Speaker
Description
Pressure provides a clean way to drive phase transitions in molecular liquids by primarily changing intermolecular distances.[1] In this poster, we discuss how chirality controls the pressure-induced liquid--solid transition of $\alpha$- and $\beta$-pinene by combining high-pressure Raman microscopy, crossed-polarized imaging, X-ray powder diffraction, and density functional theory (DFT), an approach well suited to high-pressure structural studies of molecular materials.[2] The main result is that chirality strongly affects the solidification pathway: enantiopure liquids crystallize under compression, whereas the corresponding racemic mixtures solidify without developing long-range order. For the enantiopure systems, the emergence of sharp low-frequency phonons, birefringence, and Bragg reflections is consistent with a liquid-to-crystal transition.[3,4] In contrast, the racemic samples show clear pressure-induced spectral changes and solidification, but no unambiguous spectroscopic signature of crystallinity, pointing instead to an amorphous or glass-like state.
To rationalize these observations, DFT calculations were carried out in both the gas phase and the solid state. Gas-phase calculations on isolated clusters were used to probe local intermolecular arrangements and support vibrational band assignments in the liquid, while periodic solid-state calculations under pressure were used to optimize crystal structures and reproduce the Raman-active phonons of the compressed solids. Taken together, experiment and theory suggest that chirality is not a secondary structural detail, but a determining factor in the phase-transition process. Mixing opposite enantiomers introduces competing local arrangements that can alter prenucleation association and slow down nucleation, thereby hindering the emergence of long-range order, whereas enantiopure liquids can reorganize more cooperatively into crystalline phases.[5,6] These results highlight chirality as a key parameter for understanding and controlling pressure-induced solidification in weakly interacting molecular liquids.
References:
[1] H.-K. Mao et al., Rev. Mod. Phys. 90, 015007 (2018).
[2] E. V. Boldyreva et al., J. Phys.: Conf. Ser. 121, 022023 (2008).
[3] K. Bērziņš et al., Int. J. Pharm. 592, 120034 (2021).
[4] C.-F. Chang et al., J. Phys. Chem. C 118, 2702–2709 (2014).
[5] C. Brandel et al., Chem. Eur. J. 22, 16103–16112 (2016).
[6] L. C. Harfouche et al., CrystEngComm 23, 8379–8385 (2021).