High-resolution synchrotron data collection for charge-density work at 100 and 20 K

For the measurement of very accurate high-resolution X-ray data for charge-density work, synchrotron beamlines provide a bright radiation source of outstanding properties. Most important are the very high primary intensity and the possibility of choosing hard radiation with lambdaapproximate to0.5 Angstrom or even shorter. This together with area detection and a stable low-temperature device means that accurate and large data sets can be measured in a short time. A number of data collections are reported, which were carried out at the beamlines F1 and D3 of Hasylab (DESY, Hamburg) first at 100 K and later at 15-20 K, demonstrating the quality of the measured intensities. A low temperature of around 20 K was obtained at beamline D3 with a double-stage closed-cycle helium cryostat where the standard beryllium cylinder of the vacuum chamber was replaced by a 0.1 mm Kapton film. Comparison of different data sets measured for a strychnine crystal demonstrated how I/sigma ratios favorably improve if synchrotron radiation at a low temperature of 15 K was used. Synchrotron-based studies on several biologically active compounds are briefly summarized and a synchrotron experiment of an otherwise not sufficiently diffracting crystal of a tetraphenyl barbaralane derivative is described in detail. Atomic volumes and charges of a highly substituted C-60 fullerene are reported derived from a synchrotron data set of more than 350000 reflections.