J. Phys. Chem. C 2009, 113, 15761–15767
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H-Bond Breaking in High-Pressure Urea Anna Olejniczak, Kinga Ostrowska, and Andrzej Katrusiak* Faculty of Chemistry, Adam Mickiewicz UniVersity, Grunwaldzka 6, 60-780 Poznan´, Poland ReceiVed: May 27, 2009; ReVised Manuscript ReceiVed: July 13, 2009
Hydrogen bonds NH · · · O are broken and restored, and their lengths changed by more than 1 Å in the strained crystal environment of urea, (NH2)2CO, when exposed to high pressure. Single crystals of urea phases I, III, and IV were grown in situ in a diamond-anvil cell, and their structures were determined by X-ray diffraction. At 0.48 GPa, on transformation from phase I (tetragonal space group P4j21m) to phase III (orthorhombic space group P212121), the channel voids characteristic of phase I collapse, one of the NH · · · O bonds is broken, and the H-acceptor capacity of the oxygen atom is reduced from 4 to 3. Above 2.80 GPa, in phase IV (orthorhombic space group P21212), the H-bonding pattern of phase I and fourfold H-acceptor oxygen are restored. The thermodynamic phase transitions in urea have been rationalized by a microstructural mechanism involving the interplay of pressure-induced molecular reorientations, with hydrogen bonds competing for access to lone-electron pairs of carbonyl oxygen, and by the increasing role of van der Waals interactions. None of phases I, III, and IV contain the hydrogen bond types most frequently encountered in urea cocrystals. 1. Introduction Urea, (NH2)2CO, is one of the most common and renowned chemical and biological compounds, being the chief nitrogencontaining end product of animal metabolism and widely used in chemical practice and in industry, mainly for the production of fertilizers, pharmaceuticals, and urea-formaldehyde plastics. The synthesis of urea in Friedrich Wo˝hler’s laboratory in 1828 ended the vis vitalis theory and triggered the development of organic chemistry. Urea is also a prototypical hydrogen-bonded molecular crystal,1 and it was the first organic compound studied by X-ray diffraction.2 Since then its structure has been extensively studied using X-rays,3,4 neutrons,5 spectroscopy,4,6 and theory.7 The tetragonal structure of urea, space group P4j21m, hereafter referred to as phase I, has been confirmed down to 12 K.8 In this crystal structure, one CdO carbonyl group acts as the acceptor of four NH · · · O hydrogen bonds, an extremely rare capacity connected with the protein-denaturing activity of urea. The 3D network of H-bonded urea molecules contains channel voids, which are also characteristic of numerous urea inclusion compounds9 of exceptional ferroic properties.10 However, the types of hydrogen bonds found in unsolvated urea are distinctly different to those in inclusion compounds (Scheme 1), and urea is a prominent example of the discrepancy between the H-bonding properties revealed in the unsolvated compound and its cocrystals. Of 477 urea cocrystals currently deposited in the Cambridge Structural Database (CSD), there are only 38 ureahost networks with chelated H-bonds (Scheme 1a), and 20 single two-center H-bonds in which the H-atom is directed perpendicularly to the molecular plane (Scheme 1b); both these types present in the structure of urea polymorph I, whereas the most frequent (144 structures) is a different motif of double twocenter hydrogen bonds shown in Scheme 1c. In addition, there are four structures with patterns of single two-center H-bond (Scheme 1d) and 26 cocrystals where both kinds of the double two-center patterns are present (Scheme 1c,d). For 168 cocrystals in CSD, no N-H · · · O hydrogen bonds are formed between * To whom correspondence should be addressed. E-mail: katran@ amu.edu.pl.
SCHEME 1: Main Motifs of Hydrogen Bonds Formed by Urea Molecules and Their Graph Descriptors13 a
a (a) Chelated R12(6) H-bonds and (b) two-center C12(6) H-bonds in urea phase I, (c) double two-center H-bonded R22(8) ring, most frequent in urea inclusion compounds, and (d) ring R24(8) of four two-center N-H · · · O bonds.
