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

Abstract

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.