Deciphering the hydrogen-bonding scheme in the crystal structure of triphenylmethanol: a tribute to George Ferguson and co-workers.

The crystal structure of triphenylmethanol, CHO, has been redetermined using data collected at 295 and 153 K, and is compared to the model published by Ferguson et al. over 25 years ago [Ferguson et al. (1992). Acta Cryst. C48, 1272-1275] and that published by Serrano-González et al., using neutron and X-ray diffraction data [Serrano-González et al. (1999). J. Phys. Chem. B, 103, 6215-6223]. As predicted by these authors, the hydroxy groups are involved in weak intermolecular hydrogen bonds in the crystal, forming tetrahedral tetramers based on the two independent molecules in the asymmetric unit, one of which is placed on the threefold symmetry axis of the R-3 space group. However, the reliable determination of the hydroxy H-atom positions is difficult to achieve, for two reasons. Firstly, a positional disorder affects the full asymmetric unit, which is split over two sets of positions, with occupancy factors of ca 0.74 and 0.26. Secondly, all hydroxy H atoms are further disordered, either by symmetry, or through a positional disorder in the case of parts placed in general positions. We show that the correct description of the hydrogen-bonding scheme is possible only if diffraction data are collected at low temperature. The prochiral character of the hydrogen-bonded tetrameric supramolecular clusters leads to enantiomorphic three-dimensional graphs in each tetramer. The crystal is thus a racemic mixture of S and R motifs, consistent with the centrosymmetric nature of the R-3 space group.