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

Phenol hemihydrate, C 5 H 5 OH·0.5H 2 O, 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 molecules, related by an inversion centre, are bridged by one water molecule 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 interactions between nearest neighbour phenol molecules 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.