2-Hydroxy-5-nitropyridine and 5-nitro-2-pyridone: Tautomerism, infrared, Raman, and NMR spectral interpretations, normal coordinate analysis, and DFT calculations

keto–enol tautomerism was investigated for 2‐hydroxy‐5‐nitropyridine (HNP) using quantum mechanical calculations combined with spectral (Raman, infrared, and nuclear magnetic resonance ) analyses. The results totally favor the keto‐NPO, 5‐nitro‐2‐pyridone, tautomer, which occurs as H‐bonded dimer in the solid state. Confident vibrational assignments were presented aided by normal coordinate analysis and potential energy distributions. The calculated/observed 13C/15N NMR chemical shifts verified the dominant keto‐NPO in solution.