PHYSICAL CHEMISTRY

Taking a Powder

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Science  23 May 2003:
Vol. 300, Issue 5623, pp. 1203
DOI: 10.1126/science.300.5623.1203c

Powder diffraction is widely used to determine the structures of polycrystalline materials. However, complex crystal structures (such as organic molecules with more than 30 nonhydrogen atoms) often cannot be solved in this way, because the overlap of peaks in the diffraction pattern prevents the determination of accurate diffraction intensities.

Brunelli et al. put into practice an approach first suggested 40 years ago to facilitate the determination of complex molecular structures from powder diffraction. The method is based on the direction-dependent (or anisotropic) expansion of many low-symmetry materials with temperature. This expansion shifts the relative positions of the diffraction peaks. Hence, reflections that may overlap at one temperature are resolved at another. The authors used a recently upgraded, high-resolution powder diffraction beam line to solve an organic structure with 48 nonhydrogen atoms. Data were collected in 1 day, and the structure was solved the next day, suggesting that larger, more complex structures may also be tackled in this fashion.—JFU

Angew. Chem. Int. Ed. 42, 2029 (2003).

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