Excitations Follow (or Lead?) Thermodynamic Scaling in Propylene Carbonate

Structural excitations that enable interbasin (IB) barrier crossings on a potential energy landscape (PEL) are thought to play a facilitating role in the relaxation of liquids. Here we show that the population of these excitations exhibits the same thermodynamic scaling observed for α relaxation in propylene carbonate (PC), even though they are heavily influenced by intramolecular modes. We also find that IB crossing modes exhibit a Gruneisen parameter (γG) that is approximately equivalent to the thermodynamic scaling parameter γTS. This observations suggests that the well-documented relationship between γG and γTS may be a direct result of the pressure dependence of the frequency of unstable (relaxation) modes associated with IB motion.