Direct Methods in Organic Electron Crystallography

Douglas Dorset博士由化学部门的理论和计算化学项目以及材料研究部门的聚合物项目资助，对有机和聚合物样品进行电子晶体学研究。Dorset将在x射线晶体学中广泛应用的直接相位法扩展到电子晶体学的有机结构测定。这些方法可能基于相不变和的概率估计，如在x射线晶体学中，或者也可能包括来自高分辨率电子显微镜“晶格图像”的相信息。已经研究的体系包括小有机分子、正石蜡、脂质片层、线性聚合物和大型熔融环体系。该方法将扩展到其他化合物和聚合物的研究，其中不存在x射线晶体结构。如果这项研究成功，那么Dorset将开发电子晶体学作为从头计算结构分析和细化的可靠方法。具有原子分辨率的分子结构仍然是从头算电子结构理论的主要试验台。它们还为材料性质的分子水平解释提供了重要的见解。虽然x射线晶体结构一直是这一信息的主要来源，但并不是所有化合物形成的晶体都足够大，可以用这种技术进行研究。Dorset博士使用电子衍射的方法适用于微晶样品，并且仍然提供化学结构的重要原子分辨率信息。

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.