Pressure-Induced Reversible Amorphization in Hydrogen-Bonded

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Pressure-Induced Reversible Amorphization in Hydrogen-Bonded Crystalline Phenyl Carbamate Form-I Tingting Yan, Dongyang Xi, Zhenning Ma, Xufeng Fan, and Yang Li J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b06505 • Publication Date (Web): 17 Aug 2017 Downloaded from http://pubs.acs.org on August 22, 2017

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The Journal of Physical Chemistry

Pressure-Induced Reversible Amorphization in Hydrogen-bonded Crystalline Phenyl Carbamate Form-I

Tingting Yan,*,† Dongyang Xi,‡ Zhenning Ma,† Xufeng Fan,† and Yang Li†

†

‡

School of Science, Shenyang Jianzhu University, Shenyang 110168, China

School of Material Science and Engineering, Shenyang Jianzhu University, Shenyang 110168, China

*Corresponding author. Email: [email protected]

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Abstract We characterized the high-pressure response of hydrogen-bonded crystalline phenyl carbamate (C7H7NO2, PC) form-I through in situ synchrotron X-ray diffraction (XRD) and Raman spectroscopy in a diamond anvil cell (DAC) under pressures of up to ∼13 GPa at room temperature. No evidence for the polymorphic transformation of crystalline PC form-I crystal to form-II was observed under high pressure. The evolution of the XRD patterns and Raman spectra indicated that crystalline PC form-I underwent reversible pressure-induced amorphization (PIA) at 12.7 GPa. Ab initio calculations were performed to account for the changes in molecular arrangements and hydrogen-bonded networks in PC form-I under pressure. Hirshfeld surfaces and fingerprint plots were utilized to directly compare the variations in packing patterns and intermolecular interactions. Based on the experimental and calculated results, we proposed that PIA in crystalline PC form-I is driven by the competition between close packing and long-range order. This competition is accompanied by hydrogen-bond collapse.


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Introduction Pressure-induced amorphization (PIA) has held the attention of physicists, chemists, and materials scientists since it was first reported in 1984.1 The transformation of a crystalline solid to its amorphous form at a high pressure is anomalous because amorphous solids usually crystallize under such condition.2 An amorphous solid crystallizes under high pressure because its excess free volume is distributed throughout the substance and is consequently released during crystallization. PIA is a prevalent phenomenon in numerous compounds,3–11 and bondings in PIA systems vary widely in accordance with the material system. For example, bonds in SiO2 are covalent,12 whereas those in Ca(OH)2 systems are ionic.13 C2(CN)4 systems exhibit van der Waals interactions,14 and ice exhibits hydrogen bonding.1 Meanwhile, the mechanisms that drive PIA are also expected to differ for different classes of materials, such as orientational disorder,15 high-pressure polymorphism,16–17 dimerization of tetrahedral molecules,18 and the bending of bonding angles beyond their energetic limit.19 The investigation of hydrogen-bonded crystal networks under high-pressure conditions has fundamental and practical importance in understanding the basic process of PIA. PIA in hydrogen-bonded crystals was first reported in ice1 and has been subsequently reported in other hydrogen-bonded molecular systems including pharmaceuticals, amino acid, and energetic materials.20-23 Above 4.9 GPa, drug γ-indomethacin transforms into a high-density amorphous state, which is different from the common vitreous state.24 Energetic material 4-carboxybenzenesulfonyl azide undergoes two phase transitions and eventually turns into an amorphous state above 10.5 GPa. This amorphization is attributed to the decomposition of the bent azide group.25 Hydrogen bonding is the most common and extensively investigated non-covalent interaction because of its directionality, specificity, and reversibility.26–30 The strength and properties of a given hydrogen bond can be govern by its geometric parameters. Pressure can decrease the intermolecular distance of materials and bring atoms closer to each other. This effect considerably changes the hydrogen bonding.31–33 Given that the application of pressure can reorganize crystal packing,34–36 pressure tuning can serve as a very efficient tool for the study of hydrogen-bonded systems and provide new insights into the nature of structure-property relationships, as well as enable intelligent crystal engineering. Phenyl carbamate (C7H7NO2, PC), an important pharmaceutical intermediate, has extensive pharmacological activities and applications. Traditional crystallization experiments have led to the discovery of three pure PC polymorphs: form-I, form-II, and form-III.37 Form-I is produced through repeated crystallization from methanol and acetonitile and form-II is produced through from ethyl ACS Paragon Plus Environment


