Reinvestigation of the water bending region in the spectra of gypsum-like compounds - An FTIR study

The IR spectra of gypsum (CaSO4.2H(2)O) and the related CaSeO4.2H(2)O and YPO4.2H(2)O have been reinvestigated by FTIR spectroscopy. For the components of the correlation-field doublet in the delta(HOH) region, the measured ratio of the intensities for CaSO4.2H(2)O is rather close to that derived (employing the oriented-gas model) from neutron diffraction data. From the acquired spectroscopic data for the selenate and the phosphate (no neutron data are available for either of them), the angle between the HOH bisector and the C-2 axis was estimated. The small but significant increase in the frequency separation of the components of the delta(HOH) doublet on lowering the temperature can be explained in terms of the unit cell contraction which brings the water molecules closer to each other and thus increases the dipole-dipole interactions which are believed to be the principal cause for the correlation-field splitting. (C) 1997 Elsevier Science B.V.