USE OF AREA DETECTORS IN SMALL MOLECULES CRYSTALLOGRAPHY
Ilona Turowska-Tyrk
Department of Chemistry, Wroclaw University
of Technology, Wybrzeze Wyspianskiego 27, 50-370 Wroclaw, Poland;
E-mail: ilona@kchf.wch.pwr.wroc.pl
Keywords: area detectors, data collection,
structure determination, small molecules
At the beginning of nineties area detectors were started to be used in small molecules crystallography. First small molecules structures determined by the use of the two-dimensional detectors were presented during the 51st Annual Pittsburgh Diffraction Conference in 1993 [1]. The strategy of area detector data collection and reduction for small molecules crystals was described [2, 3] and such structures were started to be published.
At present several firms produce area detectors: Siemens (SMART CCD system), Nonius (NONIUSKAPPACCD and FAST systems), Rigaku/Molecular Structure Corporation (R-Axis imaging plate detector), Marresearch (marccd and mar345 imaging plate), Area Detector Systems Corporation. In 1997 KUMA Diffraction (Poland) delivered first KM4CCD cameras. The second of them was installed at Wroc3aw University of Technology [4].
The features of the KM4CCD diffractometer are:
- 1024x1024 (512x512)
pixels CCD detector with Peltier cooling system
- conventional (point) detector near the CCD camera
- four-circle k-geometry goniometer
- fine-focus sealed tube generator
- optical camera for crystal alignment
- automatic data collection and reduction package for Windows95.
One of the most important advantages of all area detector systems over conventional (point) detector diffractometers is the ability to collect many reflections in parallel what considerably reduces total time necessary for data collection. Moreover, the data obtained from the area detectors are of very high quality. A detailed analysis of the quality aspects (in terms of R values, reflection statistics, electron density maps, internal structural consistency etc.) for several crystals examined by the use of the KM4CCD camera will be presented. A comparison with the same structures determined by the conventional point detector will be given.
The method used for data collection with area detectors, and also with KM4CCD, is rotational: for each small rotation of a crystal, a two-dimensional image (frame) of data is taken. Details of the data collection strategies will be discussed.
It is necessary to point out a possibility of novel
experiments with the area detectors: data collection for very
small or weakly diffracting crystals, fast data collection for
unstable samples, structural studies of photo-induced crystals,
electron density studies for big structures, diffuse scattering
analysis.