Pressure-Promoted CH···O Hydrogen Bonds in Formamide

ARTICLE pubs.acs.org/crystal

Pressure-Promoted CH 3 3 3 O Hydrogen Bonds in Formamide Aggregates Roman Gajda and Andrzej Katrusiak* Faculty of Chemistry, Adam Mickiewicz University, Grunwaldzka 6, 60-780 Pozna
n, Poland

bS Supporting Information ABSTRACT: The N
H 3 3 3 O bonded sheets in compressed formamide are reconstructed due to the formation of pressure-promoted C
H 3 3 3 O hydrogen bonds. Isochoric freezing of formamide in high pressure leads to a new β phase. Its structure has been determined by single-crystal X-ray diffraction at 0.88 and 1.20 GPa. The motif of N
H 3 3 3 O bonded sheets in phase α is modified to facilitate the C
H 3 3 3 O contacts and denser packing in phase β, whereas the distance between sheets increases compared to ambient pressure. Despite the strong rearrangement, phases α and β retain a remarkable similarity in the unit-cell dimensions, spacegroup symmetry P21/n, and structural fragments. Two symmetry-independent molecules in phase β differ in their H-bonding patterns: each molecule is involved in four N
H 3 3 3 O bonds; however, one of the molecules is H-bonded to four molecules within the sheet and the other to three molecules. The two molecules in phase β form similar C
H 3 3 3 O bonds, competing with the N
H 3 3 3 O bonds for access to the H-accepting CdO groups. The isothermal compressibility measured in a piston-cylinder apparatus confirms that formamide solidifies directly in phase β at 296 K.

’ INTRODUCTION The molecular structure and association of formamide (CHONH2), the simplest amide, have been of particular interest because amides are widespread in nature and have many technological applications. Formamide is the simplest compound containing the peptide linkage and the simplest liquid (m.p. 275 K) capable of forming hydrogen bonds N
H 3 3 3 O. The first X-ray diffraction studies1 on the low-temperature crystalline formamide were followed by the studies of electron-density distribution,2,3 on the liquid formamide in various conditions of pressure and temperature,4
8 high-pressure Raman spectroscopy,9 studies of structure of formamide entrapped by micelles,10 as well as theoretical calculations and simulations. 11
16 It was also reported that amorphous formamide could be prepared by vapor deposition on substrates at 77 K.17 Microsolvated formamide clusters were generated and characterized to explain the water binding preferences observed in proteins.18 The Raman spectra of formamide measured at 300 K up to 10 GPa showed that it freezes at about 0.5 GPa and that there is a solid
solid phase transition at 5.0 GPa;9 it was concluded that the crystalline phase below 5.0 GPa is the same as the lowtemperature phase at ambient pressure. Unlike urea, OdC(NH2)2, with four H-donors and one H-acceptor group per molecule, in formamide the number of H-donors and H-acceptors is balanced, when the CdO group is associated with two acceptor sites. The excess of the H-donors in urea, is considered the main reason for the exceptionally high 4-fold H-acceptor capacity of the carbonyl oxygen in its structure and of the 3-dimensional H-bonding pattern.19 r 2011 American Chemical Society

Hydrogen bonds also play a key role in the functions and properties of biopolymers and technological polymers, like polyamides.20 It was shown recently that the high strength and high bulk modulus of polyamides, including nylon, strongly depend on the orientation of polymeric molecules controlled by the hydrogen bonds. The formation of specific H-bridges, in turn, depends on the conditions of crystallization and can be water mediated.21
25 Noncovalent interactions in general, and specifically the H-bonds of amide groups, are likely to be affected by thermodynamic conditions. For example, some of the N
H 3 3 3 O hydrogen bonds in urea are broken at a moderate pressure of 0.48 GPa and then reinstalled above 2.4 GPa.19 The changes in N
H 3 3 3 O bonded pattern and molecular association in formamide crystallized in high-pressure conditions is described below. It was intended to destabilize the intermolecular interactions involving amide groups by compressing the molecules to a more close-packed arrangement and to trigger a polymorphic transformation of this simple model compound.

