Discovery of High-Pressure Polymorphs for a Typical Polymorphic

Apr 21, 2015 - (1-4) Polymorphs with various molecular arrangements and intermolecular .... planar one-dimensional molecular chains that run parallel ...
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Discovery of High-Pressure Polymorphs for a Typical Polymorphic System: Oxalyl Dihydrazide Xiao Tan,† Kai Wang,† Tingting Yan,† Xiaodong Li,‡ Jing Liu,‡ Ke Yang,§ Bingbing Liu,† Guangtian Zou,† 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 § Shanghai Synchrotron Radiation Facilities, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201204, China ‡

S Supporting Information *

ABSTRACT: The variation of pressure is an effective experimental technique to explore new polymorphs of organic crystals. At ambient condition, oxalyl dihydrazide (C2N4O2H6, ODH) exhibits five polymorphs: α, β, δ, γ, and ε. Here we report the high-pressure response of the existed five forms of ODH by in situ Raman spectroscopy and synchrotron X-ray diffraction techniques with a pressure of about 20 GPa. Highpressure experimental results show that all five polymorphs undergo phase transitions to new phases at different pressures, respectively. We propose that the special molecular conformation yields several geometric constructions for hydrogenbonding arrangements. The detailed mechanisms of the phase transition and the high-pressure behaviors of the polymorphs are analyzed by considering molecular stacking.

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new forms under high-pressure conditions.19−22 For polymorphic systems, pressure-induced phase transitions between known polymorphs are also observed in the system of paracetamol, pyrazinamide, and maleic hydrazide.23−25 Hence, high-pressure analysis of simple compounds containing flexible backbone and plenty of hydrogen bonds elucidates the intermolecular and intramolecular interactions, bonding characteristics, and packing motifs of polymorphs. One relevant aspect related to polymorphism is the number of polymorphic forms in a given molecule. Only a few molecules yield a large number of experimentally verified polymorphs. Oxalyl dihydrazide [H2N−NH−CO−CO−NH− NH2, ODH] is a kind of conformational polymorphism with five known polymorphs. There are six potential hydrogen-bond donors (N−H bonds of the NH and NH2 groups) and four potential hydrogen-bond acceptors (oxygen atoms of the C O groups and the nitrogen atoms of the NH2 groups) in the ODH molecule. The five polymorphs have been reported in the literature, namely, α, β, γ, δ, and ε.26 The crystal structures of the five polymorphs are all monoclinic with space group P21/c, and the crystallographic data of ODH polymorphs are summarized in Table 1.26 In all polymorphs, the N−N−C− C−N−N backbone of the molecule is planar and adopts the

INTRODUCTION Polymorphs are crystalline modifications with identical molecules which packed in different ways.1−4 Polymorphs with various molecular arrangements and intermolecular interactions may have different physicochemical properties, such as melting point, stability, solubility, and density.5−7 Some polymorphic systems elucidate the structure−property relationships.8−11 Studies have also focused on determination and characterization of polymorphs of active molecules (such as a drug or pigment); polymorphs with the most desirable properties for target applications are selected.12−14 Discovery, structural characterization, and physicochemical analysis of polymorphic forms accessible to a given molecule are important to develop reliable and reproducible procedures for polymorph formation.15 Hydrogen bonding is one of the most important intermolecular interactions. Given the ability of a change in pressure to modify the strength and geometric properties of a given D− H···A hydrogen bond (D and A mean donor and acceptor, respectively), it is expected that the molecular structure can be susceptible to the affects of pressure.16−18 High-pressure studies of hydrogen-bonded polymorphic compounds can provide more information about the nature of the polymorphs. Small organic molecules, pharmaceuticals, amino acids, and energetic materials have been verified that show abundant polymorphs are obtained under high pressure. For example, urea, glycine, piracetam, etc., yield two or more © 2015 American Chemical Society

Received: December 2, 2014 Revised: March 19, 2015 Published: April 21, 2015 10178

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The Journal of Physical Chemistry C Table 1. Crystallographic Parameters of ODH Polymorphs26 polymorph crystal system space group a/Å b/Å c/Å β/° Z V/Å3

(a) α form

3.6221(4) 6.8322(7) 9.1294(10) 99.298(9) 2 222.96(4)

(b) β form

(c) γ form

(d) δ form

(e) ε form

3.762(4) 11.652(5) 5.619(2) 92.793(5) 2 246.0(3)

monoclinic P21/c 5.0795(4) 14.6679(14) 7.0345(7) 114.160(6) 4 478.20(8)

