Under ambient conditions resorcinol (Res), C6H4(OH)2, favorably crystallizes from methanol and aqueous solutions as the anhydrate, in the form of polymorph α at room temperature. Anhydrous polymorph β can be obtained above 360 K. However, above 0.80 GPa the monohydrate Res·H2O is formed from the aqueous solution. The monohydrate is less stable than the duotritohydrate 3Res·2H2O, which nucleates later. The latter forms a tight passivation layer on the surface of monohydrate crystals and protects them from dissolution. Between 0.20 and 1.0 GPa the duotritohydrate is more favored than the previously reported Res polymorphs α and β. From a methanol solution above 0.40 GPa the methanol monosolvate Res·CH3OH precipitates. In Res·H2O resorcinol molecules assume the syn-syn conformation, and in 3Res·2H2O independent syn-syn and anti-anti conformers are present. The anti-anti molecule is orientationally disordered, despite the fact that usually the disorder requires extra space, while the high pressure suppresses the volume. In all three new solvates, the solvent molecules mediate the H bonding between the hydroxyl groups. The formation of solvates can be rationalized by the low potential energy of syn-syn conformers as well as the volume gain of the solvates in comparison to the summed volumes of the pure resorcinol crystal and stoichiometric amounts of the solvent. The strong preference of the analogous orcinol (5-methylresorcinol) for the monohydrate formation under normal conditions is unchanged under high pressure.
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