon can also be found in other compound with both C-H and O-H bonds, such as p-aminobenzoic and maleic hydrazide.14,16 It is noted that relative intensities of the two peaks at around 3070 cm-1 show continuous changes with increasing pressure. In addition, a new mode marked with an asterisk appears around 3143 cm-1, which is very weak and probably due to the emerging of the initially obscured peak. Upon further compression, there is no obvious change detected, except the gradually reduced intensities of these peaks with increasing pressure. The spectrum is also reversible upon complete decompression. The pressure dependence of the Raman peak positions in this region is shown in Figure 5(b). The peak positions gradual shift to high frequencies, indicating the strengthened chemical bonds and the



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more compact structure. To further investigate the structural stability and provide more structural information of BA at high pressures. We conducted angle dispersive X-ray diffraction (ADXRD) experiment, which is believed to be a direct and powerful probe for structural change. Representative ADXRD patterns of BA at various pressures up to 18.1 GPa are shown in Figure 6(a). Some peaks are rather weak, which could be due to the weak scattering of light elements in BA. With increasing pressure, all the diffraction peaks shift towards higher two-theta angles, which is due to the reduced interplanar distances as well as decrease of unit cell volume under high pressure. The intensity of (100) peak reduces gradually with pressure increasing, and eventually merges with the nearby stronger peak (-102) at 2.8 GPa. Upon further compression, there is no abnormal change observed up to the highest pressure 18.1 GPa except the gradually normal reduced intensities of peaks, which can be explained by the thinned sample at high pressures. No new peaks can be detected in this pressure range, which indicates the crystal structure of BA is stable and rules out any structural transitions reported.27,29,31 Figure 6(b) shows the pressure dependence of d-spacing values of main peaks, peaks shift gradually to lower d-spacing values without any discontinuities (abrupt changes), with (100) and (-102) peaks merging together at about 2.8 GPa. Moreover, the diffraction pattern still shows good characteristics of crystal when pressure is raised to 11.8 GPa, no obvious sign of amorphous state can be observed, which is not consistent with the crystalline-to-amorphous transition at 11.1 GPa.29 It is noted that the diffraction peaks are broadening and some peaks


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vanish into the background (such as (002)) at high pressures, suggesting that the crystallinity of BA sample is decreased. We have performed refinements of the XRD patterns, selected refinement results at ambient and high pressures (0.7, 2.8, 4.7, 8.1 and 15.0 GPa) are shown in Figures S1. Rietveld refinement of the diffraction pattern at 1 atm shows good agreement with P21/c space group, generating lattice constants a = 5.51(5) Å, b = 5.15(8) Å, c = 21.98(1) Å, β = 97.40(8)°, and unit cell volume V = 620.19(1) Å3, which is highly consistent with the literatures.24-26 The XRD patterns at high pressures can be well refined with the ambient space group P21/c, which, together with the normal changes in Raman spectra and the continuous decrease of unit-cell parameters under high pressure, rule out any crystal phase transitions within this pressure range. The experimental unit-cell parameters and atomic coordinates of BA at 1 atm, 0.7, 2.8, 4.7, 6.9, 8.1, 11.8 and 15.0 GPa are also presented in Table S1. The present XRD result can not provide evidence for the proposed phase transitions of BA in previous Raman measurements, which indicate that it is not always feasible to define a phase transition using optical spectroscopy only. Raman scattering is sensitive to vibration of bonds and groups, it is powerful to probe the local changes of chemical environment of specific groups. Compared with Raman spectra, X-ray diffraction is a direct and reliable technique to explore the molecular arrangement changes in crystal, which can help to understand the macro level of movements of molecules, such as crystal structural transformation and amorphization. The structural stability of BA is also in agreement with the single-crystal X-ray diffraction experiment.30 The comparison of diffraction patterns of BA is presented in Figure S2.



