Indications for change of dynamical regime in pure liquids

Abstract

¹³C NMR spin-lattice relaxation times T ₁ of quinoline (1) of some of its methyl derivatives (quinaldine (3), lepidine (4), 6-methylquinoline (5), 7-methylquinoline (6)) of isoquinoline (2), and of two quinoline related compounds having two nitrogen atoms in one fused sixmembered ring skeleton (quinazoline (7), quinoxaline (8)) have been measured in pure liquid form as a function of temperature. The temperature range is not the same for all the compounds. It is at least 46 K for (7) and it reaches 93 K for (4). ln T ₁ is plotted as a function of the inverse of the absolute temperature T ⁻¹. For (2), (6), (7) and (8) the data can be accounted for by only one activation energy, for (1), (3), (4) and (5) by two activation energies one above temperature T _t, one below. For all these compounds the high temperature relaxation corresponds to an energy barrier which is less than 20·5 kJ mol⁻¹, and the low temperature process (when a two energy barrier regime exists) corresponds to an energy barrier close to 30 kJ mol⁻¹. For quinoline and isoquinoline, the saturated specific heat C _s plotted as a function of _T exhibits behaviour similar to that of ln T ₁ studied as a function of T ⁻¹. The results reported here suggest the existence of a dynamical transition in the liquid state at a given temperature, a process somewhat similar to that proposed for some super-cooled liquids.