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X-Ray Crystallography and Molecular Mechanics Methods
The research emphasis in my group is on the elucidation of structures of materials in the solid state. The primary technique which we use in this work is that of diffraction, usually x-ray diffraction but occasionally also neutron diffraction. We not only use these techniques to determine the structures of a variety of chemically significant materials, but also, as a physical chemist, are as interested in the further development of such techniques.
In terms of applications, we participate in collaborative studies with many other groups whose research is described elsewhere in this document, and with other groups in chemical engineering, materials science, and physics that make up the Ames Laboratory of the Department of Energy. Of special interest is the molecular structure of materials with the potential of showing long range interactions in the solid state. One recent example is the structure of palladium nickel porphyrin compound prepared by Keith Woo's group. The structure is shown in the figure; it contains 89 non-hydrogen atoms whose coordinates had to be determined. This investigation was carried out using diffraction information obtained from a small single crystal of the material (0.36 x 0.36 x 0.10 mm). A total of 21,343 diffraction maxima were measured in this study.
As noted above, a considerable fraction of our research effort is devoted to the further development of the techniques for structure determination. Much of this is computer oriented; software for better and faster collection of data is being developed and, as well, software for improved methods of structure solution once the data have been acquired. New area detectors, i.e., detectors that provide accurate intensity information on large areas in diffraction space, have recently become commercially available. We are working on software to make most efficient use of such detectors - one example of our software efforts. Another quite different example involves the development of the theory and the accompanying computer algorithms for a new method of phasing the structure amplitudes obtained from intensity data. This algorithm was used on the palladium complex cited above and proved to be a very efficient way to determine even this complex structure.
Of late we have also been making greater use of molecular mechanics methods; in this approach atoms are adjusted in a structure to best fit empirical bond stretching, bending and related restraints. This is being used not only to help predict molecular structures, but also in combination with diffraction methods. We have been developing computer algorithms to generate alternate structures in order to determine the energies of various conformations of ring systems and especially to determine the lowest energy conformer.
We have excellent computer facilities for this work ranging from high end PC's on each desktop to more powerful workstations as well as excellent diffraction facilities including a high intensity rotating anode source for single crystal investigations.