The structures of 2-amino-4'-chlorobenzophenone and 2-amino-4'-bromobenzophenone, previously determined at room temperature in the space group Pna21, have been redetermined at low-temperature revealing two different reversible phase transitions. Additionally, high-pressure X-ray diffraction studies were conducted to allow for a comparison of the behavior of these structures in response to different external stimuli. Variable temperature analyses reveal that 2-amino-4'-bromobenzophenone transitions to the monoclinic space group Pa accompanied by a nonmerohedral twinning. The monoclinic phase exhibits approximate symmetry mimicking that of the supergroup Pna21 but with a metric symmetry that precludes true Pna21 symmetry. The structure of 2-amino-4'-chlorobenzophenone transitions to an incommensurately modulated phase upon cooling. The changes in the structure are attributed to slight conformational variations and a rearrangement of the hydrogen bonding networks in the structure in response to stimuli. The phase transition for 2-amino-4'-bromobenzophenone is classified as second order and a mechanism involving multiple coincident nucleation events is inferred from the conformational change and the incidence of twinning. The transformation of 2-amino-4'-chlorobenzophenone to an incommensurate phase is proposed to arise due to thermal contraction of the structure, which allows distinct competing hydrogen bonding networks between neighboring molecules to evolve.
Attiwell et al. (Fri,) studied this question.