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

Abstract

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.

The Synergy between Nuclear Magnetic Resonance and Density Functional Theory Calculations

“…One case is shown in Figure 8 for 1-(n-pyridinyl)butane-1,3-diones. The calculated nuclear shieldings (B3LYP/6-311++G(d,p)) of both structure and nuclear shielding calculations for the two tautomers are weighted according to their calculated energies [ 28 ].…”

“… Possible hydrogen bond in 1-(n-pyridinyl)butane-1,3-dione other than the strong one to oxygen [ 28 ]. …”

“…One case is shown in Figure 8 for 1-(n-pyridinyl)butane-1,3-diones. The calculated nuclear shieldings (B3LYP/6-311++G(d,p)) of both structure and nuclear shielding calculations for the two tautomers are weighted according to their calculated energies [28]. In cases in which other intramolecular effects, such as hydrogen bonding to nitrogen, is present, DFT calculations of the structure are invaluable (see Figure 9).…”