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The stability of two stable cis-enol forms in two categories of β-diketones, including para-substituted of trifluorobenzoylacetone (X-TFBA) and 1-aryl-1,3-diketone malonates (X-ADM, X: H, NO2, OCH3, CH3, OH, CF3, F, Cl, and NH2) has been obtained by different theoretical methods. According to our results, the energy difference between the mentioned stable chelated enol forms for the titled compounds is negligible. The theoretical equilibrium constants between the two stable cis-enol of the mentioned molecules are in excellent agreement with the reported experimental equilibrium constant. In addition, the effect of different substitutions on the intramolecular hydrogen bond strength has been evaluated. The correlation between Hammett para-substituent constants, σp. with the theoretical and experimental parameters related to the strength of hydrogen bond in p-X-TFBA and p-X-ADM molecules also investigated by means of density functional theory calculations. The electronic effects of para-substitutions on the intramolecular hydrogen bond strength were determined by NMR and IR data related to intramolecular hydrogen bond strength, geometry, natural bond orbital results, and topological parameters. These parameters were correlated with the Hammett para-substituent constants, σp. Good linear correlations between σp and the several parameters related to the hydrogen bond strength, in this study were obtained.

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Conformational stability, equilibrium constant between two stable cis-enol forms, and intramolecular hydrogen bonding (IHB) of benzoylacetone (BA) and p-substituted benzoylacetone (X-BA), where X=NO 2 , OCH 3 , CH 3 , OH, CF 3 , Cl, F, and NH 2 , have been investigated by means of density functional theory (DFT) calculations and compared with the reported experimental results. According to our calculations, the energy difference between the two stable chelated enol forms is negligible, about 0.35-1.1 kcal/mol ranges in the gas phase and different solvents. The electronic effects of p-substituted benzoylacetone on IHB strength were determined and established by NMR, IR spectra, geometry, and topological parameters with Hammett linear free energy relationships. Also, the linear correlation coefficients between σ p and selected parameters related to IHB strength, such as geometrical, topological parameters, IR and NMR spectroscopic data, and NBO results related to IHBs were considered. Good linear correlations between σ p and the mentioned parameters were obtained

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.