Exploring the Interaction of Water with Open Metal Sites in MIL-101(Cr) by 1H NMR Relaxometry and ReaxFF Molecular Dynamics Simulations

In this work, we investigated the state of water in the metal-organic framework MIL-101(Cr) by combining 1H magic angle spinning (MAS) NMR, NMR relaxometry, and molecular dynamics (MD) simulations based on a reactive force field. The MD simulations indicated that water molecules are coordinated to the open metal sites and are organized in shells. Through 1H T2 and T1 relaxation measurements at the fixed Larmor frequency of 21 MHz, water in the intergrain spaces was distinguished from that in the intragrain mesopores of MIL-101(Cr). 1H MAS NMR measurements showed that water in the mesopores is structurally ordered, as revealed by the presence of 1H-1H residual dipolar interaction. The 1H NMR relaxometric behavior of water located in the intergrain spaces of MIL-101(Cr) as a function of the magnetic field strength, determined by fast field cycling NMR relaxometry, was interpreted extending an existing model, generally applied to paramagnetic aqua ions in solution, to the case of Cr3+ ions fixed on the surface of the metal-organic framework. The model predicts the exchange of water between the first and second shell of a Cr3+ ion, giving access to the water residence time in the first shell that would be difficult to determine using other techniques. In the temperature interval of 25-70 °C, water residence times in the range of tens of nanoseconds were found and a limited accessibility of water to the open metal sites located on the external grain surface was observed.