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acetate. At 25 °C, form-I undergoes a solution-mediated phase transformation and solid-state transformation to form II. The calculated density of form-II is higher than that of form-I by 4.6% in accordance with Burger’s density rule. Form-III is produced by heating form-II, and its formation can be monitored through changes in HSM, PXRD and DSC spectra. In the present study, we selected PC form-I as a model to investigate the pressure-induced polymorphic transformations and pressure-induced disordering toward an amorphous state in an original polymorphic molecular system. Single-crystal X-ray diffraction (XRD) analysis revealed that crystalline PC form-I exhibits monoclinic symmetry with space group P21/c under ambient conditions. The unit cell parameters of form-I are a = 11.990(2) Å, b = 6.352(1) Å, c = 9.721(2) Å, β = 103.01(3)°, V = 721(2) Å3, and Z = 4.37 As shown in Figure 1, the N–H…O hydrogen bonds in form-I are linked with neighboring molecules to form hydrogen-bonded sheets. Each molecule involves two proton donors and two oxygen acceptors that can participate in the formation of four hydrogen bonds. The crystal packing of PC form-I is characterized by molecular associations in dimer formation via hydrogen bonds. PC form-I is composed of ribbons that are oriented along the c axis with alternating R22(8) and R416(16) derived from the c glide. Numerous hydrogen bonds strengthen the stability of PC form-I, which also exhibits relatively weak van der Waals interaction. Consequently, the cooperation between hydrogen bonding and van der Waals interactions determines the structural stability of form-I under high pressure. PC form-I was subjected to combined in situ synchrotron XRD and high-pressure Raman scattering under pressures of up to ∼13 GPa at room temperature. The analysis of the XRD spectra revealed that PC form-I underwent reversible PIA at 12.7 GPa. High-pressure Raman spectroscopy was performed to investigate the responses of PC form-I under high pressure. The external (intermolecular) and internal (intramolecular) modes detected through Raman spectroscopy were analyzed to provide detailed information on the motions of molecular fragments and hydrogen bonds during the PIA of PC form-I. Moreover, ab initio calculations were applied to gain insight into the behavior of hydrogen bonds in PC form-I under high pressure. Hirshfeld surfaces and fingerprint plots were used for the direct comparison of the variations in packing patterns and intermolecular interactions. Our study attempts to provide a thorough understanding of the mechanism of PIA in hydrogen-bonded crystals and of the behavior of hydrogen bonding under high pressure.


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Figure 1. Ambient unit cell and hydrogen-bonded networks of PC form-I. The dashed lines represent N−H···O hydrogen bonds.



Experimental section 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

The polycrystalline samples of PC were purchased from Sigma Aldrich. Traditional crystallization experiments were performed to obtain single crystals. PC form-I was obtained through slow evaporation method, in which saturated methanol solutions of PC were allowed to evaporate at room temperature. The sample was recrystallized twice to improve crystal quality. The crystal structure of form-I was then determined through single-crystal XRD using a suitable single crystal with dimensions of 0.10 mm×0.10 mm×0.10 mm. Meanwhile, data collection and structural analysis were performed on a Bruker AXS SMART APEX II diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) at 296(2) K. Data processing was accomplished with the SAINT processing program. Crystal structure was resolved through direct methods and refined on F2 by full-matrix least-squares techniques using the SHELXS-97 crystallographic program package. The resolved structure was in accordance with the results reported by Wishkerman and Bernstein. A single crystal was selected and ground to a grain size of approximately a few micrometers for high-pressure experiments. High-pressure measurements were performed using a symmetric DAC equipped with 400 µm diamond culets. A sample chamber with 130 µm diameter was drilled into a pre-indented T301 stainless steel gasket. The powdered sample was loaded into the chamber together with one or two small ruby balls for the determination of pressure via the R1 ruby fluorescent method.38 Liquid nitrogen was used to ensure hydrostatic pressure conditions. The ruby lines were sharp and well separated to the highest pressure. All of the experiments were performed at room temperature. In situ synchrotron XRD experiments were performed on 4W2 beamline at the High Pressure Station of the Beijing Synchrotron Radiation Facility (BSRF). The monochromatic 0.6199 Å beam with a 20 × 30 µm2 spot was adopted as the incidence light source. CeO2 was used as a standard sample for the calibration of the geometric parameters before data collection. Bragg diffraction rings were recorded using an imaging plate detector (Mar345), and data from the image plate were integrated using Fit2D software.39 In situ high-pressure Raman measurements were conducted using an Acton SpectraPro 2500i spectrometer with a diode laser at 532 nm as the excitation source. The laser was focused with a ×50 long-focal-length objective to a spot of approximately 2 µm. The laser power on the sample was maintained at 10 mW, and the Raman spectra were collected with backscattering geometry. The Raman signals were recorded using a liquid nitrogen-cooled CCD camera (Pylon, 100B) with a spectral resolution set of 1 cm−1. Gaussian and Lorentzian functions were combined for the Raman profiles. Ab initio calculations were performed with the norm-conserving pseudopotential plane-wave method based on density functional theory implemented in the Materials Studio 7.0 CASTEP ACS Paragon Plus Environment

