Institute for Condensed Matter Physics, Technische Universität Darmstadt, Hochschulstraße 6, 64289 Darmstadt, Germany.
Chair of Solid State and Materials Chemistry, Institute of Physics, University of Augsburg, Universitätsstraße 1, 86159 Augsburg, Germany.
J Chem Phys. 2023 Jul 21;159(3). doi: 10.1063/5.0159256.
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.
我们使用 2H 核磁共振（NMR）研究了具有直径约为 1.2nm 和 1.9nm 的近球形孔的规则三维网络的金属有机骨架 MFU-4l 中的水（D2O）重取向和扩散。我们观察到，旋转相关时间分别遵循 Vogel-Fulcher-Tammann 和 Arrhenius（Ea=0.48eV）关系，高于约 225K 和低于约 170K，而在两者之间的宽交叉区中，温度依赖性连续从一种行为演变到另一种行为。在常见的温度范围内，目前的 NMR 结果与以前在非常相似的骨架中关于水（H2O）的宽带介电光谱（BDS）数据完全一致。我们的一些观察结果，例如旋转-平移偶联，表明在交叉区上方观察到了类似体相的结构（α）弛豫。当冷却通过交叉区时，保留了准各向同性的重取向机制，而 2H 自旋晶格弛豫从指数变为非指数，这意味着在实验时间尺度上，低温下探测到的水动力学不再完全恢复遍历性。我们讨论了后一种效应可能是由于体相和/或受限引起的空间不均匀的水性质所致。与其他受限水的以前的 NMR 和 BDS 结果的比较表明，对于约 2nm 的受限尺寸，水的重取向更多地取决于孔径而不是孔径化学性质，而水的扩散则强烈受到孔的连通性和拓扑结构的影响。
