π‐Electron Ring‐Current Effects in Multiple Adducts of 3He@C60 and 3He@C70: A 3He NMR Study

Bis- to hexakisadducts of C60 (1–6) and mono- to tetrakisadducts of C70 (7–13) containing 3He atoms (endo-hedral helium complexes) were prepared and studied by 3He NMR spectroscopy to determine the influence of degree of functionalization and addition pattern on the chemical shift of the 3He atom. In the series of C60 derivatives, which included the previously measured 6–6 monoadduct 3He@C61H2, the 3He resonance was shifted considerably upfield relative to that of 3He@C60 (δ = −6.36) up to the bisadduct 1 (δ = −11.45). The resonances of the higher adducts 3–6, however, were shifted only slightly further up-field and all appeared in a rather narrow spectral range between δ = −11.84 and −12.26. The absence of further substantial upfield shifts was rationalized in terms of the compensation of deshielding due to the functionalization-induced decrease in the diamagnetic π-electron ring currents that extend around the fullerene sphere by the shielding that results from the weaker pentagonal-ring paramagnetic currents and the increased number of localized benzenoid substructures in 3–6. In contrast, all 3He resonances of the C70 adducts are shifted downfield relative to the signal of 3He@C70 (δ = −28.81). A monotonic relationship exists between the chemical shift and the degree of functionalization, whereby the 3He chemical shifts steadily decrease from monoadduct 7 (δ = −27.53) to the isomeric tetrakisadducts 12 (δ = −21.09) and 13 (δ = −20.68). This deshielding was explained by the reduction of the strong diamagnetic ring currents extending along the polar corannulene perimeters as a result of the functionalization at α- and β-type polar bonds in 7–13. In both series, the addition pattern was also found to have a distinct influence on the 3He chemical shift.