A RESONANCE MODEL FOR THE GAMMA(X-H-CENTER-DOT-CENTER-DOT-CENTER-DOT-Y) AND NU(X-H) VIBRATIONS IN THE BENT H-BONDS, FOR INTERPRETATION OF THE GAMMA(X-H-CENTER-DOT-CENTER-DOT-CENTER-DOT-Y) VIBRATION FIRST OVERTONE BAND SHAPES IN THE IR - THE IMIDAZOLE CRYSTAL

In this paper we attempt to explain spectral effects measured in the I R spectra of a number of the H-bonded crystals, which appear in the fr equency range corresponding to the overtone bands of the [GRAPHICS] (' 'bending-out-of-plane'') vibrations and are connected with considerabl e intensities of the overtone bands. We also attempt to explain the ob served fine structure of these bands and a strong temperature effect. We consider a simple model of coupling between the two proton vibratio ns in the H-bonds of the crystalline lattices of imidazole and adipic acid, formally similar to the Fermi resonance model. However, we rathe r exclude anharmonicity of the protonic vibrations in the H-bonds as a source of the spectral effects analyzed, ascribing spectral phenomena to the effective geometry change (i.e. bending) of the H-bonds in the crystals. Such an approach allows us to explain temperature effects i n the crystalline spectra, i.e. to predict the decrease of the couplin g energy values with growing temperatures. Model calculations demonstr ate that the appearance of the overtone band fine structure is related to the v(X-H) band fine structure and the gamma(X-H ... Y), band shap e depends on the intensity distribution in each individual v(X-H) band . The analyzed mechanism is a kind of transfer of the ''strong-couplin g'' mechanism onto the proton bending vibrations in the H-bonds, which -due to their highly harmonic nature- do not participate in this sort of interaction.