Reconciling thermal and structural data from the polymorphic transitions of malonamide

Although screening for organic polymorphs has become a hot topic in recent years, polymorph screens continue to be largely based on solution crystallization. Other routes to different crystal forms, such as grinding, annealing, and melt crystallization can also be used to look for polymorphs when chemical stability allows. Thermal analysis is often the first method for identifying polymorphic transitions in solids. However, reconciling the thermal data with the structural data is rarely straightforward without a single crystal X-ray structure. By using differential scanning calorimetry, infrared spectroscopy, and powder X-ray diffraction, the polymorphic transitions of malonamide have now been studied, and two solid state pathways, solid state grinding and annealing, are verified to independently yield the tetragonal polymorph of malonamide from the monoclinic form. In addition, melt recrystallization yields a third polymorph, which has now been confirmed to be the orthorhombic polymorph.