Resolving the Arrhenius Paradox by Isochoric Analysis of Rotational Barriers in Molecular Glasses

The Arrhenius model has long provided a framework for describing thermally activated processes in solids, linking temperature to relaxation dynamics through a fixed activation barrier. Yet in molecular glasses, it routinely produces unphysically low preexponential factors, τ_{0}^{*}, raising fundamental questions about the physical meaning of the extracted parameters. Here, we resolve this long-standing paradox by showing that the activation energy decreases linearly with temperature as a consequence of density-driven variations of the barrier. Using a constant-volume approach combining broadband dielectric spectroscopy with pressure-volume-temperature analysis, we separate thermal and density contributions and directly quantify energy barriers. The results confirm theoretical predictions made fifty years ago but never experimentally verified. Neglecting the temperature dependence of activation energy systematically inflates barrier heights and drives anomalously low τ_{0}^{*} in conventional Arrhenius fits, leading to misleading conclusions about glassy dynamics. This effect is observed for both intramolecular secondary relaxations and intermolecular processes of the Johari-Goldstein relaxation type, highlighting the generality of the mechanism. Our Letter sets a new standard for interpreting secondary relaxation processes and provides a physically justified framework for analyzing thermally activated dynamics across a wide range of amorphous materials.