Electronic Structure of C28, Pa@C28, and U@C28

Electronic structure calculations, including relativistic core potentials and the spin−orbit interaction, have been carried out on the C 28, Pa@C 28, and U@C 28 species. Excitation energies, spin−orbit splittings, the electron affinity, and the ionization potential are computed for C 28 . The ground state of C 28 is described well by the Hartree−Fock wave functions, but other states are not. The computed electron affinity and ionization potential are similar to those of C 60 . Strong metal-cage binding is found for Pa@C 28 and U@C 28, similar to that in U(C 8 H 8 ) 2 . The ground electronic states depend on the order of the lowest-energy cage π* and metal 5f orbitals, with (π*) 1 and (π*) 1 (5f) 1 found to be the ground electronic configurations for the two complexes. U@C 28 is found to be diamagnetic.