3.6608(5) 14.550(2) 5.0646(8) 119.006(9) 2 235.93(6)

5.3642(3) 3.8412(2) 12.3191(6) 108.999(3) 2 240.01(2)

center holes of 130 μm were drilled for the sample. The ruby chip was used for pressure determination using the standard ruby fluorescent technique. Liquid argon was used as the pressure-transmitting medium. All experiments were performed at room temperature. The high-pressure Raman spectra of ODH were collected using an Acton SpectraPro 2500i spectrometer equipped with a liquid nitrogen cooled CCD camera with the excitation light of 532 nm. A 500 mm focal length spectrometer and 1800 lines/ mm grating with 0.9 cm−1 resolution was used. The system was calibrated using silicon lines with an uncertainty of 1 cm−1, and all the Raman spectra were recorded with backscattering configuration. To avoid the strong first-order Raman modes of diamond at ∼1332 cm−1, the spectra were collected in ranges 40−1300 cm−1, 1350−1800 cm−1, and 3000−3600 cm−1 in several collection windows. For each spectrum, a collection time of 120 s was employed; the average laser power on the sample was maintained at ∼30 mW; and the error for Raman data was ∼1 cm−1. In situ ADXRD experiments of α- and ε-ODH were performed at the 4H2 High-Pressure Station of Beijing Synchrotron Radiation Facility. A 0.6199 Å beam with a spot size of 20 × 30 μm2 was used as the incidence light source. Ones of γ- and δ-ODH were performed using the BL15U1 beamline at the Shanghai Synchrotron Radiation Facility, where a monochromatic 0.6199 Å radiation with a spot size of 5 × 5 μm2 was used for data collection. An average acquisition time of 300 s was adopted for each pattern, considering the small cross sections for light atoms (C, H, O, and N) in ODH. Before each experiment, the CeO2 standard was used to perform the calibration of geometric parameters. The Bragg diffraction rings were recorded with an image plate area detector (Mar345) and converted into plots of intensity versus 2θ with Fit2D software. All the indexing and refinements were performed using the Reflex module in Commercial Materials Studio 5.5 software (Accelrys).

trans−trans−trans conformation. The oxygen atoms of the OC groups and the hydrogen atoms of the NH groups lie within this plane. The NH2 group is pyramidal, consistent with sp3 hybridization of the nitrogen atom, which enhances the capacity for the nitrogen atom to serve as a hydrogen-bond acceptor. Computational methods are performed to analyze of the energy rank for the known polymorphs: α > ε > γ ∼ δ > β.27,28 We anticipate that there may be a scope for ODH to exhibit more extensive polymorphism for two reasons. On one hand, ODH molecule exists in five polymorphs with energetically accessible conformations and orientations of the terminal NH2 groups in ambient conditions. On the other hand, variation in pressure has the ability to influence the geometry properties of a given structure. With this motivation, a series of high-pressure experiments on the known polymorphs have been carried out to find new ones of the ODH molecule. Herein, high-pressure in situ Raman scattering and synchrotron angle-dispersive X-ray diffraction (ADXRD) studies were carried out by using a diamond anvil cell (DAC) for the ODH polymorphs (α, β, γ, δ, and ε) of ODH at a pressure of about 20 GPa. Raman scattering spectroscopy provides structural fingerprinting because of its narrow and highly resolved bands. Synchrotron XRD measurements give the structural information on the five polymorphs under high pressure. We aim to explore new ODH forms and provide opportunities to study the origin of polymorphism, stabilities of the polymorphs, and the production of new forms in polymorphic substances.

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EXPERIMENTAL SECTION The commercially available α form of ODH (Sigma-Aldrich) was used as the raw material in all of our experiments. Four forms of ODH, α, γ, δ, and ε, were prepared as the methods described by D. M. Harris.26 High-quality crystals were chosen after detailed examination under an optical microscope and gently ground to obtain powders with a size of approximately several micrometers. The purity of the crystal was confirmed by XRD patterns, which are shown in Figure S1 (Supporting Information, SI). The powders were used for synchrotron powder XRD experiments. The β form, which was not reproducible in ref 26, was synthesized by crystallization from a solution (60 °C) containing ODH in methanol/water (3:1 v/ v), cooling rapidly to ambient temperature in our experiments. As it transformed to α form immediately when subjected grinding, the synchrotron X-ray diffraction experiment cannot be performed for this polymorph. We carried out the singlecrystal Raman measurements for the five forms to confirm the Raman spectral qualities and for consistence. The symmetric diamond anvil cell (DAC) with 400 μm culet diamonds was adopted for in situ high-pressure measurements. T301 stainless steel gaskets were preindented to a thickness of 40 μm, and

