GUEST DISCRIMINATION IN COMPLEXES OF ALKALI-METAL CATIONS WITH THE C36H36 SPHERIPHANE - AN AB-INITIO ELECTRONIC-STRUCTURE STUDY

Large-scale electronic structure calculations, carried out at the HF/6 -311G* level of theory, reveal qualitative differences among the comp lexes of the C36H36 spheriphane with alkali metal cations. Although bo th endohedral and exohedral isomers are found to be energy minima for the Li+, Na+, and K+ guests, the formation of endohedral species proce eds without a barrier only in the first two cases. In the endohedral i somers, the Li+ cation is displaced from the cage center, whereas the Na+ and K+ cations are not. The size of the guest cation is also a con trolling factor for the complexation energies, which decrease in magni tude from Li+ to K+. The endohedral isomers are predicted to be more s table than their exohedral counterparts in the gas phase, but this ord er of stabilities is expected to be reversed in solvents that form str ong complexes with the guest cations. Bonding in the inclusion complex es of C36H36 is analyzed in terms of electrostatic, polarization, and steric effects and compared to that in the endohedral complexes of the C-60 fullerene.