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 fitte d with two relaxation processes. The long relaxation time, tau(1)(T), is eq ually well described by the Vogel-Fulcher-Tammann and the mode-coupling the ory. However, the parameters differ from literature data for propylene carb onate based on the frequency of maximum dielectric loss. This low-frequency dispersion step is attributed to the essentially isotropic rotational diff usion of the molecular dipole vector with the carbonates exhibiting some in ter-molecular association. For butylene carbonate the fast relaxation proce ss, tau(2) approximate to 2 ps, partly arises from the rotation of the ethy l side chain. However, for all three liquids the observed high-frequency co ntribution in the spectra reflects the transition from molecular dynamics g overned by free rotation to long-time rotational diffusion.