TY - JOUR

T1 - Dynamics of molecules and phase transitions in the crystals of pure and binary mixtures of n-paraffins

AU - Puchkovska, G. O.

AU - Makarenko, S. P.

AU - Danchuk, V. D.

AU - Kravchuk, A. P.

AU - Baran, J.

AU - Kotelnikova, E. N.

AU - Filatov, S. K.

PY - 2002/9/2

Y1 - 2002/9/2

N2 - In the present paper, we report on temperature dependent FR-IR spectra studies of the value of Davydov splitting for CH2 rocking vibrations of pure crystalline n-paraffins CnH2n+2 (with odd number n of carbon atoms) and some isomorphic substituted binary mixtures of n-paraffins C22/C24. 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.

AB - In the present paper, we report on temperature dependent FR-IR spectra studies of the value of Davydov splitting for CH2 rocking vibrations of pure crystalline n-paraffins CnH2n+2 (with odd number n of carbon atoms) and some isomorphic substituted binary mixtures of n-paraffins C22/C24. 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.

KW - Davydov splitting

KW - IR spectra

KW - n-Paraffin

KW - Rotator crystal state

UR - http://www.scopus.com/inward/record.url?scp=0037009315&partnerID=8YFLogxK

U2 - 10.1016/S0022-2860(02)00237-5

DO - 10.1016/S0022-2860(02)00237-5

M3 - Article

AN - SCOPUS:0037009315

VL - 614

SP - 159

EP - 166

JO - Journal of Molecular Structure

JF - Journal of Molecular Structure

SN - 0022-2860

IS - 1-3

ER -