Analysis of Dielectric Loss in Two Urea Addition Compounds : The Hindered Single Axis Polar Rotator

Abstract

The dielectric behavior of two real substances is related to their molecular structure by the application of a theory of the general type proposed by Hoffman. The two real substances are the urea addition compounds of 12-bromotricosane and 16-hentriacontanone, where the polar long-chain molecules exhibit dielectric loss through rotation about their long molecular axes between orientational sites. This dielectric loss has been measured by Meakins, who proposed a qualitative theory similar to the one used here. Smith has determined the crystal structure of the urea addition lattice. The orientational sites of the polar molecules are determined by the potential energy of the crystalline field, which is estimated from the geometrical configuration of the lattice by a Lennard-Jones (6–12) potential. The dielectric loss is calculated in terms of the energy minima and maxima of the potential energy. The energy minima and the differences between the energy maxima are evaluated by a fit of the experimental dielectric loss of 12-bromotricosane in urea. These values are in good qualitative agreement with the potential energy obtained from the crystal structure. The temperature dependence of the dielectric properties of 12-bromotricosane in urea are predicted under certain assumptions. The validity of these assumptions and the applicability of the theory are investigated by considering the dielectric loss of 16-hentriacontanone in urea over a wide temperature range.