M@C78 (M = U, Th): Inherent Topological Connectivity Existed in Thermodynamically Stable Isomers and the Possibility of an Endohedral Fullerene Containing One Heptagon Ring.

ABSTRACT

The density functional theory combined with statistical thermodynamic analyses of M@C78 (M = U and Th) demonstrated that four isomers, M@D3h(24109)-C78, M@C2v(24107)-C78, M@C1(22595)-C78, and M@C1(23349)-C78, and a nonclassical isomer, M@C1(id7)-C78, containing one heptagon ring possess outstanding thermodynamic stabilities in the two M@C78 series. Especially, the M@C1(id7)-C78 isomer is the first nonclassical C78 fullerene that can exist stably. Importantly, these five fullerene cages are found to be related in the form of Stone-Wales (SW) transformations. Geometric analyses disclosed that, unlike lanthanide metals, actinide metals are more likely to bond with sumanene-type hexagonal rings when they are encapsulated in IPR C78 cages. Frontier molecular orbital analysis showed that both U and Th atoms donate four electrons to the C78 carbon cages.