Dielectric Studies of Organic Molecules and the Determination of Energy Barriers

Abstract

The relaxation times of polar molecules have been
determined by the measurement of the dielectric absorption of
dilute solutions of the polar solute molecules in a non-polar
solvent at microwave frequencies, by a bridge method. The
study was applied to molecules that contain rotating volar
groups so that a group, as well as a molecular, relaxation
time was evaluated. By measuring the relaxation times at
several temperatures the enthalpies of activation to molecular
and group rotation were determined.

Among the compounds studied was acetophenone, which
was examined in three solvents, and for which an energy barrier
to acetyl group rotation of approximately 2 kcal/mole was
obtained. Several other aromatic and also aliphatic molecules
containing acetyl groups were investigated and the resulting
relaxation time data correlated with the structures of the
compounds. An intensive study was also carried out on
aromatic molecules containing methoxy groups and the way in
which the group relaxation time and energy barrier to rotation
was affected by other substituents in the benzene ring noted.

Dielectric studies have also been made on molecules
of the type X-C,H,-X, where X is a rotating polar group, and
for these only group rotation is observed. Since group
relaxation is the only relaxation mechanism the relaxation
time can be determined with an accuracy of + 5% whereas when
molecular and group relaxation processes are both taking place the accuracy is approximately + 10% on the relaxation times.

In addition to the derivation of the relaxation times
of polar molecules the dipole moments have also been determined
and these include several for which no previous value existed.
The derived dipole moments have proved useful in assessing
the distribution of charge within the molecules.

The possible causes of the short relaxation times
of diphenylether and its substituted derivatives are discussed.
Selected molecules were chosen for study so that an estimation
of the most likely causes could be made.

Date of Award	1967
Original language	English
Awarding Institution	Aston University