Pressure-controlled aggregation in carboxylic acids. A case study on the polymorphism of bromochlorofluoroacetic acid

Pressure induces different hydrogen-bonding patterns in the polymorphs of bromochlorofluoroacetic acid, CBrClFCOOH, by affecting the balance between secondary intermolecular interactions involving halogen and oxygen atoms. In polymorph α a pattern of the molecules syn–syn H-bonded into catemers is strongly corrugated, up to the limit imposed by steric hindrances between the neighboring chain members, whereas in polymorph β the molecules are H-bonded into dimers. No phase transition between the catemeric and dimeric CBrClFCOOH polymorphs, despite the over-pressurizing phase α by over 1.3 GPa into the stability region of phase β, demonstrates that the preference for dimeric and catemeric forms of carboxylic acids may be impossible for detection as classical solid-state phase transitions, without completely dissolving or melting these compounds and avoiding their nucleation. The smaller volume of the β phase, and hence its high-pressure stability, has been rationalized by more freedom of the 0-D dimers to adjust their positions in the crystal structure, compared to the 1-D catemers. The conformational limitations of the carboxylic-acid aggregates are consistent with the survey of all carboxylic-acid structures determined so far.