Low-density preference of the ambient and high-pressure polymorphs of dl-menthol

Lower-density polymorphs of dl-menthol were nucleated and crystallized in their high-pressure stability regions. Up to 0.30 GPa, the triclinic dl-menthol polymorph α, which is stable at atmospheric pressure, is less dense than a new β polymorph, which becomes stable above 0.40 GPa, but is less dense than the α polymorph at this pressure. The compression of polymorph α to at least 3.37 GPa is monotonic, with no signs of phase transitions. However, recrystallizations of dl-menthol above 0.40 GPa yield the β polymorph, which is less compressible and becomes less dense than α-dl-menthol. At 0.10 MPa, the melting point of the β polymorph is 14°C, much lower compared with those of α-dl-menthol (42–43°C) and l-menthol (36–38°C). The structures of both dl-menthol polymorphs α and β are very similar with respect to the lattice dimensions, the aggregation of OH⋯O molecules bonded into C (i) symmetric chains, the presence of three symmetry-independent molecules (Z′ = 3), their sequence ABCC′B′A′, the disorder of the hydroxyl protons and the parallel arrangement of the chains. However, the different symmetries relating the chains constitute a high kinetic barrier for the solid–solid transition between polymorphs α and β, hence their crystallizations below or above 0.40 GPa, respectively, are required. In the structure of polymorph α, the directional OH⋯O bonds are shorter and the voids are larger compared with those in polymorph β, which leads to the reverse density relation of the polymorphs in their stability regions. This low-density preference reduces the Gibbs free-energy difference between the polymorphs: when polymorph α is compressed to above 0.40 GPa, the work component pΔV counteracts the transition to the less dense polymorph β, and on reducing the pressure of polymorph β to below 0.40 GPa, its transition to the less dense polymorph α is also hampered by the work contribution.