’ EXPERIMENTAL SECTION The freezing pressure of formamide determined by the compressibility measurements and by microscopic observation in the diamondanvil cell (DAC) is 0.43
0.44 GPa at 296 K. The compression of liquid and solid formamide was measured in a piston-cylinder apparatus, similar to that described by Baranowski and Moroz,26 at 296 K. The initial volume of the sealed cylinder was 9.8 cm3. Received: January 26, 2011 Revised: August 19, 2011 Published: September 06, 2011 4768

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reduction.30 The data were corrected for the DAC absorption, gasket shadowing, and absorption of the sample itself.31,32 The completeness of recorded data was low, due to the low crystal symmetry and disadvantageous orientation of the crystal in the DAC, with the monoclinic [y] direction along the DAC axis (Figure 1). In this orientation, the reciprocal-space is accessible only to small h-indices.27 Moreover, the accessed reciprocal-space portion was divided by a 2-fold axis and a mirror plane of Laue class 2/m, so only one-fourth of the accessed reflections were nonequivalent. We attempted to grow new formamide crystals in a different orientation; however, the preferred crystal-growth orientation persisted. Similar preferred orientations for the crystals in situ grown at high-pressure conditions were previously reported for benzene, bromine, tetrabromocarbonate, and carbon disulfide,33 although later high-pressure benzene phases were obtained by chance in new orientations.34,35 Nonetheless, the formamide structure was solved by direct methods using program SHELXS97 and well refined with SHELXL97.36 The O atoms were refined with anisotropic displacement parameters. The H-atoms were calculated from the molecular geometry, with the formyl H-atom located at C
H 0.93 Å and amide H-atoms at N
H 0.86 Å distances, the H-atoms’ Uiso = 1.2Ueq of their carriers. The same procedure was followed for the formamide crystal at 1.20 GPa. Both high-pressure experiments indicated a new phase of formamide. Although its space-group symmetry symbol is the same as that of the low temperature phase (Table 1), but the unit-cell volumes and structures are different. Therefore, the low-temperature structure has been labeled as phase α and the high-pressure structure as phase β. The final crystallographic data and refinement details are listed in Tables 1 and S1 (Supporting Information), and the structural information are deposited in the form of crystallographic information files (CIF) in the Cambridge Crystallographic Database Centre with supplementary publication numbers CCDC 806160 and 806161 for 0.88 and 1.20 GPa, respectively (copies can be obtained free of charge on request from [email protected] or www.ccdc.cam.ac. uk/data_request/cif). The molecular graphics were prepared by X-Seed.37 Program Gaussian38 was used for calculating the electrostatic potential at the B3LYP/6-311++g(d, p) level of theory and program Moliso for mapping it onto the isopotential molecular surface.39

Single crystals of formamide have been in situ pressure frozen in a diamond-anvil cell (DAC). A modified Merrill-Bassett DAC was used,27 with the gasket made of 0.3 mm steel foil, with a spark-eroded hole 0.5 mm in diameter. The pressure was calibrated with a BETSA PRL spectrometer by the ruby-fluorescence method with a precision of 0.05 GPa.28 Pressure in the DAC chamber was gradually increased until the sample froze at isothermal conditions in the polycrystalline form. By increasing temperature, all crystal grains but one were melted (Figure 1). Then temperature was slowly lowered until this single crystal grew at isochoric conditions to entirely fill the pressure chamber at room temperature. The X-ray diffraction data have been collected at 0.88 and 1.20 GPa. The DAC with the formamide sample was centered on a KUMA KM4-CCD diffractometer by the gasket-shadow method.29 The MoKα radiation from the sealed X-ray tube was graphite monochromated. The reflections were recorded in the ω-scan mode.29 The CrysAlis program was used for data collections, unit-cell refinements, and initial data

’ DISCUSSION The structure of low-temperature α-formamide, determined at 223 K by Ladell and Post1 and at 90 K by Stevens,3 is monoclinic, space group P21/n (Table 1). It is built of molecules H-bonded into sheets. There are van der Waals contacts between the sheets only. The average distance between the sheets is

Figure 1. Isochoric growth of the β-formamide single-crystal in the DAC chamber leading to the sample at 0.88 GPa (cf. Figure S1 in Supporting Information): (a) four crystal grains at 320 K; (b) a single seed at 340 K; (c) at 320 K; (d) the single crystal close to room temperature. A ruby chip for pressure calibration lies at the center of the chamber. The Miller indices of crystal faces have been indicated in panel (c).