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There are no abnormal changes observed in both XRD experiments, which indicate the stable of the BA crystal structure. To explore the compressibility of BA, the lattice parameters and unit cell volume at various pressures were calculated by fitting the diffraction patterns using Rietveld refinements. The pressure evolution of the unit-cell parameters (a, b, c, V) of BA is plotted in Figures 7(a) and 7(b). With increasing pressure, lattice parameters show monotonically decrease without abrupt changes, indicating the stability of the crystal structure. The experimental pressure-volume data are fitted by a third-order Birch-Murnaghan (BM) equation of state (EoS)48:

3B P(V ) = 0 2

 V  7 3  V  5 3   3  V  2 3   ' 0 0   −    1 + ( B0 − 4 )  0  − 1  V  V    4  V     

Where V0 is the unit cell volume at ambient pressure, V is the unit cell volume at pressure P. B0 is isothermal bulk modulus, and B0 ' is the first pressure derivative of the bulk modulus. The solid line in Figure. 7(a) represents the fitted results. Results of EoS yield the isothermal bulk modulus B0 = 41.7(6) GPa and a first pressure derivative B0 ' = 4.5(4). The unit cell volume is reduced by about 32.4% in the pressure range of 0

- 18.1 GPa, showing considerable compressibility, which is associated with the weak intermolecular interactions. We also performed simulations of the BA structures under high pressure, the comparison of unit cell volume between simulation and experiment is shown in Figure S3. The experimental data of unit cell volume show good agreement with that of simulation. Figure 7(b) shows the evolution of lattice parameters. The decreases of parameters are 13.9% for a-axis, 9.8% for b-axis, and 9.7% for c-axis from 0 to 18.1 GPa. Obviously, the compression behavior of the unit


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cell axes is anisotropic and the crystallographic a-axis is more compressible than b-axis and c-axis. The pressure dependence of β angle is also shown in Figure S4,

which shows a smooth increase and no anomalies are observed. In order to understand the anisotropic behavior, it is essential to consider the crystal structure of BA. The crystal structure of BA is built of two parallel sets of dimers and supported by two kinds of intermolecular interactions (hydrogen bond, van der Waals interactions). The BA molecules form relatively smaller angles with the b-c planes, and a-axis is almost perpendicular to all hydrogen bonds. Hydrogen bond is stronger than van der Waals interaction, thus showing much harder to be compressed along its direction. Thus, when external pressure is applied, a-axis contracts more quickly and shows larger compressibility. X-ray scattering is not sensitive to the position changes of hydrogen atoms, so the first principles calculations were used to elucidate the behaviors of hydrogen bond at high pressures. Figures 8(a) and 8(b) show the optimized BA dimer structures at 1 atm and 17 GPa, respectively. At ambient conditions, the bond of H···O is almost two times length compared with that of O-H bond. When pressure is applied, the distance between the two O atoms in a hydrogen bond is reduced, the electrostatic attraction between the proton and the O atom (H···O) increases, thus lengthening the O-H bond and reducing its restoring force, which induces the blue shift of O-H bending mode ν5 (Figure 4(a)). At the same time, the distance of H···O is decreased. When pressure is increased to 17.0 GPa, the bond distance of O-H increases to 1.157 Å, which is almost the same with H···O distance (1.216 Å). The close distances between O-H and H···O



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bonds indicate that the electron cloud of H atom is almost equally distributed between the two O atoms, and the electrostatic attraction between H and O atoms (H···O) possess a substantial covalent characteristic. The C-O and C=O in carbonyl group also show smaller difference of bond length from 0.011 Å (1 atm) to 0.005 Å (17.0 GPa). Thus, the symmetrization of hydrogen bond is expected in BA at higher pressures, which can be found in many compounds containing X-H···X hydrogen bonds.5,15,21-23 Moreover, the difference of bond length between C-O and C=O as a function of pressure is shown in Figure S5. One can see that the difference of bond length between C-O and C=O is continuous decreasing when pressure is increasing, which indicate the symmetrization of hydrogen bonds will eliminate the distinction between single and double C-O bonds. Figure 9(a) displays the calculated pressure dependence distances of O-H, H···O and O-H···O. Obviously, with increasing pressure, the hydrogen bonding (O-H···O) distance decreases. Meanwhile, the distance of covalent bond O-H increases with increasing pressure, while the distance of electrostatic interaction H···O decreases. The nearly equal O-H and H···O lengths suggests the proton moves close to the midpoint of O-H···O and the near symmetrization limit of hydrogen bonds.5,15,21 It is noticed that the distances (O-H, H···O) change quickly at the low pressure region (0 - 10 GPa), while shows very small change at higher pressures. The ratio of distances as a function of pressure is shown in Figure 9(b), which shows similar behavior with that of bond distances. Pressure increases the bond energy both for O-H and H···O, the repulsive forces between the atoms will increase, thus the slower change of bond distances at higher pressures.


