Thermodynamic and structural relationships between the two polymorphs of 1,3-dimethylurea

The title compound exists as polymorph (I), Fdd2 with Z = 8 [Pérez-Folch et al. (1997). J. Chem. Cryst. 27, 367–369; Marsh (2004). Acta Cryst. B60, 252–253], and as polymorph (II), P2₁2₁2 with Z = 2 [Martins et al. (2009). J. Phys. Chem. A, 113, 5998–6003]. We have redetermined both structures at somewhat lower temperatures [(I) at 180 K rather than room temperature; (II) at 100 K rather than 150 K]. For polymorph (I) the space group Fdd2 is confirmed rather than the original choice of Cc. The molecular structures of both polymorphs are essentially identical, with exact crystallographic twofold symmetry, approximate C_2v symmetry, and a trans orientation of the H—N—C=O moiety. In both polymorphs the molecules associate into chains of rings with graph set C(4)[R₂¹(6)] via bifurcated hydrogen-bond systems C(N—H)₂...O=C. In the polar structure (I) the chains are necessarily all parallel, whereas in (II) equal numbers of parallel and antiparallel chains are present. Further physical investigations [differential scanning calorimetry (DSC), powder investigations, solvent-induced phase conversions] were undertaken: these showed: (i) that the commercially available compound consists predominantly of polymorph (II), which on heating transforms into polymorph (I) by an endothermic reaction, so that both polymorphs are related by enantiotropism; (ii) that polymorph (I) represents the more stable modification at room temperature, where polymorph (II) is metastable, with the thermodynamic transition temperature lying somewhere between 253 K and room temperature. An apparent third polymorph, consisting of fibrous needles, was shown by powder diffraction to consist of a mixture of polymorphs (I) and (II).

Computing details

For both compounds, data collection: CrysAlis PRO, Oxford Diffraction Ltd., Version 1.171.35.15 (release 03-08-2011 CrysAlis171 .NET) (compiled Aug 3 2011,13:03:54); cell refinement: CrysAlis PRO, Oxford Diffraction Ltd., Version 1.171.35.15 (release 03-08-2011 CrysAlis171 .NET) (compiled Aug 3 2011,13:03:54); data reduction: CrysAlis PRO, Oxford Diffraction Ltd., Version 1.171.35.15 (release 03-08-2011 CrysAlis171 .NET) (compiled Aug 3 2011,13:03:54); program(s) used to solve structure: SHELXS97 (Sheldrick, 1990); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: Bruker XP; software used to prepare material for publication: SHELXL97.

Figures

(berlin) 1,3-dimethylurea

Crystal data

C3H8N2O	Dx = 1.149 Mg m^−3
Mr = 88.11	Cu Kα radiation, λ = 1.54184 Å
Orthorhombic, Fdd2	Cell parameters from 7072 reflections
a = 11.3837 (2) Å	θ = 4.5–75.7°
b = 19.6293 (4) Å	µ = 0.73 mm^−1
c = 4.56079 (13) Å	T = 180 K
V = 1019.12 (4) Å^3	Block, colourless
Z = 8	0.15 × 0.15 × 0.12 mm
F(000) = 384

Data collection

Oxford Diffraction Xcalibur, Atlas, Nova diffractometer	300 independent reflections
Radiation source: Nova (Cu) X-ray Source	299 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.017
Detector resolution: 10.3543 pixels mm^-1	θmax = 75.9°, θmin = 9.0°
ω scans	h = −14→14
Absorption correction: multi-scan CrysAlis PRO, Oxford Diffraction Ltd., Version 1.171.35.15 (release 03-08-2011 CrysAlis171 .NET) (compiled Aug 3 2011,13:03:54)	k = −24→24
Tmin = 0.668, Tmax = 1.000	l = −5→5
8220 measured reflections

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F^2 > 2σ(F^2)] = 0.029	H atoms treated by a mixture of independent and constrained refinement
wR(F^2) = 0.080	w = 1/[σ^2(Fo^2) + (0.0529P)^2 + 0.1325P] where P = (Fo^2 + 2Fc^2)/3
S = 1.13	(Δ/σ)max < 0.001
300 reflections	Δρmax = 0.08 e Å^−3
34 parameters	Δρmin = −0.12 e Å^−3
1 restraint	Absolute structure: Flack H D (1983), Acta Cryst. A39, 876-881
Primary atom site location: structure-invariant direct methods	Absolute structure parameter: −0.6 (6)

Special details

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger. The NH hydrogen was refined freely. The methyl group was refined as an idealized rigid group allowed to rotate but not tip. In the absence of significant anomalous scattering, Friedel pairs were averaged and the Flack parameter is thus meaningless.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
C1	0.5000	0.5000	0.5388 (5)	0.0523 (5)
N1	0.58420 (9)	0.46870 (6)	0.6952 (3)	0.0560 (4)
H01	0.5771 (12)	0.4699 (7)	0.881 (7)	0.058 (5)*
C2	0.67226 (12)	0.42632 (9)	0.5609 (6)	0.0757 (5)
H2A	0.6340	0.3892	0.4536	0.114*
H2B	0.7234	0.4073	0.7130	0.114*
H2C	0.7193	0.4537	0.4247	0.114*
O1	0.5000	0.5000	0.2681 (4)	0.0891 (7)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
C1	0.0544 (10)	0.0628 (9)	0.0397 (10)	0.0003 (7)	0.000	0.000
N1	0.0548 (6)	0.0683 (6)	0.0448 (7)	0.0073 (4)	0.0024 (5)	0.0014 (6)
C2	0.0640 (8)	0.0845 (9)	0.0787 (11)	0.0158 (7)	0.0134 (8)	0.0003 (9)
O1	0.0930 (12)	0.1358 (17)	0.0385 (10)	0.0295 (12)	0.000	0.000

