Dielectric relaxation studies of solid solutions of monofluorobenzaldehydes in a polymer: eyring parameters for aldehyde group rotation

Abstract

The Eyring parameters for the aldehyde group rotation in monofluorobenzaldehyde molecules dispersed in a solid polymer matrix, as determined from dielectric relaxation studies, are discussed. AM1 self-consistent field molecular-orbital calculations, with full geometry optimization, are reported for isolated monofluorobenzaldehyde molecules and their rotamers. The entropy of activation for the aldehyde group rotation in the isolated molecules is negative, and almost the same for ortho, meta and para isomers. In contrast the values obtained experimentally for the solid solutions are positive and isomer dependent. Thus, the results obtained from dielectric relaxation studies on solids cannot be used uncritically in interpreting the dynamics of intramolecular processes in gas-phase molecules. A corresponding discrepancy is found for the enthalpies of activation of internal rotation. The differences between the observed (solid-state) behaviour and the calculated (gas-phase) behaviour can be accounted for by assuming a pre-equilibrium between ‘normal’ and ‘expanded’ cavities in the polymer matrix.