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In summary, the Raman and ADXRD results provide strong evidence for the stability of BA structure. Under ambient conditions, the BA crystal structure is supported by the two dominant interactions (hydrogen bonding and van der Waals forces interactions). The structural stability can be explained by the balance between these two interactions. Increasing pressure induces the reduced distances between the adjacent BA molecules, resulting in enhanced van der Waals forces between neighboring BA molecules. Meanwhile, hydrogen bonding interactions are also strengthened due to the reduced lengths of hydrogen bonds. This process will make contributions to the total Gibbs free energy within the structure. However, for the crystal structure of BA, it is built with planar dimers. Two monomers in each dimer are linked by two hydrogen bonds, which could resist the effect of pressure along hydrogen-bonding direction. This high-pressure behavior can also be found in pyrazinamide polymorphs.49 The energy for structural transformation of BA is not high enough in this pressure range, and thus cannot cross the barriers. The present results may be helpful in exploring the stability of molecular crystals and pressure can be potentially applied in the pharmaceutical industry.



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Conclusion The vibrational and structural properties of benzoic acid have been investigated using in situ high-pressure Raman spectroscopy and synchrotron X-ray diffraction up to ~ 18.0 GPa. None of the postulated structural transitions have been confirmed from the present results. The isothermal pressure-volume relationship is well presented by the third-order Birch-Murnaghan equation of state. Axial compressibility shows obvious anisotropy, the a-axis is more compressible than b-axis and c-axis. We propose the stability is associated with the special dimer structures and hydrogen bonding interaction. We hope the present results can provide some insight for better understanding of hydrogen bonds as well as the structural stability of molecular crystals under high pressure.


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Acknowledgments The authors want to show their reverence and gratitude to Dr. Xiao-jia Chen (HPSTAR, Shanghai) and Dr. Viktor Struzhkin (Geophysical Laboratory, Carnegie Institution of Washington) for their helpful instructions and discussions. This work is supported by NSFC (No. 91227202), RFDP (No. 20120061130006) and Open Project of State Key Laboratory of Superhard Materials (Jilin University): 201506. X-ray diffraction experiments were conducted at 4W2 beamline, Beijing Synchrotron Radiation Facility (BSRF) which is supported by Chinese Academy of Sciences (No. KJCX2-SW-N03, KJCX2-SW-N20). Portions of this work were performed at the 15U1 beamline at the Shanghai Synchrotron Radiation Facility (SSRF).



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Supporting Information Available: The refinement result of XRD patterns of BA at 1atm, 0.7, 2.8, 4.7, 8.1, 15.0 GPa. The comparison of reduced unit-cell volume between calculation and experiment. The comparison of diffraction patterns between the single-crystal experiment and our experiment. The pressure dependence of monoclinic β angle. The difference of bond length between C-O and C=O as a function of pressure. The experimental unit-cell parameters and atomic coordinates of BA at 1 atm, 0.7, 2.8, 4.7, 6.9, 8.1, 11.8 and 15.0 GPa. This information is available free of charge via the Internet at http://pubs.acs.org.


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References 1. Silinsh, E. A. Organic Molecular Crystals; Springer. Berlin, 1980. 2. Hemley, R. J.; Dera, P. Molecular Crystals. Rev. Mineral. Geochem. 2000, 41, 335-419. 3. Jones, J. T. A.; Hasell, T.; Wu, X.; Bacsa, J.; Jelfs, K. E.; Schmidtmann, M.; Chong, S. Y.; Adams, D. J.; Trewin, A.; Schiffman, F., et al. Modular and Predictable Assembly of Porous Organic Molecular Crystals. Nature 2011, 474, 367-371. 4. Luzar, A.; Chandler, D. Hydrogen-Bond Kinetics in Liquid Water. Nature 1996, 379, 55-57.

