Hydrogen-Bonding Modification in Biuret Under Pressure - The

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Hydrogen-Bonding Modification in Biuret Under Pressure Gustav Michael Borstad, and Jennifer A. Ciezak-Jenkins J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b09670 • Publication Date (Web): 06 Jan 2017 Downloaded from http://pubs.acs.org on January 8, 2017

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The Journal of Physical Chemistry

Hydrogen-Bonding Modification in Biuret Under Pressure Gustav M. Borstad and Jennifer A. Ciezak-Jenkins∗ RDRL-WML-B, U.S. Army Research Laboratory, Aberdeen Proving Ground, Maryland 21005, United States E-mail: [email protected]

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Abstract Biuret (C2H5N3O2) has been studied to 30 GPa by Raman spectroscopy and 50 GPa by xray diffraction. Raman peaks exhibit shoulders and splitting that suggests that the molecules undergo reorientation in response to compression. These are observed in three pressure ranges: the first from 3 - 5 GPa, the second from 8 - 12 GPa and finally from 16 - 20 GPa. The particular modes in the sample that are observed to change in the Raman are strongly linked to the molecular vibrations involving the N-H and the C=O bond these are bonds that are most strongly coupled to the hydrogen-bonded lattice structure. The x-ray diffraction suggests that the crystal maintains a monoclinic structure to the highest pressures studied. Although there was a considerable degree of hysteresis observed in some x-ray runs, all the changes observed under pressure are reversible.


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Introduction Biuret is an important compound in biological applications and medicine. It was discovered over 150 years ago as a product of thermal decomposition either of urea or urea nitrate.1 Biuret is a component of a large class of organic substances, with investigations into their potential physiological and chemotherapeutic properties as well as for applications in plastics and resins.1 It is used as a protein replacement in cattle feed.2 The chemical transformation of the fertilizer urea to biuret can occur when urea is heated above its melting point of 132 C. These conditions can occur during the manufacturing of urea and are significant owing to the toxicity of biuret to plants, especially in their early development.3,4 Its negative effects on plant growth occurs at concentrations of biuret in urea in excess of 2% in many plants and even at concentrations of under 1% in others, such as citrus and pineapple.4 Although biuret has been known for a long time and its importance is attested by its appearance in fundamental reactions and many applications, its structural and thermodynamic properties have proven challenging to characterize5 and conflicting results have been reported in the literature regarding even fundamental properties such as its melting temperature.6 Biuret appears alongside compounds such as water, ammonia, and urea in many important systems, but it is the least understood of these compounds and thus the limiting factor in the understanding of processes.6 The presence of hydrogen bonding among the molecules in the biuret crystal also reflects its importance. Hydrogen bonding is of interest because of the insight it provides into the behaviors and properties of some of the most fundamental elements and chemicals and because of its potential to be harnessed in applications, such as pharmaceuticals, hydrogen storage, and tunable sensitivity of energetic materials, among others. Biuret provides an interesting intermediary between urea and urea nitrate. It is a larger molecule than urea, and it contains the same amount of nitrogen per structural unit as urea nitrate. Additionally, biuret is a molecular crystal, while urea nitrate is an ionic crystal. Additionally, urea nitrate has two more oxygen atoms and one less carbon per structural unit compared to biuret. As a result, it is natural to compare the high pressure behavior of biuret to urea 3 ACS Paragon Plus Environment


