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 complexes of the C36H36 spheriphane with alkali metal cations. Although both endohedral and exohedral isomers are found to be energy minima for the Li+, Na+, and K+ guests, the formation of endohedral species proceeds without a barrier only in the first two cases. In the endohedral isomers, 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 controlling factor for the complexation energies, which decrease in magnitude from Li+ to K+. The endohedral isomers are predicted to be more stable than their exohedral counterparts in the gas phase, but this order of stabilities is expected to be reversed in solvents that form strong complexes with the guest cations. Bonding in the inclusion complexes 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.