5. Goncharov, A. F.; Manaa, M. R.; Zaug, J. M.; Gee, R. H.; Fried, L. E.; Montgomery, W. B. Polymerization of Formic Acid under High Pressure. Phys. Rev. Lett. 2005, 94, 065505.

6. Jeffrey, G. A.; Maluszynska, H. A Survey of Hydrogen Bond Geometries in the Crystal Structures of Amino Acids. Int. J. Biol. Macromol. 1982, 4, 173-185. 7. Dalladay-Simpson, P.; Howie, R. T.; Gregoryanz, E. Evidence for A New Phase of Dense Hydrogen above 325 Gigapascals. Nature 2016, 529, 63-67. 8. Drozdov, A. P.; Eremets, M. I.; Troyan, I. A.; Ksenofontov, V.; Shylin, S. I.

Conventional Superconductivity at 203 Kelvin at High Pressures in the Sulfur Hydride System. Nature 2015, 525, 73-76. 9. Wang, L.; Liu, B.; Li, H.; Yang, W. G.; Ding, Y.; Sinogeikin, S. V.; Meng, Y.; Liu, Z.; Zeng, X.; Mao, W. L. Long-Range Ordered Carbon Clusters: A Crystalline Material with Amorphous Building Blocks. Science 2012, 337, 825-828. 10. Akaishi,

M.;

Kanda,

H.;

Yamaoka,

S.

Synthesis

of

Diamond

from

Graphite-Carbonate System under Very High Temperature and Pressure. J. Cryst. Growth. 1990, 104, 578-581.

11. Tomkowiak, H.; Olejniczak, A.; Katrusiak, A. Pressure-Dependent Formation and Decomposition of Thiourea Hydrates. Cryst. Growth Des. 2012, 13, 121-125.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

12. Katoh, E.; Yamawaki, H.; Fujihisa, H.; Sakashita, M.; Aoki, K. Raman Study of Phase Transition and Hydrogen Bond Symmetrization in Solid DCl at High Pressure. Phys. Rev. B 2000, 61, 119. 13. Ninet, S.; Datchi, F.; Klotz, S.; Hamel, G.; Loveday, J. S.; Nelmes, R. J. Hydrogen Bonding in ND3 Probed by Neutron Diffraction to 24 GPa. Phys. Rev. B 2009, 79, 100101. 14. Yan, T.; Wang, K.; Duan, D.; Tan, X.; Liu, B.; Zou, B. P-Aminobenzoic Acid Polymorphs under High Pressures. RSC Adv. 2014, 4, 15534-15541. 15. Bhatt, H.; Murli, C.; Mishra, A. K.; Verma, A. K.; Garg, N.; Deo, M. N.; Chitra, R.; Sharma, S. M. Hydrogen Bond Symmetrization in Glycinium Oxalate under Pressure. J. Phys. Chem. B 2016, 120, 851-859. 16. Wang, K.; Liu, J.; Yang, K.; Liu, B.; Zou, B. High-Pressure-Induced Polymorphic Transformation of Maleic Hydrazide. J. Phys. Chem. C 2014, 118, 8122-8127. 17. Bridgman, P. W. Polymorphic Transitions of Solids under Pressure. Proc. Am. Acad. Arts Sci. 1916, 52, 91-187.

