Direct Methods in Organic Electron Crystallography

布法罗医学基金会的道格拉斯·多塞特博士得到了化学部理论和计算化学项目以及材料研究部聚合物项目的资助，以继续开发用于有机化合物和聚合物电子结晶学的直接方法。他正在与Herb Hauptman(因相关工作与Jerome Karle分享1985年诺贝尔化学奖)合作，为电子晶体数据开发一种直接相化方法，用于从头计算确定未知晶体结构。正在采取一系列措施来提高这项技术的可靠性，包括：1)提高自动化程序的优值系数，使其更能抵抗动态散射；2)开发更可靠的定相程序；3)采用多层计算以改进精确度；4)更好地理解晶相测定的局限性；以及5)更多地使用较高电压源，以确保数据更接近单次散射极限。应用包括：1)聚四氟乙烯的取向薄膜；2)小分子，如C60和C70、铜酞菁和铜全氟酞菁，以及3)酯、醚、胺和酮基取代的烷烃链。到目前为止，使用最广泛的结构工具是X射线结晶学。它被广泛使用的原因之一是，已经开发出用于自动解决许多系统的相位问题的技术。尽管如此，X射线结晶学的局限性主要源于难以获得合适尺寸的晶体。在X射线衍射法存在问题的情况下，电子结晶学应该提供X射线衍射法的补充选择。多塞特的工作旨在尝试将电子结晶学技术改进到像现在的X射线结晶学一样自动化和易于使用的程度。

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.