Theoretical Insight into Sc₂C₇₆: Carbide Clusterfullerene Sc₂C₂@C₇₄ versus Dimetallofullerene Sc₂@C₇₆

In terms of density functional theory in combination with a statistical thermodynamic method, we have investigated the Sc₂C₇₆ species including dimetallofullerenes Sc₂@C₇₆ and carbide clusterfullerenes Sc₂C₂@C₇₄. Two dimetallofullerenes, Sc₂@C_s(17490)-C₇₆ and Sc₂@T_d(19151)-C₇₆, possess the lowest relative energies but exhibit poor thermodynamic stability within the fullerene-formation region (500-3000 K). In contrast, four carbide clusterfullerene isomers, Sc₂C₂@D_3h(14246)-C₇₄, Sc₂C₂@C_2v(14239)-C₇₄, Sc₂C₂@C₂(13333)-C₇₄, and Sc₂C₂@C₁(13334)-C₇₄, have excellent thermodynamic stability when considering the temperature effect. The Sc₂C₂@D_3h(14246)-C₇₄ isomer, which satisfies the isolated-pentagon rule (IPR), was characterized by its crystallographic structure; however, the other three non-IPR structures with two pairs of pentagon adjacencies are predicted for the first time. In particular, Sc₂C₂@C₂(13333)-C₇₄ and Sc₂C₂@C₁(13334)-C₇₄ are linked by a single Stone-Wales transformation. Meanwhile, bonding critical points and Mayer bond orders in the four isomers were analyzed to disclose the unique interactions between the inner clusters and cages. Additionally, the structural characteristics, ¹³C and ⁴⁵Sc NMR chemical shifts, and IR spectra of the four stable isomers are introduced to assist experimental identification and characterization in the future.