Global and Local Shielding/Deshielding Variation in C60 and C70 Fullerenes Upon Reduction. Evaluation of Aromaticity, 13C-NMR and Anisotropy Patterns

Abstract

Fullerenes are statically pleasant species featuring symmetric cages, which can be modified upon reduction. Here, we theoretically account for the variation of 13C-NMR patterns in C60 and C70 upon six-fold reduction and the overall variation of the enabled shielding/deshielding regions induced by π and σ electrons according to different orientations of the external field and the related anisotropy. Our results show a significant modification of the chemical shift given by the main variation of the σ33 (or δ33) shielding component under the principal axis system (PAS) of the chemical shift anisotropy (CSA) at the representative carbon nucleus. For C606− a shielding cone property is enabled from any orientation, accounting for a significant spherical aromatic character. In contrast, in C706−, a shielding cone is reserved only for an axial-oriented field, with a deshielding cone behavior obtained from the complementary equatorial orientations. The overall anisotropy shows an inner isotropic region for C60 and C606−, with a continuous anisotropic outer contour for the latter. In contrast, C70 and C706− both show larger anisotropies, given the lesser spherical shape in addition to the modified π-surface. Such information is useful for further rationalizing the implementation of magnetic anisotropic molecular devices into fullerene-based materials.