Geometric parameters (Å, º)

C1—O1	1.235 (3)	N1—H01	0.85 (3)
C1—N1^i	1.3434 (16)	C2—H2A	0.9800
C1—N1	1.3434 (16)	C2—H2B	0.9800
N1—C2	1.4394 (19)	C2—H2C	0.9800
O1—C1—N1^i	122.07 (10)	N1—C2—H2A	109.5
O1—C1—N1	122.07 (10)	N1—C2—H2B	109.5
N1^i—C1—N1	115.87 (19)	H2A—C2—H2B	109.5
C1—N1—C2	122.36 (16)	N1—C2—H2C	109.5
C1—N1—H01	116.6 (10)	H2A—C2—H2C	109.5
C2—N1—H01	120.3 (10)	H2B—C2—H2C	109.5
O1—C1—N1—C2	−7.54 (14)	N1^i—C1—N1—C2	172.46 (14)

Symmetry code: (i) −x+1, −y+1, z.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N1—H01···O1^ii	0.85 (3)	2.06 (3)	2.850 (2)	154.4 (14)

Symmetry code: (ii) x, y, z+1.

(bonn) 1,3-dimethylurea

Crystal data

C3H8N2O	Dx = 1.200 Mg m^−3
Mr = 88.11	Mo Kα radiation, λ = 0.71073 Å
Orthorhombic, P21212	Cell parameters from 6268 reflections
a = 10.8522 (6) Å	θ = 3.8–30.7°
b = 4.9102 (3) Å	µ = 0.09 mm^−1
c = 4.5766 (3) Å	T = 100 K
V = 243.87 (3) Å^3	Tablet, colourless
Z = 2	0.60 × 0.50 × 0.20 mm
F(000) = 96

Data collection

Oxford Diffraction Xcalibur, Eos diffractometer	371 independent reflections
Radiation source: Enhance (Mo) X-ray Source	367 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.031
Detector resolution: 16.1419 pixels mm^-1	θmax = 27.8°, θmin = 3.8°
ω scans	h = −14→14
Absorption correction: multi-scan CrysAlis PRO, Oxford Diffraction Ltd., Version 1.171.35.15 (release 03-08-2011 CrysAlis171 .NET) (compiled Aug 3 2011,13:03:54)	k = −6→6
Tmin = 0.827, Tmax = 1.000	l = −6→6
11695 measured reflections

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F^2 > 2σ(F^2)] = 0.029	H atoms treated by a mixture of independent and constrained refinement
wR(F^2) = 0.077	w = 1/[σ^2(Fo^2) + (0.0386P)^2 + 0.0474P] where P = (Fo^2 + 2Fc^2)/3
S = 1.14	(Δ/σ)max < 0.001
371 reflections	Δρmax = 0.13 e Å^−3
34 parameters	Δρmin = −0.14 e Å^−3
0 restraints	Absolute structure: Flack H D (1983), Acta Cryst. A39, 876-881
Primary atom site location: structure-invariant direct methods	Absolute structure parameter: 10 (3)

Special details

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger. The NH hydrogen was refined freely. The methyl group was refined as an idealized rigid group allowed to rotate but not tip. In the absence of significant anomalous scattering, Friedel pairs were averaged and the Flack parameter is thus meaningless. The compound is achiral and crystallizes only by chance in a Sohncke space group.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
N1	0.42512 (9)	0.6643 (3)	0.8735 (3)	0.0245 (3)
H01	0.4320 (14)	0.659 (4)	1.067 (4)	0.032 (4)*
O1	0.5000	0.5000	0.4456 (3)	0.0298 (4)
C1	0.5000	0.5000	0.7168 (4)	0.0206 (4)
C2	0.34378 (11)	0.8573 (3)	0.7324 (3)	0.0278 (3)
H2A	0.3904	0.9634	0.5885	0.042*
H2B	0.3088	0.9802	0.8793	0.042*
H2C	0.2771	0.7590	0.6337	0.042*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
N1	0.0255 (5)	0.0295 (5)	0.0186 (5)	0.0054 (5)	0.0004 (4)	0.0004 (5)
O1	0.0360 (7)	0.0369 (8)	0.0165 (6)	0.0063 (7)	0.000	0.000
C1	0.0200 (7)	0.0236 (8)	0.0183 (7)	−0.0021 (7)	0.000	0.000
C2	0.0237 (5)	0.0287 (6)	0.0308 (7)	0.0044 (5)	−0.0036 (6)	0.0002 (7)

Geometric parameters (Å, º)

N1—C1	1.3512 (14)	N1—H01	0.890 (19)
N1—C2	1.4474 (17)	C2—H2A	0.9800
O1—C1	1.241 (2)	C2—H2B	0.9800
C1—N1^i	1.3512 (14)	C2—H2C	0.9800
C1—N1—C2	121.39 (12)	N1—C2—H2A	109.5
O1—C1—N1^i	122.06 (8)	N1—C2—H2B	109.5
O1—C1—N1	122.06 (8)	H2A—C2—H2B	109.5
N1^i—C1—N1	115.87 (15)	N1—C2—H2C	109.5
C1—N1—H01	117.5 (11)	H2A—C2—H2C	109.5
C2—N1—H01	121.0 (12)	H2B—C2—H2C	109.5
C2—N1—C1—O1	−2.36 (13)	C2—N1—C1—N1^i	177.64 (13)

Symmetry code: (i) −x+1, −y+1, z.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N1—H01···O1^ii	0.890 (19)	2.038 (19)	2.8576 (17)	152.6 (16)

Symmetry code: (ii) x, y, z+1.