Table 1. Selected Crystal Data of Low-Temperature α-Formamide and High-Pressure β-Formamide Phases (cf. Table S1 in Supporting Information) α-phasea 0.1 MPa 90 K

α-phaseb 0.1 MPa 223 K

β-phasec 0.88 GPa 296 K

β-phasec 1.2 GPa 296 K

crystal system space group

monoclinic P21/n

monoclinic P21/n

monoclinic P21/n

monoclinic P21/n

a (Å)

3.604(2)

3.69(1)

3.5914(7)

3.5586(7)

b (Å)

9.041(3)

9.180(25)

18.852(7)

18.49(4)

c (Å)

6.994(2)

6.87(2)

6.2966(9)

6.248(1)

β (°)

100.50(5)

98.00(25)

93.659(14)

93.80(2)

Z, Z0

4, 1

4, 1

8, 2

8, 2

V (Å3) Dx (g cm
3)

224.68 1.331

230.39 1.298

425.44(19) 1.406

410.3(9) 1.430

R[F2 > 2σ(F2)]

0.042

0.053

S

1.15

1.11

formamid at pressure and temperature

a

Ref 2, Stevens et al. (1978). b Ref 1, Ladell and Post (1954). c This work. 4769

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Crystal Growth & Design approximately 3.0 Å, but the closest contact between a carbon atom of one layer and a nitrogen atom of another is 3.39 Å. Within the sheets, pairs of formamide molecules are double H-bonded about inversion centers into nearly planar dimers, characteristic of amides, enclosing eight-membered rings. Their graph symbol is R22(8), according to the graph notation for H-bonds aggregates.40
42 Each of the so-formed dimers is surrounded by four R46(16) H-bonded rings (Figure 2). The sheets are corrugated due to the tilts of the R22(8) rings and puckering of the R46(16) rings. There are two symmetry-independent N
H 3 3 3 O hydrogen bonds in α-formamide (Table 2): (i) bond N
H(2) 3 3 3 O involving the syn H(2) atom (with respect to the oxygen position) is slightly longer — two such H-bonds and two molecules related by a center of inversion form a dimer and the R22(8) ring; and (ii) bond N
H(1) 3 3 3 O involving the anti H(1) atom is slightly shorter — four such H-bonds and two N
H(2) 3 3 3 O bonds enclose the large R46(16) ring, involving six molecules. The H-bonded sheets are parallel to the (101) crystal plane (Figure 2). The space-group symmetry of formamide phases α and β is the same, but the b parameter of the high-pressure phase β is doubled, the c parameter is over 0.5 Å shorter, and the monoclinic angle β is a few degrees smaller (Table 1). Thus, there is a clear relationship between the lattices of phases α and β, apparently due to the analogies in the motifs of the H-bonded sheets. The molecular geometry in phases α and β and in the gaseous phase are consistent within experimental errors (Table S3, Supporting Information) and consistent with the dimensions determined by electron diffraction43 and microwave spectroscopy.44 However, several elements of intermolecular H-bonding motifs in phases α and β have changed. Molecules of phases α and β alike form N
H 3 3 3 O bonds binding the molecules into layers. Of two types of N
H 3 3 3 O hydrogen bonds in α-formamide, these involving the syn-H atoms join the molecules into centrosymmetric dimers, and the anti-H atoms link the dimers further into a chicken-wire motif of the sheets. There are two symmetry-independent molecules, denoted (a) and (b), and four independent N
H 3 3 3 O bonds in β-formamide (Figure 2 and Table 3): (i) the syn-H atom of molecule (b) is responsible for the formation of H-bonded dimers and R22(8) rings, analogous to those in phase α, and these H-bonds also participate in the formation of R34(12) rings; (ii) the anti-H atom of molecule (b) participates in enclosing H-bonded rings R34(12) and R46(16); (iii) the syn-H atom of molecule (a) links the dimers into chains along crystal direction [101] and encloses rings R34(12) and R46(16); and (iv) the anti-H atom of molecule (a) participates in the formation of rings R34(12) and R46(16). Such a significant reconstruction of the sheets results in a modest change of the unit-cell dimensions, except for the doubling of parameter b (Table 1 and Figure 2). A unit-cell doubling, such as in polymorph β, is usually connected rather with small displacements of atoms45 than with substantial reconstructions of the structure (Figure 2). The transformation of formamide phases can be conveniently described as a modification of the patterns of H-bonded rings R22(8) and R46(16) in the sheets of phase α. In the sheets of phase β, there is an additional row of R34(12) rings inserted on both sides of each row of rings R22(8) and R46(16), as illustrated in Figure 3. It is plausible that this structural reconstruction originates from the differences in the mutual arrangement of the R22(8) and R46(16) rings, constituting the preserved fragments of phase α and extending along direction [101] in phases α and β, which allows the formation of additional CH 3 3 3 O contacts across the R46(16)

