How Accurate Do X‑ray Data Need To Be To Obtain a Reliable Order of Stability for Polymorphs? The Case Study of p‑Hydroxyacetophenone Polymorphs

Technical progress in crystallographic instruments allows contemporary crystallographers to more routinely conduct excellent quality low-temperature diffraction measurements. As a result, even crystal structures that have been extensively studied at ambient conditions start to reveal their more complex nature. The importance of lowtemperature measurements is discussed based on the new, modulated structures found for polymorphs of -hydroxyacetophenone (HAP). Diffraction data for this polymorphic system have been collected in the temperature range from 40 K to 330 K. At 123 K, form II of HAP appears to be a new, commensurately modulated structure with a modulation vector q = [0.4, 0.0, 0.0]. The structure can be solved and refined with 10 molecules in the asymmetric unit. A further decrease of temperature to 40 K resulted in the discovery of a new, incommensurately modulated phase with four independent molecules in the P21 monoclinic space group with the modulation vector q = [0.37, 0.17, 0.0]. Consequences of modulation for the relative stability of polymorphs will be discussed.