Dielectric studies of orientational disorder in D- and L-systems of camphor and camphoric anhydride

Abstract

Dielectric properties of low-temperature solid phases of mixtures of optical antipodes of camphor and camphoric anhydride have been measured and related to their phase diagrams determined calorimetrically. Two distinct mechanisms of dipolar relaxation emerge for camphor: one for mixtures with compositions around the pure optical isomer, and the other for those mixtures around the racemic composition. A composite relaxation process involving both orientational and substitutional disordering modified by short-range order effects applies in the case of compositions around the racemic mixture. A simple relaxation process associated with orientational disordering is dominant in the phase transition of mixtures around the optical isomer. The two relaxation processes account for the different patterns of dipolar relaxation phenomena observed. The overall disordering transition has a strong second-order character for compositions around the racemic mixture. In contrast, only one relaxation process involving orientational disordering is dominant for all compositions in binary mixtures D- and L-camphoric anhydride. The transition is predominantly first-order in character for all compositions of D- and L-camphoric anhydride and for mixtures around the pure isomer in camphor. Substitutional disordering appears to be responsible for much of the dielectric absorption observed in low-temperature phases of the binary systems studied. In camphoric anhydride, the presence of a large chirodiastaltic energy leads to phase separation and limited coupling between orientational and substitutional disordering processes.