Mechanism of the first-order phase transition of an acylurea derivative: observation of intermediate stages of transformation with a detailed temperature-resolved single-crystal diffraction method

The process of the first‐order solid‐to‐solid phase transition of 1‐ethyl‐3‐(4‐methylpentanoyl)urea (1) was observed by means of a detailed temperature‐resolved single‐crystal diffraction method, which resembles watching a series of stop‐motion photographs. The transition consists of two elementary processes, one supramolecular and the other molecular. Crystal structures from before and after the phase transition are isostructural. The straight‐ribbon‐like one‐dimensional hydrogen‐bonding structure is formed and stacked to form a molecular layer. The geometry of the layer is retained during the phase transition. The relative position of the layer with its neighbours, on the other hand, changes gradually with increasing temperature. The change is accelerated at the temperature representing the start of the endotherm seen in the DSC curves of (1). The structural variation yields void space between the neighbouring layers. When the void space grows enough that the crystal is unstable, the 3‐methylbutyl group on the last of the molecules turns into a disordered structure with drastic conformational changes to fill up the void space. The phase transition process is well supported with simple force‐field calculations. A crystal of 1‐(4‐methylpentanoyl)‐3‐propylurea (2), which shows no solid‐to‐solid phase transitions, was also analysed by the same method for comparison.