ACCURATE BRAGG INTENSITIES FOR URACIL - A THERMAL PARAMETER REFINEMENT

Accurate and extensive three-dimensional intensity data for a single crystal of uracil have been collected by diffractometry using MoKā x-radiation. The average precision in the Bragg intensities is less than 2°/o. Secondary extinction was eliminated by powder-diffraction measurements. Multiple diffraction could not be detected. The interaction of thermal parameters with scattering factors is analyzed with a simple model. A refinement of the anisotropic thermal parameters, based on reflections for which (sin0)/A exceeds 648 Å-1, led to values which are significantly different from refinement based on all data. The carbon-atom parameter differences are consistent with recent studies of combined x-ray and neutron-diffraction analysis. © 1969, Walter de Gruyter. All rights reserved.