Mixed-metal
uranium-based endohedral clusterfullerenes, Sc2UX@C80 (X = C, N), which were recently reported in experiments,
have been investigated considering heptagon-containing isomers by
density functional theory calculations in conjunction with statistical
thermodynamic analysis. The triplet Sc2UC@I
h
(31924)-C80 and quartet Sc2UN@I
h
(31924)-C80, named after the spiral number (31924), are found to be
thermodynamically stable and satisfy aromaticity rules. Furthermore,
the restricted movements of the Sc2UX (X = C, N) cluster
in I
h
(31924)-C80 have been demonstrated via ab initio molecular dynamics simulations.
The six-electron transfer from the inner cluster to the cage results
in the electronic structures (Sc2UX)6+@C80
6– (X = C, N), which were also confirmed
by natural bond orbital analysis. On the basis of the frontier molecular
orbitals, the oxidation states of uranium in Sc2UC@C80 and Sc2UN@C80 are +IV and +III, respectively,
with residual electrons in 5f orbitals of U. The chemical bond between
U and C (N) of the inner cluster is characterized as a double bond
(single bond) by an analysis of the Mayer bond orders. There are covalent
interactions between the inner cluster and outer cage, which is clarified
by the quantum theory of atoms in molecules. IR spectra of the optimal
isomers have also been simulated, which show the clear difference
between Sc2UX@C80 (X = C, N). These findings,
together with simulated results, are expected to supply useful information
in future experiments of mixed-metal uranium-based endohedral clusterfullerenes.