MULTINUCLEAR MAGNETIC-RESONANCE AND X-RAY-DIFFRACTION STUDIES OF AMINONITROPYRIDINES

The N-15 NMR spectra for 21 aminonitropyridines were measured and their chemical shifts assigned. The H-1 and C-13 NMR chemical shifts and spin-spin coupling constants were also determined for 16 compounds of this series. In order to relate the structural properties of nitramino groups and their N-15 NMR chemical shifts in 2- and 4-nitramino-3-nitropyridines, which differ remarkably from all other amino groups studied, low-temperature H-1 NMR, O-17 NMR, comparative INEPT and IR spectroscopic studies were carried out. In addition, the x-ray crystal structure of 2-nitramino-3-nitropyridine was determined. Comparative spectroscopic studies showed that the nitramino derivatives exhibit different characteristics to the other compounds studied. Based on the x-ray structural data, the nitramino group differs markedly from separate aryl-bound amino and nitro groups. The nitramino group does not exhibit prototropic tautomerism in the crystalline state. This is in agreement with the small variation in the N-15 NMR chemical shifts of the pyridine nitrogen, which excludes the prevalence of pyridinium ions. Among the four NMR nuclei studied, N-15 seems to possess the best predictive power regarding the exceptional properties of the nitramino group.