Low-Frequency Motions of Water Molecules in Hydrated Salts of Transition Metals
The motions of water molecules in a number of salts, including Co(NO3)2 • 2H2O, Co(NO3)2 • 6H2O, Cu(NO3)2 • 3H2O, CuSO4 • H2O, CuSO4 • 5H2O, and UO2(NO3)2 • 6H2O, have been investigated by the inelastic scattering of cold neutrons. For each salt, one or more bands are observed in the region of neutron energy gain from 400 to 800 cm−1 and are assigned to the librational modes of the water molecules. Low-energy peaks in the region near 200 cm−1 are also obtained in each case and attributed to “optical” translations of the water molecules in the lattice. In general, split librational bands are observed for the lower hydrates, with the higher-energy component assigned to wagging and rocking modes of the water molecules, and that at lower energies to the torsional modes. The librational bands are broadened in the higher hydrates with some indication of splitting. This is to be expected, because of the increase in different “types” of water molecules, as well as coupling effects. The data are compared with recent infrared results.