MS26-P04 Exploring modulation in molecular crystals at low temperatures Technical progress in crystallographic instruments allows contemporary crystallographers to more routinely conduct low-temperature diffraction measurements. Additionally, modern diffractometers have stronger sources and more sensitive detectors than they had a few years ago. As a consequence, even crystal structures that have been extensively studied at ambient conditions start to reveal their more complex nature. The importance of low-temperature measurements is discussed for polymorphs of 4’-hydroxyacetophenone (HAP)1 .
For this enantiotropic polymorphic system, we collected single crystal X-ray diffraction data in the temperature range from 30 K to 330 K. At 123 K orthorhombic 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 in a supercell with 10 molecules in the asymmetric unit. Interestingly, further lowering of temperature down to 30K resulted in a discovery of a new, incommensurately modulated phase with 4 independent molecules and crystal lattice of the P21 space group and modulation vector q = [0.37, 0.17, 0.0]. Consequences of modulation for the relative stability of polymorphs will be discussed.References:
 Bernardes, C. E. S.; Piedade, M. F. M.; Minas, M. E. 2008, Cryst. Growth Des. 8 (7), 2419–2430.Keywords: polymorphism, commensurate modulation, incommensurate structure