Non‐additive carbon‐13 NMR substituent chemical shifts. II—1,3‐disubstituted benzenes

Aromatic 13C chemical shifts are reported for an extensive set (15 common substituents) of 1,3‐disubstituted benzenes measured at low concentration in deuteriochloroform. The substituent chemical shift data show systematic non‐additivity. Data for the para‐ and two meta‐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 meta‐oriented substituents, and (b) the shift‐charge ratio term extends past the ipso‐position. These results are discussed in terms of local paramagnetic shielding contributions to 13C shifts.