Dielectric Studies of Molecular Interaction and Group Reorientation

Abstract

Molecules in liquids are constantly reorienting owing to the Brownian Motion. A measure of the time taken for reorientation of a dipolar molecule is the dielectric relaxation time (Τ). If a molecule is non-rigid relaxation time is shorter than expected but if it interacts it is longer than expected. An apparatus for the measurement of the dielectric constant and loss of solutions at microwave frequencies and procedures for analysing the data into one or two relaxation times using a computer are described. The main body of the thesis discusses the relaxation processes of hydroxylic compounds in solutions. If the OH group is not prevented from interacting, its relaxation time is dependent upon the electron donor capacity of the solvent. Phenols with intramolecular hydrogen ponds of weak or medium strength are found to undergo OH group relaxation but those with a strong hydrogen bond are rigid. The data are explained in terms of an asymmetric energy barrier. In molecules having a para nitro substituent mesomeric effects are found to account for the long group relaxation time. Phenols with intramolecular hydrogen ponds should not readily form intermolecular hydrogen bonds. o-Nitrophenol and 8—hydroxyquinoline seem to interact with p-dioxan, and dinitrophenols also with p-xylene*put these interactions do not directly involve the OH group. Phenols with weak or medium intramolecular hydrogen bonds interact strongly with a strong electron donor, 1,4—diazabicyclo-(2,2,2)-octane (DBO). A strong interaction is even found for 2,4,6—trichlorophenol which is intramolecularly hydrogen bonded in both preferred OH configurations. Finally some systems of chloroform with various electron donor molecules were examined with a view to finding formation constants and dipole moments of the complexes. .The method is shown to be potentially valuable. In an appendix, relationships between the mean relaxation time and distribution parameter and the contributing relaxation times and weight factors are considered.