On energy models of orientational disorder of molecules in plastic crystals

A model approach to the description of the orientational disorder of molecules in plastic (orientationally disordered) crystals has been presented in this paper. It is based on the use of the thermodynamic parameters of the transition from ordered to plastic crystal—TtrsTtrs, ΔtrsSΔtrsS, and ΔtrsCpΔtrsCp. Mathematical expressions were derived for connecting them with the number of non-equivalent orientations and the energy parameter of molecular disorder (the energy difference between the lowest and the following neighboring energy level of molecular orientations). A number of essentially different energy distributions of rotational orientations are discussed. The considered models of the energy states of molecules in plastic crystals were verified on the basis of the experimental calorimetric data for a large group of organic compounds.