Resolving Confined Li Dynamics of Uranyl Peroxide Capsule U.

We obtained a kerosene-soluble form of the lithium salt [UO(O)(OH)] phase (Li-U), by adding cetyltrimethylammonium bromide surfactant to aqueous Li-U. Interestingly, its variable-temperature solution Li 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-U-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-U in both the aqueous and the solid-state phases. Density functional theory calculations suggest that the two experimental Li NMR chemical shifts are due to Li cations coordinated within the square and hexagonal faces of the U cage, and they can undergo exchange within the confined environment, as the solution is heated. Very different than U in aqueous media, there is no evidence that the Li cations exit the cage, and therefore, this represents a truly confined space.