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(OC(NH2)2)7,8 and urea nitrate ([HOC(NH2)2]+[NO3] )9 under pressure. Both urea and urea nitrate exhibit rich phase diagrams. Urea undergoes several phase transitions under 10 GPa.7,8,10 It undergoes a phase transition from Phase I (tetragonal, space group P-421m, Z=2) to Phase III (orthorhombic, space group P212121, Z=4) at 0.48 GPa.8 Phase II is a high pressure phase occurring above 0.60 GPa and 373 K. At room temperature, Phase III undergoes a phase transition to Phase IV (orthorhombic, P212121, Z=4) at 2.80 GPa.8 A transition to Phase V (monoclinic, space group Pmcn, Z=4) was observed in deuterated urea above 7.5 GPa at room temperature.10 For phases III and IV, changes in the hydrogen bonding constitutes the major alteration in the crystal structure.8 Urea nitrate does not exhibit the variety in the phase diagram at low pressure, which is exhibited by urea. Nevertheless, it has an interesting irreversible phase transition in the pressure range of ∼9 to 15 GPa9 from the initial P21/c space group to a Pc space group. The recovered Pc phase has a volume ∼11% smaller than the P21/c phase. This is due to the transformation from a two-dimensional hydrogen-bonded network in the low-pressure phase into a three-dimensional network in high-pressure. Biuret does not have either the energetic content or density (1.45 g/cm3) that urea nitrate does at ambient conditions (density of 1.66 g/cm3). Thus, it is intriguing to consider whether a denser form of biuret might increase its energy density substantially at moderate pressures (10 - 20 GPa) becoming similar to urea nitrate. For instance, the N-H··· O hydrogen bonds in biuret have the potential to form hydrogen bonds with the NH and NH2 groups. Under pressure, the hydrogen atoms might be increasingly transferred to the oxygen, leading to a charge transfer process similar to that which takes place in the ionic urea nitrate. Two other similar molecules are biurea ((H2N)C(O)N(H)N(H)C(O)(NH2))11 and thiourea dioxide ((NH)(NH2)CSO2H)).12 In contrast to urea and urea nitrate, biurea and thiourea dioxide have not been as extensively studied under pressure. Biurea has been studied to pressures of 5 GPa, and undergoes a reversible phase transition in the pressure range of 0.6-1.5 GPa.11 The lowpressure phase belongs to the monoclinic C2/c space group, while the higher pressure phase is proposed to belong to monoclinic P2/n space group. Changes in the hydrogen bonding was suggested as one of the main characteristics of this phase transition. Finally, thiourea dioxide


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undergoes a reversible phase transition at 3.7 GPa from a low pressure phase belonging to the orthorhombic Pnma space group, while the higher-pressure phase has orthorhombic symmetry and is proposed to belong to the space group Pbam.12 Changes in the vibrational spectra suggest that distortions in the hydrogen bonding were a significant component in the phase transition. The study of a molecular crystals such as biuret could also shed light on the nature of polymerization of molecular materials composed of N, C, O, and H atoms into extended phases.13 Consider the case of thiourea (SC(NH2)2), which is structurally similar to urea except for the substitution of oxygen with sulfur. Nevertheless, these two molecules have divergent chemical behavior.5 Thiourea has been observed to release H2S resulting in either cyanamide (CN2H2) or spontaneously polymerize into dicyandiamide (NCNC(NH2)2 or (NCN(H)C(NH)NH2).5 In contrast, urea does not easily lose the analogous H2O, but instead transforms into biuret (releasing one NH3 molecule per two urea molecules) or triuret (releasing two NH3 molecules per three urea molecules). Thus both urea and thiourea tend to form larger, chain-like molecules, one loses ammonia and the other hydrogen sulfide. Since the low-Z elements under pressure tend to behave like higher Z elements in their period at lower pressures, this suggests that biuret has the potential for various chemical changes under pressure. This kind of transformation would also be paralleled by that of urea nitrate to a layered structure under pressure, as previously mentioned.9 These reactions and transformations may lead provide pathways to polymeric forms of nitrogen. Nitrogen has tremendous potential to provide a clean energetic material without sacrificing performance14 as calculations have shown that extended polymeric phases of nitrogen could have detonation pressures exceeding that of HMX by a factor of ten.15 A goal of high pressure and synthetic chemistry14 has been the reduction of the pressure required to synthesize polynitrogen, which require high pressure and temperatures (∼ 1-2 Mbar and 2,000 K).16,17 In addition, to date it has proven difficult to recover the polymeric phases to low pressures. While there have been reports of recovered samples to low pressure and/or low temperature, reproducibility has remained a poor.15,18 A poly-nitrogen material synthesized from N, C, O and H would also provide a benchmark for what fraction of the pure N2 performance could be obtained.



