NMR Chemical Shifts. 2. Interpretation of the Carbon Chemical Shifts in Monocyclic Aromatic Compounds and Carbenes

The physical basis of the well-known linear correlation between 13C chemical shift and charge density in monocyclic aromatic ions has been investigated. Structures of the ions were calculated at the MP2/6-31G* level, and their chemical shifts were calculated using GIAO and IGAIM at the B3LYP/6-311+G(3df,2p) level using the MP2 geometries. The calculated tensor components of the shifts are in good accord with the experimental data. The linear correlation of chemical shift with charge was extended to CH3 + and planar CH3 -, and the physics underlying this correlation has also been found to explain shielding trends for singlet carbenes. The tensor components along the in-plane axes for CH3 - are close to the maximum diamagnetic shielding for carbon. The corresponding components for CH3 + are 700 ppm downfield from CH3 -, largely resulting from paramagnetic current densities about the in-plane axes. On the other hand, the tensor components about the out-of-plane axis, resulting from magnetically induced current densities in the plane of these ions, are all essentially the same. The chemical shifts for these species are not directly related to the charge at carbon but instead depend on the difference in the occupancies of the p orbitals normal to the field direction. This is further supported by calculations of the shielding in C4+, C2+, and C4-, all of which are more shielded than TMS.