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Figure 2. Autostereographic projections of N
H 3 3 3 O hydrogenbonded formamide molecules in the structures of (a) phase α at 90 K, (b) phase β at 0.88 GPa (larger portions of the sheets are shown in Figure S2). The sheets in both phases are parallel to the (101) plane. Hydrogen bonds N
H 3 3 3 O are indicated with dashed lines, and graph descriptors of the N
H 3 3 3 O bonded rings are shown in green.

rings in high pressure. It can be seen in Figures 2 and 3 that the R46(16) ring in phase β is narrower and that the trans-annular carbonyl groups and H atoms are closer than in phase α. This is consistent with the observation of pressure-promoted CH 3 3 3 O bonds, evidenced by spectroscopic and X-ray diffraction methods, in a series of organic compounds.46
48 Also the C
H 3 3 3 O angles fulfill the criteria for the contacts to be classified as hydrogen bonds.49,50 4770

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Table 2. Intermolecular Hydrogen Bonds Involving anti-H(1) and syn-H(2) Atoms in Low-Temperature α-Formamidea at 90 K, and the Shortest CH 3 3 3 O Contact Across the R46(16) Ring D
H 3 3 3 A 0

a

N1
H(1) 3 3 3 O1 N1
H(2) 3 3 3 O100 C1
H(3) 3 3 3 O1000

H 3 3 3 A (Å)

D 3 3 3 A (Å)

D
H 3 3 3 A (deg)

symmetry code

1.90(6)

2.885(3)

167(2)

1.5
x, 0.5+y, 1.5
z

1.94(5) 3.99(6)

2.948(3) 4.904(4)

176(2) 152(2)

1
x, 2
y, 2
z
x,
y, 1
z

The results according to Stevens et al., 19782; the results at 223 K by Ladell and Post, 19541 are compiled in Table S2, Supporting Informations.

Table 3. Dimensions of Hydrogen Bonds in β-Formamide at 0.88 and 1.20 GPa (cf. Table 2)a D
H 3 3 3 A

H 3 3 3 A (Å)

D 3 3 3 A (Å)

D
H 3 3 3 A (deg)

symmetry codes

N1a
H1aanti 3 3 3 O1b N1a
H2asyn 3 3 3 O1ai

2.02

2.859(13)

165.3


x
0.5, y
0.5, 0.5
z

2.12

2.965(12)

167.1

0.5 + x,
0.5
y, z
0.5

2.89

3.660(11)

141.6

x
0.5,
y
0.5, z + 0.5

2.04

2.903(12)

176.8

N1b
H2bsyn 3 3 3 O1biv C1b
H3b 3 3 3 O1av

2.12

2.976(14)

170.7


x,
y,
z

2.85

3.737(15)

