Absorbing a Little Water: The Structural, Thermodynamic, and Kinetic Relationship between Pyrogallol and Its Tetarto-Hydrate

The anhydrate and the stoichiometric
tetarto-hydrate of pyrogallol
(0.25 mol water per mol pyrogallol) are both storage stable at ambient
conditions, provided that they are phase pure, with the system being
at equilibrium at aw (water activity)
= 0.15 at 25 °C. Structures have been derived from single crystal
and powder X-ray diffraction data for the anhydrate and hydrate, respectively.
It is notable that the tetarto-hydrate forms a tetragonal structure
with water in channels, a framework that although stabilized by water,
is found as a higher energy structure on a computationally generated
crystal energy landscape, which has the anhydrate crystal structure
as the most stable form. Thus, a combination of slurry experiments,
X-ray diffraction, spectroscopy, moisture (de)­sorption, and thermo-analytical
methods with the computationally generated crystal energy landscape
and lattice energy calculations provides a consistent picture of the
finely balanced hydration behavior of pyrogallol. In addition, two
monotropically related dimethyl sulfoxide monosolvates were found
in the accompanying solid form screen.