INDICATIONS FOR CHANGE OF DYNAMICAL REGIME IN PURE LIQUIDS

C-13 NMR spin-lattice relaxation times T-1 of quinoline (1) of some of its methyl derivatives (quinaldine (3), lepidine (4), 6-methylquinoli ne (5), 7-methylquinoline (6)) of isoquinoline (2), and of two quinoli ne related compounds having two nitrogen atoms in one fused six-member ed ring skeleton (quinazoline (7), quinoxaline (8)) have been measured in pure liquid form as a function of temperature. The temperature ran ge is not the same for all the compounds. It is at least 46 K for (7) and it reaches 93 K for (4). In T-1 is plotted as a function 1 For (2) , (6), (7) and (8) the data can be of the inverse of the absolute temp erature T-1. For (2), (6), (7) and (8) the data can be accounted for b y only one activation energy, for (1), (3), (4) and (5) by two activat ion energies one above temperature T-t, one below. For all these compo unds the high temperature relaxation corresponds to an energy barrier which is less than 20.5 kJ mol(-1) and the low temperature process (wh en a two energy barrier regime exists) corresponds to an energy barrie r close to 30 kJ mol(-1). For quinoline and isoquinoline, the saturate d specific heat C-s plotted as a function of T exhibits behaviour simi lar to that of ln T-1 studied as a function of T-1. The results report ed here suggest the existence of a dynamical transition in the liquid state at a given temperature, a process somewhat similar to that propo sed for some supercooled liquids.