C-13 NMR-STUDY OF THE SUBSTITUENT EFFECTS IN ORTHO-SUBSTITUTED NITROBENZENES

C-13 NMR chemical shifts of 22 ortho-substituted nitrobenzenes are reported. Except for those on C-3 and C-5, the effects of the interacting nitro group and the substituent are non-additive. The chemical shifts of the ipso, ortho and para carbons in the spectra and those of monosubstituted benzenes are linearly dependent. Multiparameter correlation analysis shows a predominant field/inductive effect on the ipso compared with both the ortho and para carbon chemical shifts. It shows that the Reynolds non-electronic factors evaluate the substituent chemical shifts of the ipso and ortho positions much better than the Charton steric parameters. Moreover, both in ortho- and para-substituted nitrobenzenes the substituent exerts almost the same shift on the ipso carbon atom. This suggests that the studied compounds, except those carrying very bulky groups, show a high tendency to be planar.