The unusual phases of anhydrous and hydrated pinacol.

The structure of highly twinned pinacol (2,3-dimethyl-2,3-butanediol) monohydrate, the existence of which has been known since 1922, has been determined, and the structures of anhydrous pinacol and its two other known hydrates have been reinvestigated. All the phases are unusual. The anhydrous phase is exceptional among molecular crystals in having molecules located on three different symmetry sites (1, \overline 1 and 2). A hexagonal form of pinacol originally described as a second polymorph has been shown to be a solvate of uncertain composition that is very loosely packed. Pinacol hexahydrate, which was originally reported as tetragonal and highly disordered, appears to be described better as having an orthorhombic structure that is both disordered and twinned; the diffraction pattern at 90 K shows structured diffuse scattering that suggests short-range correlations of disordered molecules. The occurrence of this unusual set of structures is attributed to the combination of the hydrogen-bonding requirements of the pinacol molecule with its small size and limited conformational flexibility.