Hydrated {Mo 72Fe 30} clusters: Low-frequency hydrogen modes and self-aggregation

Abstract

Using incoherent quasi-elastic and inelastic neutron scattering, we have investigated the hydrogen relaxational dynamics and hydrogen vibrational modes in the polyoxomolybdate specie [Mo 72Fe 30O 252(CH 3COO) 12[Mo 2O 7(H 2O)] 2[H 2Mo 2O 8(H 2O)](H 2O) 91]· ≈ 150 H 2O. The translational dynamics of the water molecules in the compound is profoundly different from that of bulk water at the same temperature showing a non-Debye relaxation behavior. The temperature dependence of the relaxation time can be described in terms of an Arrhenius law, indicating that the dynamics is triggered by the breaking of the bonds connecting the crystal water molecules with the hydrophilic nanocapsule surfaces. Inelastic neutron scattering spectra confirm the attenuation of water translational modes with respect to the bulk water case due to the strong destructuring effect imposed by the nanocage interface and the enhancement of the highest frequency librational mode as already found in hydrated Vycor or Gelsil matrix. Small angle X-ray scattering on freshly prepared aqueous solution evidences the presence of nanocapsule structures proper of the monomer (2.6 nm in diameter) that coexist with a small amount of oligomers. After 1 month the polyoxomolibdate specie self-assembles in a supramolecular structure with a polydisperse distribution of dimensions spanning from the monomer to the “blackberry” vesicular structure already reported in literature.