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. NMR and aromatic properties of smaller fullerenes from Stone‐Wales transformation‐connected D6h‐ and D2d‐C36 shows a shift from a planar‐like aromatic to a nonaromatic cage.