A TEST OF THE SUITABILITY OF CCD AREA DETECTORS FOR ACCURATE ELECTRON-DENSITY STUDIES

The performance of the Siemens SMART CCD (charge-coupled device) area detector has been tested to assess its suitability for accurate electr on-density (ED) determination. 92043 diffraction intensities (14701 un ique reflections, 65 h experiment) have been collected on a reference crystal of methyl hydropyrimidin-1-yl)methyl]-3-thiophenecarboxylate ( C-30- H30N6O3S) at T = 120 K and compared with those (51485, 14699 uni que, 600 h) obtained from a previous collection at T = 18 K on the sam e crystal using a diffractometer equipped with a conventional detector . Results from spherical and multipolar refinements (agreement factors , standard uncertainties of refined variables and geometries after cor rection for thermal motion) have also been compared. The lambda/2 cont amination, which affects area detectors but not well tuned conventiona l detectors, has been carefully investigated and proved to be a neglig ible source of errors (which can anyway be easily corrected). The enco uraging results of this test prove that area detectors are also well s uited for charge-density studies, offering a cheap and fast data-colle ction mode, without loss of accuracy, which can be exploited for ED st udies on large systems.