Dielectric Studies of Organic Molecules and Energy Barriers for Conformational Changes

Abstract

A bridge method has been used to measure the dielectric absorption of liquids and solutions at microwave frequencies. The dielectric data are discussed in terms of apparent dipole moments and relaxation processes contributing to the dielectric absorption. The relaxation times of some heterocyclic compounds are shown to be dependent upon the nature of the solvent, and the degree of shielding of the heterocyclic nitrogen atom. For quinoline, isoquinoline and phthalazine the relaxation time is independent of the direction of the molecular dipole moment, and molecular re-orientation occurs mainly by rotation about the axis perpendicular to the plane of the ring. The dipole moments of 1,4-thioxan, 1,4-dithian and1,4-dioxan indicate a small proportion of a boat form, the dielectric absorption of the first is characterised by two relaxation times, one of which may be associated with the interconversion of two conformational isomers. For cyclopentanone, cycloheptanone and cyclooctanone the dipole moments are smaller than those obtained from classical methods, which suggests that they are flexible rings, and for cyclopentanone a second relaxation process is identifiable with an interconversion between pseudo rotational arrangements of the molecule. A series of haloethanes have been measured in cyclohexane and p-xylene solutions and the relaxation times show them to form weak intermolecular bonds with the latter. The distribution coeficients observed, using both solvents, for the di- and trisubstituted compounds may be associated with intramolecular relaxation. Several symmetrically substituted alkylbenzenes are found to absorb in the microwave region and possible explanations are forwarded. These results appear to offer an alternative to the hypothesis that an intramolecular relaxation process contributes to the dielectric absorption of some monoalkylbenzenes. A detailed study of the latter confirms the existence of a short relaxation time process, and the results have been used to critically examine the intramolecular process and alternative explanations are considered,