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 3̄ 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.