A simple N,N′-diaryl urea derivative was found to form four different anhydrous crystal forms (I−IV°), 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 I, 34.5 kJ mol−1 for the most stable form IV°). The crystal structures of all forms show the same conformer for forms I−III and a second conformer in form IV°. 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.
Edkins, K., Qureshi, N., Yufit, D. S., Howard, J. A. K., & Steed, J. W. (2009). Hydrogen Bonding Is Not Everything: Extensive Polymorphism in a System with Conserved Hydrogen Bonded Synthons. Crystal Growth and Design, 10(2), 880-886. https://doi.org/10.1021/cg901224f