the urea molecules, and for 77 entries no atomic coordinates were deposited. Moreover, the planar and C2V symmetric urea molecule revealed in phase I is inconsistent with the energetically more stable C2 and Cs symmetric conformations with the NH2 groups twisted off from the molecular plane, as observed by microwave spectroscopy6 and postulated by theory.11 Although there were suggestions of possible transformations of phase I at about 200 K,12 no structural evidence was ever reported. The metastable molecular conformation and the voids present in phase I are prerequisites for the high susceptibility of the structure to elevated pressure. Indeed, in 1916, Bridgman discovered that urea transforms to phase II above 373 K and 0.60 GPa, and to phase III at 293 K above 0.48 GPa.14 Several
10.1021/jp904942c CCC: $40.75 2009 American Chemical Society Published on Web 08/11/2009
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studies aimed at determining the high-pressure phases of urea were undertaken. In a neutron powder-diffraction study, the compressibility of deuterated urea (ND2)2CO was measured to 4.5 GPa, and it was established that phase III above 0.48 GPa is orthorhombic, space group P212121 with the unit cell containing four molecules (Z ) 4).15 In a subsequent neutron powder-diffraction study, phase IV of (ND2)2CO, space group P21212, Z ) 2, stable between 2.80 and 7.20 GPa and phase V (Pmcn, Z ) 4) above 7.20 GPa were revealed16 and confirmed in yet another neutron-diffraction experiment.17 In a later X-ray diffraction and spectroscopic study on urea (NH2)2CO, the symmetry of phase III was described as consistent with space groups P212121 and Pna21 and coexistence of domains with distinct spectral features was observed within phases IV and V.4 However, these reports4,15-17 included neither unit-cell dimensions nor atomic coordinates, presumably because of the experimental difficulties and ambiguities caused by a low resolution of neutron powder high-pressure data and the destructive character of the I/III phase transition. The high-pressure structure of urea was also tackled theoretically. Density functional theory was applied to calculate changes in the urea molecule subjected to pressure of up to 10 GPa in the restrained crystal environment of phase I.18 In a phenomenological approach, Go´ra and Parlin´ski19 employed symmetries determined experimentally by high-pressure neutron-diffraction studies on deuterated urea powder,15,16 as well as other unpublished neutron-diffraction results, to a cluster of 16 molecules in a supercell (2*2*2 phase I unit cells) with periodic boundary conditions at 0 K. Their calculations yielded the transition to the phase of space group P21212, Z ) 2, at 0.10 GPa, and another transition to space group P212121, Z ) 4, at 1.20 GPa. This sequence of phase transitions is the reverse of that observed experimentally at 296 K. Their theoretically computed unit-cell dimensions and atomic coordinates were reported, but to our knowledge this is the only structural information of the urea high-pressure phases published until now and for this reason these results cannot be verified experimentally. Because of the lack of experimental data, these structures have been used for discussing urea polymorphs in subsequent studies. Also, a monoclinic urea polymorph, space group P21/m and Z ) 2, was obtained above 535 GPa from theoretical calculations,7f where the NH · · · O hydrogen-bonded urea molecules were arranged into planar sheets and each CdO oxygen atom was fourfold coordinated and each NH2 group was involved in two hydrogen bonds. However, in that model, the molecular dimensions were hardly affected by such a huge pressure, while the intermolecular H · · · H contacts were squeezed to distances smaller than 1 Å. Thus, while there is a pressing need for understanding the molecular structure, H-bonding, and the association of urea molecules at varied environments and thermodynamic conditions, there are conflicting fragmentary experimental and theoretical reports and no experimental structural information is available on the high-pressure urea polymorphs, neither (NH2)2CO or (ND2)2CO. Hence, this single-crystal X-ray diffraction study has been undertaken to determine reliable structures of the urea polymorphs and to understand the H-bonding properties of urea molecules in varied crystal environments and thermodynamic conditions. 2. Experimental Section To circumvent the destructive phase transitions and high crystal strain, single crystals of urea were grown in situ in isochoric or isothermal conditions in a modified Merrill-Bassett
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Figure 1. Two stages of urea crystal isochoric growth in phase I: (a) a thin plate at 373 K and (b) this crystal at 0.47 GPa/296 K. The ruby chip for pressure calibration is located close to the gasket upper edge.
Figure 2. Isochoric growth of the urea crystal in phase III: (a) at 333 K the single crystal very thin plate is hardly visible in the polarized light and (b) the sample crystal at 1.48 GPa/296 K. The ruby chip for pressure calibration is on the upper right side of the DAC chamber.
