that was reflective of phenol where the molecules are packed as linear chains at
pressure [80]. However, 3-chloro- and 3-fluorophenol crystallised in the same
polymorph at low temperature and at high pressure. Of the two polymorphs
known for 4-chlorophenol (solid at 293 K), we found that Form II was more stable
at high pressure and transformed to Form I on decompression, but in each case the
structures possess packing attributed to a ‘bulky’ group. 2-Fluorophenol shows very
interesting behaviour at high pressure. It crystallises in a new polymorph at 0.36 GPa
compared with low temperature, but the molecules were arranged in a fourfold helix
Fig. 7 (a) A typical hydrogen-bonded arrangement for molecules with ‘bulky’ R-groups as
displayed by phenol at ambient pressure. Multiple molecules mimic a high-symmetry screw-axis
(this case) or crystallisation of one molecule around a high-symmetry screw-axis (2-chlorophenol);
(b) the high-pressure polymorph of phenol crystallised with one molecule in the asymmetric unit
around a 2 1 -screw axis, typical of the crystallisation of a ‘thin’ R-group; (c) the crystallisation of
methanol at high pressure where it displays another type of packing where there is a 2-1-2-1
arrangement
Crystallography Under High Pressures
161