Calorimetric, infrared and ESR studies of the plastically crystalline state of organic compounds

Abstract

Enthalpy changes of the crystal-plastic and plastic-liquid transitions are related to the temperature range of stability of the plastic phase. Thermodynamics of the plastic state of binary mixtures have been examined. Infrared correlation times, τ_c, and activation energies have been measured for a few molecules in the plastic state. Molecular tumbling times, τ_t, have also been measured employing ESR spectra of a spin-probe. Plots of log τ_c(τ_t) versus 1/T are continuous through the plastic-liquid transition. Activation energies for molecular motion seem to vary in the same direction as the ΔH of the plastic-crystal transition. Infrared correlation times of solute molecules in binary solutions in the plastic and the liquid states show interesting variations with solute concentration.