Theoretical Insight into Thermodynamically Optimal U@C₈₄: Three-Electron Transfer Rather Than Four-Electron Transfer

Four-electron transfer from U to the fullerene cage commonly exists in U@C₂ (2n < 82) so far, while four- and three-electron transfers, which depend on the cage isomers, simultaneously occur in U@C₈₂. Herein, detailed quantum-chemical methods combined with statistical thermodynamic analysis were applied to deeply probe into U@C₈₄, which is detected in the mass spectra without any further exploration. With triplet ground states, novel isomers including isolated-pentagon-rule U@C₂(51579)-C₈₄ and U@D₂(51573)-C₈₄ as well as nonisolated-pentagon-rule U@C(51365)-C₈₄ were identified as thermodynamically optimal. Surprisingly, there were unexpected three-electron transfers, which directly led to one unpaired electron on the cage, in all of the three isomers. Significant covalent interactions between the cage and U successively weakened for U@D₂(51573)-C₈₄, U@C₂(51579)-C₈₄, and U@C(51365)-C₈₄. Besides, the IR absorption spectra were simulated as a reference for further structural identification in the experiment. Last but not least, the potential reaction sites were predicted to facilitate further functionalization and thus achieve promising applications for U@C₈₄.