Ab inifio theoretical predictions of Cz8, C28H4, Cz8F4, (Ti@C&14,

Recent experiments have demonstrated that Czs is the smallest fullerene cage that successfully traps elements in its inside. In this work, we have studied the electronic structures, equilibrium geometries, and binding energies of the title molecules at the self-consistent field (SCF) Hartree-Fock level of theory employing basis sets of double-zeta quality. The empty Czs fullerene is found to have a 5A2 open-shell ground state and behaves as a sort of hollow superatom with an effective valence of 4, both toward the outside and inside of the carbon cage. The theoretical evidence suggests that C&H4 and C2sF4 should be stable molecules. The possibility of simultaneous bonding from the inside and outside of the Czs shell, as in (Ti@C2s)H4, is also explored. Our calculations show that the binding energy of the M@Czs species is a good indicator of the success in experimentally trapping the metal atoms (M) inside the fullerene cage. Based on these results, we propose that elements with electronegativities smaller than 1.54 should form endohedral fullerenes larger than a minimum size which depends on the ionic radius of the trapped atom. This qualitative model, correctly reproduces the available experimental evidence on endohedral fullerenes.