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

The principal values of the 13C 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 δ33 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 δ33 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 δ33 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.