The stability of endohedral clusterfullerenes is generally understood in terms of the metal cluster shape, cage structure, and metal–cage interactions, with the electronic state of the internal cluster mostly neglected. Herein, theoretical calculations reveal that the (Ti₃C₃)⁶⁺ unit of recently synthesized Ti₃C₃@I_h(7)-C₈₀ exhibits a superatomic state with a perfect closed-shell 1S²1P⁶1D¹⁰ electronic configuration in accordance with the famous jellium model. This “trapped superatom” features considerable aromaticity and hyperconjugation interactions never reported for other clusterfullerenes. Besides the localized two-center two-electron (2c-2e) Ti–C/C–C bonds, it also has two 3c-2e Ti–C–Ti bonds. Furthermore, the ring strain of the cyclopropane-like C₃ core is effectively released upon the metal coordination. All these factors greatly stabilize the (Ti₃C₃)⁶⁺ cluster, showing the critical role of metal-to-cage charge transfer and cage encapsulation in enhancing the stability of this exotic metal cluster.

中文翻译：

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在富勒烯笼中发现超原子

通常从金属簇的形状，笼结构和金属-笼的相互作用来理解内面簇簇富勒烯的稳定性，而内部簇的电子态则被忽略。在此，理论计算表明，最近合成的Ti ₃ C ₃ @ I _h（7）-C ₈₀的（Ti ₃ C ₃）⁶⁺单元表现出具有完美的闭壳1S ² 1P ⁶ 1D ¹⁰的超原子态。电子配置符合著名的果酱模型。这种“被困的超原子”具有相当大的芳香性和高共轭相互作用，这是其他簇富勒烯从未有过的报道。除了局部的双中心双电子（2c-2e）Ti-C / CC键，它还具有两个3c-2e Ti-C-Ti键。此外，环丙烷状C ₃核的环应变在金属配位时被有效地释放。所有这些因素极大地稳定了（Ti ₃ C ₃）⁶⁺团簇，显示了金属-笼式电荷转移和笼式封装在增强这种奇异金属簇的稳定性方面的关键作用。