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RESULTS AND DISCUSSION The stabilities of the five known polymorphs (i.e., α, β, γ, δ, and ε) of ODH are determined by the competition between different types of hydrogen bond and van der Waals forces between π-stacked molecules.26 In the α polymorph, the molecules are arranged in two-dimensional sheets parallel to the (1 0 −2) plane, and the sheets are not perfectly flat. The molecules within the sheets are linked by hydrogen-bonded rings involving N−H···O and N−H···N hydrogen bonds. Each molecule is involved in four cyclic hydrogen-bonded arrays of this type. The out-of-plane N−H bonds of the NH2 group form N−H···O hydrogen bonds with the oxygen atoms of OC groups in adjacent sheets. For the β polymorph, the structure comprises planar one-dimensional molecular chains that run 10179

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the Raman modes of each polymorph are summarized in Table 2.29 Raman spectra depend on the crystal structures of the five polymorphs. In particular, the connection and strength of the hydrogen bonds greatly influence the NH stretching vibrations. As can be seen in the 3000−3600 cm−1 region of Raman spectra, there are three Raman peaks with medium intensities for α, β, and ε forms, while γ and δ forms have a similar profile with two obvious Raman modes. In α, β, and ε forms, two N− H···O hydrogen bonds form between NH2 and CO groups, and a N−H···N hydrogen bond forms between NH and NH2. For the frequencies of NH stretching modes, these three forms have an order of β (3306 cm−1) > ε (3297 cm−1) > α (3274 cm−1), indicating the strength of the N−H···N hydrogen bond: α > ε > β. N···N distances of the hydrogen bonds also have the order of β (3.010 Å) > ε (2.942 Å) > α (2.919 Å), in accordance with the strength of the hydrogen bonds. However, for γ and δ forms, NH serves as one of the proton donors for the N−H···O hydrogen bond, and the NH2 group provides two protons for N−H···O and N−H···N hydrogen bonds. The frequency of the NH stretching mode of γ form is 3289 cm−1, while the one of δ is 3285 cm−1, revealing a stronger N−H···O hydrogen bond formed in δ form, which is also in agreement with the distance between N and O atoms: δ (2.848 Å) < γ (2.860 Å). For the five ODH polymorphs, the β form has higher N−H stretching vibrational frequencies, reflecting the weaker N−H··· O hydrogen bonds in the β form. It also has the lowest vibrational frequencies for some bending modes, such as the NH in-plane bending (β (1520 cm−1) < ε (1541 cm−1) ∼ α (1542 cm−1) ∼ γ (1543 cm−1) ∼ δ (1545 cm−1)) and NH2 outof-plane bending (β (1154 cm−1) < α (1166 cm−1) < δ (1174 cm−1) ∼ γ (1175 cm−1) < ε (1182 cm−1)). Therefore, we suspect that the hydrogen bonds formed in β form are the weakest in the five polymorphs, which is in accordance with the instability of the β form. Conversely, the α form has the lowest vibrational frequencies for NH stretching modes, compared with the other four forms. Considering the maximum density and the stability under ambient conditions, it is speculated that the strongest hydrogen bonding connections have formed in α form. Different forms may exhibit different behaviors under high pressure, especially for ODH polymorphs. The evolution of lattice modes in ODH under high pressure can give essential information on the symmetry change. Figure 3(a)−(e) summarizes the pressure dependence of lattice modes. The lattice modes of five polymorphs exhibit blue shifts with different rates before they suffer significant changes. These blue shifts indicate that intermolecular distances are contracted with increasing pressure. Phase transitions can be detected with the appearance of new modes (marked by the up arrows) for all the five polymorphs according to Raman spectra at the pressure of 12.0, 14.0, 11.2, 10.6, and 6.0 GPa for α, β, γ, δ, and ε, respectively. For the γ form, the spectra at 9.5 and 11.2 GPa are essentially different, which reflects that the γ form undergoes a phase transition to a new form at the pressure of ∼10 GPa directly. Phase transition regions can be observed obviously for the other four polymorphs (α, β, δ, and ε). During the processes, the Raman spectra (red lines) clearly show the coexistence of the original and the high-pressure form, in which process the original modes vanish and the new ones appear gradually with the increasing pressure. All the new modes shift to higher wavenumbers gradually without any discontinuity upon compression to 20 GPa, which is the highest pressure

