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

The results of dielectric relaxation experiments on benzonitrile, propylene carbonate and butylene carbonate, performed between 228.15 K and 338.15 K in the frequency range 0.1 less than or equal to nu/GHz less than or equal to 89 are presented. The spectra of the three liquids can be formally fitted with two relaxation processes. The long relaxation time, tau(1)(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 maximum dielectric loss. This low-frequency dispersion step is attributed to the essentially isotropic rotational diffusion of the molecular dipole vector with the carbonates exhibiting some inter-molecular association. For butylene carbonate the fast relaxation process, tau(2) approximate to 2 ps, partly arises from the rotation of the ethyl side chain. However, for all three liquids the observed high-frequency contribution in the spectra reflects the transition from molecular dynamics governed by free rotation to long-time rotational diffusion.