Dynamics of benzonitrile, propylene carbonate and butylene carbonate: the influence of molecular shape and flexibility on the dielectric relaxation behavior of dipolar aprotic liquids

Abstract

The results of dielec. relaxation expts. on benzonitrile, propylene carbonate and butylene carbonate, performed between 228.15 K and 338.15 K in the frequency range 0.1 ≤ ν/GHz ≤ 89 are presented. The spectra of the three liqs. can be formally fitted with two relaxation processes. The long relaxation time, τ1(T), is equally well described by the Vogel-Fulcher-Tammann and the mode-coupling theory. However, the parameters differ from literature data for propylene carbonate based on the frequency of max. dielec. loss. This low-frequency dispersion step is attributed to the essentially isotropic rotational diffusion of the mol. dipole vector with the carbonates exhibiting some inter-mol. assocn. For butylene carbonate the fast relaxation process, τ2 ≈ 2 ps, partly arises from the rotation of the Et side chain. However, for all three liqs. the obsd. high-frequency contribution in the spectra reflects the transition from mol. dynamics governed by free rotation to long-time rotational diffusion.