Figure 3. Urea crystal in phase IV, isothermally grown from a methanol/ethanol/water (16:3:1) mixture at 296 K: (a) one seed of the orthorhombic bipyramide lying on the (1j1j1j) face and (b) the crystal at 3.10 GPa/296 K. The small ruby chip for pressure calibration is by the upper vertex of the urea crystal.
diamond-anvil cell (DAC).20 DACs equipped with diamond anvils mounted on steel backing plates with conical windows were applied.21 The high-pressure study on urea phases I and III was performed on the single crystals obtained in the DAC from the saturated solution of urea in water (Figures 1 and 2). The crystal at phase IV was obtained from a methanol/ethanol/ water mixture, 16:3:1 in volume (Figure 3). Pressure in the DAC was calibrated by the ruby-fluorescence method,22 using a BETSA PRL spectrometer, with an accuracy of 0.05 GPa. All the high-pressure X-ray measurements were carried out at 296 K. The single-crystal data were measured with a KUMA KM4CCD diffractometer; CrysAlis software23 was used for the data collections24 and the preliminary reduction of the data. The intensities were corrected for the effects of DAC absorption, sample shadowing by the gasket, and sample absorption,25 and the reflections overlapping with diamond reflections were eliminated. Owing to these precise corrections, the space group
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TABLE 1: High-Pressure Crystal Data of Urea Polymorphs I, III, and IV polymorph pressure (GPa) temperature (K) crystal system space group unit cell dimensions (Å) a b c V (Å3) Dx (g cm-3) completeness (%) final R1/wR2 (I > 2σI) R1/wR2 (all data)
I
III
III
IV
0.47(5) 296(1) tetragonal P4j21m
0.80(5) 296(1) orthorhombic P212121
1.48(5) 296(1) orthorhombic P212121
3.10(5) 296(1) orthorhombic P21212
5.589(6) 5.589(6) 4.680(5) 146.1(3) 1.365 46.6 0.065/0.243 0.071/0.257
3.624(11) 8.272(4) 8.844(4) 265.09(18) 1.505 35.7 0.054/0.119 0.058/0.122
3.539(7) 8.234(16) 8.790(18) 256.14(9) 1.558 28.2 0.034/0.087 0.040/0.090
3.414(3) 7.360(8) 4.606(10) 115.7(3) 1.723 41.9 0.035/0.112 0.058/0.145
symmetries were unequivocally determined, and structures were straightforwardly solved by direct methods and refined by fullmatrix least squares.26 Anisotropic temperature factors were generally applied for non-hydrogen atoms. The H-atoms were located from the molecular geometry (dN-H ) 0.86 Å), and their Uiso’s were constrained to 1.2 times Ueq of their carrier atoms. The selected crystal data are listed in Table 1. The unit-cell dimensions are plotted in Figure S1 in the Supporting Information. The structural details are listed in Table S1 in the Supporting Information and deposited in the Cambridge Structural Database (CCDC 731958, 731959, 731960, 731961). Structural drawings were prepared using the X-Seed interface of POV-Ray.27 In this report, we applied the urea phase labeling introduced by Bridgman14 and extended by Weber et al.,15,16 where phase II occurs above 373 K and 0.60 GPa. 3. Results and Discussion The morphologies of urea crystals reflect the changing hierarchy of intermolecular interactions in their structures. Phase I and III samples quickly grow along the columns of chelated hydrogen bonds, coinciding with the [001] direction (Figures 1 and 2). This is not the case in phase IV (Figure 3), which indicates that in its structure no specific interactions favor a faster rate of growth in one particular direction. While the molecular dimensions are consistent within errors (Table S2 in the Supporting Information), pressure considerably modifies the H-bonding association of the urea molecules, as shown in Figure 4. In phase I, the carbonyl oxygen interacts with three neighboring molecules, forming four O · · · H bonds (Figure 4a, Table 2). In phase III, one of the two-center H-bonds is broken and the CdO group interacts with two molecules via three O · · · H bonds (Figure 4b). In phase IV, the O-atom is fourfold coordinated again, similarly as in phase I (Figure 4c). The hydrogen bonds lengths H · · · O, N · · · O and CdO · · · H,
