Phosphate ion vibrations in dihydrogen phosphate salts of the type M(H2PO4)2·2H2O (M = Mg, Mn, Co, Ni, Zn, Cd): Spectra–structure correlations

Abstract

The infrared, far-infrared and Raman spectra of acid phosphate salts of the type M(H2PO4)2·2H2O (M=Mg, Mn, Co, Ni, Zn, Cd) in both protiated and deuterated forms have been recorded at ambient and liquid nitrogen temperature. The vibrational spectra have been interpreted with respect to the vibrations of the H2PO4− ions. Strong vibrational couplings are established between the PO4 stretching modes and POH bending modes as well as between the PO4 bending modes and the water librations. The magnitude of the correlation field splittings A–B and g–u of the vibrational modes have been estimated. The influence of the metal ions on the frequencies of the stretching vibrations of the PO bonds has been discussed and some trends regarding the frequency variation within the M(H2PO4)2·2H2O series have been found. It has been established that the extent of the energetic distortions of H2PO4− with respect to the PO bond lengths revealed by the spectroscopic data correlates with the extent of the geometric distortion shown by the structural data.