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Biuret is a white crystalline powder, which belongs to the monoclinic crystal family with eight molecules per unit cell and lattice parameters a = 9.20 Å, b = 6.60 Å, c = 15.4 Å, and β ≈ 90°5 and a density at ambient pressure of 1.45 g/cm3. The space group was identified as either C2/c or Cc based upon the presence of the {hkl} reflections only when (k + l) was even and of the {h0l} reflections when both h and l were even. Nevertheless, this large unit cell with 96 atoms made it difficult to extract more precise information such as the atomic positions. Consequently, what is known of the molecular structure of isolated molecules and in crystals has been provided by spectroscopy, mostly on compounds containing biuret. These studies include infrared and xray studies on biuret hydrate,19,20 infrared spectroscopy and x-ray diffraction studies on biurethydrogen peroxide21 and infrared and visible absorption spectra of biuret-copper complexes22 and computational studies.23

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More recently a study of four biuret-containing complexes were

characterized using UV, FTIR and Raman spectroscopies, X-ray powder diffraction and thermogravimetric analysis.28 Additionally, biuret has been found to form co-crystals with cyanuric and glutaric acids.29 Several possible molecular geometries have been proposed for biuret, including some that assign C=N bonds (with formal charges to the oxygen) and O-H bonds.1,5 The two most common molecular geometries are those that assign either cis or trans isomers which minimize the formal charges. Nevertheless, with the hydrogen-bonding in the crystal, it is not necessary to press the distinction too far, with O··· H bonding occurring in the form of hydrogen bonding. Biuret has been reported to exist in both the cis and trans isomers in the gas phase as well as in the crystals.30 The cis (trans) denotes that the C=O groups are on the same side (opposite sides) of the molecule (see Figure 1. Based upon the infrared gas spectra and theoretical calculations, it has been suggested that isolated molecules of biuret exist as a mixture of 3:2 ratio of trans to cis in equilibrium at 400 K.24 It is possible that a mixture of both isomers might exist in the crystal.


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(a)

(b) 1

Figure 1: The isolated molecule biuret in its (a) trans and (b) cis conformers. While the lowest energy conformer is the trans conformer, it is possible that in the biuret crystal, cis conformer molecules are present. Molecules were drawn in VESTA.31

Experimental Details The experiments were performed in symmetric diamond anvil cells (DACs) with culets of 300 µm. Rhenium was used as the gasket material and was indented to final thickness of 40 to 50 µm. Then using an electrical discharge machine (EasyLabs), a cylindrical hole was drilled in the center of the indented area to make a chamber for the sample. Biuret (Acros Organics, 97%) crystals were loaded into the cell sample chamber as received. Additionally, a small sphere of ruby was added to the sample chamber to permit the measurement of the sample pressure with the fluorescent shift of ruby.32 The DAC with the loaded sample was placed under vacuum to help remove any water that may have been absorbed during the loading process. Then neon was loaded as a pressure medium using a high pressure gas loading device at 25,000 psi at ambient temperature.33 The Raman spectroscopy was performed with a dichroic micro-Raman system in a backscattering geometry using a 532-nm cw diode laser (Coherent Sapphire) as the excitation source. An IsoPlane SCT320 spectrograph with an air-cooled PIXIS 400BR eXcelon CCD and the LightField software (Princeton Instruments) were used to collect the Raman spectra. Spectra were collected in the range of 0 to 30 GPa at room temperature from 75 to 4000 cm-1. Collection times of 180 s with a laser power of 10 mW were typical. The sample was not observed to react with the 7 ACS Paragon Plus Environment


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laser at any pressure. The background was subtracted from the spectra using the Adaptive Penalized Least Squares algorithm34 and implemented in the R programing language35 with aid of algorithms in Baseline Wavelet package.36 X-ray diffraction was performed at the Advanced Light Source at Lawrence Berkeley National Laboratory. The diffraction patterns were collected using a MAR345 image plate and the geometric parameters of this detector were calibrated using a LaB6 sample. The x-ray beam energy was 25 keV, and all patterns were collected at room temperature. Diffraction patterns were collected from 1.8 to 50 GPa. Collection times of 120 seconds were typical. Once again, the pressure was determined from the shift of the ruby fluorescent line.32 LeBail refinement was performed to extract the lattice parameters using Jade 9.0 software.

