Disappearing and Reappearing Polymorphism in <i>p</i>-Methylchalcone

One stable form (Form I) and two metastable forms (Forms II and IV) of p-methylchalcone have been obtained, with Form II having previously been reported to be a disappearing polymorph. To prevent the conversion of Form II, which has a melting point of 89–91 °C, into the stable form it was produced in conditions free from seeds of the stable form. Form IV was discovered serendipitously as a single crystal that appeared in the preparation of a powder of Form II. Forms I and II have also been characterized by optical microscopy, differential scanning calorimetry (DSC), variable temperature powder X-ray diffraction, and spectroscopic analysis. The crystal structures of these three forms have been solved, Forms I and IV by single-crystal methods and Form II from synchrotron powder diffraction. This investigation highlights the fact that the challenges of reproducing experimental results (i.e., the production of a particular crystal form) may be overcome by a systematic search for new conditions under which previously obtained results can be regained.