18. Olejniczak, A.; Ostrowska, K.; Katrusiak, A. H-Bond Breaking in High-Pressure Urea. J. Phys. Chem. C 2009, 113, 15761-15767. 19. Zieliński, W.; Katrusiak, A. Hydrogen Bonds NH···N in Compressed Benzimidazole Polymorphs. Cryst. Growth Des. 2012, 13, 696-700. 20. Gajda, R.; Katrusiak, A. Pressure-Promoted CH···O Hydrogen Bonds in Formamide Aggregates. Cryst. Growth Des. 2011, 11, 4768-4774. 21. Xu, W.; Greenberg, E.; Gregory, K. R.; Moshe, P. P. Pressure-Induced Hydrogen Bond Symmetrization in Iron Oxyhydroxid. Phys. Rev. Lett. 2013, 111, 175501. 22. Goncharov, A. F.; Struzhkin, V. V.; Somayazulu, M. S.; Hemley, R. J.; Mao, H. K. Compression of

Ice to 210 Gigapascals: Infrared Evidence for A Symmetric

Hydrogen-Bonded Phase. Science 1996, 273, 218-220. 23. Loubeyre, P.; LeToullec, R.; Wolanin, E.; Hanfland, M.; Hausermann, D. Modulated Phases and Proton Centring in Ice Observed by X-ray Diffraction up to 170 GPa. Nature 1999, 397,503-506.


Page 24 of 47

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


24. Sim, G. A.; Robertson, J. M.; Goodwin, T. H. The Crystal and Molecular Structure of Benzoic Acid. Acta Crystallogr. 1955, 8, 157-164. 25. Feld, R.; Lehmann, M. S.; Muir, K. W.; Speakman, J. C. The Crystal Structure of Benzoic Acid: A Redetermination with X-rays at Room Temperature; A summary of Neutron-Diffraction Work at Temperatures Down to 5 K. Z. Kristallogr, 1981, 157, 215-231.

26. Bruno, G.; Randaccio, L. A Refinement of the Benzoic Acid Structure at Room Temperature. Acta Cryst. B 1980, 36, 1711-1712. 27. Horsewill, A. J.; McDonald, P. J.; Vijayaraghavan, D. Hydrogen Bond Dynamics in Benzoic Acid Dimers as A Function of Hydrostatic Pressure Measured by Nuclear Magnetic Resonance. J. Chem. Phys. 1994, 100, 1889-1894. 28. Brougham, D. F.; Horsewill, A. J.; Ikram, A.; Ibberson, R. M.; McDonald P. J.; Pinter-Krainer, M. The Correlation Between Hydrogen Bond Tunneling Dynamics and the Structure of Benzoic Acid Dimers. J. Chem. Phys. 1996, 105, 979-982. 29. Wang, Z. P.; Tang, X. D.; Ding, Z. J. Raman and Photoluminescence Spectroscopy Study of Benzoic Acid at High Pressures. J. Phys. Chem. Solids, 2005, 66, 895-901. 30. Cai, W.; Katrusiak, A. Pressure Effects on H-Ordering in Hydrogen Bonds and Interactions in Benzoic Acid. CrystEngComm, 2012, 14, 4420-4424. 31. Tao, Y. Dreger, Z. A.; Gupta, Y. M. High Pressure Effects on Benzoic Acid Dimers: Vibrational Spectroscopy. Vib. Spectrosc. 2014, 73, 138-143. 32. Mao, H.; Xu, J.-A.; Bell, P. Calibration of the Ruby Pressure Gauge to 800 kbar under Quasi-hydrostatic Conditions. J. Geophys. Res. 1986, 91, 4673-4676. 33. Hammersley, A. P.; Svensson, S. O.; Han fland, M.; Fitch, A. N.; Hausermann, D. Two-Dimensional Detector Software: From Real Detector to Idealised Image or Two-Theta Scan. High Pressure Res. 1996, 14, 235-248. 34. Clark, S. J.; Segall, M. D.; Pickard, C. J.; Hasnip, P. J.; Probert, M. J.; Refson, K.; Payne, M. C.Z. First Principles Methods Using CASTEP. Kristallogr. 2005, 220, 567-570.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

35. Troullier, N.; Martins, J. L. Efficient Pseudopotentials for PlaneWave Calculations. Phys. Rev. B 1991, 43, 1993-2006.

