Acta Crystallographica Section A

Foundations of Crystallography

Volume 60, Part 5 (September 2004)

research papers

Acta Cryst. (2004). A60, 371-381 [doi:10.1107/S0108767304015120]

The charge density of urea from synchrotron diffraction data

H. Birkedal, D. Madsen, R. H. Mathiesen, K. Knudsen, H.-P. Weber, P. Pattison and D. Schwarzenbach

Abstract: 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 sin [bold theta] [bold lambda] 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.

Keywords: charge density; urea; non-centrosymmetric structures; synchrotron radiation.


	PDF file (316.8 kbytes) Anisotropic peak broadening, topological properties of electron density, dipole moment of urea

	PDF file (130.5 kbytes) Le Bail fits for urea

	PDF file (153.0 kbytes) Anisotropic peak broadening in urea

	PDF file (52.7 kbytes) Critical points and bond paths in urea

	PDF file (72.4 kbytes) Laplacian of charge density of urea

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