Accurate Bragg intensities for uracil: a thermal parameter refinement

Abstract

Accurate and extensive three-dimensional intensity data for a single crystal of uracil have been collected by diffractometry using MoK[unk] x-radiation. The average precision in the Bragg intensities is less than 2%. 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 (sinθ)/λ 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.