Direct Methods in Organic Electron Crystallography

Douglas Dorset得到了化学学部理论与计算化学项目和材料研究学部聚合物项目的共同资助，继续研究电子晶体学的直接方法。研究计划的总体目标是继续将直接相位方法应用于电子晶体学。目前项目的具体目标是：1)改进收集更精确的电子衍射强度数据的方法；2)优化处理欠采样倒易格的直接方法的使用；3)开发改进程序，以避免与多重散射问题相关的困难。虽然该提案主要侧重于方法开发，但也提出了各种应用于小型有机系统，如三苯、TCNQ、芘、四苯和蒽，以及聚合物系统，如对氧苯甲酸晶须晶体、蜘蛛拖线丝和聚四氟乙烯定向薄膜。x射线衍射对化学和材料科学领域产生了巨大的影响。它提供了在原子水平上对分子结构的直接实验探测。然而，对于许多重要的有机化合物，只能生长出不适合x射线衍射研究的小晶体。电子晶体学为研究这类化合物提供了一种强有力的替代方法，只要它能克服许多技术上的困难。Dorset在这一领域的工作旨在改进这种替代传统x射线晶体学的方法，使其精确且相对容易使用。

Douglas Dorset is jointly supported by a grant from the Theoretical and Computational Chemistry Program in the Chemistry Division and the Polymers Program in Division of Materials Research to continue his studies of direct methods for electron crystallography. The overall thrust of the research program is to continue the application of direct phasing methods to electron crystallography. Specific goals of the current project are: 1) to improve methods for collecting more accurate electron diffraction intensity data; 2) to optimize the use of direct methods for handling undersampled reciprocal lattices; and 3) to develop refinement procedures which circumvent the difficulties associated with multiple scattering problems. Although the proposal is primarily focused on methodology development, various applications are also proposed on small organic systems such as triphenylene, TCNQ, pyrene, quaterrylene and anthanthrene, and polymeric systems such as whisker crystals of p-oxybenzoate, spider drag line silk, and oriented films of poly(tetrafluoroethylene). X-ray diffraction has had a tremendous impact on the fields of Chemistry and Material Science. It provides a direct experimental probe of molecular structure at the atomistic level. However, for many important organic compounds, only small crystals can be grown that are unsuitable for investigation by X-ray diffraction. Electron crystallography presents a powerful alternative for studying such compounds providing that a number of technical difficulties can be overcome. Dorset's work in this area is directed at improving this alternative to conventional X-ray crystallography to the point where it is precise and relatively easy to use.