Dynamics of molecules and phase transitions in the crystals of pure and binary mixtures of n-paraffins

Abstract

In the present paper, we report on temperature dependent FT-IR spectra studies of the value of Davydov splitting for CH 2 rocking vibrations of pure crystalline n-paraffins C n H 2 n+2 (with odd number n of carbon atoms) and some isomorphic substituted binary mixtures of n-paraffins C 22/C 24. It was shown that the temperature dependencies of Davydov splitting value are characterized by the number of irregularities (sharp decreasing), which correspond to phase transitions into the high-temperature (hexagonal) state for pure n-paraffins or different rotator crystalline states for the mixtures. A statistic and dynamic model is proposed which provides adequate description of the observed effect. In the framework of this model, two different mechanisms are responsible for the temperature behavior of the splitting value of vibrational modes. In addition to thermal expansion of crystals under heating, the damping of vibrational excitons on orientational defects of different nature takes place breaking the translational symmetry of the crystal lattice. Genesis of such defects is connected with excitation of librational and molecular rotational degrees of freedom at the crystal polymorphic transitions into different rotator crystalline states. Theoretical analysis of the effect of resonance dynamical intermolecular interaction on the spectra of intramolecular vibrations of the crystals was performed in terms of stochastic equations with account for mentioned phase transitions. Computer simulation of such dependence was performed for pure n-paraffin compounds. Good agreement between the experimental and computer simulation results was obtained.