36. Pfrommer, B. G.; Côte, M.; Louie, S. G.; Cohen, M. L. Relaxation of Crystals with the Quasi-Newton Method. J. Comput. Phys. 1997, 131, 233-240. 37. Klausberger, G.; Furić, K.; Colombo, L. Vibrational Spectra and Normal Mode Calculations of Benzoic Acid Single Crystals. J. Raman Spectrosc. 1977, 6, 277-281. 38. Colombo, L.; Furić, K. Low-Frequency Raman Spectrum of Benzoic Acid Single Crystals. Spectrochim. Acta. A 1971, 27, 1773-1784. 39. Kolesov, B. A. Unusual Behavior of Benzoic Acid at Low Temperature: Raman Spectroscopic Study. Spectrochim. Acta. A 2015, 142, 320-323. 40. Stepanian, S. G.; Reva, I. D.; Radchenko, E. D.; Sheina, G. G. Infrared Spectra of Benzoic Acid Monomers and Dimers in Argon Matrix. Vib. Spectrosc 1996, 11, 123-133. 41. Harvey, K. B.; McQuaker, N. R. Low Temperature Infrared and Raman Spectra of the Ammonium Halides. J. Chem. Phys. 1971, 55, 4390-4396. 42. Boldyreva, E. High-Pressure Diffraction Studies of Molecular Organic Solids. A Personal View. Acta Crystallogr., Sect. A 2008, 64, 218-231. 43. Li, S.; Wang, K.; Zhou, M.; Li, Q.; Liu, B.; Zou, G.; Zou, B. Pressure-Induced Phase Transitions in Ammonium Squarate: A Supramolecular Structure Based on Hydrogen-Bonding and π-Stacking Interactions. J. Phys. Chem. B 2011, 115, 8981-8988. 44. Torabi, A.; Song, Y.; Staroverov, V. N. Pressure-Induced Polymorphic Transitions in Crystalline Diborane Deduced by Comparison of Simulated and Experimental Vibrational Spectra. J. Phys. Chem. C 2013, 117, 2210-2215. 45. Hamann, S. D.; Linton, M. The Influence of Pressure on the Infrared Spectra of Hydrogen-Bonded Solids. III. Compounds with NH···X Bonds. Aust. J. Chem.

1976, 29, 1641-1647.


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46. Mishra, A. K.; Murli, C.; Sharma, S. M. High Pressure Raman Spectroscopic Study of Deuterated γ-Glycine. J. Phys. Chem. B 2008, 112, 15867-15874. 47. Fillaux, F.; Limage, M. H.; Romain, F. Quantum Proton Transfer and Interconversion in The Benzoic Acid Crystal: Vibrational Spectra, Mechanism and Theory. Chem. Phys. 2002, 276, 181-210. 48. Birch, F. The Effect of Pressure Upon the Elastic Parameters of Isotropic Solids, According to Murnaghan's Theory of Finite Strain. J. Appl. Phys. 1938, 9, 279-288. 49. Tan, X.; Wang, K.; Li, S. R.; Yuan, H. S.; Yan, T. T.; Liu, J.; Yang, K.; Liu, B. B.; Zou, G. T.; Zou and Zou, B. Exploration of the Pyrazinamide Polymorphism at High Pressure. J. Phys. Chem. B, 2012, 116, 14441-14450.



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Figures Figure 1. Crystal structure and hydrogen-bonded networks of BA at ambient conditions in the ac-plane. The hydrogen bonds are marked as dashed lines.


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Figure 2. Experimental and calculated Raman spectra of BA at ambient conditions. The omitted spectral regions are due to the lack of spectroscopic features. All the assignments of the Raman modes are labeled above each band. Bands related to hydrogen-bonding vibrations are labeled with νi (i=1-5).



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Figure 3. (a) Selected Raman spectra of lattice modes in BA at various pressures in the range of 50 - 400 cm-1. (b) Peak positions in lattice and hydrogen-bonding region versus pressure. Peaks related to O-H···O vibrations are labeled with ν1, ν2, and ν3. The peaks marked with asterisks are the new observed peaks, and the peaks marked with down-facing arrows represent their decreasing intensities with increasing pressure.