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program suite.40 Exchange-correlation effects were treated within the generalized gradient 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

approximation of PW91. Geometry optimizations, including lattice constants and atomic positions, were performed using the BFGS algorithm. Meanwhile, Vanderbilt-type pseudopotentials were adopted with a plane-wave cut-off energy of 530 eV and 1.0 scaling factor. The k-point was of fine quality with 0.05 Å−1 separation.



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Results and Discussion The point group symmetry of the PC form-I crystal (Z = 4) is C2h (2/m). The mechanical representation of this symmetry is M = 51Ag + 51Au + 51Bg + 51Bu showing three acoustic modes Γacoustic = Au + 2Bu and 309 optic modes Γoptic = 51Ag + 50Au + 51Bg + 49Bu The group theoretical classification of the 204 optical modes shows that the Raman-active modes belong to 51Ag + 51Bg symmetry. The remaining infrared-active modes belong to 50Au + 49Bu symmetry. Some of the Raman modes cannot be observed in our experiments because of their very weak intensities and limited splitting between correlation components.

Figure 2. Synchrotron XRD patterns of PC form-I under different pressures (a). Diffraction images of PC form-I compressed under 12.7 GPa (b) and decompressed under ambient pressure (c). The synchrotron XRD patterns and diffraction images of PC form-I under different pressures are presented in Figure 2. All of the diffraction peaks have shifted to high angles because of the decreasing interatomic distances and unit cell volume with increasing pressure. The peaks become broader and less intense under increasing pressure. Moreover, some of the peaks merge together. This behavior indicates the progressive evolution of the system into a disordered state. At 12.7 GPa, the observed diffraction peaks have drastically changed into a single broad peak, a pattern that is indicative of a non-crystalline structure. Meanwhile, under the same pressure, the synchrotron XRD images have disappeared, confirming pressure-induced amorphization (Figure 2b). When pressure is decreased to ambient pressure, the diffraction pattern and image returne to their initial state; this ACS Paragon Plus Environment

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response indicates the reversibility of the PIA.

Figure 3. Representative Raman spectra of PC form-I at various pressures in the region of 30−430 cm−1. To understand the detailed variations in the motions of molecular fragments and hydrogen bonds, it is necessary to conduct in situ high-pressure Raman spectroscopy. The Raman spectrum of PC form-I obtained at ambient pressure agrees well with that reported in the literature.37 Assignments for the Raman modes of PC form-I are based on the ab initio calculated results. Figure 3 shows the evolution of Raman spectra ranging from 30 to 430 cm−1. The low-frequency Raman spectra of crystalline states are composed of phonon peaks that correspond to lattice vibrations and reflect long-range crystal order. The spectrum at 0.1 GPa consists of eight external modes (59, 67, 74, 85, 99, 120, 148, and 154 cm−1). At 0.8 GPa, the mode marked with a downward-facing arrow becomes so weak and loses its intensity absolutely at 1.5 GPa. In the plot of 2.1 GPa, the O−C=O out-of-plane bending mode displays an abrupt red shift. A new mode, which is indicated by an asterisk, shifts to the margin of the traceable spectrum as the pressure is increased to 2.9 GPa. With further compression beyond 4.7 GPa, the two external modes with downward-facing arrows in the 130−200 cm−1 region gradually lose their intensities and eventually vanish into the scattering background. Overall, the Raman bands progressively broaden while decreasing in intensity. These features continue up to 12.6 GPa, at which all the external modes and O−C=O and O=C−N out-of-plane bending modes disappear completely. The disappearance of well-defined external modes indicates the loss of long-range order.41–42 Thus, the high-pressure Raman investigation confirms the conclusion from the X-ray study that the crystal lattice of PC form-I becomes ACS Paragon Plus Environment


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disordered and eventually becomes amorphous at 12.7 GPa. After the pressure is released, the Raman spectrum remains the same as the initial pattern, indicating that the amorphous state is reversible.