CdO · · · N, and N-H · · · O listed in Table 2 and plotted in Figures 5 and 6 all illustrate that the hydrogen bonds of phases I and IV are very similar and that the largest transformations of the hydrogen bonds proceed within the phase III region. One of the hydrogen bonds is broken and dimensions of other hydrogen bonds change dramatically in phase III, and then in phase IV they revert to dimensions similar to those in phase I. It is characteristic that only for three H-bonds within phase III the angular dimensions CdO · · · H and CdO · · · N correspond to the most favored directions of the H-bond formation,28 that is, between 120° and 150°, and are outside this range for phases I and IV. In phase I, each molecule forms eight hydrogen bonds, six hydrogen bonds in phase III, and eight again in phase IV. It will be shown below that the transformation of hydrogen bonds is directly connected to the crystal strain generated by the high pressure. The structures of phases I, III, and IV are analogous, in that the molecules are hydrogen-bonded into a 3D network with chelate bonds running along the [001] direction, the CdO bonds of molecules are exactly (in phases I and IV) or approximately (in phase III) aligned along [001], and the crystal retains the unit cell, albeit doubled along [001] in phase III and strongly strained in phases III and IV. The doubling of the cIII parameter is due to the zigzag opposite tilts of the molecules (Figures 7 and 8), whereas the orthorhombic strain of the crystal lattice in phases III and IV is due to the collapse of channels (the strain matrices are given in the Supporting Information). The main hydrogen-bonding motif in urea phase I, the chelate H-bonded chains running down and up the [001] direction, involves the anti H-atoms being directed toward the loneelectron pairs of the oxygen atom in the molecular plane (the anti and syn positions of the amine H-atoms are related to the carbonyl oxygen). In phases I and IV, these chelate Hanti-bonds have shorter N · · · O distances than the two-center hydrogen
Figure 4. Coordination of carbonyl oxygen in urea: (a) phase I at 0.47 GPa, (b) phase III at 0.80 GPa, and (c) phase IV at 3.10 GPa. Hydrogen bonds are indicated by dashed lines. Only the symmetry-independent atoms are labeled for each of the phases (cf. Supporting Information).
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TABLE 2: Dimensions of Hydrogen Bonds (Å, deg) in Phases: I at 0.47 GPa, III at 0.80 and 1.48 GPa, and IV at 3.10 GPaa polymorph I at 0.47 GPa
III at 0.80 GPa
III at 1.48 GPa
IV at 3.10 GPa
a
D-H · · · A N(1)-H(1a) · · · O N(1)-H(1b) · · · Oii N(1)-H(1b) · · · N(1)iii N(1)-H(1b) · · · N(1)iv N(1)-H(1a) · · · Oi N(1)-H(1b) · · · Oii N(2)-H(2b) · · · Oii N(2)-H(2a) · · · N(2)iv N(1)-H(1a) · · · Oi N(1)-H(1b) · · · Oii N(2)-H(2b) · · · Oii N(2)-H(2a) · · · N(2)iv N(1)-H(1a) · · · Oi N(1)-H(1b) · · · Oii N(1)-H(1b) · · · N(1)iii N(1)-H(1b) · · · N(1)iv i
H· · ·A
D· · ·A
D-H · · · A
symmetry code
2.14 2.20 3.13 3.14 2.05 2.05 2.32 2.51 2.02 2.04 2.30 2.49 2.18 2.14 2.60 2.61
2.978(9) 2.960(8) 3.465(8) 3.465(8) 2.896(6) 2.864(6) 3.065(5) 3.354(8) 2.866(6) 2.846(6) 3.044(6) 3.328(10) 2.997(6) 2.913(13) 3.072(11) 3.072(11)
165.78 147.54 105.81 105.18 169.47 157.18 144.69 167.10 169.04 156.5 144.38 166.80 158.07 150.15 115.67 114.67
(i) y - 1, 1 - x, 1 - z (ii) x, y, 1 + z (iii) y - 1, 1 - x, -z (iv) -y, 1 - x, -z (i) x - 0.5, 0.5 - y, -z (ii) 0.5 - x, 1 - y, z - 0.5 (iii) 0.5 - x, 1 - y, z - 0.5 (iv) 0.5 + x, 1.5 - y, -z (i) x - 0.5, 0.5 - y, -z (ii) 0.5 - x, 1 - y, z - 0.5 (iii) 0.5 - x, 1 - y, z - 0.5 (iv) 0.5 + x, 1.5 - y, -z (i) x - 0.5, 1.5 - y, 2 - z (ii) x, y, 1 + z (iii) 0.5 + x, 1.5 - y, 1 - z (iv) x - 0.5, 1.5 - y, 1 - z
The syn-hydrogen atom (with respect to the CdO bond) is marked by “a” added to the label and the anti-hydrogen by “b” (cf. Figure 4).
