Polymorphism in 3-acetyl-4-hydroxy-2H-chromen-2-one

A new polymorph (denoted polymorph II) of 3-acetyl-4-hydroxy-2H-chromen-2-one, C11H8O4, was obtained unexpectedly during an attempt to recrystallize the compound from salt-melted ice, and the structure is compared with that of the original polymorph (denoted polymorph I) [Lyssenko & Antipin (2001). Russ. Chem. Bull. 50, 418-431]. Strong intramolecular O-H center dot center dot center dot O hydrogen bonds are observed equally in the two polymorphs [O center dot center dot center dot O = 2.4263 (13) angstrom in polymorph II and 2.442 (1) angstrom in polymorph I], with a slight delocalization of the hydroxy H atom towards the ketonic O atom in polymorph II [H center dot center dot center dot O = 1.32 (2) angstrom in polymorph II and 1.45 (3) angstrom in polymorph I]. In both crystal structures, the packing of the molecules is dominated and stabilized by weak intermolecular C-H center dot center dot center dot O hydrogen bonds. Additional pi-pi stacking interactions between the keto-enol hydrogen-bonded rings stabilize polymorph I [the centres are separated by 3.28 (1) angstrom], while polymorph II is stabilized by interactions between alpha-pyrone rings, which are parallel to one another and separated by 3.670 (5) angstrom.