Figure 4. Selected Raman spectra of PC form-I in the region of 470−1840 cm−1 under various pressures. The Raman spectra of PC form-I ranging from 470 to 1840 cm−1 are depicted in Figure 4. The internal modes are related to the vibrations of a specific group and can be used to explore local variations in the chemical environment around specific groups. Several variations in the spectrum can be detected for these internal modes. Two new vibrational bands emerge at 778 and 1628 cm-1 at 0.8 GPa. CO (975 cm-1) and CH (1587 cm-1) out-of-plane bending modes lose their intensities above 1.5 GPa. In the curve of 2.1 GPa, the ring out-of-plane bending, NH2 wagging, and CH out-of-plane bending modes show asymmetry and marked twofold splitting. Meanwhile, the NH out-of-plane bending and CH in-plane bending modes evolve into triplet bands. Moreover, the CH out-of-plane bending mode at 1426 cm-1 shows a sudden redshift. Simultaneous variations in the Raman spectra at 2.1 GPa suggest the overall rotation of molecules. The splitting of the existing modes and appearance of new modes reveal that crystalline symmetry decreases and that the local chemical environments around C, N, and O atoms have changed upon pressure application. As pressure increases to 4.7 GPa, the ring breathing mode at 1066 cm-1 evolves into the doublet bands. The behaviors of the modes that correspond to ring vibrations appear complex because of enhanced π-stacking interaction. The Raman bands progressively broaden with decreasing intensity up to 12.6 GPa. Almost all the internal modes disappear at this point. The broadening of internal modes with ACS Paragon Plus Environment

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decreasing intensity results from the random arrangement and distortion within and between molecules.

Figure 5. Frequency shifts of various modes of PC form-I ranging from 30 to 1840 cm−1 as a function of pressure. Linear fits are performed for clarity. Figure 5 illustrates the frequency shifts of the external and internal modes ranging from 30 to 1840 cm−1. The mode at 60 cm−1 initially shifts to a high frequency and then shifts to a low frequency after 8.6 GPa. Under the lattice dynamical viewpoint, the stability limit of a crystal lattice is reached as the frequency of any external mode decreases. Thus, the red shift indicates that the crystal structure gradually becomes unstable.43–46 Apart from this mode, other external modes exhibit substantial blue shifts throughout the entire pressure region, as expected for organic crystals. The blue shift is attributed to the decreased intermolecular distances and enhanced intermolecular coupling.47 Similarly, Raman bands in the internal mode region also gradually shift toward high frequencies, except for the sudden red-shifts of γ(OCO) and γ(CH). The increases in frequency may be explained by the shortened interatomic distances and increased effective force constants under crystal compression.48 Notably, the pressure-induced frequency shifts of the C=O stretching mode involved in molecular associations via hydrogen bonds exhibit a blue shift throughout the whole pressure region. This blue shift indicates that the N–H…O hydrogen bonds in PC form-I are strong and continuously strengthen with increasing pressure.49–52 The typical hydrogen bond can be denoted as D–H…A, where D and A denote the donor and acceptor, respectively. Pressure can shorten the D…A distance and then contract the H…A distance of weak and moderate hydrogen bonds by increasing the electrostatic attraction between H and A. The D–H distance is then extended, resulting ACS Paragon Plus Environment


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in the red shift of D–H stretching modes. By contrast, the blue shift can be traced to strong hydrogen bonds derived from the sustained enhancement of hydrogen bonds under high pressure. In addition, the frequency shifts of the C=O stretching mode are higher than those of the other internal modes. Large frequency shifts are related to molecular flexibility, and the behavior of the C=O stretching mode indicates strong hydrogen bonding that is responsible for the long-range orientation of dimers.24

