Water Dynamics in NH2-MIL-125: Insights from a Combined 1H NMR Relaxometry and Computational Investigation

The dynamics of water confined in a microporous metal-organic framework was investigated by 1H fast field-cycling nuclear magnetic resonance (NMR) relaxometry, exploring time scales ranging between 10 ?s and 0.1 ns in the 25-80 °C temperature interval. The data were interpreted within a dynamic model where molecules bind to the surface hopping among preferential binding sites. The bound molecules are also subject to local faster reorientations. Numerical analysis of the data allowed the characteristic times associated with hops and local anisotropic reorientations to be determined together with their activation energies, as derived through Arrhenius fits. The values of the activation energies, 16 ± 2 and 4.5 ± 0.5 kJ/mol, respectively, were rationalized within the model. 1H magic-angle spinning NMR was used to quantify the water loading level and to obtain evidence on the presence of bound water molecules as required by the dynamic model, whereas molecular simulations were conducted to obtain complementary information on relevant properties, such as the porosity of the matrix, the water binding sites, self-diffusion, and interaction energies in the confined space.