Order–disorder phase transitions in 9-hydroxyphenalenone

A heat capacity study of 9-hydroxyphenalenone reveals three phase transitions at 254.9, 379.8, and 385.5 K, followed by melting at 475.5 K. The observed entropy change at each transition can be fitted by an orientational disorder model. The broad transition at 379.8 K is ascribed to the onset of multiple disorder in both symmetry-independent molecules. Complete deuteration of the hydroxyl groups increases the lower phase transition temperature by 7.6 K, decreases that of both upper phase transitions by 2.4 K and decreases the melting point by 2.3 K. The effect of deuteration on the transition temperatures appears to be linearly dependent on the extent of isotopic replacement.