Figure 6. Raman spectra of PC form-I in the CH and NH stretching region: (a) evolution of Raman spectra at different pressures; (b) frequency shifts of these modes. Although the positions of hydrogen atom cannot be inferred from XRD data, high-pressure Raman spectroscopy can provide detailed insights into the properties of hydrogen-bonded systems. In Figure 6, we present the Raman spectra in the C−H and N−H stretching region and frequency shifts of PC form-I. At 0.1 GPa, three C−H and two N−H stretching modes are detected. The NH stretching mode at 3185 cm−1 corresponds to symmetric stretching vibrations, whereas the mode at 3260 cm−1 corresponds to asymmetric stretching vibrations. At 2.1 GPa, a new C–H stretching mode appears at 3100 cm−1, and the νs(NH) mode shows an abrupt red shift. The νas(NH) and νs(NH) modes lose their intensities above 1.5 and 2.9 GPa, respectively. In addition to the influence of background scattering, the disappearance of the N−H stretching vibration is attributed to highly close packing, which can enhance interactions around amino groups and reduce vibration flexibility. These results indicate that hydrogen-bonded networks have drastically changed and collapsed into a highly disordered network. Furthermore, the modes related to N−H···O hydrogen bonds, including N−H out-of-plane bending, O−C=O out-of-plane bending, NH2 wagging, and C=O stretching modes, ACS Paragon Plus Environment

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also indicate various responses to pressurization. In addition, the disappearance of these modes at 12.6 GPa indicates the complete collapse of hydrogen-bonded networks.24 Upon the release of pressure, the Raman spectrum returns to its initial state. The two N–H stretching modes in Figure 6b undergo blue shifts with increasing pressure and confirm the presence of strong hydrogen bonds in the PC form-I. In addition, disorder induces the flattening of Raman spectrum and the loss of intensity. Table 1. Ab initio calculated changes in bond lengths (A˚) for hydrogen bonds; positive and negative values indicate elongation and shortening, respectively. N−H···O Pressure (GPa)

∆r(N−H)

∆r(H1···O)

∆r(H2···O)

2

−0.001

−0.143

−0.171

4

−0.002

−0.196

−0.236

6

−0.004

−0.202

−0.257

8

−0.010

−0.143

−0.272

10

−0.012

−0.285

To gain further insight into the high-pressure behavior of hydrogen bonding, we performed ab initio calculations. The calculated pressure-induced changes in the N−H and H···A distances are shown in Table 1. Positive and negative values indicate elongation and shortening, respectively. From our calculations, the N−H bond smoothly decreases on pressurization, and this decrease corresponds to the blue shifts of N−H stretching in the Raman spectra.53 Meanwhile, the H1···O distance is shortened and is then extended after 6 GPa. The elongation verifies that the molecules undergo remarkably distortion and rotation under pressure. Above 8 GPa, the N−H1···O hydrogen bonds entirely collapse.



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Figure 7. Hirshfeld surfaces mapped with dnorm and fingerprint plots for PC form-I at ambient pressure (a1) (b1) and at 10 GPa (a2) (b2). To understand the changes in packing patterns and intermolecular interactions under pressure, we obtained and analyzed the Hirshfeld surfaces and fingerprint plots of PC form-I under ambient and high pressure (Figure 7). The blue regions on the Hirshfeld surfaces arise from long contacts, whereas the red regions arise from short contacts.54 With further pressurization to 10 GPa, the blue regions decrease and the red regions consistently increase with close packing. The decrease in the maximum values of de under ambient pressure (2.551 Å) and 10 GPa (2.196 Å) indicate the overall shortening of these lengthened contacts with increasing pressure. The two long spikes in both plots represent N−H···O hydrogen bonds. The lower of the two sharp features corresponds to the hydrogen bond acceptor and the other to the hydrogen bond donor.55 The N−H···O spikes become less pronounced as the rest of the plot moves toward the origin. The contribution of the H···O interaction negligibly changes from 27.1% at ambient pressure to 26.9% at 10 GPa. The diffused set of points marked with a red circle between the two spikes, with the shortest contact near 2.6 Å under ambient pressure, arise again from short C−H···H−C contacts across the centrosymmetric hydrogen-bonded dimer. The distance of C−H···H−C contacts is compressed from 2.6 Å to 1.8 Å, and the contributions are reduced from 44.1% at ambient pressure to 35.5% at 10 GPa. Moreover, the contributions of other contacts simultaneously change with increasing pressure. These alterations include H···N contacts from 2.9% to 2.8%, H···C contacts from 29.3% to 23.3%, and C···O contacts from 2.0% to 1.8%.

