Fermi resonance in Raman and infrared spectra of Ni(CH₃COO)₂·4H₂O and its deuterated analogue

The doublet at 962–906 cm −1 observed in the room temperature spectra of Ni(CH 3 COO) 2 . 4H 2 O is attributed to the ν 4 state of the acetate group undergoing strong Fermi‐type anharmonic interaction with a two‐phonon hybrid state associated with a water libration exhibiting characteristic hypsochromic shift and hyperchromic effect with decreasing temperature. In the deuterated analogue another doublet at 706–666 cm −1 is associated with the v 5 state of the acetate group and another two‐phonon hybrid state, also associated with a D 2 O libration, exhibiting hypsochromic shift but the hypochromic effect. The Fermi interaction enhances in the hydrate and diminishes in the deuterate as the temperature is lowered.