Dynamics of hydration water in CaCl2 complexes

We report in this Letter the measurement of unusually well-defined vibrational modes of water. Calcium chloride forms complexes made of different amounts of water, namely 1/3, 2 or 4 water per CaCl2 molecule. Based on the crystallographic structure, we performed normal mode calculations from first principles, and simulated the neutron scattering spectra. A very good agreement between calculation and experiment, without any parameter refinement, confirms and completes the intuitive assignment of vibrational excitations of water. A closer look at the calculation enables us to investigate the anharmonicity of the system and the dispersion of the excitations along the hydrogen bonds.