Charge Densities Based on Synchrotron/CCD Experiments
T. Koritsanszky and R. Flaig
Institute for Crystallography, Free University of Berlin Takustr. 6, 14195, Berlin, Germany
The extraction of the solid state electron density from X-ray diffraction data has been a troublesome, timeconsuming procedure, because it has relied on serial collection of Bragg intensities detected by scintillation counters. Due to the applicability of modern charge-coupled device (CCD) area detectors to accurate intensity registration, the method is being revolutionized and the data collection time of several weeks or even months needed for the conventional technique can now be reduced to days. The combination of a CCD detector with synchrotron radiation source seems to open new perspectives for the experimental charge density determination [1].
In spite of a few very promising results there are numerous cases for which the experimental setup outlined above delivers data of only moderate quality. Thus, it is an important issue to find the optimal experimental parameters and the best data reduction strategy that can lead to a set of reflections suitable for charge density determination.
The lecture is a report on such applications to small molecules of biological activity, such as amino acids (D,L-Proline, D,L-Glutamic acid and L-Asparagine), a dipeptide (Glycil-L-Threonine) and a potent antithrombotic pharmacological product.
The data were collected at the synchrotron beam line D3 of the storage ring DORISIII at HASYLAB/DESY in Hamburg using a wavelength of around 0.5 A and a liquid nitrogen cooling device. For each data sets at least a four-fould redundancy was achieved and the averaging of the equivalent reflections led to internal residuals of 4-5%. For comparison, we have performed also conventional measurements for most of the compounds listed above.
The data were interpreted within the rigid pseudoatom formalism [2] as implemented to the computer program mackage XD [3]. The results were analysed in terms of the topological properties [4] of the static electron densities extracted and compared to those calculated from the wave functions at the Hartree-Fock level of theory [5].