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

• Tautomeric equilibrium in solution of kynurenic acid derivatives. • Use of NMR to study the behavior of kynurenic acid derivatives. • Correlation between experimental and computationally calculated NMR d values. 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. [Display omitted] [ABSTRACT FROM AUTHOR]