Results and discussion Raman Spectroscopy As already mentioned, the biuret has twelve atoms, and there are 8 molecules per unit cell, with systematic absences of the diffraction lines indicating a monoclinic structure belonging either to the Cc or the C2/c space groups with the molecules located at general lattice positions.5 It has been found that the trans conformer is the minimum energy structure, at least at the SCF/3-21G* level of theory.25 Furthermore, further computational studies of biuret found that real frequencies were predicted only for non-planar forms of biuret, although the energies of the planar and nonplanar forms are very similar.27 This result rules out the C2v (for cis) and the Cs space groups for the symmetry of biuret molecule. While it is possible that the cis belongs to C2 point group, it seems most likely that the isolated biuret molecules have C1 symmetry, regardless of whether they are cis or trans. Even though the trans form of the molecules is favored energetically for the isolated molecule, there are cases of the molecule assuming a higher energy isomer in the crystal, such as cyanoacetohydrazide.37 Intermolecular forces, such as hydrogen-bonding, can often lead to


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considerable distortions to the molecule in crystals relative to their minimum energy configuration as isolated molecules. Additionally, distortions to the molecules in crystals may also be expected when the molecule lies on a site of lower symmetry than the symmetry of the molecule. Since the biuret molecules likely lie on C1 sites in the crystal, any symmetry elements that the isolated molecules might possess are likely to be lifted. Thus, it is possible that the molecules could have either cis or trans or mixture thereof in the crystal. As a result, the number and character of the Raman active modes will be considered separately for each of the two possible space groups by factor group analysis.38 In both cases, there would be 144 vibrational modes (3N, where N is the number of atoms per primitive cell or 48 atoms). For the Cc (C4s ) space group, the Raman-active modes transform as, Γtot = ΓRaman = 72A' + 72A''. For the C2/c (C62h) space group, the irreducible representation is, assuming that the atoms lie on C1 sites (the other sites only have two equivalent sites in the unit cell) Γtot = 36Ag + 36Au + 36Bg + 36Bu. From group theory, it appears that the Raman active modes are as follows: ΓRaman = 36Ag + 36Bg. As will shortly be shown by our experimental data, this many vibrational modes are not observed. This may be explained by the fact that many of the fundamental modes may lie near in frequency and thus are not resolved. If the coupling between the molecules in the unit cell is weak, and as a result, the Raman spectrum to a good approximation corresponds to that of an isolated biuret molecule. In this case, many of the expected modes in the crystal (144 for Cc and 72 for C2/c, would be very close to each other in frequency as they would represent combinations (often in-phase and out-of-phase) of similar molecular motions for molecules on different lattice sites. Thus, the 30 molecular modes would provide a good initial approximation that would be split, perhaps immeasurably. All the vibrational modes would be Raman active in 9 ACS Paragon Plus Environment


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any molecular symmetry that biuret could possess in either the trans or cis conformers. Additionally, many of the fundamental modes may have small Raman cross-sections and scatter light weakly. In the present study, the assignments of the modes have been taken from the literature.24,26 In Figure 2, the ambient spectrum of biuret is shown along with the different types of vibrational motions approximately where they lie in vibrational frequency. To a very high degree of approximation, a true separation exists between the high-frequency N-H modes and all the other vibrational motions. Aside from this, the molecular motions do not separate as neatly as the labels in Figure 2 might indicate. Many peaks have considerably mixing of components. Nevertheless, the labeling provides the general regions when the different atomic motions dominate, with more details are included in the discussion below. Upon the application of pressure, the modes shift continuously with pressure, suggesting that no chemical bonds are dissociated and no phase transitions. As commonly occurs in samples subjected to pressure, the peaks of the vibrational spectrum weaken and broaden. Most of the peaks shift to higher frequencies with pressure, although two of the peaks corresponding to the N-H stretching exhibit red-shift. The most significant changes in the Raman spectra of biuret consist of the appearance of low-frequency shoulder on the peaks near 1000 and 1500 cm-1.