parallel to the [1 0 1] direction. The only planar HBs are the N−H···N from the NH groups, which link the synthons to form molecular “ribbons”, while two types of N−H···O HBs lie out of plane, between adjacent ribbons, to link the ribbons to form 3-dimensonal structure. In the γ polymorph, the molecules are linked by N−H···O and N−H···N hydrogen bonds to form quasi-2-dimensional layers. Two N−H···N hydrogen bonds are located out of the molecular layer and assemble the adjacent layers to lead to a sheet-like hydrogenbonded network, although this sheet is not completely flat. The δ polymorph is structurally similar to γ, but it contains only two (N−H···N and the N−H···O) hydrogen bonds in the ab molecular plane and an N−H···O bond out of the ab plane. Molecules assemble by N−H···O hydrogen bonds to form ribbons along the c-axis. The ribbons are linked with N−H···N hydrogen bonds to form a molecular plane. The “out-of-plane” N−H bonds of the two NH2 groups that point outward form an N−H···A interaction with an OC group in an adjacent sheet. The ε phase is quite different from the others: it shows a “grid-like ” shape, in the ab plane, that arises from the alternate arrangement of molecular ribbons perpendicular to each other. For each ribbon there are two different intermolecular N−H··· O HBs, one in-plane and one out-of-plane (the latter between parallel ribbons). The N−H···N hydrogen bonds from the NH groups and from the nitrogen of the NH2 group link adjacent ribbons along c-axis. Molecular packings in different polymorphs are shown in Figure 1.

Figure 1. Crystal structures of ODH polymorphic forms: α, β, δ, γ, and ε forms.

Raman spectra provide fingerprint-type molecular information and can be used to identify and characterize polymorphs since vibrational information is specific to the chemical bonds and symmetry of crystal structures. It is the first time to summarize the Raman spectra for the five room-condition polymorphs. For the ODH polymorphs, Raman spectra exhibit different vibrational frequencies for the same mode, as presented in Figure 2. NH stretching, NH2 symmetric, and asymmetric stretching vibrations locate in the region of 3000− 3400 cm−1. The bending vibrations including in-plane, out-ofplane, wagging, and scissoring vibrations of NH and NH2 groups are mainly identified in 1100−1650 cm−1 region. The frequency range for skeleton vibrations is 250−1100 cm−1, involving in-plane bending and stretching of the N−N bond; rocking, in-plane bending, and out-of-plane bending of the NCO bond; and CC stretching modes. Lattice modes, which reflect the collective motions of all atoms in the unit cell, are located in the range of 50−250 cm−1. The profiles of the five lattice modes are essentially different and can be used to distinguish the five polymorphs. The detailed assignments of 10180

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Figure 2. Raman spectra (30−1800 cm−1 and 3100−3400 cm−1) of the five polymorphs at ambient conditions.

(j), in which the discontinuity of the slope caused by the phase transitions can be seen obviously. As the Raman spectrum is a fingerprint of a crystal structure, the high-pressure Raman experimental results have given the conclusion that five new forms are obtained by the method of increasing pressure according to the ten different Raman spectra of the lattice region. The internal modes are sensitive to high pressure and can be used to probe local variations of the chemical environment around molecules and atoms. When a phase transition is induced, selection rules change, and this will come to light through the appearance of new features in the spectra because either forbidden excitations become Raman active or degeneracies are lifted.30 Figure 4(a)−(e) illustrates the selected Raman spectra ranging from 200 to 1300 cm−1 and 1350 to 1800 cm−1 of the five ODH polymorphs with increasing pressure. For all the five polymorphs, great changes can be detected in the profiles with increasing pressure: vanishing and splitting of the modes of the original phase and new peaks replacing old ones in a range of pressures for the five polymorphs. The modes of new Raman spectra exhibit a blue shift without any discontinuity upon compression to ∼20 GPa. This implies that the new forms are stable under the pressure of 20 GPa. The pressure frequency shifts of the Raman modes for the five forms are depicted in Figure 4(f)−(j). The discontinuities of the shifts are consistent with the proposed phase transitions. Detailed information can be obtained from the Raman spectra evolutions.31−34 The vibrational modes of the five polymorphs exhibit blue shift with different rates, which implies the bonds within different chemical environments have different responses to pressure. For example, out-of-plane bending of N−N bonds has the following shift rates, δ (10.34) > α (8.51) > ε (4.91) > γ (4.59) > β (3.84), and that for the new forms has the rank, β′ (3.46) > ε′ (3.16) > γ′ (2.07) > α′ (1.79) > δ′ (1.35) (the units of the parameters are cm−1/GPa). On the whole, we can see that the linear pressure coefficients in the high-pressure forms are smaller than the ones in their counterparts, suggesting new phases adopt more efficient

