Dielectric Studies of Factors Influencing the Energy Barriers to Molecular and Group Reorientation

Abstract

Dielectric relaxation times and energy barrier parameters of polar organic molecules were determined in dilute solution at microwave frequencies using a coaxial line apparatus, which has not previously been employed for low loss measurements, and a bridge technique. A study has been made of factors affecting the reorientating processes of rotatable polar groups substituted mainly on aromatic molecules, where two mechanisms, molecular and intramolecular, are present. Previous work has shown that acetyl] and methoxy groups possess a mobility which reduces the observed relaxation time to a value below that of molecular relaxation. The size of reduction depends on the resultant dipole moment direction, as illustrated by the results of the p-halo and p-alkylacetophenones, and also on the magnitude of the intramolecular relaxation time, a tendency was observed for acetyl reorientation to lengthen with increasing conjugation to the parent molecule. Viscous solution studies further clarified these intramolecular processes. Placing a single chlorine atom ortho to an acetyl group was found to increase its mobility, while inserting two neighbouring groups, as in 2,4,6-trimethylacetophenone and 9-acetylanthracene appeared to prevent acetyl rotation completely. Of the o-substituted alkoxy benzenes examined an intramolecular process was only detected in o-fluoroanisole. Measurement of cholesteryl- methylether provided an accurate value of the aliphatic methoxy relaxation time (and, therefore, a minimum value for the aromatic parameter), since complete separation of molecular and intramolecular absorption occurred. A similar, but smaller study to the acetyl compounds was carried out on aldehyde derivatives. Analysis of results proved difficult as the aldehydes' dielectric behaviour was found to be dependent on the nature of the solvent. An internal mechanism was detected in the p-halobenzaldehydes, but not in benzaldehyde nor 2-naphthaldehyde. A series of ortho and meta halo substituted iodobenzenes were measured for comparison with the analogous acetyl and alkoxy derivatives, and the results, with others, used to suggest a possible correlation between a molecule's moment of inertia and its relaxation time.