Theoretical Insight into Sc₂O@C₈₄: Interplay between Small Cluster and Large Carbon Cage

Very recently, a series of endohedral fullerenes Sc₂O@C_2n (n = 35-47) were facilely produced. However, only two of them have been further characterized so far. Theoretically, we studied another discandium oxide endohedral fullerene without any characterizations, Sc₂O@C₈₄, which is the second most-abundant species in terms of relative heights of all mass spectrum peaks. Two thermodynamically stable isomers with isolated pentagon rule-obeying cages were found, namely, Sc₂O@C_2v(51575)-C₈₄ and Sc₂O@C₁(51580)-C₈₄. This is the first case that an endohedral fullerene containing the C_2v(51575)-C₈₄ cage acts as the lowest-energy isomer, and it is the first report of a clusterfullerene containing the C₁(51580)-C₈₄ cage. The endohedral Sc₂O cluster can keep its ideal structure after encapsulation, while both C₈₄ cages have deformed dramatically. Orbital analysis suggests that nucleophilic and oxidization reactions of both isomers should take place on the cage, while regioselectivity of Sc₂O@C_2v(51575)-C₈₄ and Sc₂O@C₁(51580)-C₈₄ is different due to their different characteristics of the highest occupied orbital distribution. Two-dimensional electron localization function and Laplacian of electron density maps unambiguously indicate strong electrostatic interactions exist between one scandium atom and the oxygen one. Meanwhile, overlaps of occupied metal atom orbitals and the cage ones along with Mayer bond order analysis identify that covalent interactions between a scandium atom and each C₈₄ cage cannot be neglected. At last, ¹³C NMR, UV-vis-NIR, and IR spectra of both Sc₂O@C₈₄ isomers were simulated theoretically. Because of their structural difference, all spectra between two isomers are significantly divergent. Consequently, these spectra are helpful to distinguish Sc₂O@C_2v(51575)-C₈₄ and Sc₂O@C₁(51580)-C₈₄ in further experimental characterizations.