Figure 5. Hydrogen bond lengths (a) H · · · O and (b) N · · · O in the structures of urea phases I, III, and IV. The vertical dotted lines mark the magnitudes of pressure of transitions between phases I/III and III/ IV. The lines joining the points were drawn to guide the eye only, the double guiding lines mark two symmetry-dependent H-bonds. The chelated H-bond dimensions are indicated with open circles (light blue and navy blue) and the two-center ones with full circles (purple and red lines). All the dimensions involving H-atoms at 0.1 MPa were calculated for the H-atoms located from molecular geometry using the same criteria to avoid errors in the comparisons. These plots showing the enhanced lengths of the H-bonds are presented in Figure S2 in the Supporting Information.
bonds involving the syn H-atoms, running along [110] and [1j10] directions. However, in phase I the chelate H-bonds are more strained, as their N-H · · · O angles are smaller, and the Hanti · · · O distances longer, than those of the two-center bonds. The motif of two-center Hsyn-bonded molecules is most affected on the urea crystal transforming to phase III (Figures 4, 5, and 8): one
Figure 6. Angular dimensions of the hydrogen bonds in urea polymorphs: (a) angle C-O · · · H, (b) angle C-O · · · N, and (c) angle N-H · · · O (cf. Figure 5). For plots with enhanced H-bonding dimensions, see Figure S3 in the Supporting Information.
of the two-center hydrogen bonds is broken and the other becomes the shortest of all the hydrogen bonds. This results from the molecular tilts, which release the strain in one of the two-center hydrogen bonds at the cost of the other. The molecular tilts in phase III also differentiate the chelate H-bonds,
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Figure 7. Molecular packing of urea crystals in phase I at 0.47 GPa, phase III at 0.80 GPa and phase IV at 3.10 GPa viewed along [001]. The hydrogen bonds are indicated by the dashed lines.
Figure 8. Urea crystal structures in phases I, III, and IV, as projected along the [010] direction (cf. Figure 7).
one of which becomes less strained and shorter, and the other more strained and longer. At the III/IV phase transition the molecules resume the twofold crystal symmetry and exact alignment along [001], as observed in phase I. Because of the absence of molecular tilts, the unit-cell parameter cIV is two times shorter than cIII and is similar to cI (see Figure S1 and the strain tensors in Supporting Information). The hydrogen bond dimensions also become similar to those in phase I. The difference in N · · · O length between the chelate and two-center hydrogen bonds is more pronounced in phase IV than in phase I, and the Hanti · · · O distance of the chelate bonds becomes shorter than the Hsyn · · · O distance in the two-center Hanti-bonds (Figure 5). The similarity between phases I and IV can be clearly seen in Figures 7 and 8: the molecular packing of phase IV is the collapsed version of phase I with the molecules arranged in a similar manner, but inclined by angle 2 arctg(a/b) ) 49.77° at 3.10 GPa (instead of the 90° inclination in phase I), and with the channel voids eliminated. In all urea phases, the molecular arrangement is governed mainly by NH · · · O hydrogen bonds. However, also some H · · · N contacts are present (Table 2). In phase I, the H · · · N distances
are much longer than the sum of van der Waals radii,29 but in phases III and IV they become commensurate with this sum. At 0.80 GPa (phase III), the shortest H · · · N distance is that of 2.51 Å that links molecules into zigzag chains along the (001) planes; at 1.48 GPa this distance is reduced to 2.485 Å. At 3.10 GPa (phase IV), the two shortest H · · · N contacts, 2.597 and 2.610 Å, are longer than those in phase III. Thus, the NH · · · N contacts are long and unlikely to play a major role in the structural transformations of urea between phases I and IV. Whereas phase IV can be considered as a collapsed version of phase I, phase III has a considerably more complex structure. It appears that when the channel voids collapse at the I/III phase transition, the two-center Hsyn-bonds (less strained than the chelate Hanti-bonds, which in phase I show the N-H · · · O angles that are ca. 20° smaller; see Figure 6) start to compete with the chelate Hanti-bonds for access to the lone-electron pairs of the oxygen atom. One two-center Hsyn-bond and one chelate Hantibond become significantly shorter than their counterparts when the molecules are tilted. It can be observed for all pressure ranges investigated, spanning phases I, III, and IV, that the length of hydrogen bonds correlates with their angular dimen-
