Resolving Confined ⁷Li Dynamics of Uranyl Peroxide Capsule U₂₄

We obtained a kerosene-soluble form of the lithium salt [UO₂(O₂)(OH)₂]₂₄ phase (Li-U₂₄), by adding cetyltrimethylammonium bromide surfactant to aqueous Li-U₂₄. Interestingly, its variable-temperature solution ⁷Li NMR spectroscopy resolves two narrowly spaced resonances down to -10 °C, which shift upfield with increasing temperature, and finally coalesce at temperatures > 85 °C. Comparison with solid-state NMR demonstrates that the Li dynamics in the Li-U₂₄-CTA phase involves only exchange between different local encapsulated environments. This behavior is distinct from the rapid Li exchange dynamics observed between encapsulated and external Li environments for Li-U₂₄ in both the aqueous and the solid-state phases. Density functional theory calculations suggest that the two experimental ⁷Li NMR chemical shifts are due to Li cations coordinated within the square and hexagonal faces of the U₂₄ cage, and they can undergo exchange within the confined environment, as the solution is heated. Very different than U₂₄ in aqueous media, there is no evidence that the Li cations exit the cage, and therefore, this represents a truly confined space.