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

## TL;DR

This paper studies how two similar chemical compounds change their crystal structures under low temperature and high pressure.

## Contribution

The study reveals new insights into phase transitions involving twinning and incommensurate modulation in halobenzophenones.

## Key findings

- 2-amino-4′-bromobenzophenone transitions to a monoclinic phase with nonmerohedral twinning at low temperatures.
- 2-amino-4′-chlorobenzophenone transitions to an incommensurately modulated phase upon cooling.
- Phase transitions are linked to hydrogen bonding network rearrangements and conformational changes.

## Abstract

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.