Abstract
Isotopic dilution studies of water molecules in weakly bound crystalline hydrates reveal a strong dependence of the infrared spectra on temperature. The infrared stretching and bending bands of dilute HDO in NaClO4⋅H2O and LiI⋅3H2O show multiple bands at low temperature which broaden and move together as the temperature is raised. In contrast, only single narrow bands are seen in the very similar LiClO4⋅3H2O. The potential in which the waters move is calculated from a simple electrostatic model. The stretching frequencies are taken to be a function of the librational coordinate of the water and the observed frequencies are then averages over the libration. This simple prescription for calculating the observed bands is that given by Redfield theory. We conclude that the observed band collapse in NaClO4⋅H2O is due primarily to the increase with temperature of the in-plane water libration. At higher temperatures, the 180° flipping motion also contributes to the collapse. For LiI⋅3H2O, the motion is a complex one involving locally ordered domains of the water molecules in the crystal.
Published Version
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