Application of Hammett equation to intramolecular hydrogen bond strength in para-substituted phenyl ring in some β-diketones and schiff bases

The stability of stable cis-enol forms in three categories of β-diketones, including para-substituted of benzoylacetone (X-BA), trifluorobenzoylacetone (X-TFBA) and 1-aryl-1,3-diketone malonates (X-ADM), and 2-(((1 phenylethyl)imino)methyl)phenol ( X-PIMP), as a Schiff base with chiral carbon, has three conformers that, X=H, NO2, OCH3, CH3, OH, CF3, F, Cl, and NH2, have been obtained by theoretical methods. According to the theoretical results, the energy difference between the mentioned stable chelated enol forms for the titled compounds is negligible. In addition, the effect of electronic different substitutions on the intramolecular hydrogen bond (IHB) has been evaluated. The correlation between experimental Hammett s para function, (p) with the theoretical and experimental parameters related to the strength of hydrogen bond in p-X-BA, p-X-TFBA, p-X-ADM and p-X-PIMP molecules also investigated by means of DFT calculations at B3LYP/6-311++G(d,p) level of theory. The electronic effects of para-substitutions on the IHB strength were determined by NMR and IR data related to IHB strength, geometry, NBO results, and topological parameters. These parameters were correlated with the Hammett para-substituent constants, σp [1-3]. Good linear correlations between σp and the several parameters, related to the hydrogen bond strength, were obtained and showed in Table 1, to our knowledge as (Parameters=p+ const.), equation.