Calculation of carbon-13 n.m.r. shielding constants of carbocations

Carbon‐13 n.m.r. shielding constants of isobutene molecules and t‐butyl carbenium ions are calculated using the NDDO quantum chemical approximations. The theoretical resonance shifts between corresponding nuclei of the two structures are compared with experimental values published by Olah et al. The results are discussed with respect to excitation energies which are obtained including Fischer‐Kollmar‐parametrization.