Predictable Disorder versus Polymorphism in the Rationalization of Structural Diversity: A Multidisciplinary Study of Eniluracil

Detailed analysis of X-ray diffraction data from four single crystals of eniluracil, prepared under different crystallization conditions, confirms a picture in which the crystals exhibit different degrees of disorder, which is also suggested by the computed low energy crystal structures. Since several of these crystal structures that effectively differ by an interchange of the oxygen and hydrogen atoms on C(4) and C(6) are essentially equi-energetic, growth errors that may be difficult to reverse are practically inevitable. The structural variations observed for the crystals of eniluracil studied are more appropriately described in terms of variable degrees of disorder rather than polymorphism. Analysis of the computed crystal energy landscape for interchangeable hydrogen-bonded (or other strong) motifs is, therefore, shown to be a valuable complement to X-ray diffraction and solid-state NMR for understanding and characterizing disorder in organic solid state systems. In the case of eniluracil, this detailed picture probably accounts for the challenges in devising a robust production process for this anticancer agent in the 1990s. The specific nature of the disorder accounts for different structures being obtained from powder X-ray diffraction data of different samples, and the possibility of publishable single crystal X-ray refinements also being interpreted as polymorphism rather than disorder.