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sions. Hydrogen bond N(2)-H(2b) · · · Oiii, which is broken in the phase III structure, becomes strongly bent as its O-H · · · Niii angle decreases to nearly 140°, and angles C-O · · · Hiii to less than 70° and C-O · · · Niii to less than 80°. Thus, the phase III region can be described as a competition zone, where owing to molecular tilts the NH2 donor groups are more conveniently or less conveniently positioned with respect to the lone-electron pairs of the acceptor carbonyl oxygen, which strengthens or weakens the hydrogen bonds. The molecular tilts are eliminated at the III/IV phase transitions at 3.10 GPa, where the more compact coplanar arrangement is enforced in the compressed crystal by van der Waals interactions. In the compressed environment of phase IV, the angular dimensions of the H-bond (Figure 6) are similar to those in phase I, and the H-bonds lengths (Figure 5) also become similar. 4. Conclusions The high-pressure structures of urea polymorphs I, III, and IV crystallized in situ in a DAC and determined by X-ray diffraction are consistent with Bridgman’s compressibility measurements14 and preliminary information from neutrondiffraction studies on (ND2)2CO powders,15,16 but different from theoretical structures.19 The theoretical estimates of the unitcell dimensions for tetragonal space group P4j21m at 0 GPa were a ) 5.558 Å, c ) 4.7062 Å; for orthorhombic space group P21212 at 0.13 GPa, a ) 6.0896 Å, b ) 4.8217 Å, c ) 4.7069 Å; and for space group P212121 at 1.39 GPa, a ) 5.8585 Å, b ) 4.6080 Å, and c ) 9.2160 Å (cf. Table 1). The strong transformations in the hydrogen bond geometry and the breaking of one two-center Hanti-bond can be explained by the molecular rearrangements induced by pressure. The onset of molecular rearrangements is induced by the collapse of the channel voids on the transformation to phase III, and then by the enforced close-packing alignment of the molecules on transformation to phase IV. The molecular reorientations in urea phases I, III, and IV and the changes of hydrogen bond dimensions illustrate the importance of the angular dimensions and directionality of hydrogen bonds for the polymorphism and solid-state phase transitions, similarly to that described for the proton-disordering phase transitions in the structures of O-H · · · O bonded ferroelectrics.30 There are still several unanswered questions about hydrogen bonding and the high-pressure behavior of urea. It is possible that the high-temperature and high-pressure urea phase II discovered by Bridgman14 and the presently determined phase IV are the same phase, since the structures of phases I and IV are so closely related. Other questions would concern the structure of high-pressure phase V and still higher-pressure phases, which have not been accessed in our study. It is likely that new H-bonding motifs exist in phase V, above 7.20 GPa. It is noteworthy that the urea crystals in several respects are analogous to H2O ices: the ambient pressure structure is governed by hydrogen bonds forming a 3D network, the H-bonded aggregates enclose voids in the crystal, and both urea and H2O are prone to forming inclusion compounds (hydrates). The differences in their H-bonding properties are that the H-bonds in urea are heteroconjugated, and homoconjugated in H2O ices, and that the number of H-donors exceeds H-acceptor sites in urea, whereas they are balanced in H2O. Both urea and H2O have rich phase diagrams, and it is now evident that homoconjugated bistable OH · · · O bonds are not essential for this richness. Despite the formation of voids, the ∂(mp)/∂p is positive for urea, albeit very small in
