H-1, C-13 AND O-17 NMR-STUDY OF SUBSTITUTED NITROPYRIDINES

H-1, C-13 and O-17 NMR spectra for 22 substituted nitropyridines were measured and their H-1 NMR spectra were analysed. The most significant variations in the NMR parameters are found for isomeric hydroxy derivatives, owing to the possibility of keto-enol tautomerism. The prevalence of the keto form is observed in 2- and 4-hydroxy derivatives, while the 3-hydroxy derivative exists in its enol form. Among the three nuclei studied, O-17 seems to be the best nucleus for probing the keto-enol tautomerism. No correlation is observed between the torsion angle of the nitro group and its O-17 NMR chemical shift. Molecular mechanics calculations were performed to clarify the torsional energetics of the nitro group and the preferences for keto-enol tautomerism.