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Figure 4. (a) Selected Raman spectra of internal modes in BA at various pressures in the range of 400 - 1900 cm-1. (b) Peak positions in internal region versus pressure. The combination of O-H···O stretching and (CC) ring bending vibrations is marked by ν4, and the O-H bending mode is denoted as ν5. The peaks marked by up-facing and down-facing arrows represent their increasing and decreasing intensities with increasing pressure.



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Figure 5. (a) Selected Raman spectra of C-H ring stretching vibration region in BA at various pressures in the range of 2800 - 3400 cm-1. (b) Peak positions of vibrations versus pressure.


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Figure 6. (a) Representative X-ray diffraction patterns of BA crystal at various pressures. The background was subtracted. (b) Variation of the d-spacing of main peaks at high pressures. The peak marked with a down-facing arrow represents the decreasing intensity with increasing pressure.



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Figure 7. Pressure dependence of (a): unit cell volume (V), the solid line is the fitting data by third-order Birch-Murnaghan equation of state; and (b): lattice parameters (a, b, c), the curves serve as guides to the eyes.


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Figure 8. The calculated structures and bond distances of BA dimers at (a) 1 atm; (b) 17 GPa (with the distances shown in Å).



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Figure 9. (a) Variation of O-H, H···O and O-H···O distances as a function of pressure. (b) The bond distance ration of H···O and O-H versus pressure. The curves serve as guides to the eyes.


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Table of Contents Graphic



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Figure 1. Crystal structure and hydrogen-bonded networks of BA at ambient conditions in the ac-plane. The hydrogen bonds are marked as dashed lines. 43x25mm (300 x 300 DPI)


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Figure 2. Experimental and calculated Raman spectra of BA at ambient conditions. The omitted spectral regions are due to the lack of spectroscopic features. All the assignments of the Raman modes are labeled above each band. Bands related to hydrogen-bonding vibrations are labeled with νi (i=1-5). 54x36mm (300 x 300 DPI)



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Figure 3. (a) Selected Raman spectra of lattice modes in BA at various pressures in the range of 50 - 400 cm-1. (b) Peak positions in lattice and hydrogen-bonding region versus pressure. Peaks related to O-H•••O vibrations are labeled with ν1, ν2, and ν3. The peaks marked with asterisks are the new observed peaks, and the peaks marked with down-facing arrows represent their decreasing intensities with increasing pressure. 92x103mm (300 x 300 DPI)


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Figure 4. (a) Selected Raman spectra of internal modes in BA at various pressures in the range of 400 1900 cm-1. (b) Peak positions in internal region versus pressure. The combination of O-H•••O stretching and (CC) ring bending vibrations is marked by ν4, and the O-H bending mode is denoted as ν5. The peaks marked by up-facing and down-facing arrows represent their increasing and decreasing intensities with increasing pressure. 50x31mm (300 x 300 DPI)



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Figure 5. (a) Selected Raman spectra of C-H ring stretching vibration region in BA at various pressures in the range of 2800 - 3400 cm-1. (b) Peak positions of vibrations versus pressure. 92x104mm (300 x 300 DPI)


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Figure 6. (a) Representative X-ray diffraction patterns of BA crystal at various pressures. The background was subtracted. (b) Variation of the d-spacing of main peaks at high pressures. The peak marked with a down-facing arrow represents the decreasing intensity with increasing pressure. 100x66mm (300 x 300 DPI)



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Figure 7. Pressure dependence of (a): unit cell volume (V), the solid line is the fitting data by third-order Birch-Murnaghan equation of state; and (b): lattice parameters (a, b, c), the curves serve as guides to the eyes. 99x56mm (300 x 300 DPI)


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Figure 8. The calculated structures and bond distances of BA dimers at (a) 1 atm; (b) 17 GPa (with the distances shown in Å). 65x66mm (300 x 300 DPI)



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Figure 9. (a) Variation of O-H, H•••O and O-H•••O distances as a function of pressure. (b) The bond distance ration of H•••O and O-H versus pressure. The curves serve as guides to the eyes. 101x59mm (300 x 300 DPI)


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Table of Contents Graphic 48x30mm (300 x 300 DPI)


High Pressure Structural Investigation of Benzoic Acid: Raman

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