Three possible isomers of C-121. i.e. [6,6]-[6,6] (1, D-2d), [6,6]-[6,5] (2, C-s), and [6,5]-[6,5] (3, C-2), were examined by ab initio MO calculations and a hybrid method of Hartree-Fock and DFT. The geometry optimizations gave closed-bond structures (methanofullerene) for all 6,6-bridge and open-bond structures (homofullerene) for all 6,5-bridge structures. Calculations of energies revealed that 2 is the most stable, while 1 is slightly less stable than 2 by Delta(DeltaG(f)degrees (298.15 K)) = 2.521 kJ mol(-1), and 3 is much less stable than 2 by Delta(DeltaG(f)degrees (298.15 K)) = 21.777 kJ mol(-1). That is explained by the difference of energy between a methanofullerene and a homofullerene, and the strain energy of the spirojunction at the central bridging carbon atom of C-121 The predicted IR and Raman spectra of 1, 2, and 3 indicate that all of 1, 2, and 3 have the same six cage-cage vibrational modes. By comparing the predicted C-13 NMR spectra of 1, 2, and 3, and the experimental C-13: NMR spectra of C-121 isolated by HPLC, we concluded that the structure of the C-121 is 2, which is the first C-60 dimer that has both a methanofullerene and a homofullerene moiety.