Resonant Scattering of Light Atoms - Measuring Methods and Applications

The paper describes measuring methods of weak resonant scattering signals occurring in X-ray diffraction pattern of a crystal containing light atoms (C, N, O, F) in its unit cell. Difficulties resulting from the large distance on the energy scale between the K-absorption edges of light atoms and contemporary available energy range of X-ray sources for diffraction experiments may be overcome using mixed synchrotron radiation and sealed tube measurements. Techniques such as high resolution synchrotron radiation diffraction experiment, low resolution azimuthal scan, top reflection azimuthal scan are discussed and their applications are presented. Several enantiomer correctness indicators, evaluating the confidence level of absolute structure determination, are applied for crystals containing oxygen or nitrogen as anomalous scatterers. Resonant scattering of light atoms, which are fundamental constituents of organic molecules and polymers, carries the information about the absolute structure. Growing importance of stereospecific drugs which follows recent recognition of drug-receptor interaction mechanism increases the demand for determination of drug molecule handedness. Investigation of absolute structure for molecules in their original shape (without the introduction of heavy anomalous scatterers) is potentially a vast application field of the described methodology.