Non-coincidence effect and orientational dynamics in aromatic molecules

The ring breathing vibrations of aromatic molecules show an unusual non-coincidence effect, due to a poorly defined resonant coupling. This kind of non-coincidence effect has been compared within and between two couples of isotopomers, light and deuterated toluene and o-xylene, in pure substances and in dilutions with CCl4, in a temperature range of 282-350 K. Analogous trends are shown between the physical and molecular properties, such as single particle orientational times, hydrodynamic volumes and the non-coincidence effect itself. For each couple, the vibrational and rotational dynamics, together with structural properties, can be described within a unique framework. For the first time, the dynamic trends of similar isotopomer couples have been explored, showing obvious similarities and unsuspected differences between and within each couple of the chosen molecular liquids. In particular, the hydrodynamic volume values have been calculated and interpreted within the free volume theory.