Strong-coupling mechanism for interpretation of the IR spectra of the hydrogen bonded imidazole crystal

In this work we have attempted to provide a new interpretation of some complex spectral effects, recently observed in the high frequency region of the IR spectra of hydrogen bonded imidazole crystals. The subjects of our interest were effects corresponding to the intensity distribution, the linear dichroism, the influence of temperature and the isotopic substitution of deuterium in imidazole molecules, measured in the nu(N-H) and the nu(N-D) band ranges in the crystalline spectra. Based on model calculations it was shown that the ''strong-coupling'' model was able to quantitatively reproduce the nu(N-H) and nu(N-D) band shapes in the spectra of the imidazole crystal. This fact can be considered as a basis for an explanation of the isotopic, polarization and temperature effects in the spectra. The Fermi resonance effects have been found to be of a minor importance. The key property of the crystal responsible for the abnormal spectral properties of its H-bonds seems to be the statistical disorder in the crystalline lattice, due to bending of the H-bonds. This structural effect, along with the very specific selection rules in the IR for the centrosymmetric H-bonded dimers and the correlation field interactions in the crystal, were considered to be the main source of the spectral effects analyzed.