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The charge density of urea is studied using very high precision single-crystal synchrotron-radiation diffraction data collected at the Swiss–Norwegian Beam Lines at ESRF. An unprecedented resolution of 1.44 Å−1 in sinθ/λ is obtained at 123 K. The optimization of the experiment for charge-density studies is discussed. The high precision of the data allowed the refinement of a multipole model extending to hexadecapoles and quadrupoles on the heavy and H atoms, respectively, as well as a liberal treatment of radial functions. The topological properties of the resulting electron density are analysed and compared with earlier experimental results as well as with periodic Hartree–Fock calculations. The properties of the strongly polarized C—O bond agree with trends derived from previous experimental results while the ab initio calculations differ significantly. The results indicate that the description of the C—O bond requires more flexible basis sets in the theoretical calculations. The calculated integrated atomic charges are much larger than the observed ones. It is suggested that the present experimental results provide new target values for validation of future ab initio calculations. The molecular dipole moment derived from the integrated atomic properties is the same as the one obtained from the multipole model even though the individual atomic contributions differ. Comparison with literature data for urea in solution and the gas phase yields a dipole enhancement in the solid of about 1.5 D. The thermal expansion of urea is determined using synchrotron powder diffraction data. With decreasing temperature, an increasing anisotropic strain is observed.

Supporting information

cif

Crystallographic Information File (CIF) https://doi.org/10.1107/S0108767304015120/xc5013sup1.cif
Contains datablocks urea, I

pdf

Portable Document Format (PDF) file https://doi.org/10.1107/S0108767304015120/xc5013sup2.pdf
Anisotropic peak broadening, topological properties of electron density, dipole moment of urea

pdf

Portable Document Format (PDF) file https://doi.org/10.1107/S0108767304015120/xc5013sup3.pdf
Le Bail fits for urea

pdf

Portable Document Format (PDF) file https://doi.org/10.1107/S0108767304015120/xc5013sup4.pdf
Anisotropic peak broadening in urea

pdf

Portable Document Format (PDF) file https://doi.org/10.1107/S0108767304015120/xc5013sup5.pdf
Critical points and bond paths in urea

pdf

Portable Document Format (PDF) file https://doi.org/10.1107/S0108767304015120/xc5013sup6.pdf
Laplacian of charge density of urea

hkl

Structure factor file (CIF format) https://doi.org/10.1107/S0108767304015120/xc5013Isup7.hkl
Contains datablock I

CCDC reference: 1047783

Computing details top

Data collection: KM6-CH software; cell refinement: KM6-CH software; data reduction: XD_RED; program(s) used to refine structure: VALRAY.

Figures top
[Figure 1]
[Figure 2]
[Figure 3]
[Figure 4]
[Figure 5]
(I) top
Crystal data top
CH4N2OF(000) = 64
Mr = 60.06Dx = 1.368 Mg m3
Tetragonal, P421mSynchrotron radiation, λ = 0.5996 (1) Å
Hall symbol: P -4 2abCell parameters from 17 reflections
a = 5.5780 (6) ÅT = 123 K
c = 4.6860 (7) ÅBlock, colourless
V = 145.80 (3) Å30.23 × 0.13 × 0.09 mm
Z = 2
Data collection top
KM6-CH six-circle κ
diffractometer operated as standard four-circle κ
diffractometer with detector circle in the vertical plane to profit from the linear polarization of the synchrotron beam.
817 reflections with I > 2σ(I)
Radiation source: bending magnetRint = 0.014
Si(111) double crystal monochromatorθmax = 59.8°, θmin = 3.7°
ω–scanh = 1515
3942 measured reflectionsk = 150
1045 independent reflectionsl = 130
Refinement top
Refinement on F2Primary atom site location: structure-invariant direct methods
Least-squares matrix: fullSecondary atom site location: difference Fourier map
wR(F2) = 0.010Hydrogen site location: inferred from neighbouring sites
S = 1.17All H-atom parameters refined
1045 reflections w = 1/[σ2(Fo2]
77 parameters(Δ/σ)max < 0.001
0 restraints
Crystal data top
CH4N2OV = 145.80 (3) Å3
Mr = 60.06Z = 2
Tetragonal, P421mSynchrotron radiation, λ = 0.5996 (1) Å
a = 5.5780 (6) ÅT = 123 K
c = 4.6860 (7) Å0.23 × 0.13 × 0.09 mm
Data collection top
KM6-CH six-circle κ
diffractometer operated as standard four-circle κ
diffractometer with detector circle in the vertical plane to profit from the linear polarization of the synchrotron beam.
817 reflections with I > 2σ(I)
3942 measured reflectionsRint = 0.014
1045 independent reflections
Refinement top
wR(F2) = 0.01077 parameters
S = 1.170 restraints
1045 reflectionsAll H-atom parameters refined
Special details top

Refinement. Charge density refinement. H positional and displacement parameters fixed at values from neutron study (Swaminathan et al. (1984) Acta Cryst. B40, 300–306.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/Ueq
C0.00.50.32821 (3)0.01238 (2)
O0.00.50.59634 (10)0.01527 (2)
N0.14468 (7)0.64468 (7)0.17901 (10)0.02273 (3)
H10.2557 (4)0.7557 (4)0.2841 (4)0.0365 (6)
H20.1431 (4)0.6431 (4)0.0348 (3)0.0333 (5)
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
O0.01958 (4)0.01958 (4)0.00665 (3)0.00164 (8)0.0000.000
C0.01519 (4)0.01519 (4)0.00675 (4)0.00035 (6)0.0000.000
N0.02931 (6)0.02931 (6)0.00956 (4)0.01570 (7)0.00002 (3)0.00002 (3)
H10.0440 (11)0.0440 (11)0.0216 (7)0.0222 (8)0.0031 (9)0.0031 (9)
H20.0430 (10)0.0430 (10)0.0140 (6)0.0158 (8)0.0019 (8)0.0019 (8)
Geometric parameters (Å, º) top
O—C1.2565 (5)N—H11.005 (2)
C—N1.3384 (4)N—H21.0020 (15)
O—C—N121.49 (2)C—N—H2120.78 (13)
C—N—H1119.16 (12)H1—N—H2120.06 (17)

Experimental details

Crystal data
Chemical formulaCH4N2O
Mr60.06
Crystal system, space groupTetragonal, P421m
Temperature (K)123
a, c (Å)5.5780 (6), 4.6860 (7)
V3)145.80 (3)
Z2
Radiation typeSynchrotron, λ = 0.5996 (1) Å
µ (mm1)?
Crystal size (mm)0.23 × 0.13 × 0.09
Data collection
DiffractometerKM6-CH six-circle κ
diffractometer operated as standard four-circle κ
diffractometer with detector circle in the vertical plane to profit from the linear polarization of the synchrotron beam.
Absorption correction
No. of measured, independent and
observed [I > 2σ(I)] reflections
3942, 1045, 817
Rint0.014
(sin θ/λ)max1)1.442
Refinement
R[F2 > 2σ(F2)], wR(F2), S ?, 0.010, 1.17
No. of reflections1045
No. of parameters77
H-atom treatmentAll H-atom parameters refined
Δρmax, Δρmin (e Å3)?, ?

Computer programs: KM6-CH software, XD_RED, VALRAY.

 

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