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

Abstract

Four-electron transfer from U to the fullerene cage commonly exists in U@C 2n (2n < 82) so far, while four-and threeelectron transfers, which depend on the cage isomers, simultaneously occur in U@C 82 . Herein, detailed quantum-chemical methods combined with statistical thermodynamic analysis were applied to deeply probe into U@C 84 , which is detected in the mass spectra without any further exploration. With triplet ground states, novel isomers including isolated-pentagon-rule U@C 2 (51579)-C 84 and U@ D 2 (51573)-C 84 as well as nonisolated-pentagon-rule U@C s (51365)-C 84 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@D 2 (51573)-C 84 , U@C 2 (51579)-C 84 , and U@C s (51365)-C 84 . 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@C 84 .

Discovery of Non-Isolated-Pentagon-Rule Fullerenes from Computational Characterization of U2O@C72

“…However, their presence is suggested to be determined by kinetic factor, namely, the formation and yield cannot be well explained/predicted by thermodynamic calculations. In addition, Zhao et al’s recent series of theoretical calculations regarding different U-based EMFs all indicate that the possibility of any heptagon-containing isomer can be completely ignored. ,, Thus, it is reasonable that we did not consider such nonclassical isomers as the candidate cages in this work.…”

“…Thus, the pure functional (BP86) yields different results from the hybrid functionals (M06-2X, B3LYP). Since the three functionals are all popular and perform reliably in the recent computational characterizations of U-containing EMFs, − , we decided to include all the three isomers in the following discussions. This prevents the possible omission of any minor U 2 O@C 72 isomer produced in the experiments .…”

“…In addition, Zhao et al's recent series of theoretical calculations regarding different U-based EMFs all indicate that the possibility of any heptagoncontaining isomer can be completely ignored. 18,27,28 Thus, it is reasonable that we did not consider such nonclassical isomers as the candidate cages in this work.…”

“…Full geometry optimizations were conducted by employing the M06-2X, BP86, , and B3LYP , exchange-correlation functionals. The three functionals have been largely employed for characterizing the possible structures of U-containing EMFs. − , The standard 6-31G* all-electron basis set and the Stuttgart/Dresden quasi-relativistic effective core potential (ECP) and corresponding basis set (SDD) were employed for C/O and U, respectively. Harmonic vibrational frequency calculations at the same level of theory were then carried out to verify that all the obtained stationary points are true energy minima on the potential energy surface.…”

“…In this regard, the strong metal-cage interactions in actinide EMFs are very promising to stabilize the strained pentagon adjacencies (PAs) by large charge transfer and effective coordination. Indeed, mono-EMFs U@ C 2 (10612)-C 72 , U@ C 1 (17418)-C 76 , U@ C 1 (28324)-C 80 , U@ D 3 (31921)-C 80 , and U@ C s (51365)-C 84 were all demonstrated to violate IPR either experimentally or theoretically. We recently also found that the experimentally reported endohedral clusterfullerene (ECF) U 2 O@C 76 could be U 2 O@ C s (17490)-C 76 or U 2 O@ C 2 v (19138)-C 76 according to the density functional theory (DFT) calculations .…”