Very low H–O–H bending frequencies. III. Fourier transform infrared study of cobalt potassium phosphate monohydrate and manganese potassium phosphate monohydrate

Abstract

Fourier transform infrared (FTIR) spectra were recorded, at room temperature (RT) and at the boiling temperature of liquid nitrogen (LNT), for two compounds of the MKPO 4·H 2O type (where M∈{Co, Mn}) and of their deuterated analogues. The studied systems exhibit very large red shift of the δ(HOH) frequency (compared to that of gaseous water). In the FTIR spectra of the protiated cobalt compound, no band of significant intensity is found in the prevalent δ(HOH) region (above 1600 cm −1) and, instead, a band of appreciable intensity (which disappears on deuteration) is found some 100 cm −1 lower than the gas–phase value for this mode. In the case of MnKPO 4·H 2O, two bands of almost equal intensities appear (above and below 1600 cm −1). These findings may be rationalised in terms of a Fermi resonance between the low-lying δ(HOH) mode and the overtone of the water librational modes, the resonance being much more pronounced in the case of MnKPO 4·H 2O than in CoKPO 4·H 2O.