Influence of endohedral confinement on the electronic interaction between He atoms: a He2@C20H20 case study.

The electronic interaction between confined pairs of He atoms in the C(20)H(20) dodecahedrane cage is analyzed. The He-He distance is only 1.265 A, a separation that is less than half the He-He distance in the free He dimer. The energy difference between the possible isomers is negligible (less than 0.15 kcal mol(-1)), illustrating that there is a nearly free precession movement of the He(2) fragment around its midpoint in the cage. We consider that a study of inclusion complexes, such as the case we have selected and other systems that involve artificially compressed molecular fragments, are useful reference points in testing and extending our understanding of the bonding capabilities of otherwise unreactive or unstable species. A key observation about bonding that emerges uniquely from endohedral (confinement) complexes is that a short internuclear separation does not necessarily imply the existence of a chemical bond.