13C Substituent chemical shifts of the carbonyl sites in the side-chains of meta-and para-substituted benzenes of the type XC,H,COZ have been measured. Analysis of this data using the dual substituent parameter method shows that inductive effects are predominant, The reverse inductive contribution observed is explained in terms of a xpolarization mechanism. Critical support for this mechanism is obtained from additional series where the carbonyl is complexed with Lewis acids. The concepts of ' extended ' and ' localized ' n-polarization are discussed. -2.23 -0.

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Aromatic uC chemical shifts are reported for an extensive set (15 common substituents) of l,%-disubstituted benzenes measured at low concentration in deuteriochloroform. The substituent chemical shift data show systematic non-additivity. Data for the paru-and two rneta-positions to the variable substituent were analysed using the recently developed non-linear resonance (DSP-NLR) and standard (DSP) dual substituent parameter equations. It is shown for these positions that mutual substituent interaction is responsible for the non-additive shifts and that (a) this interaction has both an inductive (polar) and a resonance component, even for rnefu-oriented substituents, and (b) the shift-charge ratio term extends past the &so-position. These results are discussed in terms of local paramagnetic shielding contributions to =C Shifts.