Resolving Confined ⁷Li Dynamics of Uranyl Peroxide Capsule U₂₄

Abstract

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.

Transformation of Uranyl Peroxide Studtite, [(UO2)(O2)(H2O)2](H2O)2, to Soluble Nanoscale Cage Clusters

“…6 The uranyl ions in the U 24 cluster are each coordinated by two bidentate peroxide groups in a cis arrangement and two hydroxyl groups that define an equatorial edge of the hexagonal bipyramid (Figure 1f). 30 These are linked into rings of four and six uranyl ions, giving an overall sodalite-topology cage. The transformation from solid studtite to dissolved U 24 clusters clearly involves changes in the local coordination environments about uranyl, and the mechanisms of this transformation warrant further study, such as by using nuclear magnetic resonance and 17 O-labeled peroxide.…”

“…In studtite, each uranyl ion is coordinated by two peroxide in a trans arrangement, and by two H 2 O groups, also in a trans arrangement (Figure b) . The uranyl ions in the U 24 cluster are each coordinated by two bidentate peroxide groups in a cis arrangement and two hydroxyl groups that define an equatorial edge of the hexagonal bipyramid (Figure f) . These are linked into rings of four and six uranyl ions, giving an overall sodalite-topology cage.…”

The Role of the Organic Solvent Polarity in Isolating Uranyl Peroxide Capsule Fragments

“…Soluble uranyl peroxide molecules including monomers [UO 2 (O 2 ) 3 ] 4– ( U 1 ), ,, rings [(UO 2 ) 5 (O 2 ) 10 ] 10– ( U 5 ), and fully formed capsules (many compositions; most commonly observed include [(UO 2 )­(O 2 ) 1.5 ] 28 28– ( U 28 ), ,− [(UO 2 )­(O 2 )­OH] 24 24– ( U 24 ), − and [(UO 2 )­(O 2 )­OH] 60 60– ( U 60 )) all carry large anionic charges. From monomers to rings to capsules, these anions decrease in charge per uranyl with increasing nuclearity.…”

Recent advances in the liquid‐phase 6,7Li nuclear magnetic resonance

“…The 7 Li NMR spectra of the Li‐U 24 surfactant encapsulated species: (a) NMR experiment of Li‐U 24 ‐CTA at 25°C and (b) experimental (left) and simulated (right) variable‐temperature NMR data from 25°C to 100°C with a coalescence temperature of 85°C. Reproduced with minor editing privilege from Xie et al [ 49 ] with the permission of Springer Nature…”