Innermolekulare Beweglichkeit und dielektrische Relaxation bei drei polaren Molekülgruppen

Abstract

lntermolecular Rotation and, Dielectric Relaxation of Three Polar Groups Benzene derivations mono and para-disubstituted by chloromethyl, methoxy and acetyl groups have been investigated. Their complex dielectric constant was measured in dilute mesitylene solutions from -30° to 70 °C and also in benzene and carbon disulfide solutions at 20 °C for frequencies between 0.3 and 135 GHz. Already, chlorobenzene as a rigid molecule shows un-symmetrical absorption curves with flatter slopes at higher frequencies, where the influence of far infrared (FIR) absorption bands becomes noticeable. This additional absorption at millimetre waves can be approximated by a Debye-term. In a first approximation, its time constant is characterized by solvent and temperature and, therefore, has also been taken into account in deriving the relaxation time distribution from the absorption curves of the other molecules. For the discussion it has been assumed that molecules with free rotating groups in para position should have their absorption maximum at higher frequencies than the mono derivatives. Under these conditions relaxation time distributions of molecules with the CH2Cl-group can be derived from the measured absorption curves in such a way that the microwave absorption alone corresponds to the Budo model of a rotating group. The same interpretation may be given for molecules with the OCH3-group. The acetyl group must be considered rigid. Moreover, the paper discusses the activation energies for group rotation, the very high contribution to the FIR-absorption by the orientation polarisation of the investigated groups and the relaxation times of rigid molecules with different orientation of their dipole moment with respect to the long molecular axis