Table 2. Tentative Assignment for Raman Bands of Five Oxalyl Dihydrazide Polymorphsa frequencies (cm−1) form α

form β

form γ

form δ

form ε

ν(NH) νs(NH2) νas(NH2) ν(CO)

3274 3229 3190 1700

3289 3180 1684

3285 3254 3173 1679

3297 3262 3200 1675

δ(NH2) δ(NH)+ν(CN) ρ(NH2)+ω(NH2) ν(CN)+δ(NH) ρ(NH2) ν(NN)

1638 1542 1375 1298 1166 1037

1639 1543 1318 1302 1182 1037

1600 1545 1327 1284 1174 1030

1613 1541 1333 1309 1175 1044

δ(NCO) ν(CC) π(NH)+ω(NH2) π(NCO) π(NN) ρ(NCO) δ(NN)

930 809 764 531 445 396 310 273 205 141 106 100 89

3306 3263 3197 1705 1691 1625 1520 1320 1307 1154 1025 1013 917 828 755 519 397 386 311 256 186 167 144 130 106 59

947 818 707 522 408 375 288 245 158 128 117 106 76

939 815 718 517 414 359 304 259 147 92 73

938 839 787 522 406 375 310 269 190 160 141 139 103

assignment

lattice modes

a ν, stretching; δ, in-plane bending; π, out-of-plane bending; ρ, rocking; ω, wagging; and s, symmetric; as, asymmetric; the error for Raman data is ∼1 cm−1.

used in the experiments. The modified forms are named with α′, β′, γ′, δ′, and ε′, and the Raman spectra of these pure forms are drawn with blue lines. The pressure dependences of the lattice modes for the five forms are summarized in Figure 3(f)− 10181

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Figure 3. (a)−(e) Representative Raman spectra of the five polymorphs of ODH between 40 and 350 cm−1 at various pressures; the spectra drawn with black lines represent the existed forms, the red ones represent the new forms, and the blue ones represent the mixture. (f)−(j) Pressure dependence of the lattice modes for the five polymorphs in the range of 40−350 cm−1. The shadows represent the phase-transition regions.

respectively. For the five original conformers, different N−H stretching vibrations display red or blue shifts under pressure. In general, red shifts can be attributed to the corresponding weak hydrogen bonds, of which the strength can be enhanced with increasing pressure. In weak and moderate hydrogen bonds, pressure can shorten the distance between D and A (D for donor, and A for acceptor), followed by contraction of the H···A distance because of the increasing electrostatic attraction between H and A. The subsequent extension of the D−H distance results in a red shift of the D−H stretching modes, whereas the blue shift can be traced to strong hydrogen bonds because of the continuous enhancement of hydrogen bonds at high pressure.35,36 When the pressure was increased, the position and/or orientation of the molecules of each polymorph changed, which provokes a modification of the bonding scheme, further to generate new hydrogen-bond interactions. Then, new NH and NH2 stretching modes appear in the region of 3000−3400 cm−1 for the five polymorphs. Of the five new spectra, the ones of α and ε forms seem not to experience serious changes around the phase transition, reflecting the distortion of portions of hydrogen bonds, while the rest are all totally different from the old ones, which demonstrates the reconstructive nature of the hydrogen bonds through the phase transitions for the three forms. For α form, both CO stretching modes and NH2 stretching modes did not experience obvious changes across the phase transition, and only a new N−H stretching mode appeared at 3260 cm−1. This reveals that the CO···H−N hydrogen bond almost remains

crystal packing and are hard to compress. Moreover, the vibrations belonging to N−N bonds (out-of-plane bending and stretching modes) of the five polymorphs have different behaviors when the phase transitions occur. For the α form, there are no new peaks appearing across the phase transition, while for the other four forms, new Raman modes replace the old ones. That implies that the α form did not change a lot in the N−N bond across the phase transition, but the N−N bonds of the other four forms may go through significant distortion such as the torsion of the N−N bonds or the changes of the bonds length. In-plane bending, out-of-plane bending, and rocking of N−CO bonds also reflect the variation in molecular skeletons under pressure. In the process of phase transition, almost all the vibrational modes about N−CO bonds lose their intensities, and new Raman modes appeared, indicating the changes in the length or/and the angle of the N− CO bonds. Moreover, the vibrations of NH and NH2 exhibit significant changes: new modes substituting old ones for the five polymorphs. It demonstrates the surroundings of the NH− NH2 or the NH−NH2 groups themselves changed a lot after the phase transition for all five polymorphs. They are not just affected by the N−N−C backbone but also influenced by the strength of the hydrogen bonds, which will be discussed below. Apart from the aforementioned modes, the dependence of N−H stretching vibrations also plays an important role in monitoring the structural change and hydrogen bonds at high pressure. Figure 5(a)−(e), (f)−(j) reveal the N−H stretching modes and the pressure dependence of peak positions, 10182

