Temperature study of resonance intermolecular interaction in normal long-chain carboxylic acid crystals using IR absorption spectra

In the present paper we report on temperature dependent FTIR spectra studies of Davydov splitting value for the in-phase CH2 rocking vibrations of crystal methylene chains of n-carboxylic acids CH3(CH)nK2COOH with odd (nZ15,17,19) and even (nZ10,14,16,22) numbers of carbon atoms, n, in the temperature region from 100 K to the crystal melting temperature. A statistic-dynamic model is proposed which provides an adequate description of the observed dependences. In the framework of this model, two different mechanisms are responsible for the temperature changes of the splitting value of vibrational modes. In addition to the thermal expansion of crystals under heating, the damping of vibrational excitons on orientational defects of different nature takes place. Genesis of such defects is connected with the excitation of conformational, librational and rotational degrees of freedom of H-bonded molecular dimers of studied acids at different temperatures. The theoretical analysis of the effect of the resonance dynamical intermolecular interaction on the spectra of intramolecular vibrations of the crystals was performed in terms of stochastic equations, taking into account mentioned mechanisms. The explicit expression for the theoretical dependence of Davydov splitting value on temperature was obtained. Computer simulation of such dependence was performed for crystalline long-chain n-carboxylic acids. Good agreement between the experimental and computer simulation results was obtained. q 2005 Elsevier B.V. All rights reserved.