160.5


x
1.0,
y, 1.0
z

N1a
H1aanti 3 3 3 O1b N1a
H2asyn 3 3 3 O1ai

2.04 2.08

2.876(17) 2.930(14)

165.3 170.2


x
0.5, y
0.5, 0.5
z 0.5 + x,
0.5
y, z
0.5

2.82

3.620(14)

145.1

x
0.5,
y
0.5, z + 0.5

1.90

2.751(18)

173.4

2.12

2.968(17)

170.0


x,
y,
z

2.81

3.698(17)

159.2


x
1.0,
y, 1.0
z

0.88 GPa

C1a
H3a 3 3 3 O1aii N1b
H1banti 3 3 3 O1aiii

1.20 GPa

C1a
H3a 3 3 3 O1aii N1b
H1banti 3 3 3 O1aiii N1b
H2bsyn 3 3 3 O1biv C1b
H3b 3 3 3 O1av

Letters “a” and “b” denote atoms of molecules (a) and (b); amide H1 is anti with respect to the oxygen and H2 is syn, which has been indicated by superscripts for clarity, and the formyl hydrogen is labeled H3. The Roman numbers indicate atoms transformed according to listed symmetry codes. Contact C1a
H3a 3 3 3 O1aii is formed across the R46(16) ring, and contact C1b
H3b 3 3 3 O1av is across ring R34(12). All dimensions are given for H-atomic positions idealized to typical X-ray diffraction positions (dC
H = 0.93 and dN
H = 0.86 Å). a

Figure 3. Schematic views of the patterns of R22(8), R34(12), and R46(16) rings (shown as green hexagons, blue dodecagons, and red hexadecagons, respectively) within NH 3 3 3 O bonded sheets in formamide phases (a) α; and (b) β (cf. Figure 2). Both drawings are based on projections of the non-H atoms in the H-bonded sheets.

The H-bonded sheets are strongly corrugated (Figure 4): in phase α the sheets are uneven, with bumps and holes associated with the parts of the R46(16) rings sticking out to the opposite sides of the sheets, and the R22(8) dimers being the planar tilted parts of the sheets. In phase β the distortion from planarity is like a wave modulation with a period of b. It appears that this corrugation allows the molecules to be more densely packed along the sheets, rather than to reduce the distance between the

sheets. The wavy sheets in phase β are more tightly packed with molecules than the uneven sheets in phase α. The interlayer distances, corresponding to the distances between planes (101), are 2.944(2) Å for phase α at 90 K, and for phase β 3.031(1) Å at 0.88 GPa, and 3.001(1) Å at 1.20 GPa. The area of the (101) plane, parallel to the sheets, per one molecule is 18.906 Å2 in phase α and 17.546 Å2 in the high-pressure phase β. Thus, the significant volume contraction, from V/Z equal to 57.60 Å3 in phase α to 53.18 Å3 in phase β, is mainly due to the decreased area (per one molecule) of the sheets, which can be attributed, at least partly, to the formation of medium R34(12) rings and narrowing of the large R46(16), both these rings spanned across by the CH 3 3 3 O bridges. The catemers of formamide molecules running along the crystal direction [101] on the edges of the R34(12) rings in phase β are the most bent and protruding fragments of the sheets, while the R22(8) and R46(16) rings constitute their planar parts. It shows that the incorporation of R34(12) rings is essential for the wavy corrugation of sheets and for the unusual mode of compression of formamide, where most compressed is the area along the strongest interactions, that is, H-bonded sheets, while the direction along seemingly the softest forces between the sheets expands on transition to the highpressure phase β. The strong corrugation of H-bonded sheets in phase β is reflected in the values of torsion angles along the edges of the H-bonded rings, that is, along covalent bonds and hydrogen bonds C
N 3 3 3 OdC (Tables S4 and S5 in Supporting Information). The most nonplanar H-bonds, with torsion angles C
N 3 3 3 OdC over 70°, are within the R34(12) ring, 4771

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Figure 5. Formamide molecule and its electron-density envelope at 0.0004 au decorated with the electrostatic-potential calculated for optimized geometry with program Gaussian38 at the B3LYP/ 6-311++g(d, p) theory level. The color scale ranges from
155 kJ/ mol (red) to 160 kJ/mol (purple).

