On the 13C-NMR chemical shift anisotropy patterns and aromatic character in strained fullerenes: Computational analysis of D6h/D2d-C36 fullerene

The NMR characterization of small C36 hollow fullerene exposes a stable cage with structural features based on its strained curved π-surface in a D6h-symmetry. Our results indicate that planar-like aromatic properties of the D6h-C36 isomer decrease to a nonaromatic cage for D2d-C36 after Stone-Wales transformation of an equatorial [6.6] bond. This is given by the decrease of conjugation in the strained π-surface after Stone-Wales transformation. Calculated nuclear shielding shows that the main shielding component is always oriented perpendicularly to the cage π-surface despite the larger curvature degree, as a result of the sp2 character of carbon atoms, resembling C60. Moreover, the D6h- and D2d-C36 cages differ in their charge distribution properties, leading to charge accumulation in the strained cap of the former that leads to a polymerization-prone cage, which contrasts with that expected for the D2d structure. Hence, structural rearrangements of these non-Hirsch aromatic fullerenes show that their inherent global behavior requires considering the structural features besides electron counting rules.