ELECTRONIC-STRUCTURE OF C-28, PA-AT-C-28, AND U-AT-C-28

Electronic structure calculations, including relativistic core potenti als 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 splittin gs, the electron affinity, and the ionization potential are computed f or C-28. The ground state of C-28 is described well by the Hartree-Foc k wave functions, but other states are not. The computed electron affi nity and ionization potential are similar to those of C-60. Strong met al-cage binding is found for Pa@C-28 and U@C-28, similar to that in U( C8H8)(2). The ground electronic states depend on the order of the lowe st-energy cage pi and metal 5f orbitals, with (pi*)(1) and (pi*)(1)(5 f)(1) found to be the ground electronic configurations for the two com plexes. U@C-28 is found to be diamagnetic.