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Figure 4. (a)−(e) Evolutions of Raman spectra of the five polymorphs of ODH at high pressure in the following ranges: 200−1300 cm−1 and 1350− 1800 cm−1; the spectra drawn with black lines represent the existing forms, the red ones represent the new forms, and the blue ones represent the mixture. (f)−(j) Frequency shifts of major internal modes ranging from 200 to 1800 cm−1 as a function of pressure, and the shadows represent the phase-transition regions. 10183

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Figure 5. (a)−(e) Evolutions of Raman spectra of the five polymorphs of ODH in the range 2900−3600 cm−1 at selected pressures: the spectra drawn with black lines represent the existing forms, the red ones represent the new forms, and the blue ones represent the mixture. (f)−(j) Frequency shifts of major internal modes ranging from 2900 to 3600 cm−1 as a function of pressure. The vertical dotted line stands for the beginning of the variations in Raman spectra.

high-pressure powder XRD measurements to verify the phase transitions and explore the possible space group of the new polymorphs. It is worthy of attention that the β-form is transformed to α-form under application of shear force; hence, we can only obtain the other four powder X-ray diffraction patterns under high pressure. The selected powder XRD patterns of the ODH polymorphs at various pressures are shown in Figure 6. With the pressure increasing, new Bragg peaks (up arrows, Figure 6) exist for the samples at different pressures, thereby revealing the onset of phase transitions. The diffraction patterns show the coexistence of the original and the high-pressure phases for α, δ, and ε forms and the direct phase transition without transition range for the γ form, which are consistent with the evolution observed in the Raman spectra. The transition pressure points or regions of four ODH polymorphs were determined from the XRD patterns as follows: from 13.0 to 14.2 GPa for α form, 11.2 GPa for γ form, from 10.6 to 14.9 GPa for δ form, and from 6.9 to 10.0 GPa for ε form. The new XRD patterns are confirmed from the distinct features in contrast to the original counterparts. The observed d-values of selected diffraction peaks as a function of pressure for the four polymorphs, (a) α, (b) γ, (c) δ, and (d) ε forms, are provided in the Supporting Information. Some errors for the results between Raman and synchrotron experiments are caused by the experimental details and the sample. Although the high-pressure powder X-ray diffraction measurement could

the same, but a weaker N−H···N hydrogen bond forms, which may influence the NH2 groups, resulting in the appearance of new bending modes. New NH2 stretching modes emerge in low wavenumbers in β′ form (∼3060 and 3150 cm−1) and ε′ form (∼2980 and 3075 cm−1), indicating strong hydrogen bonds have formed in these new forms. At the same time, a new vibration for CO stretching modes of β′ form and ε′ form can be detected in lower wavenumbers, revealing that the N− H···OC hydrogen bond formed by NH2 and CO groups may still exist in new forms. For the γ′ form, six new stretching modes can be observed in the 2950−3450 cm−1 region. The emergence of novel N−H modes suggests that there is a substantial rearrangement in the hydrogen bonds. New NH and CO stretching modes with higher frequencies can be observed indicative of weakening of the CO···H−N hydrogen bond. However, for δ′ form, old N−H stretching modes disappeared, and new one appears at 3350 cm−1, indicating the formation of weak hydrogen bonds or free N−H stretching modes. However, the new CO stretching mode is not detected in higher wavenumbers. Two CO stretching modes appear at 1645 and 1657 cm−1, revealing that CO and NH2 groups may serve as the proton acceptor and donor in new CO··· H−N hydrogen bonds. For the five new polymorphs, new stretching modes exhibit red or blue shifts due to the different strengths of the new hydrogen bonds. Since the high-pressure Raman experiences have revealed the new phases under the pressure above 10 GPa, we performed 10184

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Figure 6. Representative XRD patterns of four forms of ODH at high pressure. The wavelength for data collection is 0.6199 Å; the patterns drawn with black lines represent the existing forms; the red ones represent the new forms; the blue ones represent the mixture; and the green ones represent the simulated XRD.

