Structural analysis of Kynurenic acid and derivatives by NMR spectroscopy and DFT calculations

The 4-quinolinone derivatives present a keto-enol tautomerism, which can be studied using different analytical techniques, including NMR. In this work, the spectroscopic study in solution of numerous derivatives with different substitution patterns in positions 1-4 of the quinolone rings, and their correlation with simulated spectra by density functional theory-gauge independent atomic orbitals method (DFT-GIAO) approximation is presented. Polyfunctionalized derivatives where the tautomeric equilibrium is blocked by N- or O-substitution allow us to observe the different behavior in chemical shifts by NMR. The study of some derivatives in the solid state by ss-NMR and X-ray Crystallography is also included.