Water Dynamics
in Metal–Organic Frameworks:
Effects of Heterogeneous Confinement Predicted by Computational Spectroscopy
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Abstract
The behavior of water confined in
MIL-53(Cr), a flexible metal–organic
framework (MOF), 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