Phenol hemihydrate: redetermination of the crystal structure by neutron powder diffraction, Hirshfeld surface analysis and characterization of the thermal expansion.

Phenol hemihydrate, C5H5OH·0.5H2O, crystallizes in the space group Pbcn, Z = 8. The previously published crystal structure [CSD refcode PHOLHH; Meuthen & von Stackelberg (1960 ▸). Z. Elektrochem. 64, 387-390] is shown to be incorrect. Pairs of phenol mol-ecules, related by an inversion centre, are bridged by one water mol-ecule via O-H⋯O hydrogen bonds; an extended R 4 4(8) hydrogen-bonded motif links these inversion dimers into chains parallel to the c axis. Packing of the chains is achieved by weaker T-shaped C-H⋯π inter-actions between nearest neighbour phenol mol-ecules in the bc plane. Analysis of the thermal expansion and parameterization with a Debye model in terms of the linear elastic moduli shows that the c axis is ∼3 times stiffer than the two orthogonal directions.