Absolute structure from single crystal X-ray data even when the resonant atoms are disordered.

What is it about?

It can be impossible to find an atomic model to fit disordered regions in a crystal structure. The SQUEEZE procedure is a well known alternative, which accounts for the disordered region using continuous electron density derived from a difference Fourier transform. Although this can account very well for the disordered part of the structure, it does not include contributions for resonant X-ray scattering which are the basis for determining the absolute structure (and hence absolute stereochemistry of the crystal). This paper proposes and tests an extra correction (called HUG) to the calculated SQUEEZE correction in order to account for the resonant scattering, and shows that in some cases the absolute structure can then be determined.

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Why is it important?

Reliable determination of absolute structure by X-ray diffraction is an important technique. The benefit is lost if the resonant scattering atoms, which make the technique possible, are disordered within the overall order of the crystal structure. The correction makes it possible to obtain the benefits of working with strongly resonant scatterering atoms even if there is a tendency for them to disorder (e.g. if they are part of a solvent molecule).