Tautomerism of pyridinylbutane-1,3-diones: An NMR and DFT study

The three possible 1-(n-pyridinyl)butane-1,3-diones (nPM) have been synthesized. Structures, tautomerism, and conformations are investigated by means of DFT calculations. ¹ H and ¹³ C NMR spectra are assigned, and deuterium isotope effects on ¹³ C chemical shifts have been measured. Analysis of the isotope effects leads to the equilibrium constants of the keto-enol tautomers. Some interesting differences are seen between the three compounds and the phenyl analogs. The isotope effects can also rank the hydrogen bonds of the compounds, with the one with nitrogen in the three positions of the pyridine ring as the weakest. Structures, conformers, energies, and NMR nuclear shieldings are calculated using DFT calculations at the B3LYP/6-311++G(d,p) level.