Spectral Effects of Resonance Dynamic Intermolecular Interaction for Crystalline Carboxylic Acids at Temperature Phase Transitions

摘要

In the present paper we report on temperature dependent FTIR spec-tra studies of Davydov splitting value for the in-phase CH_2 rocking vibrations of methylene chains in crystalline n-carboxylic acids CH_3(CH_2)_(n-2)COOH with odd (n = 15, 17, 19) and even (n = 10, 14, 16, 22) numbers n of carbon atoms in the temperature region from 100 K to the crystal melting point. The analysis of obtained temperature dependencies allows to determine the fol-lowing regularities. For all acids in the region of low temperatures the Davydov splitting value practically does not depend on temperature. When temperature in-creases for acids with odd number n = 10, 14, 16 of carbon atoms the splitting value decreases to zero without sharp changes. For acids with odd number and even num-ber n = 22 of carbon atoms, the slow decreasing of the splitting value and then sharp decreasing to zero in the nearest region to the crystal melting point takes place. A statistic-dynamic model is proposed which provides an adequate description of the observed effects. In the frameworks of this model two different mechanisms are responsible for the temperature changes of the vibrational modes splitting value. In addition to the thermal expansion of crystals at heating, the damping of vibra-tional excitons on orientational defects of different nature takes place. Genesis of such defects is related to the excitation of conformational, librational and rotational degrees of freedom of H-bonded molecular dimers at different temperatures. Theoretical analysis of resonance dynamical intermolecular interaction effect on the intramolecular vibrations spectra of the crystals was performed in the terms of stochastic equations with account of such mechanisms. The explicit expressionfor the theoretical dependence of Davydov splitting value on temperature was ob-tained. Computer simulation of such dependence was performed for crystalline normal chain carboxylic acids. Good agreement between the experimental data and computer simulation results takes place.