Figure 4. Projections along the [101] directions within H-bonded sheets in (a) α-formamide at 90 K; (b) β-formamide at 0.88 GPa.

characteristic of phase β. The presence of ring R34(12) within the sheet apparently releases the strains in other rings, which become more flat in phase β than in phase α (Tables S3 and S4). In this study, the β phase was directly crystallized from liquid and did not transform from the α phase in a solid
solid phase transition. The molecular dynamics simulations for liquid formamide revealed a balance of R22(8) ring-dimers and linear chain aggregates of formamide molecules and that pressure enhances the ring-dimer formation at the expense of chains.8 This direction of changes is reversed in the solid-state aggregation, in this respect that the number of R22(8) dimers decreases by half in β-formamide. It is plausible that this difference is caused by the CH 3 3 3 O bonds formed outside the R22(8) ring dimers within the sheets of highpressure phase β, as they lead to the formation of the R34(12) rings, necessary for releasing strains induced in the narrowed R46(16) rings. The similar unit-cell dimensions of phases α and β can be attributed to the layered crystal structures built of H-bonded sheets of phase α modified by the insertion of R34(12) rings in phase β. Moreover, the R22(8) dimers preserve the symmetry of inversion-center, which requires that the two molecules involved be parallel. This arrangement is consistent with the rigid and nearly planar double NH 3 3 3 O bonding that does not allow significant distortions within the dimer. The variation in mutual orientation of N
H 3 3 3 O bonded molecules within the sheets in phases α and β has been illustrated in Figure S3: the most puckered are the arrangements in phase α, and the molecules H-bonded to molecule (a) in phase β. The molecular neighborhood around symmetry-independent molecule (b) in phase β is most flat. The electrostatic potential mapped on the electronic density isosurface for the isolated molecule of formamide is shown in Figure 5. There are two distinct regions of opposing charge in the molecule, which are closely located in the formamide

Figure 6. The isothermal compression of formamide at 296 K, measured in the piston-cylinder press (blue circles) and by X-ray diffraction (red circles). The critical values of pressure (Pc) and volume change (ΔV) associated with isothermal freezing of formamide, directly into phase β, are indicated.

aggregates and illustrate the role of electrostatic matching and the contribution of electrostatic attraction to the cohesion forces in molecular crystals.51
54 It can be observed that in phase α the oxygen is approached by the amide H atoms at CdO 3 3 3 H angles 119.2° (syn-H0 ) and 125.7° (anti-H0 ). These angles are close to the favored directions of formation of H-bonds along the lone pairs of the carbonyl oxygen. In phase β at 0.88 GPa, the amide H-bonds are strained, as the CdO 3 3 3 H angles assume 102.3° and 144.7° in molecule (a), and 120.7° and 144.4° in molecule (b), respectively. It is thus plausible that these strains accommodate the formyl H(3) atom approaching the carbonyl oxygens at the CdO(1a) 3 3 3 H angle 97.6° in molecule (a), and at angle CdO(1b) 3 3 3 H of 80.4° in molecule (b). The shortest of the NH 3 3 3 O bonds in both phases α and β involve hydrogen H1 in position anti. For both molecules in phase β, the formation of C
H 3 3 3 OdC contacts is accompanied by the increase of the CdO 3 3 3 H(1) angles, which shifts the H(1) atoms closer to the most negative electrostatic potential on the molecular surface (Figure 5), whereas angles CdO 3 3 3 H(2) (syn) decrease their opening. 4772

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Crystal Growth & Design The compressibility measurement in the piston-cylinder press (Figure 6) suggests that the isothermal freezing of liquid formamide leads directly to the β-phase. The freezing pressure of 0.445 GPa at 296 K has been observed in the isothermal compression of formamide as a discontinuity associated with the volume change at the liquid
solid boundary. No other discontinuity in the compression has been detected to 1.52 GPa. The direct freezing in the β-formamide structure was also confirmed by the microscopic observations of the habits of crystals nucleating in the isothermal crystallizations in the DAC (Figures 1 and S1 in the Supporting Information).