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Figure 2: An ambient Raman spectrum of biuret with assignments of the different vibrational modes to the general regions to which they correspond. A more detailed assignment of the vibrational modes is given in the text and in the Supporting Information. Note the ν corresponds to stretching, δ to bending, δS to scissoring, ρ to rocking, ω to wagging, and τ to torsion. Lattice Modes Figures 3 and 4 summarize the Raman results on biuret. The diamond has a strong vibrational peak near 1334 cm-1 that is much stronger than the peaks from the sample. Additionally, the broad peak from 2200 to 2700 cm-1 is the diamond two-phonon line. The low-frequency peaks (under 200 cm-1) correspond to the vibration of the lattice. All these peaks shifted to higher wavenumbers with increasing pressure. There is one peak near 200 cm-1 that appears to split around 3.3 GPa. Aside from this peak, the same qualitative features appear in the spectra from 0.2 GPa until 20 GPa. After 20 GPa, there is a pronounced broadening and weakening of the peaks, consistent with the common behavior observed in materials under pressure. This weakening and broadening suggests that the long-range order of the molecular crystal may be breaking down, as the molecules may be rearranging slightly in the lattice or the 11 ACS Paragon Plus Environment


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stresses may be causing distortions to the crystals as the density increases. Additionally, the shifting and separating of the peaks that are strongly overlapping at low pressure also contributes to the observed weakening of the lattice modes. The appearance of additional peaks near 75 cm-1 should not be taken as evidence of a phase transition, since these peaks are most likely merely shifting across the cutoff wavenumber of the dichroic filter. The overall uniform shift suggests no evidence of phase transitions until at least 20 GPa, and quite likely until 30 GPa. This is consistent with the x-ray diffraction results, as will be seen in what follows. Modes from 400 to 1800 cm-1 The peaks in the region from 400 to 1300 cm-1 are all considerably weaker than the lattice modes at low pressure. With pressure, these peaks gain in intensity overall with respect to the lattice modes, and the intensity of the peak near 950 cm-1 increases relative to the lattice modes and overtakes the lattice modes in intensity by 15 GPa. There is a weak broad peak near 280 cm-1 that persists over the pressure range. It appears to gain in intensity around 5.1 GPa and most likely corresponds to C-N-C and N-C-N bending modes. Two peaks around


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Figure 3: Representative Raman spectra in all the frequency ranges with peaks from biuret. Note the appearance of shoulders on the peaks centered at 710, 950, and 1500 cm-1, A peak at 620 cm-1 gains strength with pressure in addition to a peak near to 1780 cm-1 at 20 GPa. The strong features visible near 1330 and 1400 cm-1 are due to the diamond anvils. 430 and 450 cm-1 appear as two singlets at low-pressures, but broaden considerably between 3.3 and 7.8 GPa. These peaks correspond to C=O bending, NCN, and N-C stretching and possibly to NH2 wagging and torsions.26 There is a peak around 620 cm-1 that is either absent or very weak at ambient and low pressures (Figures 3 and 4). Nevertheless, the intensity of this feature increases and becomes well-resolved by 5 GPa. It continues to become more intense relative to the other peaks until at least 11 GPa. After this pressure, it at least maintains a constant relative intensity with respect to the other peaks. Vibrational motions in this frequency range are assigned to wagging and torsions of N-H as well as C=O bending.26 A similar feature was observed in the spectra of biurea at 550 cm-1.11 This peak was difficult to discern at low pressures, but becomes clearly resolved after 1.1 GPa.



