Solid-State ²H NMR Analysis for Hierarchical Water Clusters Confined to Quasi-One-Dimensional Molecular Nanoporous Crystals

Water confined to the quasi-one-dimensional hydrophilic molecular nanoporous crystal of {[Co^II(H₂bim)₃](TMA)·20H₂O}_<i>n</i> (<b>1</b>) (H₂bim = 2,2'-biimidazole, TMA^3- = trimesate) forms dynamic water molecule clusters (WMCs) with a hierarchical three-layered hydrogen-bonding (H-bonding) structure and a time-averaged structure in the melting state due to interactions with the walls of the ∼1.6 nm nanopores. This was first revealed by measuring the solid-state ²H NMR spectra of a single crystal of {[Co^III(D₂bim)₃](TMA)·20D₂O}_<i>n</i> (<b>1'</b>), which revealed distinct hierarchical peaks of H₂O H-bonding interactions. In addition, the frozen WMCs in <b>1</b> exhibit a premelting state, retaining some ice structures, just before melting through a first-order phase transition during the heating process. Measurements of the spin-lattice relaxation time (<i>T</i>₁) in solid-state ²H NMR revealed rapid rotation motions for water molecules in these premelting WMCs, resulting in a correlation time τ closer to that of bulk liquid water.