Pressure-Dependent Crystallization Preference of Resorcinol Polymorphs

Polymorph α of resorcinol, at ambient pressure stable to 365 K when it transforms to polymorph β, is exceptionally resistant to high pressure. The crystals of polymorph α can be compressed to over 4 GPa without transforming to the β phase. We have performed high-pressure recrystallization of resorcinol aqueous and methanol solutions, and they yielded polymorph α below 0.5 GPa and polymorph β above this pressure. Our single-crystal X-ray diffraction studies on resorcinol polymorphs in a diamond-anvil cell reveal the structural origins of the phase transition. The high pressure changes the angular dimensions of bistable hydrogen bonds OH···O, which destabilizes the H-atoms and the structure of polymorph α above 0.5 GPa, consistent with the calorimetric and NMR results. The high-temperature, high-pressure polymorph β achieves the more dense packing through the changed conformation of one of the hydroxyl groups and the considerable twisting of the hydrogen bonds necessary for the formation of additional C–H··...