Direct Methods in Organic Electron Crystallography

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Dr. Douglas Dorset is supported by a grant from the Theoretical and Computational Chemistry Program in the Chemistry Division and the Polymers Program in the Division of Materials Research to perform electron crystallography of organic and polymeric samples. Dorset will extend the direct phasing methodology which has been widely used in X-ray crystallography to organic structure determination by electron crystallography. These methods may be based on the probabilistic estimate of phase invariant sums, as in X-ray crystallography, or may also include phase information from high resolution electron microscope "lattice images". Systems which have already been studied include small organic molecules, n-paraffins, lipid lamellae, linear polymers, and large fused ring systems. The methodology will be extended to the investigation of other compounds and polymers for which no X-ray crystal structures exist. If this research is successful then Dorset will have developed electron crystallography as a reliable method for ab initio structure analysis and refinement. %%% Molecular structures with atomic resolution continue to be the primary test bed for ab initio electronic structure theories. They also provide important insights on the molecular level interpretation of material properties. Although X-ray crystal structures have been the primary source of this information, not all chemical compounds form large enough crystals to be studied by this technique. Dr. Dorset's approach which uses electron diffraction is applicable to microcrystalline samples, and still provides vital atomic resolution information on chemical structures.