A simple N,N'-diaryl urea derivative was found to form four different anhydrous crystal forms (I-IV degrees), which can be crystallized in pure form by several techniques from solution as well as from the melt. These polymorphs were characterized by thermomicroscopy, differential scanning calorimetry, Fourier-transform infrared spectroscopy and single crystal and powder X-ray diffraction. The polymorphs were found to be purely monotropically related with large differences in their heats of fusion (11.1 kJ mol(-1) for the least stable form 1, 34.5 kJ mol(-1) for the most stable form IV degrees). The crystal structures of all forms show the same conformer for forms I-III and a second conformer in form IV degrees. However, the hydrogen bonding motifs in all of the polymorphs are the same and thus the compound can be seen as a model for the importance of the entire crystal packing arrangement to the overall energy and stability of the crystal form, as opposed to just a few dominant interactions.
