2H NMR studies on the dynamics of supercooled water in a metal-organic framework

We use 2H nuclear magnetic resonance (NMR) to study water (D2O) reorientation and diffusion in the metal-organic framework MFU-4l, which features a regular three-dimensional network of nearly spherical pores with diameters of 1.2 and 1.9 nm. We observe that the rotational correlation times follow Vogel-Fulcher-Tammann and Arrhenius (Ea = 0.48 eV) relations above ∼225 K and below ∼170 K, respectively, whereas the temperature dependence continuously evolves from one to the other behavior in the broad crossover zone in between. In the common temperature range, the present NMR results are fully consistent with previous broadband dielectric spectroscopy (BDS) data on water (H2O) in a very similar framework. Several of our observations, e.g., rotational-translational coupling, indicate that a bulk-like structural (α) relaxation is observed above the crossover region. When cooling through the crossover zone, a quasi-isotropic reorientation mechanism is retained, while 2H spin-lattice relaxation evolves from exponential to nonexponential, implying that the water dynamics probed at low temperatures does no longer fully restore ergodicity on the time scale of this experiment. We discuss that the latter effect may result from bulk-like and/or confinement-imposed spatially heterogeneous water properties. Comparison with previous NMR and BDS results for water in other confinements reveals that, for confinement sizes around 2 nm, water reorientation depends more on the pore diameter than on the pore chemistry, while water diffusion is strongly affected by the connectivity and topology of the pores.