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

Abstract

Abstract Bis‐ to hexakisadducts of C 60 ( 1–6 ) and mono‐ to tetrakisadducts of C 70 ( 7–13 ) containing 3 He atoms (endo‐hedral helium complexes) were prepared and studied by 3 He NMR spectroscopy to determine the influence of degree of functionalization and addition pattern on the chemical shift of the 3 He atom. In the series of C 60 derivatives, which included the previously measured 6–6 monoadduct 3 He@C 61 H 2 , the 3 He resonance was shifted considerably upfield relative to that of 3 He@C 60 (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 3 He resonances of the C 70 adducts are shifted downfield relative to the signal of 3 He@C 70 (δ = −28.81). A monotonic relationship exists between the chemical shift and the degree of functionalization, whereby the 3 He 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 3 He chemical shift.