High-resolution X-ray diffraction data collected at 20 K are interpreted in terms of the rigid-pseudoatom formalism to derive the electron density and related properties, such as the electrostatic potential and electric moments, of the crystalline d,l-aspartic acid. The refinement models applied are restricted via rigid-bond type constraints to reduce possible bias in the mean-square displacement amplitudes due to inadequacies in the thermal deconvolution. The density and its Laplacian extracted from the data is analyzed in terms of the topological properties of covalent bonds and nonbonded interactions. The results are compared to those calculated at the Hartree−Fock level of theory and to those obtained experimentally for analogous molecules. The comparison must consider the differences in the locations of the bond critical points of the densities in question, that is, how the bond polarity manifests itself in the distribution of charge obtained by different methods. One of the key questions to the reliability of experimental pseudoatomic densities seems to be whether the treatment of the X-ray data can be standardized so as to reduce model inadequacies, especially those related to the derivation of monopole populations.

A 1-day x-ray diffraction experiment on dl-proline monohydrate was performed at 100 kelvin with synchrotron radiation and a charge-coupled device area detection technique. The accuracy of the charge density distribution and of the related electronic properties extracted from these data is comparable or even superior to the accuracy obtained from a 6-week experiment on dl-aspartic acid with conventional x-ray diffraction methods. A data acquisition time of 1 day is comparable to the time needed for an ab initio calculation on the isolated molecules. This technique renders larger molecular systems of biological importance accessible to charge density experiments.

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Cm. 2CsHg0 (100 K): hexagonal space group P63, a = 23.694(4), c = 10.046(2) A, V = 4884(2) A3, D, = 1.903 g cm-3, Z = 6, F(OO0) = 2856, A(CuKa) = 1 S4178 A, , LI = 0.84 mm-'. c60 2C8H10 (20 K): hexagonal space group P63, a = 23.67(1), c = 10.02(1) A, V = 4862(6) A?, D, = 1.912 g cm-3, Z = 6, F(OO0) = 2856, L(CuKa) = 1.54178 A, ,LI = 0.84 mm-'.The structures were determined by Patterson syntheses and rigid-body refinements. The c 6 0 molecules show two orientations with one molecular centre in common. The solvent molecules are disordered too. Static disorder could not be overcome or influenced by cooling down. A coordination number of 10 was found for the fullerene molecules.Die Strukturen wurden durch Patterson-Synthesen und Rigid-Body-Verfeinerngen bestimmt. Die Cm-Molekule zeigen zwei Orientierungen mit einem gemeinsamen Molekulzentrum. Die Losungsmittelmolekule sind ebenfalls fehlgeordnet. Tiefe Temperaturen konnten die statische Fehlordnung nicht verhindern oder beeinflussen. Fur die Fulleren-Molekule wurde eine Koordination von 10 gefunden.Kristalldaten: siehe oben *) Additional material to this paper can be ordered refering to the no. CSD-404137 and CSD-404138, the names of the authors and the journal ciitation from the Fachinformationszentrum Karlsruhe, Gesellschaft fur wissenschaftlich-technische Information mbH, D-76012 Karlsruhe, Federal Republic of Germany.