Grant: Research Tools and Instruments - Category 1 (<$150,000)

Single-crystal X-ray crystallography is one of the most powerful techniques for establishing the detailed, three-dimensional structures of molecules - information that is pivotal for making important connections between molecular structures and chemical reactivity. In this technique, a single crystal of the compound is irradiated with a beam of X-rays, giving rise to thousands of diffracted X-ray beams. Structure determination then involves measuring the individual intensities of these diffracted beams and mathematically transforming the data to obtain positions of all atoms in the molecule. ) A major recent breakthrough in crystallographic instrumentation has been the development of powerful X-ray 'microsources' that combine high beam intensity, low power usage, and virtually maintenance-free operation. Operating at only 30 W, these microsources can produce an X-ray beam comparable in intensity to that for a 'rotating-anode' source operating at 6 kW (the previous standard for laboratory X-ray sources), but without the rotating-anode system's need for frequent expensive repairs. ) Our proposal is to upgrade our existing X-ray diffractometer system, presently featuring a molybdenum-radiation 'sealed-tube' source and state-of-the-art charge-coupled-device-based X-ray detector, by addition of a copper-radiation microsource. This will yield a highly-flexible 'dual-wavelength' system. The more intense microsource will allow more weakly-diffracting crystals to be studied, and will be valuable for determination of absolute stereochemistry, which is normally difficult for molecules containing only lighter chemical elements. For compounds containing heavy atoms in which the exact composition is uncertain, copper-wavelength data can be used initially to identify atom types, followed by the final refinement using higher-resolution molybdenum-wavelength data. The molybdenum source will remain in service for use with larger crystals containing heavier elements, which more strongly absorb X-rays produced by a copper source.