Olejniczak et al. magnitude as remarked by Bridgman in 1916,14a whereas the negative ∂(mp)/∂p of ice Ih is commonly associated with the formation of voids in its structure. It is characteristic that the N-H · · · O′ hydrogen bonds are ideally coplanar with the molecular plane in phases I and IV, owing to the crystal symmetry, and nearly coplanar in phase III for both symmetry-independent NH2 groups, including the broken NHsyn · · · O bond. It shows that the inconsistency of the planar NH2 configuration in the crystalline state and the twisted NH2 groups in the gas phase, as evidenced by microwave spectroscopy6 and postulated by theoretical computations for isolated molecules,11 are not due to the symmetry restrictions in the crystal. Despite the relatively low pressure range of the I/III phase transition, the evidenced changes in NH · · · O dimensions for urea are the highest of all structures of biologically important compounds investigated thus far.31-33 Acknowledgment. This research is partially supported by the Ministry of Science and Information Technology, Grant No. NN204195633, and graduate students program (K.O.). Supporting Information Available: Detailed crystallographic information on phase I at 0.47 GPa, phase III at 0.80 GPa, phase III at 1.48 GPa, and phase IV at 3.10 GPa (Table S1); atomic coordinates (Tables S3-S6, respectively); bond lengths and angles (Table S7-S10); anisotropic displacement parameters (Tables S11-S14); hydrogen coordinates and isotropic displacement parameters (Tables S15-S18); strain tensors for the urea lattice transformations between phases I and III, phases I and IV, and phases III and IV. Crystallographic data in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Kitaigorodskii, A. I. Molecular Crystals and Molecules; Academic Press: New York, 1973. (2) (a) Becker, K.; Jancke, W. Z. Phys. Chem. 1921, 99, 242–266. (b) Becker, K.; Jancke, W. Z. Phys. Chem. 1921, 99, 267–274. (c) Mark, H.; Weissenberg, K. Z. Phys. 1923, 16, 1–22. (d) Hendricks, S. B. J. Am. Chem. Soc. 1928, 50, 2455–2464. (e) Wyckoff, R. W. G. Z. Kristallogr. 1930, 75, 529–537. (f) Wyckoff, R. W. G. Z. Kristallogr. 1932, 81, 102–109. (g) Wyckoff, R. W. G.; Corey, R. B. Z. Kristallogr. 1934, 89, 462–468. (3) (a) Vaughan, P.; Donohue, J. Acta Crystallogr. 1952, 5, 530–535. (b) Sklar, N.; Senko, M. E.; Post, B. Acta Crystallogr. 1961, 14, 716–720. (c) Caron, A.; Donohue, J. Acta Crystallogr. 1964, 17, 544–546. (d) Mullen, D.; Hellner, E. Acta Crystallogr., Sect. B 1978, 34, 1624–1627. (e) Scheringer, C.; Mullen, D.; Hellner, E.; Hase, H. L.; Schulte, K.-W.; Schweig, A. Acta Crystallogr., Sect. B 1978, 34, 2241–2243. (f) Swaminathan, S.; Craven, B. M.; Spackman, M. A.; Stewart, R. F. Acta Crystallogr., Sect. B 1984, 40, 398–404. (g) Zavodnik, V.; Stash, A.; Tsirelson, V.; de Vries, R.; Feil, D. Acta Crystallogr., Sect. B 1999, 55, 45–54. (h) Birkedal, M.; Madsen, D.; Mathiesen, R.; Knudsen, K.; Weber, H.-P.; Pattison, P.; Schwarcenbach, D. Acta Crystallogr., Sect. A 2004, 60, 371–381. (4) Lamelas, F. J.; Dreger, Z. A.; Gupta, Y. M. J. Phys. Chem. B 2005, 109, 8206–8215. (5) (a) Worsham, J. E.; Levy, H. A.; Peterson, S. W. Acta Crystallogr. 1957, 10, 319–323. (b) Pryor, A. W.; Sanger, P. L. Acta Crystallogr., Sect. A 1970, 26, 543–558. (c) Guth, H.; Heger, G.; Klein, S.; Treutmann, W.; Scheringer, C. Z. Kristallogr. 1980, 153, 237–254. (6) (a) Godfrey, P. D.; Brown, R. D.; Hunter, A. N. J. Mol. Struct. 1997, 413-414, 405–414. (7) (a) Caron, A.; Donohue, J. Acta Crystallogr., Sect. B 1969, 25, 404. (b) Spackman, M. A.; Weber, H.-P.; Craven, B. M. J. Am. Chem. Soc. 1988, 110, 775–782. (c) Spackman, M. A.; Byrom, P. G. Acta Crystallogr., Sect. B 1997, 53, 553–564. (d) Spackman, M. A.; Byrom, P. G.; Alfredsson, M.; Hermansson, K. Acta Crystallogr., Sect. A 1999, 55, 30–47. (e) de Vries, R. Y.; Feil, D.; Tsirelson, V. Acta Crystallogr., Sect. B 2000, 56, 118–123. (f) Me´reau, R.; Desmedt, A. J. Phys. Chem. B 2007, 111, 3960–3967. (8) Swaminathan, S.; Craven, B. M.; McMullan, R. K. Acta Crystallogr., Sect. B 1984, 40, 300–306.
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