The essential role of symmetry in understanding He-3 chemical shifts in endohedral helium fullerenes

The He-3 atom is an excellent NMR probe, particularly when enclosed in endohedral helium fullerenes. The He-3 chemical shift, delta(He-3), in fullerenes spans a range from ca. -50 to +10 ppm, and changes sensitively between different cages, isomers, and external substituents. Reduction of the fullerenes to anions changes the delta(He-3) dramatically and unexpectedly, particularly for the most symmetric and also the most abundant C-60 and C-70 cages. While the He-3 atom is shielded by similar to 43 ppm upon charging the He@C-60 to He@C-60(6-), it is correspondingly deshielded by similar to 37 ppm in the He@C-70/He@C-70(6-) pair. Here, we show that such puzzling differences in delta(He-3) relate to the high symmetry of the host fullerene cages. While similar shielding is induced at the He-3 atom by the core orbitals of different cages, the symmetry of the cage allows or quenches large paramagnetic, i.e., deshielding orbital interactions of frontier orbitals upon charging of the cage, which is directly responsible for the large observed chemical shift range of endohedral He-3.