Solid Phase Transitions of the Cyclohexane Derivatives and the Model of Energy States of Molecules in Plastic Crystals

The present paper summarizes the results of the measurements of thermodynamic properties of phase transitions of some cyclohexane derivatives in the condensed state and provides the analysis of the changes of dynamic behaviour of molecules during transitions from rigid crystal to plastic crystalline state. These results are discussed on the basis of the main trends of thermodynamic properties of solid state transitions in the series of the compounds having the similar structure. The analysis of the data obtained allows to develop new concept of energy states of molecules in plastic crystals. The combination of the results of calorimetric and spectral measurements with the data obtained by the method of statistical thermodynamics for the series of cyclohexane derivatives leads to the conclusion of the existence of internal rotation, ring inversion, and rotation of molecules as a whole in the nodes of the plastic crystal lattice.