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@C2n (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@Cs(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@Cs(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.