The discovery of new crystal forms of 5-fluorocytosine consistent with the results of computational crystal structure prediction

A computational search for low-energy crystal structures of 5-fluorocytosine was performed in conjunction with a manual crystallization screen to determine whether solid forms other than the known monohydrate structure could be discovered. The predicted low-energy structures were based on a hydrogen-bonded ribbon, which was observed in two newly determined anhydrous polymorphic crystal structures, a hemipentahydrate, and two solvates, as well as the known monohydrate. An alternative ribbon structure present in some less-stable predicted structures was found in a disordered monohydrate structure. The anhydrous crystal structure prediction was a success, by the criteria of the international crystal structure prediction blind tests. Thus we can rationalize the crystal structure behavior of 5-fluorocytosine as having a strongly preferred two-dimensional ribbon structure, which exhibits versatile methods of packing, leading to polymorphism and a number of closely related solvate structures.