Theoretical Study On Structural Properties Of Superatoms Based On Endo-ionic Interaction

Superatoms can exhibit molecular orbitals characteristics similar to those of electron arrangement with the shell structure of atoms.By altering their composition,size and electronic states,superatoms provide a broader range of choices than natural elements.Therefore,superatoms hold the promise of breaking the shackles of traditional material system construction based on a limited number of elemental species in nature.and using them as building blocks with atomic-level precision could lead to the construction of new materials and devices,opening new opportunities for development.To this end,it is necessary to gain a profound understanding of their electronic structure and interaction mechanism at the atomic level is necessary.In this paper,we propose a novel type of superatom based on“endo-ionic interaction”by analyzing a series of endohedral metallofullerene（EMF）structures containing magnesium embedded in typical fullerene C₂₀ in various charge states.Through this analysis,we have gained a comprehensive understanding of the electronic structure characteristics and interaction mechanisms of the Mg@C₂₀ superatom cluster.Furthermore,we have conducted calculations and characterization of the ultraviolet-visible（UV-Vis）spectroscopy of these charged structures,providing valuable references for further research and experimental observation of corresponding structures.Previous studies have shown that fullerenes of varying sizes can serve as ideal carriers for superatoms,encapsulating atoms or small molecules and exhibiting a range of electronic and structural properties.In particular,the second main group alkaline earth metals can act as good electron donors,with their s valence electrons readily hybridizing with other orbitals within the molecule to form+2 valence ion system.In a confined environment,the alkaline earth metal Mg atom can be protected,and through electronic rearrangement,the whole system exhibits an extremely complex electronic structure and even exhibits unique molecular spectra and other properties.Based on the above,we designed a series of small-sized EMF superatoms composed of light elements with the general formula Mg²⁺@[C₂₀]^（n-2）,where n=4,2,0,-2,and-4,by embedding Mg atom into the smallest fullerene C₂₀.It was found that the embedding of magnesium effectively stabilized the C₂₀ cage structure,and structures were theoretically predicted to stabilize EMF superatomic structures,all of which adhere to the distinct 18-electron principle.Specifically,the electronic configuration of these superatomic clusters is1S²1P⁶1D¹⁰1F^4-n,where n=+4,+2,0,-2,and-4,corresponding to 18,20,22,24,and26 valence electrons,respectively.In addition to the valence electrons that satisfy the18-electron principle,the additional electrons（4-n）occupy 1F superatomic molecular orbitals（SAMOs）.To gain insight into the stability of these superatomic clusters,we performed energy level analysis,energy decomposition analysis（EDA）,and extended transition state-natural orbitals for chemical valence（ETS-NOCV）.The results clearly demonstrate significant ionic interactions between the encapsulated metal atom and the carbon cage,contributing to the system’s energetic stabilization via charge transfer from the central metal atom to the carbon cage.This mechanism of superatomic stabilization,termed"endo-ionic interaction",is intriguing.Furthermore,we explored the UV-Vis optical properties of these clusters,it was found that the spectrum shows significant changes as the superatomic electrons in the clusters increase gradually from 18 e to 26e.The gradual red-shifted of the absorption peaks with increasing superatomic electrons and the similarity of the main source of the orbital transitions in the near-ultraviolet region provide a theoretical basis for detecting this series of Mg²⁺@[C₂₀]^n-2 clusters experimentally.In summary,this paper demonstrates that embedded Mg atom in C₂₀ can form a series of stable small-sized EMF superatomic structures of with different charge states,which adhere to the distinct 18-electron principle,based on first-principles calculations.An in-depth analysis revealed that the proposed“endo-ionic”interaction contributes to the stabilization of these structures.Considering that C₂₀ is the smallest carbon-based fullerene,these theoretically predicted superatomic structures hold promises as typical small-sized EMF superatoms for fundamental theoretical exploration and for applications in various fields of research.