Structures of the monofluoro- and monochlorophenols at low temperature and high pressure.

2-Fluorophenol, 3-fluorophenol and 3-chlorophenol were recrystallized from frozen solids at 260, 263 and 283 K. All compounds were also crystallized by the application of high pressure (0.36, 0.12 and 0.10 GPa). While 3-fluorophenol and 3-chlorophenol yielded the same phases under both conditions, different polymorphs were obtained for 2-fluorophenol. 4-Chlorophenol was crystallized both from the melt and from benzene to yield two different ambient-pressure polymorphs; crystallization from the melt at 0.02 GPa yielded the same phase as from benzene at ambient pressure. 3-Fluorophenol is unusual in forming a hydrogen-bonded chain along a 2(1) screw axis. Such behaviour is usually only observed for small alcohols, but here it appears to be stabilized by intermolecular C-H...F hydrogen-bond formation. 3-Chlorophenol is a more typical large alcohol and emulates a fourfold screw axis with two independent molecules positioned about a 2(1) axis, although there are significant distortions from this ideal geometry. The two phases of 4-chlorophenol consist of chains or rings connected by C-Cl...H interactions. The low-temperature and high-pressure polymorphs of 2-fluorophenol consist of chains of molecules connected through OH...OH hydrogen bonds; while inter-chain C-H...F interactions are significant at high pressure, there are none in the low-temperature form.