Theoretical and experimental studies of the charge density in urea

Abstract

X-ray structure amplitudes [F~o[ for urea at 123 K have been derived experimentally for 318 reflections, IF[ > 3tr(Fo), sin 0/h < 1.15 ,~-~. Corresponding amplitudes [F'ol have also been calculated for a simulated crystal structure with the charge density derived from a 6-31G** wavefunction for the isolated urea molecule. The nuclear positional and anisotropic thermal parameters at 123 K were those obtained by neutron diffraction. The [F~[ were placed on an 'absolute' scale with the [F'ol. Simple pseudoatom models for the charge density have been fitted by least squares using the set of ]FX[ and IF'o] as observations in separate refinements. Maps of the resulting static deformation density distribution are similar to each other and to the map derived directly from the wavefunction. Thus the pseudoatom models are efficient, although there are limitations which are attributed primarily to the restricted nature of the radial functions. The most significant differences between the experimental and theoretical model maps are close to the H atoms and may be related to H-bonding effects in the crystal. [Crystal data: a5-578(1), c = 4.686(1)~, tetragonal, P42~m with two molecules per unit cell lying in special positions having point symmetry 2mm.]