Rational synthesis, enrichment, and (13)C NMR spectra of endohedral C(60) and C(70) encapsulating a helium atom.

Endohedral fullerenes encapsulating a helium atom, i.e., He@C(60) and He@C(70), at occupation levels of 30% were prepared by rational chemical synthesis. The existence of weak interactions between the inner helium and the outer fullerene cages was demonstrated by experimental and computational investigations.