Disappearing disorder.

Single crystal X-ray diffraction is the method of choice to resolve difficult solid-state problems. This is exemplified by analysis of three disordered structures that cease to be disordered depending on the circumstances. Interesting research questions of relevance for the pharmaceutical industry, including possible issues of intellectual property, emerge from disappearing disorder phenomena. Its relationship to polymorphism is discussed, and the term 'archetype structure' that includes both polymorphs and disorder components is recommended for use. X-ray structural studies reported here benefit from refinement with aspherical scattering factors that improve accuracy and precision of the results, thereby providing the confidence required in interpreting comparably small residual electron density features. Full understanding of the mechanism of how disorder can come into existence and disappear with temperature or time (pressure being another factor not exploited here) requires computing the relative energy differences of archetype structures by ONIOM cluster or solid-state computations. [ABSTRACT FROM AUTHOR]