Anomalous scattering for small-molecule crystallography

Anomalous dispersion techniques in small-molecule crystallography have been used in recent years with the advent of intense tunable X-radiation from synchrotron X-ray sources. By tuning the wavelength of the radiation close to the absorption edges of specific elements, it is possible to identify and distinguish between atoms which are close together in atomic number, even when two or more such atoms occupy a single site. Anomalous dispersion techniques can also be used to distinguish between valence states of different sites of an element by the valence contrast method or to determine the coordination geometry of an atom. In some cases, when the coordination of an atom is asymmetric, the absorption-edge position may depend on the orientation of the crystal. Finally, anomalous dispersion techniques can be used for ab initio structure solution from powder diffraction data. The properties of microporous materials often depend on the incorporation of metal atoms into the framework. Two or more metal atoms close together in atomic number may be incorporated, sometimes occupying a single site. In one study the site of an Ni atom in the aluminophosphate NiAPO was determined by anomalous dispersion techniques and data from the NSLS. Also, the location of the isomorphously substituted cobalt over two crystallographically different zinc sites was determined in the zincophosphate CoZnPO-CZP, in a five-wavelength study using data from ELETTRA.