Rational synthesis, enrichment, and C-13 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.