A computational and experimental search for polymorphs of parabanic acid - a salutary tale leading to the crystal structure of oxo-ureido-acetic acid methyl ester

A computational search to predict the crystal structure of parabanic acid produced the known P21/c crystal structure as the global minimum in the lattice energy. However, there are many hypothetical structures only 2-6 kJ mol(-1) less stable than the known form, which are within the energy range of possible polymorphism and have reasonable mechanical properties and relative growth rates. The harmonic intermolecular frequencies and the attachment energy estimate of relative growth rates suggest that the known polymorph is thermodynamically and kinetically favoured, but the possibility of other polymorphs cannot be excluded. A simultaneous experimental search for new polymorphs found crystals with a new morphology and X-ray powder pattern when a solution of parabanic acid in methanol was left to evaporate. Eventually, the structure was shown by single crystal X-ray diffraction to be that of oxo-ureido-acetic acid methyl ester. Thus, under the conditions of recrystallisation from methanol, parabanic acid had undergone a previously unreported ring-opening reaction, and had not crystallised as a new polymorph as had seemed likely prior to single crystal characterisation. The combination of the experimental and theoretical studies indicates that new polymorphs of parabanic acid are unlikely to be found readily.