Water Dynamics in Metal-Organic Frameworks: Effects of Heterogeneous Confinement Predicted by Computational Spectroscopy

The behavior of water confined in MIL-53(Cr), a flexible metal organic framework (MOP), is investigated through computational infrared spectroscopy. As the number of molecules adsorbed inside of the pores increases, the water OH stretch band of the linear infrared spectrum grows in intensity and approaches that of bulk water. To assess whether the water confined in MIL-53(Cr) becomes liquid-like, two-dimensional infrared spectra (2DIR) are also calculated. Confinement effects result in distinct chemical environments that appear as specific features in the 2DIR spectra. The evolution of the 2DIR line shape as a function of waiting time is well described in terms of the orientational dynamics of the water molecules, with chemical exchange cross peaks appearing at a time scale similar to the hydrogen bond rearrangement lifetime. The confining environment considerably slows the hydrogen bond dynamics relative to bulk as a result of the competition between water-framework and water-water interactions.