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 (lambda)~0.5  … 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 C60 fullerene are reported derived from a synchrotron data set of more than 350000 reflections.