Molecular and group relaxation studies in parasubstituted benzenes in various media

Abstract

Dielectric absorption studies have been made on molecules of the type [Formula: see text][Formula: see text] where X—Y is —OCH3, —COCH3, —CH2Cl, and —CH2CN. The molecules have been dissolved in (a) a polystyrene matrix and (b) in the viscous liquid o-terphenyl. For 1,4-dimethoxybenzene the group process dominates in both media, and estimates have been made of the activation enthalpy for group relaxation. For 1,4-dimethoxybenzene in benzene, decalin, Nujol, and o-terphenyl solutions, however, there appears to be a small contribution from the molecular process. No such contribution was detected in the matrix studies on this solute molecule. For 1,4-diacetylbenzene, 1,4-bis(chloromethyl)benzene, and 1,4-bis(cyanomethyl)benzene in o-terphenyl solution it again seems that the group and molecular processes overlap. However, in polystyrene the enthalpies of activation which emerge are characteristic of the group relaxation process as determined by the nmr approach. Altogether, the work illustrates that o-terphenyl may be a useful solvent in helping to decide whether a solute has contributions from an intramolecular motion.