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 {[CoII(H2bim)3](TMA)·20H2O}n (1) (H2bim = 2,2'-biimidazole, TMA3- = 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 {[CoIII(D2bim)3](TMA)·20D2O}n (1'), which revealed distinct hierarchical peaks of H2O H-bonding interactions. In addition, the frozen WMCs in 1 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 (T1) 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.