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The peak around 710 cm-1 begins as a singlet, and then exhibits a shoulder on the lowfrequency side, possible as early as 7.8 GPa. This shoulder becomes more distinguishable at higher pressures. This feature most likely corresponds to the superposition of the C=O bending, C-N stretching, NCN bending, and possibly NH wagging modes.26 The peak around 950 cm-1 becomes more prominent relative to the other peaks over the pressure range. It shows a clear shoulder by 9 GPa (not shown in Figure 3, and it continues until 30 GPa. This peak corresponds to the C-N stretching and bending.26 The broad peak centered around 1130 cm-1 shows a very slight blue shift over the pressure range. It contains components of NH2 rocking and C-N and C=O stretching motions.26 There is a collection of peaks from 1400 to 1700 cm-1 at ambient pressure. These include four separate peaks at 1510, 1550, 1616, and 1680 cm-1. The peak at 1616 cm-1 shows a redshift with pressure. By 11.1 GPa, all of these peaks except one have disappeared into the background. The remaining peak, which corresponds to the one at 1510 cm-1 at ambient, appears to have a shoulder on the lower frequency side, which becomes clearer as the frequency separation increases with pressure. Additionally, there are two peaks to the high-frequency side that appear to emerge at 20 GPa. One is essentially a shoulder off the main peak and the other is at 1780 cm-1 and appears to emerge from the background and strengthens in intensity with pressure. These atomic motions that dominate in these most are C=O stretching and the bending and scissoring motions of the NH and NH2 groups.26 In particular, the stretching of the C=O groups is likely the main contribution to the new peak that appears at 1780 cm-1 after 20 GPa. The strong peaks that between 1500 and 1600 cm-1 are most likely dominated by NH scissoring, bending, C=O stretching and C-N stretching. The peaks near 1200 and 1240 cm-1 are most likely satellite bands due to the diamond and are considered spurious in the present study. This is supported by their clear absence in the initial ambient and recovered spectrum (Figures 2 and 3). The N-H stretching modes


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The N-H stretching mode occur between 3200 and 3400 cm-1 and appear in three bands. Two overlap slightly, with peaks at 3190 and 3270 cm-1, and the other peaks is at 3420 cm-1. The highest frequency branch corresponds to normal modes dominated by the asymmetric stretching of the N-H bonds in the NH2 groups. In the literature, two different assignments are given for the two other, lower-frequency branches. The branch at 3270 cm-1 is assigned to motion by the N-H symmetric stretching bonds in the NH2 groups and the branch at 3190 cm-1 to the stretching of the NH group by Korolevich et al.24 This assignment is reversed by Sullivan et al. with the branch at 3270 cm-1 being assigned to the stretching of the NH group and that at 3190 cm-1 to the symmetric stretching of the NH2 group.26 This difference in the mode assignments does not affect the following analysis. The broadness of these peaks arises from the range of N-H bond lengths due to the hydrogen bonding. The two lowest-frequency branches exhibit shifts to lower frequencies as seen in Figure 4. Figure 3 shows that the intensities of these peaks decrease quickly, disappearing around 8 GPa. The highest frequency branch appears to weaken in the pressure range from 0.2 until 3.3 GPa. With continued application of pressure, the intensity increases until at least 8 GPa and clear splitting in observed at 5.1 GPa. With increasing pressure, this branch also weakens and by 13 GPa, has disappeared into the background. This mode exhibits a weak redshift from to 4 GPa. After this pressure it splits into two. The higher frequency peak exhibits little pressure dependence while the lower frequency peak exhibits a weak redshift. The hydrogen bonding in biuret is due to the interaction between the protons of the hydrogen atoms on the NH and NH2 and the oxygen atoms on the carbonyl groups. The redshifting of the N-H vibrational modes is due to the attractive hydrogen bonding with oxygen atoms.11,39 In this case, the N-H bond length increases as the strength between the O···H increases. This redshifting effect is more pronounced in the single N-H bond and symmetric stretching NH2 modes than in the asymmetric NH2 stretching modes. Note also that the hydrogen modes in biurea exhibit similar pressure dependence.11 With the exception of the mode at 950 cm-1, all the peaks that show shoulders and splitting or that appear with the application of pressure (such as the peaks around 620 and 1780 cm-1) are



