Investigation of Davydov Splitting in the ir Spectra of Crystalline Dicarboxylic Acids

In the present paper the collective (vibrational) excitations in molecular crystals were studied. Since the corresponding molecular interactions are characterized by a small radius, the investigation results are important for the molecular and nanoelectronics problem solving.

The paper includes the investigation results of resonance (Davydov) splitting of the methylene СН₂ group rocking vibrations in the IR absorption spectra of even homologues of crystalline dicarboxylic НООС(СН₂)_nСООН acids (crystal space group P2₁/a). Temperature dependence of the Davydov splitting value for the series of rocking vibration bands of methylene groups in the spectral range 700-1100 cm^–1 was investigated for homologues with the number of carbon atoms n = 4-10 using the polarized IR spectroscopy in the wide temperature range 100-300 K. Interpretation of the series of rocking vibration bands of methylene groups in the IR absorption spectra of even homologues of crystalline dicarboxylic acids is performed. Based on the theoretical calculation of normal modes and the assignment of observed absorption bands to vibrations of different symmetry types it was shown, that in the spectra of the studied acids the series of rocking vibration bands may be interpreted as the vibration of (n – 2) methylene groups connected by the collective interaction, in contrast to the case of normal paraffins where all the methylene groups are involved in rocking vibrations. Dependence of the Davydov splitting value on the number of methylene groups is analyzed, and shown that this value increases proportionally to the methylene chain length.