Structural Features of the Thymol–Carvacrol Equimolar Mixture: X‑Ray Scattering and Molecular Dynamics

We present a structural characterization of a low-transition-temperature
mixture (LTTM), consisting of thymol and carvacrol, at an equimolar
ratio. Carvacrol and thymol are natural regioisomers of terpenes.
When combined at an equimolar ratio, they form a liquid mixture at
room temperature, with supercooling capability and glass transition
at ca. 210 K. Using small- and wide-angle X-ray scattering and molecular
dynamics, we describe the structural complexity within this system.
X-ray scattering reveals a low-Q peak at around 0.6 Å–1, indicating the existence of mesoscale structural heterogeneities,
likely related to the segregation of polar moieties engaged in hydrogen
bond (HB) interactions within an aromatic, apolar matrix. These polar
interactions are predominantly a result of HBs involving thymol as
the HB donor species. The liquid structure is also driven by O–H···π
interactions, prevalently due to the ability of the carvacrol π-site
to engage in this type of weak interaction as a HB acceptor. Besides,
dispersive interactions affect the local arrangement of molecules,
with a propensity of carvacrol rings to orient their first neighbors
with a perpendicular orientation, while thymol tends to induce a closer
approach of other thymol molecules with a preferential parallel alignment.
Overall, we observed a complex structural arrangement driven by the
interplay of both conventional and weak hydrogen bond interactions,
with the aromatic nature of the compounds playing a pivotal role in
shaping the system’s architecture. Carvacrol and thymol, despite
being very similar compounds, are characterized by distinctly different
behavior in terms of the interactions they engage in with their neighbors,
likely due to the different steric hindrance experienced by their
hydroxyl groups, which are close to either a small methyl or a bulky
isopropyl group, respectively. Such observations can provide useful
hints to develop new solvents with tailored properties.