Figure 8. Calculated crystal structures of PC form-I (a) at ambient pressure and (b) at 10 GPa. The crystal structures and hydrogen-bonded networks of PC form-I under ambient and high ACS Paragon Plus Environment

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pressure are illustrated in Figure 8. As shown in the figure, at 10 GPa, the molecules exhibit 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

considerable distortions, rotations, and close packing. With increasing pressure, the distances between the molecules are dramatically reduced (5.07 Å at 0 GPa, 4.48 Å at 10 GPa). Meanwhile, the hydrogen-bonded networks are markedly distorted. Given the experimental and calculated results, we propose the mechanism that drives the PIA of PC form-I, as follows. Hydrogen bonding and van der Waals interaction are the dominant interactions within crystalline PC form-I under ambient pressure. Amino and carboxyl groups form hydrogen-bonded networks through multiple hydrogen bonds. The formation of such a long-range ordered crystal structure leads to a large proportion of empty space. Molecules in the crystals tend to be closely packed under high pressure. With increasing pressure, the Gibbs free energy in the system increases from -9.30911 keV at ambient pressure to -9.27161 keV at 10 GPa because hydrogen bonding and van der Waals interactions strengthen under increasing pressure. The increased energy could cause the rotation of molecular fragments and the distortion of the hydrogen-bonded networks. At a sufficiently high pressure, PIA occurs to achieve packing and to reduce the free energy of the system. Consequently, we attribute the PIA in PC form-I to the competition between close packing and long-range order, accompanied by the collapse of the hydrogen-bonded networks.24,56 The release of pressure restores the distorted hydrogen bonds and deformed molecules. This response indicates that amorphization from the involvement of hydrogen bonds and van der Waals interactions related to small energy barriers is easily reversed and overcome when pressure is released.



Conclusion 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

We monitored the changes in synchrotron XRD patterns and Raman spectra to explore the high-pressure behavior of hydrogen-bonded crystalline PC form-I. Experimental results confirmed that PC form-I underwent a reversible PIA at 12.7 GPa. In addition, transformation between the two PC polymorphs did not occur upon the application of pressure. Ab initio calculations were used to provide details of the variations in molecular arrangements and hydrogen-bonded networks of PC form-I under different pressures. Hirshfeld surfaces and fingerprint plots were used to compare the changes in packing patterns and intermolecular interactions. Given the experimental and calculated results, we attributed the PIA of PC form-I to the competition between close packing and long-range order, accompanied by the collapse of hydrogen-bonded networks.


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Acknowledgement 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

This work is supported by NSFC (Grant Nos. 11604224 and 11604308), project LJZ2016031 and LJZ2016030 supported by the Education Department of Liaoning Province, and project XKHY2-105 and XKHY2-101 supported by Shenyang Jianzhu University Discipline Content Education.



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TOC Graphic


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44x35mm (300 x 300 DPI)



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Figure 1. Ambient unit cell and hydrogen-bonded networks of PC form-I. The dashed lines represent N−H•••O hydrogen bonds. 75x64mm (300 x 300 DPI)


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Figure 2. Synchrotron XRD patterns of PC form-I under different pressures (a). Diffraction images of PC form-I compressed under 12.7 GPa (b) and decompressed under ambient pressure (c). 69x58mm (300 x 300 DPI)



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Figure 3. Representative Raman spectra of PC form-I at various pressures in the region of 30−430 cm−1. 110x149mm (300 x 300 DPI)


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Figure 4. Selected Raman spectra of PC form-I in the region of 470−1840 cm−1 under various pressures. 89x45mm (300 x 300 DPI)



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Figure 5. Frequency shifts of various modes of PC form-I ranging from 30 to 1840 cm−1 as a function of pressure. Linear fits are performed for clarity. 92x102mm (300 x 300 DPI)


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Figure 6. Raman spectra of PC form-I in the CH and NH stretching region: (a) evolution of Raman spectra at different pressures; (b) frequency shifts of these modes. 74x67mm (300 x 300 DPI)



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Figure 7. Hirshfeld surfaces mapped with dnorm and fingerprint plots for PC form-I at ambient pressure (a1) (b1) and at 10 GPa (a2) (b2). 71x57mm (300 x 300 DPI)


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Figure 8. Calculated crystal structures of PC form-I (a) at ambient pressure and (b) at 10 GPa. 85x45mm (300 x 300 DPI)


Pressure-Induced Reversible Amorphization in Hydrogen-Bonded

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