¹H, ¹³C and ¹⁷O NMR study of substituted nitropyridines

Abstract

Abstract 1 H, 13 C and 17 O NMR spectra for 22 substituted nitropyridines were measured and their 1 H 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, 17 O 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 17 O NMR chemical shift. Molecular mechanics calculations were performed to clarify the torsional energetics of the nitro group and the preferences for keto–enol tautomerism.