Specificity of 15N NMR chemical shifts to the nature of substituents and tautomerism in substituted pyridine N-oxides

H-1, C-13, and N-15 NMR chemical shifts have been measured for 2-aminopyridine N-oxide (1), its eleven derivatives (2-10, 13, 14), and 3-Cl and 3-Br substituted 4-nitropyridine N-oxides (11, 12). delta(N-15) of pyridine ring nitrogen in 2-acetylaminopyridine N-oxides are 5.9-11.5 ppm deshielded from those in 2-aminopyridine N-oxides. When amino and acetylamino substituents are in 4-position, delta(N-15) of ring nitrogen is 21.3 ppm deshielded in the acetylated derivative. The strong resonance interaction between 2-amino and 5-nitro groups reflects in the decrease of amino nitrogen shielding about 5.3-17.9 ppm. Also, H-1 and C-13 NMR spectral data are in agreement with N-15 NMR results reflected as deshielded amino protons and carbons C-2 and C-5. The pyridine nitrogen chemical shift in all amino- and acetylamino derivatives vary between -101.2 and -126.7 ppm, which has been connected with the tautomeric balance in our earlier studies.