New Insight into U@C: Missing U@(31921)-C and Nuanced Enantiomers of U@(28324)-C.

Triplet U@(28324)-C, violating the isolated pentagon rule, is experimentally recognized as the stable isomer for uranium-based endohedral monometallofullerene U@C. Here we first verified that triplet U@(31921)-C, following the isolated pentagon rule, was to be another thermodynamically stable isomer via density functional theory in conjunction with statistical thermodynamic analysis. U@(31921)-C was probably missing in the previous experiment and would be a promising isomer in the to-be experiment because of its excellently thermodynamic stability. In addition, the anomalous metal position was revealed in U@(31921)-C and U@(28324)-C. Four-electron transfer from U to C was also revealed for the two isomers. Thus, two unpaired 5f electrons were still in the U for U@(31921)-C and U@(28324)-C. Moreover, the covalent interactions between U and C in U@(31921)-C were stronger than those in U@(28324)-C. The electrostatic interactions preponderated in the interaction energy Δ between U and C for U@(28324)-C, and the orbital interactions dominated in the Δ for U@(31921)-C. The electrophilic and nucleophilic reactivities were also analyzed for U@(31921)-C and U@(28324)-C. Electronic circular dichroism spectra were simulated to distinguish the two enantiomers of U@(28324)-C. We are hopeful that this investigation will be valuable for further identification of the two enantiomers in future experiments.