condition. However, for the ε form, three new diffraction peaks appear when the phase transition occurs. The new form may reduce its symmetry to the subgroup of P21/c such as P21, Pc, and P-1. After many attempts, the P-1 space group turns out to be the best one to refine the pattern at 12.1 GPa. The result for the refinement is a = 5.860(2) Å, b = 5.252(1) Å, c = 6.500(0) Å, α = 105.96(2)°, β = 89.83(1)°, γ = 102.81(1)°, and V = 187.2(2) Å3. There are still two molecules in the new unit cell, but the symmetry reduced when the phase transition happened. In summary, we proposed that the four new forms might be packing in P21/c, Pmnb, P21/n, and P-1, according to the analysis and Pawley refinement results. To verify the rationality of the proposed structures, we also examine the known highpressure structures which are transformed from the P21/c space group in the Cambridge Crystallographic Database Centre (CCDC). Almost all the phase transitions happened within P21/c space group.37−41 Two cases did not: the space groups of the new forms are in P-1 and Cmc21, respectively.42,43 The best refinements at the pressure above 10.0 GPa for the pure new forms are shown in Figure 7. Structural data relating to the four new ones of ODH are summarized in Table 3. However, further high-pressure single-crystal diffraction and neutron diffraction studies are required to provide more accurate information on the new structures. The predominant intermolecular interactions within the ODH polymorphism crystals are hydrogen bonding and van der Waals interactions. The phase transition can be interpreted by the disturbed balance of these two factors. Increasing pressure decreases the distances between ODH molecules within the 3D structure. During this process, hydrogen bonds

not be performed for β form, we suspect that it is another new form under high pressure according to the Raman spectra. The high-pressure synchrotron patterns verify four phase transitions for ODH polymorphs. However, the transition points of the four forms are all above 10 GPa, and the quality of XRD patterns continues to diminish with further application of pressure. This makes resolving the exact new structures quite difficult. The reduction or growth of the number of Bragg reflections suggests an increase or decrease of the symmetry of the crystal structure. We can see from Figure 6 two or three new diffraction peaks appear, while some lose their intensities for α′ and δ′ forms. Then we suspect these two forms may retain 2/m point group. When we try to find the possible unit cells, automatic indexing using DICVOL91 gives P21/c and P21/n for them. The lattice parameters for α′ and δ′ are a = 11.340(3) Å, b = 8.663(2) Å, c = 3.574(1) Å, β = 113.18(3)°, V = 322.8(3) Å3, and a = 10.438(10) Å, b = 3.693(3) Å, c = 9.499(9) Å, β = 86.83(4)°, V = 365.6(8) Å3, respectively. The phase transitions do not change their space group. However, the unit cells of the new forms are double their size of the forms before phase transitions, and the volume of the molecules in the two forms reduced 27% and 23%, respectively. The unit cells of the new forms are “klassengleich” supercells of the forms before phase transitions. For the γ′ form, the number of the reflection seems to reduce compared with γ form, and it may transform to higher-symmetry orthorhombic structure. The indexing result for the γ′ form is Pmnb, with the lattice parameters of a = 14.084(0) Å, b = 5.275(2) Å, c = 4.494(1) Å, and V = 333.9(4) Å3. Z remains 4, and the volume of the new form reduced ∼30% compared with the value of the γ form at ambient 10185

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pressure behavior depends on the crystal structure. Here we focus on the crystal structures of the five original polymorphs. As described above, the structure of the ε form significantly differs from the other polymorphs because it contains ribbons that run in essentially perpendicular directions to create a gridlike pattern when viewed in projection along the c-axis. Under pressure conditions, the grid-like structure of the ε form is not as stable as the sheet-like parallel ribbon structures of the other polymorphs. Hence, the ε form undergoes the phase transition at ∼6 GPa, which is lower by at least 4 GPa than the other four forms. Another of concern is about the phase transition region. There are transition regions for α, β, δ, and ε forms across the phase transition, while the γ form transforms to the γ′ form directly. The space group of the original five polymorphs is P21/c; however, there are two molecules within the unit cell for α, β, δ, and ε forms, while this Z number for γ form is 4. This is because the molecules of the aforementioned four forms (α, β, δ, and ε) all lie on a crystallographic inversion center; however, in the case of γ form, the molecule is noncentrosymmetric, and the asymmetric unit comprises the whole molecule. As a result, there are symmetric hydrogen-bonded connections with the centrosymmetric molecules and asymmetric ones with the noncentrosymmetric molecules. So, the symmetric hydrogenbonded connections will lead to the high energy kinetic hindrance that prevents the rapid transition to high-pressure phase. Therefore, the phase transitions of the four forms extend over a range of pressures, while the γ form does not. Examples of molecules that have four or more experimentally verified polymorphs are certainly quite rare. The ODH molecule forms five crystal structures under ambient conditions, and the five polymorphs all transform to new forms under high pressure; hence, a total of ten polymorphs have been discovered for this molecule. Results reveal that ODH is particularly susceptible to polymorphism. Two crucial factors on phase transitions are important to form various modes of molecular aggregation in crystalline states. One is the pressureinduced and low-energy conformational changes that result in significantly different orientational characteristics of potential hydrogen-bond donors (N−H bonds of NH and NH2 groups) and hydrogen-bond acceptors (oxygen atoms of the CO group and nitrogen atoms of NH2 groups). The other is the rotation of the molecular skeleton, especially the NH2 group around the NH−NH2 bond, which is associated with a relatively low energy barrier. Thus, different conformations of NH−NH2 end groups are accessible in the solid state.

