Polymorphism of N,N''-diacetylbiuret studied by solid-state 13C and 15N NMR spectroscopy, DFT calculations, and X-ray diffraction

The molecular configuration and crystal structure of solid polycrystalline N,N"-diacetylbiuret (DAB), a potential nitrogen-rich fertilizer, have been analyzed by a combination of solid- and liquid-state NMR spectroscopy, X-ray diffraction, and DFT calculations. Initially a pure NMR study ("NMR crystallography") was performed as available single crystals of DAB were not suitable for X-ray diffraction. Solid-state C-13 NMR spectra revealed the unexpected existence of two polymorphic modifications (alpha- and beta-DAB) obtained from different chemical procedures. Several NMR techniques were applied for a thorough characterization of the molecular system, revealing chemical shift anisotropy (CSA) tensors of selected nuclei in the solid state, chemical shifts in the liquid state, and molecular dynamics in the solid state. Dynamic NMR spectroscopy of DAB in solution revealed exchange between two different configurations, which raised the question, is there a correlation between the two different configurations found in solution and the two polymorphic modifications found in the solid state? By using this knowledge, a new crystallization protocol was devised which led to the growth of single crystals suitable for X-ray diffraction. The X-ray data showed that the same symmetric configuration is present in both polymorphic modifications, but the packing patterns in the crystals are different. In both cases hydrogen bonds lead to the formation of planes of DAB molecules. Additional symmetry elements, a two-fold screw in the case of a alpha-DAB and a c-glide plane in the case of beta-DAB, lead to a more symmetric (alpha-DAB) or asymmetric (beta-DAB) intermolecular hydrogen-bonding pattern for each molecule.