About the nature of phase transition in pure n-paraffin crystals

Abstract

It is known that in long-chain aliphatic crystals a first order phase transition from three-dimensional translationally symmetric phase to one-dimensional translationally symmetric (hexagonal) one may occur in the temperature range below the melting point. During this transition, the molecules might be involved in torsion rotations around their long axes. It is clear that any distortions in periodic location of the atoms in the plane perpendicular to the long axis of the molecules must result in some peculiarities in the intermolecular interactions. In particular, one could expect changes in resonance dynamic intermolecular interaction (Davydov splitting of vibrational excitons) in orientationally disordered phase. Theoretical analysis of manifestation of resonance dynamical intermolecular interaction in the spectra of intramolecular vibrations in these crystals has been carried out in terms of stochastic equations with taking into consideration the above mentioned phase transition. We obtained the explicit expression for theoretical dependence of Davydov splitting value on temperature. We showed that at the transition to the high-temperature (hexagonal) phase, the absorption bands, which correspond to Davydov splitting components rapidly approach each other. This spectral effect is connected to the damping of vibrational excitons at their interaction with orientational defects in the crystal lattice. Such defects arise due to the thermal activation of molecular torsional-rotation degrees of freedom during the crystal transition to one-dimensional phase. Computer simulation of such dependence has been performed for some aliphatic compounds. Good agreement between the experimental and computer simulation results has been obtained. Theoretical approach developed in the present paper for resonance dynamical intermolecular interaction near the transition from three-dimensional to one-dimensional phase of crystalline n-paraffins is of general nature and may be applied to the description of some specific features observed in the vibrational spectra of rotary crystals.