Figure 7. Pawley refinements of the ADXRD patterns for (a) α′ (15.1 GPa), (b) γ′ (11.5 GPa), (c) δ′ (14.9 GPa), and (d) ε′ (12.1 GPa). The violet lines denote the differences between the observed (black) and the simulated (blue) profiles.

become stronger, and the total Gibbs free energy increases. Upon further compression, the ODH crystals cannot support the increasing Gibbs free energy any longer. Thus, the rotation and/or sliding of the ODH molecules and the reconstruction/ torsion of hydrogen bonds reduce the free energy, which result in the phase transitions at various pressures for each polymorph. Raman spectra and synchrotron showed the changes of the molecular functional groups and the crystal structures. Two points are worth mentioning with respect to the phase transition. One is that the transition pressures of α (12.0 GPa), β (14.0 GPa), γ (11.3 GPa), and δ (10.6 GPa) are above 10 GPa; by contrast, ε is transformed to ε′ form at ∼6 GPa. High-

Table 3. Crystallographic Parameters of Pawley Refinement for New High-Pressure ODH Polymorphs polymorph crystal system space group pressure/GPa a/Å b/Å c/Å α/° β/° γ/° Z V/Å3

α′ form monoclinic P21/c 15.4 11.340(3) 8.663(2) 3.574(1) -113.18(3) -4 322.8(3)

γ′ form Orthorhombic Pmnb 11.5 14.084(0) 5.275(2) 4.494(1) ---4 333.9(4) 10186

δ′ form monoclinic P21/n 14.9 10.438(10) 3.693(3) 9.499(9) -86.83(4) -4 365.6(8)

ε′ form monoclinic P-1 12.1 5.860(2) 5.252(1) 6.500(0) 105.96(2) 89.83(1) 102.81(1) 2 187.2(2) DOI: 10.1021/jp512035s J. Phys. Chem. C 2015, 119, 10178−10188

Article

The Journal of Physical Chemistry C

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CONCLUSION We performed high-pressure Raman and synchrotron XRD analyses for the five forms of ODH at ∼20 GPa. Five new forms have been discovered by exerting pressure to the original counterparts. Although previous works have predicted that other ODH polymorphs could be determined, the molecule already possesses five polymorphs under ambient conditions; each polymorph yields a new form under high pressures to obtain a total of ten explored polymorphs. Several potential hydrogen-bond donors and acceptors as well as low-energy conformational changes in NH−NH2 crucially contribute to the molecular conformation of ODH, which leads to numerous crystal polymorphs. Thus, pressure effectively changes the crystal structures of the ODH molecule. The method can be potentially applied in the pharmaceutical industry, especially in exploration of new drugs, manufacturing, and selection of polymorphs according to their properties.

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ASSOCIATED CONTENT

* Supporting Information S

Figures S1 and S2. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ jp512035s.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by NSFC (Grant Nos. 91227202 and 11204101), RFDP (Grant No. 20120061130006), National Basic Research Program of China (Grant No. 2011CB808200), and China Postdoctoral Science Foundation (Grant No. 2012M511327). Angle-dispersive XRD measurements were performed at 4W2 beamline, Beijing Synchrotron Radiation Facility (BSRF) which is supported by Chinese Academy of Sciences (Grant Nos. KJCX2-SW-N20, and KJCX2-SW-N03), and BL15U1 of Shanghai Synchrotron Radiation Facility (SSRF).

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DOI: 10.1021/jp512035s J. Phys. Chem. C 2015, 119, 10178−10188