Theoretical Study On The Structures And Bonding Characteristics Of Endohedral Fullerenes

The confinement effect of fullerenes and the charge transfer between outer cages and internal metal species not only stabilize the metal clusters and fullerenes themselves,but also make the endohedral metallofullerenes(EMFs)have the physical and chemical properties unexpected for pristine fullerenes.In this dissertation,the geometric structures of various endohedral metallofullerenes are calculated by density functional theory(DFT),and their electronic structures,bonding properties,and theoretical spectra are disclosed.First,our theoretical calculations revealed that the(Ti₃C₃)⁶⁺unit of recently synthesized Ti₃C₃@I_h(7)-C₈₀ exhibits superatomic state with a perfect closed-shell1S²1P⁶1D¹⁰ 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 stain of the cyclopropane-like C₃ core is effectively released upon the metal coordination.All these factors greatly stabilize the(Ti₃C₃)⁶⁺cluster itself,showing the critical role of metal-to-cage charge transfer and cage encapsulation in enhancing the stability of this exotic metal cluster.Second,density functional theory calculations characterized that the recently synthesized U₂O@C₇₆ could be U₂O@C_s(17490)-C₇₆ or U₂O@C_2v(19138)-C₇₆,whose cages have two or one pentagon adjacencies(PAs)and thus both violate the isolated pentagon rule(IPR).It is noteworthy that they are the first actinide-based clusterfullerenes bearing non-IPR outer cages.They are also the first Cs(17490)-and C_2v(19138)-C₇₆-based oxide clusterfullerenes.Moreover,U₂O@C_2v(19138)-C₇₆is the first example of a hexavalent metal cluster within the C_2v(19138)-C₇₆ cage.Interestingly,although trapped by the two same-sized cages,the U₂O unit exhibits a bent and a perfect linear configuration,respectively,indicative of the crucial role of cage shape in steering the internal cluster configuration.Their electronic structures can be formally described as(U₂O)⁶⁺@C₇₆^6-with primary electrostatic attractions and secondary covalent interactions between cluster and cage.Significantly,bonding analyses reveal that the encaged U₂O moiety may only features two three-center two-electron(3c-2e)U-O-U bonds with completely absent common two-center bonds.Finally,by using density functional theory,we encapsulated a Cr₂ unit in several fullerene cages with different sizes and topology structures.The results of bonding analysis show that the confinement effect,electronic effect,special fragment and large size of the carbon cages can not make the Cr₂ diatom form the desired metal-metal quintuple bond in the fullerene cages,and all the EMFs have a Cr-Cr single bond at most.