Unravelling the Disordered Hydrogen Bonding Arrangement in Solid Triphenylmethanol

In the crystal structure of triphenylmethanol (Ph3COH), the molecules form hydrogen-bonded tetramers, with the oxygen atoms positioned approximately at the corners of a tetrahedron. In the average crystal structure, three of the Ph3COH molecules (denoted as “basal”) in the tetramer are related to each other by a 3-fold rotation axis and the fourth molecule (denoted as “apical”) lies on this axis. Previously, the dynamic properties of the hydrogen-bonding arrangement have been probed through solid-state 2H NMR spectroscopic investigations of Ph3COD. There are two dynamically distinguishable types of hydroxyl hydrogen atoms (deuterons), denoted d and d3, with relative populations in the ratio 1:3. Each d1 deuteron undergoes three-site 120° jumps by rotation about the C−OD bond, with equal populations of the three sites involved in this motion. Each d3 deuteron undergoes two-site 120° jumps by rotation about the C−OD bond, with equal populations of the two sites involved in this motion. In the present paper, we focus on structural characterization of the hydrogen-bonding arrangement in solid triphenylmethanol. Single-crystal neutron diffraction has been used to determine accurate positions of the hydroxyl hydrogen atoms in the hydrogen-bonding arrangement, single-crystal X-ray diffraction has extended previous structural studies across a wider temperature range, and high-resolution solid-state 13C NMR spectroscopy has provided insights into local structural aspects of the hydrogen-bonded tetramers. Importantly, the neutron diffraction results show that, for both the apical and basal molecules in the average crystal structure, the hydroxyl hydrogen atoms are disordered among three sites, corresponding closely to the geometries expected for O−H···O hydrogen bonds. The populations of the three hydroxyl hydrogen sites for each molecule are statistically equal, and there is thus an apparent dilemma in bringing together information on the spatial distribution of the hydroxyl hydrogens conveyed by the average crystal structure and information on the spatial distribution of the hydroxyl hydrogens implicated by the dynamic model determined from 2H NMR spectroscopy. Through a detailed discussion of the different ways in which structural information is averaged in the different techniques (neutron diffraction, X-ray diffraction, 2H NMR, and 13C NMR) that have been applied in our studies of solid triphenylmethanol, these apparent difficulties can be completely reconciled