’ CONCLUSIONS The transformation between formamide phases α and β — exhibiting close analogies in their symmetry, lattice dimensions, and H-bonding patterns — can be attributed to pressure effects on the balance between intermolecular interactions. In phase β, new CH 3 3 3 O contacts, formed across the collapsed NH 3 3 3 O bonded R46(16) rings and across rings R34(12), have dimensions characteristic of hydrogen bonds. No such contacts are present in α-formamide, while in the β phase the CH 3 3 3 O contacts compete with NH 3 3 3 O bonds for the access to CdO groups, the only H-acceptors in this structure. Similarly, formation of new interactions lead to new phases of ethynylbenzene,51 dimethyl sulfoxide (DMSO),52 acetonitrile,53 and halomethanes,54 where new arrangements were induced by pressure, with intermolecular contacts all consistent with the electrostatic-potential matching between molecular surfaces. It is also characteristic of the β-formamide structure that there are many H 3 3 3 H contacts commensurate with the sums of van der Waals radii (2.4 Å), and its space filling is much more tight than in phase α. The formation of high-pressure formamide phase β is essentially different than the formation of urea phase IV. The three-dimensional N
H 3 3 3 O bonded network of phase I remains intact in ureaIV, and this is the volume of the crystal that abruptly changes as the voids within the H-bonded network of urea-I collapse. The H-bonding directions between formamide molecules are very flexible, which is consistent with the distribution of electrostatic potential on molecular surface. These strongly polar molecules are prone to form H-bonding with other polar molecules. It appears that elevated pressure promotes C
H 3 3 3 O bonds, which are common in organic compounds at ambient pressure.55
59 In formamide, the involvement of formyl H-atom shifts the balance between the H-donors and acceptors, and may be significant for the formamide aggregates with other molecules. The association of formamide molecules into H-bonded sheets, cyclomers, and dimers was observed in formamide
isonicotinamide cocrystal,60 where dimers comprising two isonicotinamide and two formamide molecules are further H-bonded into larger rings and sheets. Formamide can also associate with water, which can affect polymerization of formamide, for example, in the production of nylon. The polymorphism and transformations of H-bonding patterns in formamide illustrate the new mechanisms involving C
H 3 3 3 O contacts responsible for the transformations of molecular solids under pressure.61,62 ’ ASSOCIATED CONTENT

bS

Supporting Information. Sequences of photographs illustrating the growth of β-formamide in the DAC (Figure S1), table of selected experimental and crystal data (Table S1); molecular

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dimensions in formamide α and β and in the gas state (Table S2); hydrogen bonds in low-temperature α-formamide (Table S3); tables of intermolecular torsion angles, measured along the hydrogen bonds, in α-formamide (Table S4) and β-formamide (Table S5); X-ray diffraction and crystal data in the crystallographic information file (CIF) format. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This study was supported by the TEAM Grant 2009-4/6 of the Foundation for Polish Science. ’ REFERENCES (1) Ladell, J.; Post, B. Acta Crystallogr. 1954, 7, 559–564. (2) Stevens, E. D.; Rys, J.; Coppens, P. J. Am. Chem. Soc. 1978, 100, 2324–2328. (3) Stevens, E. D. Acta Crystallogr. B 1978, 34, 544–551. (4) Miyake, M.; Kaji, O.; Nakagawa, N.; Suzuki, T. J. Chem. Soc., Faraday Trans. 2 1985, 81, 277–281. (5) Bellissent-Funel, M. C.; Nasr, S.; Bosio, L. J. Chem. Phys. 1997, 106, 7913–7919. (6) Ohtaki, H.; Katayama, N.; Ozutsumi, K.; Radnai, T. J. Mol. Liq. 2000, 88, 109–120. (7) Ohtaki, H. J. Mol. Liq. 2003, 103
104, 3–13. (8) Radnai, T.; Megyes, T.; Bak
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