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dominated by NH and NH2 motions as well as C=O stretching and bending. This suggests that one of the main effect of the application of pressure is to alter the N-H···O hydrogen bonds between the NH and NH2 groups and the CO groups. The splitting and appearance of new peaks suggests changes in the orientation, lengths and strengths of these bonds, which is consistent with modification to the hydrogen bonding. Unfortunately, the N-H stretching modes become very weak by about 13 GPa, and thus cannot provide insight into the changes observed in the sample after that pressure. Nevertheless, the behavior of these NH and NH2 groups can be inferred from the other modes mentioned above to which they contribute. There are three pressures at which changes in Raman spectra are observed to take place. The first is around 3-5 GPa, where one new lattice peak emerges near 200 cm-1, and the peak around 620 cm-1 begins to gain considerable intensity (Figures 3 and 4). Additionally, the peak near 1610 cm-1 disappears around 5 GPa and the rate of the redshift of the NH stretching mode increases. Finally, the N-H asymmetric stretching modes to split into two after 5 GPa. The second pressure range where changes in the Raman spectra occur is between 8 and 12 GPa, with the appearance of shoulders on the low-frequency sides of the peaks at 710 and 950 cm-1. The peaks near 250, 450, and 1700 cm-1 disappear in this pressure range. At pressures slightly below 8 GPa, the lower branch of the N-H bonds ate 3100 cm-1 disappear, and the higher frequency branch begins to show splitting. Finally, in the pressure range of 16 to 20 GPa, peaks appear near 1500 and 1800 cm-1 as seen in Figure 4. Note that most these modes mentioned that undergo changes possess strong contributions from N-H and C=O vibrations. These modes would be especially sensitive to changes in the hydrogen-bonding. Thus it is likely that the Raman is indicating small changes in the hydrogen bonding, with reorientation of, and distortions to the molecules. The peak at 950 cm-1 is dominated by C-N stretching and bending vibrations. It possibly does not have components from the N-H or C=O vibrations. Nevertheless, it is not surprising that changes to the hydrogen bonding might affect the internal NC vibrations. The lower spectra (in blue in Figure 3) represents the recovered sample measured with the diamonds removed. The high degree of similarity between the recovered spectrum and initial spectrum at 0.2 GPa indicate that the vibrational characteristics of the recovered sample are


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almost identical to the initial sample. There appears to be essentially no irreversible changes to the biuret in terms of its bonding. This is seen from the good agreement in frequencies of the modes on compression and decompression in Figure 4. This reversibility is consistent with the only changes having to do with minor distortions and reorientations of molecules. On the other hand, there does appear to be some hysteresis at the level of the crystal. This is suggested by the broad band corresponding to the lattice modes on release that were much better resolved upon compression (see Figure 3). This topic will be further examined in the discussion on the x-ray diffraction.

Figure 4: The pressure-dependence of the peaks in the Raman spectrum. Three regions are noted in gray that correspond to the pressures at which the most significant changes take place in the sample, as explained in the text.



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Figure 5: Select powder diffraction pattern of biuret under pressure. A LeBail refinement of the biuret powder pattern with the space group C2/c is shown at 1.8 GPa.

Powder X-Ray Diffraction Figure 5 shows the powder diffraction patterns with increasing pressure and a representative LeBail fitting at 1.8 GPa. There are no clear discontinuities observed in the x-ray diffraction


