Does water suffer from claustrophobia? The local structural and dynamic properties of water in confined (300–4000 nm) spaces were characterized by an NMR study of the size‐dependent relaxation phenomena, which in turn reflect changes in water mobility and proton transfer. The results are important for understanding fluidics in extended nanospaces and in implementing micro‐/nanofluidic devices.

Electrochemical, complexation, and electronic properties of uranyl(VI) and -(V) carbonato complexes in an aqueous Na2CO3 solution have been investigated to define the appropriate conditions for preparing pure uranyl(V) samples and to understand the difference in coordination character between UO22+ and UO2+. Cyclic voltammetry using three different working electrodes of platinum, gold, and glassy carbon has suggested that the electrochemical reaction of uranyl(VI) carbonate species proceeds quasi-reversibly. Electrolysis of UO22+ has been performed in Na2CO3 solutions of more than 0.8 M with a limited pH range of 11.7 < pH < 12.0 using a platinum mesh electrode. It produces a high purity of the uranyl(V) carbonate solution, which has been confirmed to be stable for at least 2 weeks in a sealed glass cuvette. Extended X-ray absorption fine structure (EXAFS) measurements revealed the structural arrangement of uranyl(VI) and -(V) tricarbonato complexes, [UO2(CO3)3]n- [n = 4 for uranyl(VI), 5 for uranyl(V)]. The bond distances of U-Oax, U-Oeq, U-C, and U-Odist are determined to be 1.81, 2.44, 2.92, and 4.17 A for the uranyl(VI) complex and 1.91, 2.50, 2.93, and 4.23 A for the uranyl(V) complex, respectively. The validity of the structural parameters obtained from EXAFS has been supported by quantum chemical calculations for the uranyl(VI) complex. The uranium LI- and LIII-edge X-ray absorption near-edge structure spectra have been interpreted in terms of electron transitions and multiple-scattering features.

The U(VI) complex with a pentadentate Schiff base ligand (N,N'-disalicylidenediethylenetriaminate = saldien(2-)) was prepared as a starting material of a potentially stable U(V) complex without any possibility of U(V)O(2)(+)...U(V)O(2)(+) cation-cation interaction and was found in three different crystal phases. Two of them had the same composition of U(VI)O(2)(saldien) x DMSO in orthorhombic and monoclinic systems (DMSO = dimethyl sulfoxide, 1a and 1c, respectively). The DMSO molecule in both 1a and 1c does not show any coordination to U(VI)O(2)(saldien), but it is just present as a solvent in the crystal structures. The other isolated crystals consisted only of U(VI)O(2)(saldien) without incorporation of solvent molecules (1b, orthorhombic). A different conformation of the coordinated saldien(2-) in 1c from those in 1a and 1b was observed. The conformers exchange each other in a solution through a flipping motion of the phenyl rings. The pentagonal equatorial coordination of U(VI)O(2)(saldien) remains unchanged even in strongly Lewis-basic solvents, DMSO and N,N-dimethylformamide. Cyclic voltammetry of U(VI)O(2)(saldien) in DMSO showed a quasireversible redox reaction without any successive reactions. The electron stoichiometry determined by the UV-vis-NIR spectroelectrochemical technique is close to 1, indicating that the reduction product of U(VI)O(2)(saldien) is [U(V)O(2)(saldien)](-), which is stable in DMSO. The standard redox potential of [U(V)O(2)(saldien)](-)/U(VI)O(2)(saldien) in DMSO is -1.584 V vs Fc/Fc(+). This U(V) complex shows the characteristic absorption bands due to f-f transitions in its 5f(1) configuration and charge-transfer from the axial oxygen to U(5+).

To examine properties of pentavalent uranium, U(V), we have carried out electrochemical and spectroelectrochemical studies on UO(2)(saloph)L [saloph = N,N'-disalicylidene-o-phenylenediaminate, L = dimethyl sulfoxide (DMSO) or N,N-dimethylformamide (DMF)]. The electrochemical reactions of UO(2)(saloph)L complexes in L were found to occur quasireversibly. The reduction processes of UO(2)(saloph)L complexes were followed spectroelectrochemically by using an optical transparent thin layer electrode cell. It was found that the absorption spectra measured at the applied potentials from 0 to -1.650 V versus ferrocene/ferrocenium ion redox couple (Fc/Fc(+)) for UO(2)(saloph)DMSO in DMSO have clear isosbestic points and that the evaluated electron stoichiometry equals 1.08. These results indicate that the reduction product of UO(2)(saloph)DMSO is [U(V)O(2)(saloph)DMSO](-), which is considerably stable in DMSO. Furthermore, it was clarified that the absorption spectrum of the [U(V)O(2)(saloph)DMSO](-) complex has a very small molar absorptivity in the visible region and characteristic absorption bands due to the 5f(1) orbital at around 750 and 900 nm. For UO(2)(saloph)DMF in DMF, the clear isosbestic points were not observed in the similar spectral changes. It is proposed that the UO(2)(saloph)DMF complex is reduced to [U(V)O(2)(saloph)DMF](-) accompanied by the dissociation of DMF as a successive reaction. The formal redox potentials of UO(2)(saloph)L in L (E(0), vs Fc/Fc(+)) for U(VI)/U(V) couple were determined to be -1.550 V for L = DMSO and -1.626 V for L = DMF.