Carbon-13 Shift Tensors in Polycyclic Aromatic Compounds. 8. A Low-Temperature NMR Study of Coronene and Corannulene

Abstract

The principal values of the 13 C chemical shift tensors were measured for coronene and corannulene, both at room temperature and at approximately 100 K. At room temperature the molecules are moving, resulting in a motionally averaged powder pattern. A comparison of the principal values between the room temperature motionally averaged pattern and the low-temperature static pattern provides experimental information about the orientation of the principal axis system of the shift tensor for the bridgehead carbons in these molecules. For corannulene, the orientation of δ 33 component was determined to lie at an angle of 13°from the rotation axis (the 5-fold symmetry axis of the molecule) for the inner bridgehead carbons and at an angle 26°from this same rotation axis for the outer bridgehead carbons. These orientations are in good agreement with the angles necessary to place the δ 33 component along the p orbitals involved in π-bonding at these carbons. In the case of coronene, the differences between the principal values at the two temperatures indicate there is an angle of 14°between the axis of rotation and the δ 33 component for both the inner and outer bridgehead carbons. This indicates that the motion is not constrained to simple in-plane rotation, but must also have an out-of-plane component. Quantum chemical calculations of the shielding tensors were also completed using both experimental and optimized molecular geometries. The results of the calculations are in good agreement with the experimental findings.

Counter-Rotating Ring Currents in Coronene and Corannulene

“…Computed carbon shieldings and shifts are shown in Table 3. Excellent agreement with experiment is found for the isotropic shifts,28, 29 whereas component shifts, as also noted for the GIAO calculations reported in reference 28, are in only qualitative agreement with those extracted from solid‐state 13 C NMR measurements; tensor components are generally more sensitive than mean shieldings to basis and level of theory 30. In the calculated shifts for 2 , the main change observed on going from planar to bowl shapes is a shift downfield of the hub carbon atoms, which is compatible with the expected re‐hybridization on bending 23, 31.…”

Birch Reduction of Graphite. Edge and Interior Functionalization by Hydrogen

“…29,30 An appreciation for just how shielded the aromatic carbons are in these two materials can be gained by considering chemical shifts in model compounds. 32 The very shielded aromatic carbon nuclei in graphite 29,30 and hydrogenated graphene clearly relax very slowly for the reasons noted above. 32 The very shielded aromatic carbon nuclei in graphite 29,30 and hydrogenated graphene clearly relax very slowly for the reasons noted above.…”

13C Chemical‐shift tensors in an analogous series of heterosubstituted polycyclic aromatic compounds

“…In a single-crystal study of naphthalene 25 the agreement between theory and experiment for the orientations was good. The chemical-shift tensor component orientations in sp 2 carbons follow a general trend. 26 The υ 11 component of C-H carbons lies in the aromatic plane near the C-H bond.…”