EFFECT OF THE ORTHO-NITRO GROUP ON THE C-13-NMR CHEMICAL-SHIFTS OF SUBSTITUTED PYRIDINES

The C-13 NMR spectra of 40 vicinally substituted 2- and 3-nitropyridines were recorded. The observed chemical shifts are, in general, non-additive. These results are discussed in terms of the effects of 2-, 3- and 4-substitutents on the ring carbons and of the position of the fixed nitro group and the substituent. The chemical shifts of the para carbon, with respect to the substitutent, follow well the dual substituent parameter approach, which shows their electronic origin. However, the substituent chemical shifts of both ipso and ortho positions can be rationalized, in terms of multiple parameter substituent correlation analysis, only on a approximate level.