Which Do Endohedral Ti₂C₈₀ Metallofullerenes Prefer Energetically:  Ti₂@C₈₀ or\nTi₂C₂@C₇₈? A Theoretical Study

Abstract

Four possible isomers of the Ti₂C₈₀ metallofullerene are discussed in detail at the B3LYP DFT level of\ntheory: two isomers in Ti₂@C₈₀ formula with two Ti atoms encapsulated inside a C₈₀ cage and the other two\nin Ti₂C₂@C₇₈ formula with a Ti₂C₂ cluster involved inside a C₇₈ cage. In the encaged Ti₂C₂ cluster, there are\nend-on and side-on C₂ bridging modes into the two Ti atoms. The optimized end-on cluster has a linear\nTi−C−C−Ti array, whereas the side-on cluster has a butterfly-like structure where the two Ti atoms and the\nC₂ unit do not lie in a plane. DFT calculations show that the Ti₂C₂@C₇₈ molecule with the end-on Ti₂C₂\ncluster is energetically most favorable in the four isomers. Stabilities of the Ti₂C₈₀ molecules are essentially\ndominated by Ti binding sites inside fullerene cages. The Ti atoms bind over the hexagon rings in preference\nto a junction between hexagon and pentagon rings. In the Ti₂C₂@C₇₈ molecules, orbital interactions between\nthe Ti₂C₂ cluster and the outer cage play a significant role in determining the C₂ bridging modes into the\ndititanium center and their relative stabilities.