PI-ELECTRON RING-CURRENT EFFECTS IN MULTIPLE ADDUCTS OF HE-3-AT-C-60 AND HE-3-AT-C-70 - A HE-3 NMR-STUDY

Bis- to hexakisadducts of C-60 (1-6) and mono- to tetrakisadducts of C -70 (7-13) containing He-3 atoms (endohedral helium complexes) were pr epared and studied by 3He NMR spectroscopy to determine the influence of degree of functionalization and addition pattern on the chemical sh ift of the 3He atom. In the series of C-60 derivatives, which included the previously measured 6-6 monoadduct He-3@C61H2, the He-3 resonance was shifted considerably upfield relative to that of He-3@C-60 (delta = -6.36) up to the bisadduct 1 (delta = -11.45), The resonances of th e higher adducts 3-6, however, were shifted only slightly further upfi eld and all appeared in a rather narrow spectral range between delta = -11.84 and -12.26, The absence of further substantial upfield shifts was rationalized in terms of the compensation of deshielding due to th e functionalization-induced decrease in the diamagnetic pi-electron ri ng currents that extend around the fullerene sphere by the shielding t hat results from the weaker pentagonal-ring paramagnetic currents and the increased number of localized benzenoid substructures in 3-6. In c ontrast, all He-3 resonances of the C-70 adducts are shifted downfield relative to the signal of He-3@C-70 (delta = -28.81), A monotonic rel ationship exists between the chemical shift and the degree of function alization, whereby the 3He chemical shifts steadily decrease from mono adduct 7 (delta = -27.53) to the isomeric tetrakisadducts 12 (delta = -21.09) and 13 (delta = -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 alpha- and beta-type polar bonds in 7-13. In both series, the addition pattern was also found to have a distinct influence on the He-3 chemi cal shift.