Complex Polymorphic System of Gallic AcidFive Monohydrates, Three Anhydrates, and over 20 Solvates

We report the structure of the fifth monohydrate of gallic
acid
and two additional anhydrate polymorphs and evidence of at least 22
other solvates formed, many containing water and another solvent.
This unprecedented number of monohydrate polymorphs and diversity
of solid forms is consistent with the anhydrate and monohydrate crystal
energy landscapes, showing both a wide range of packing motifs and
also some structures differing only in proton positions. By aiding
the solution of structures from powder X-ray diffraction data and
guiding the screening, the computational studies help explain the
complex polymorphism of gallic acid. This is industrially relevant,
as the three anhydrates are stable at ambient conditions but hydration/dehydration
behavior is very dependent on relative humidity and phase purity.