The influence of Fermi resonance and of isotopic substitution on the infrared spectra of hydrogen bonded crystals

A theoretical approach, which takes into account both the Fermi resonance effect and strong anharmonic interaction between high-frequency v(XH) stretching vibra-tions and the lattice phonons, has been used to explain the IR spectral profiles of NaHF2 , HCrO2 and HCoO2 crystals and their deuterated analogues. Good correlation between the theoretical and experimental spectra has been obtained. It is shown that qualitative changes in the IR spectra of the crystals arising with isotopic substitution are mainly due to the change of the resonance conditions. It is found that in DCrO2 and DCoO2 crystals Fermi resonance is strong, whereas in HCrO2 and HCoO2 materials it is weak.