Nonplanarity and its Consequences

Due to its high symmetry, 8 exhibits only one signal in its H NMR spectrum (7.81 ppm), and three NMR lines at 127.2 ppm (10 methine rim carbons), 130.8 ppm (5 quaternary rihi carbon atoms), and 135.8 ppm (5 quaternary hub carbon atoms). The proton resonance of 8 is significantly shifted upfield, as compared to coronene 9 (8.9 ppm), which may reflect an influence of the curvature of the carbon network on the ring current. 

The formation of unusually light colored charge-transfer complexes of 8 with both picric acid and trinitrobenzene may be attributed to the decrease of the interaction between the donor and acceptor molecules due to the curvature of the former.  

In spite of this substantial curvature, corannulene undergoes rapid bowl-to-bowl inversion in solution as demonstrated by the dynamic NMR behavior of its dimethyl carbinol derivative a single signal for the methyl groups appears at ambient temperature while the diastereotopic methyls become observable separately below -64 °C. A barrier to inversion was determined to be AG = 10.2 kcal/mol. In a similar fashion, a barrier of 11.4 kcal/mol was determined for isdpropylcoran-nulene.  

The partially hydrogenated ring of dihydtocorannulene constitutes a 1,3-cyclo-hexadiene ring, a system that has been well-studied with respect to its geometry and the conformational preferences of substituents. However, the curvature of the corannulene surface introduces an additional stereochemical factor that makes the conformational analysis especially interesting. 1,3-Cyclohexadiene (23) and 9,10-dihydrophenanthrene (24) serve as models they are structurally similar systems, and their stereochemistry and conformational preferences are well documented in the literature. In both cases, the reduced ring adopts a nonplanar, semi-chair conformation of symmetry. 

The geometry of the 1,3-cyclohexadiene ring can be described by the torsion angle a which represents the conformation along the linkage (C -C, -Cj- 

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