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

The 3 He atom is an excellent NMR probe, particularly when enclosed in endohedral helium fullerenes. The 3 He chemical shift, δ ( 3 He), 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 δ ( 3 He) dramatically and unexpectedly, particularly for the most symmetric and also the most abundant C 60 and C 70 cages. While the 3 He atom is shielded by ∼43 ppm upon charging the He@C 60 to He@C 60 6- , it is correspondingly deshielded by ∼37 ppm in the He@C 70 /He@C 70 6- pair. Here, we show that such puzzling differences in δ ( 3 He) relate to the high symmetry of the host fullerene cages. While similar shielding is induced at the 3 He 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 3 He.