Realizing predicted crystal structures at extreme conditions: The low-temperature and high-pressure crystal structures of 2-chlorophenol and 4-fluorophenol

A crystal of 2-chlorophenol was grown from the liquid at ambient pressure by laser-assisted zone refinement; 4-fluorophenol was crystallized from ethanol. Different polymorphs were obtained at high pressure by compression of the liquids in a Merrill-Bassett diamond-anvil cell (crystallization pressures 0.12 and 0.28 GPa, respectively). The structures of all phases are characterized by OH---OH hydrogen-bond formation. In the ambient-pressure polymorph of 2-chlorophenol, a hydrogen-bonded chain is formed about a 32 screw-axis; the ambient-pressure phase of 4-fluorophenol contains hexameric rings located on 3 sites. In crystallizing in high-symmetry space groups, these two compounds conform to typical behavior for bulky monoalcohols. By contrast, at high-pressure both compounds form zigzag chains disposed about 21 screw-axes, behavior more characteristic of small monoalcohols. The halophenol moiety thus behaves as a bulky group at ambient pressure but a small group at high pressure. We show that Crystal Structure Prediction methodologies reproduce all four phases, even though the potentials used were developed using ambient-pressure data. This is especially encouraging as the ambient-pressure phase of 2-chlorophenol contains three molecules in the asymmetric unit, while the high-pressure phase of 4-fluorophenol is disordered.