Th@C₈₄ Revisited: Lost in the Corners of One's Own Cage

The energetic ordering of actinide endohedral fullerene isomers depends sensitively on proper exploration of their intramolecular potential energy surfaces. In a previous theoretical study, we predicted that the Th@C_s(10)-C₈₄ isomer is the lowest-energy structure, which is in apparent contradiction with recent experiments identifying Th@C₂(8)-C₈₄ and Th@C_s(15)-C₈₄ as the preferentially formed isomers. Here, we theoretically reevaluate all 24 isolated-pentagon-rule Th@C₈₄ isomers using systematic sampling of the thorium positions inside the carbon cage. We show that the experimentally observed isomers are indeed the lowest-energy structures when the conformational space is properly explored. The earlier misassignment is traced to convergence into higher-energy local minima caused by strong, directional Th-cage covalent interactions. Bonding analysis confirms the formal Th(IV) oxidation state, and the revised UV-vis-NIR spectra are consistent with the experimental results. Our results emphasize the importance of extensive conformational sampling for the reliable theoretical characterization of actinide endohedral fullerenes.