Correlation between quasielastic Raman scattering and configurational entropy in an ionic liquid

Low-frequency (5-200 cm(-1)) Raman spectra are reported for the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate, [bmim]PF6, in glassy, supercooled liquid, and normal liquid phases (77-330 K). Raman spectra of [bmim]PF6 agree with previous results obtained by optical Kerr effect spectroscopy and molecular dynamics simulation. Both the superposition model and the coupling model give reasonable fit to low-frequency Raman spectra of [bmim]PF6. The configurational entropy of [bmim]PF6 has been evaluated as a function of temperature using recently reported data of heat capacity. The calculated configurational entropy is inserted in the Adam-Gibbs theory for supercooled liquids, giving a good fit to non-Arrhenius behavior of viscosity and diffusive process, with the latter revealed by a recent neutron scattering investigation of [bmim]PF6. There is a remarkable linear dependence between intensity of quasielastic Raman scattering and configurational entropy from 77 K up to the melting point of [bmim]PF6. This correlation offers insight into the nature of dynamical processes probed by low-frequency Raman spectra of ionic liquids.