Direct Methods in Organic Electron Crystallography

Douglas Dorset博士，布法罗医学基金会，由化学部门的理论和计算化学计划以及材料研究部门的聚合物计划资助，继续他在有机化合物和聚合物的电子晶体学直接使用方法的开发方面的研究。他正在与赫伯·豪普特曼（赫伯·豪普特曼与杰罗姆·卡尔因相关工作共同获得1985年诺贝尔化学奖）合作，开发一种电子晶体学数据的直接相位法，用于从头计算未知晶体结构的测定。目前正在采取一系列措施来提高该技术的可靠性，包括：1)改进更能抵抗动态散射的自动化程序的优值；2)制定更可靠的分阶段程序；3)结合多层计算提高精密度；4)更好地理解晶体学相测定的局限性；5)增加高压源的使用，确保数据更接近单次散射极限。应用包括：1)聚四氟乙烯定向薄膜；2) C60和C70等小分子，酞菁铜和全氟酞菁铜；3)被酯、醚、胺和酮基取代的烷烃链。到目前为止，最广泛使用的结构工具是x射线晶体学。它被广泛使用的原因之一是，许多系统已经开发出了自动解决相位问题的技术。尽管如此，x射线晶体学仍有局限性，主要是由于难以获得合适尺寸的晶体。电子晶体学应该在x射线衍射有问题的情况下提供一种补充的替代方法。Dorset的工作是试图改进电子晶体技术，使其像现在的x射线晶体学一样自动化和易于使用。

Dr. Douglas Dorset, Medical Foundation of Buffalo, 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 continue his research in the development of direct methods for use in electron crystallography of organic compounds and polymers. He is working with Herb Hauptman (who shared the 1985 Nobel Prize in Chemistry with Jerome Karle for related work) to develop a direct phasing method for electron crystallographic data for the ab initio determination of unknown crystal structures. A number of steps are being taken to improve the reliability of the technique including: 1) improving a figure of merit for automated procedures that are more resistant to dynamical scattering; 2) developing more reliable phasing procedures; 3) incorporating a multislice calculation to improve refinement; 4) developing a better understanding of the limitations of crystallographic phase determination; and 5) increasing the use of higher voltage sources to ensure data closer to the single scattering limit. Applications include: 1) oriented films of poly (tetrafluorotheylene) ; 2) small molecules such as C60 and C70, copper phthalocyanine and copper perfluorophthalocyanine, and 3) alkane chains substituted with ester, ether, amine and keto groups. By far the most widely used structural tool is X-ray crystallography. One of the reasons for it's widespread use is the fact that techniques have been developed for automatically solving the phase problem for many systems. In spite of this, there are limitations to X-ray crystallography arising primarily from the difficulty of obtaining crystals of suitable size. Electron crystallography should provide a complementary alternative to X-ray diffraction for cases where the latter technique is problematic. Dorset's work is directed at attempting to improve the electron crystallographic technique to the point where it will be as automated and as easy to use as X-ray crystallography now is.