Low-Temperature and High-Pressure Phase Transitions in Two 2-Amino-4′-halobenzophenones: Incommensurate Modulation and a Case of Temperature-Induced Twinning

The structures of 2-amino-4'-chlorobenzophenone and 2-amino-4'-bromobenzophenone, previously determined at room temperature in the space group <i>Pna</i>2₁, 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 <i>Pa</i> accompanied by a nonmerohedral twinning. The monoclinic phase exhibits approximate symmetry mimicking that of the supergroup <i>Pna</i>2₁ but with a metric symmetry that precludes true <i>Pna</i>2₁ 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.