Correlation of substituted aromatic β-diketones' characteristic protons chemical shifts with Hammett substituent constants

Influence of dibenzoylmethane's substituents in meta and para positions on chemical shift values of tautomers' characteristic protons was investigated in four solvents with ¹H NMR spectroscopy: acetone-d₆, benzene-d₆, CDCl₃ and deuterated dimethyl sulfoxide (DMSO-d₆). It was proved that the influence of substituents on chemical shifts strongly depends on the kind of the solvent; the greatest changes were observed in benzene-d₆ and the smallest in CDCl ₃. In acetone-d₆ and DMSO-d₆, the influence of substituents on chemical shifts is similar and the most regular. It allowed a fair correlation of chemical shifts of para-substituted dibenzoylmethane derivatives' characteristic protons with Hammett substituent constants in these solvents. In CDCl₃, characteristic protons' chemical shifts were near ¹H NMR spectroscopy measurement error limits, and, therefore, correlation with Hammett substituent constants in this solvent was unsatisfactory. In benzene, although the changes of chemical shifts are the most evident, the changes are also the most irregular, and, therefore, correlation in this solvent failed completely. Results of meta-substituted derivatives were much more irregular, and their correlation with Hammett substituent constants was poor in all investigated solvents.