Thermodynamic Studies of Orientational Disorder in π-Molecular Compounds

Abstract

The heat capacities and thermodynamic functions from 5 to 300 K for naphthalene-pyromellitic dianhydride (N-PMDA) and pyrene-pyromellitic dianhydride (P-PMDA) are reported. From the featureless heat-capacity curve of N-PMDA, it has been concluded that the naphthalene moieties executed anharmonic librations in a thermodynamically-ordered crystal and not reorientational motion in a disordered crystal. Two minor anomalies have been observed in the heat capacity of P-PMDA. A small peak near 155 K may arise from the disordering of the PMDA molecules while an irreproducible transition observed near 250 K has been tentatively ascribed to a disordering motion of the pyrene molecules. The agreement of the P-PMDA heat capacity results of this study with previously-published data is discussed.