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patterns. The diffraction peaks due to the neon pressure transmitting medium are indicated by the black circles. Thus there is no clear evidence of phase transitions over this pressure range. In order to confirm this, it would be necessary to perform a Rietveld analysis on the data. The large, eight-molecule unit cell and the quality of the diffraction patterns, however, make Rietveld analysis difficult, even at ambient.5 As a result, LeBail analysis was performed on the patterns to extract the lattice parameters as shown in Figures 5 and 6. The β angle of the unit cell increases sharply with pressure from ambient to about 5 GPa. After this it exhibits a slow, monotonic increase with pressure until 50 GPa. The other lattice parameters decrease with pressure rapidly until about 25 GPa and more gradually after this. From ambient to 25 GPa, the a parameter changes by 9%, the b by 8% and the c by 6%. The total change in the lattice parameters over the pressure range is 11% for a, 12% for b and 10% for c. Thus the a lattice parameter changes the most from ambient to 25 GPa, and the least from 25 to 50 GPa. The differences in the rate of change of the lattice parameters suggest the hydrogen bonds may be arranged in sheets parallel to the planes along a and b lattice directions. This observation is consistent with seeing the splitting in the vibrational modes as due to the rearranging of the hydrogen bonds, and resulting a more compact, stiffer material. The volume changed by about 23% from ambient to 25 GPa and by 30% from ambient to 50 GPa. Also note the hysteresis upon release that is observed in Runs 2 and 4. This hysteresis appears to affect a and c lattice parameters to a greater extent than the b lattice parameter and the β angle. This residual strain in the lattice is consistent with the observed changes to the lattice parameters in the sample upon release 3. The x-ray data also indicates that the original ambient phase of biuret is recovered upon the release of pressure (even with the hysteresis). This result suggests that the changes observed in the Raman are at the level of molecular structure related to the distortions and molecular reorientation with very minor in influences on the unit cell. Thus the combined Raman and x-ray diffraction results indicate that the bonding in the crystal and the crystal structure have not changed. From this, it can be concluded that biuret is much more stable than urea or urea nitrate under pressure.



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Figure 6: Plots of the pressure dependence of the (a) unit cell volume and (b) lattice parameters. The compression data is shown in the closed symbols and the decompression data is shown in the open symbols. The volume shows considerable hysteresis upon release in two of the runs. This hysteresis was most significant in the along the a lattice and to a lesser extent along the c direction. It did not have a major impact along the b direction nor on the β angle.

Comparison between Biuret and Similar Compounds under Pressure In passing, it is interesting to compare to the behavior of biuret under pressure with other similar compounds such as urea, urea nitrate, biurea and thiourea dioxide. The prominence of the hydrogen bonding in these materials and alterations to the hydrogen-bonded networks under pressure make it plausible that similar behavior would be observed in biuret. It is also similar to urea, biurea, and thiourea dioxide in that the changes under pressure are all reversible. In this regard, it also has a clear contrast with the ionic urea nitrate and under the pressure range in the present study, no evidence of any chemical nor transitions to polymeric phases were observed. It is different than these other compounds in that it did not undergo a clear change in crystal symmetry. It is also necessary, however, to take into account that biuret is a low-symmetry crystal with a larger unit cell. This increases the challenge of identifying changes to the crystal structure, especially if subtle.


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Conclusion In conclusion, Raman spectroscopic data (to 30 GPa) and powder x-ray diffraction (to 50 GPa) has been presented on biuret. To the best of our knowledge, this is the first study of biuret under high pressure. Raman evidence has been presented to support the contention of a rearrangement of the molecules in the unit cell. The first indication of such a reorientation occurs between the pressures of 3 and 5 GPa, the second in the pressure range of 8 to 12 GPa and the final one occurs in the pressure range between 16 and 20 GPa. All these correlate with the appearance and disappearance of peaks and/or the splitting of peaks. Many of the peaks that are involved in these changes correlate to the vibrational motions corresponding to the N-H and C=O bonds. Changes affecting these modes likely are associated with modifications to the hydrogen bonding sublattice. Thus, it is suggested that these changes are related to molecular distortions and reorientations related to hydrogen bonding. No clear evidence of a phase transition was observed. Thus, it is probable that biuret remains a molecular crystal over the pressure range studied. Upon the release of pressure, biuret returns to its starting material.

Supporting Information Available The following files are available free of charge. •

Assignment of Vibrational Frequencies: Table listing the frequencies of the vibrational modes in biuret along with assignments of these modes.

Acknowledgement During this project, coauthor G.M.B. was supported in part by an appointment to the Postdoctoral Research Program at the US Army Research Laboratory administered by the Oak Ridge Associated Universities. Portions of this work were performed at the Advanced Light Source, which is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.



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TOC Graphic


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Hydrogen-Bonding Modification in Biuret Under Pressure - The

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