Polymorphism and crystal structure of 2R,4S,6-fluoro-2-methyl-spiro[chroman-4,4'-imidazoline]-2',5-dione (M79175)

Two polymorphic forms, alpha and beta, of 2R,4S,6-fluoro-2-methyl-spiro-[chroman-4,4'-imidazoline]-2',5-dione (1, M79175) were studied by X-ray crystallography and solid-state infrared spectroscopy. The molecular and crystal structures of the beta-form were determined by single-crystal X-ray diffraction analysis. The molecules in the beta-form crystal are arranged orderly along the b-axis, and the plate-like moieties (chroman ring) of 1 are stacked by van der Waals forces. The molecular packing structure of each crystalline form was investigated by comparing wave numbers of hydantoin rings from which solid-state infrared photoacoustic spectra (FT-IR-PAS) were obtained. From the FT-IR-PAS data of both forms, it became clear that the hydrogen bond in an alpha-form crystal is stronger than that in the beta-form crystal. The cohesion of the polymorphic crystal system is mainly due to hydrogen bonds and van der Waals forces. Based on the polymorphism of 1, it was clarified that thermodynamic stability depends on the mode of hydrogen bonding, while the enthalpy of fusion results from van der Waals forces.