Accurate charge densities in days - use of synchrotrons, image plates and very low temperatures.

Extensive synchrotron (28 K) and conventional sealed-tube (9 K) X-ray diffraction data have been collected on tetrakis(dimethylphosphinodithioato-S,S')thorium(IV), [Th(S(2)PMe(2))(4)]. The use of very low temperatures, well below those obtained with liquid-nitrogen cooling, is crucial for the accuracy of the data. This is due to minimization of temperature-dependent systematic errors such as TDS and anharmonicity, and extension and intensification of the data in reciprocal space. Comparison of structural parameters derived separately from the sealed-tube data and the synchrotron data shows good agreement. The synchrotron data are markedly superior when comparing refinement residuals, standard uncertainties (s.u.'s) of the data and s.u.'s of the derived parameters. However, the study suggests that there are still small uncorrected systematic errors in the data. The very large extent [(sinstraight theta/lambda)(max) = 1.77 Å(-1)] of the synchrotron data and the very low temperature at which they were collected makes it possible to separate anharmonic effects from electron-deformation effects even with only an X-ray data set at a single temperature. The electron density shows a large polarization of the outer Th core of d-type symmetry. This deformation is successfully modelled with contracted multipolar functions, which are only slightly correlated with anharmonic expansions in reciprocal space when using the full extent of the data. In the data collection more than a factor of 100 in speed is gained by use of image-plate area detectors at the synchrotron source compared with conventional sequential measurements. Thus accurate, very low temperature synchrotron-radiation diffraction data can now be measured within days, which makes electron-density studies of compounds beyond the first transition series more frequently within reach.

The Use of Imaging Plates in Electron-Density Mapping with Synchrotron X-rays.

Imaging-plate synchrotron data have been applied in the charge-density analyses of sodium nitroprusside and hexaamminechromium(III) hexacyanochromate(lll), collected at 100 and 50 K, respectively, and photons of wavelengths 0.656 and 0.394 A at the SUNY X3 beamline at NSLS. The electron-density maps show good agreement between chemically equivalent sections, while the multipole aspherical atom refinements lead to chemically reasonable population parameters, with trends reproduced in the available theoretical calculations. The results indicate that the time required for charge-density mapping with diffraction data can be greatly reduced by the application of the new technology.

A Multiwavelength Monochromator for the X3A1 Fixed-Angle Station at the National Synchrotron Light Source.

A new revolving monochromator intercepting 2 mrad of the radiation fan at the State University of New York (SUNY) X3 beamline at the National Synchrotron Light Source (NSLS) is described. The design allows for easy interchange between four different monochromator crystals. The photons are reflected in the horizontal plane to provide a monochromatic beam at a fixed-angle diffraction station. The triangular crystals can be bent for horizontal focusing.