Tautomeric Equilibria Studies by UV-Vis Spectroscopy in β-diketones

In general, β-dicarbonyl compounds may exist in several tautomeric forms. It is well known that the cis-enol form of β-diketones is characterized by a strong intramolecular hydrogen bond. Benzoylacetone (BA) and trifluorobenzoylacetone (TFBA) can be considered as representative examples of chemically asymmetric β-diketone with different substitution at β positions. We study the theoretical and experimental UV absorption spectra to understand the absorption bands for tautomerism study in the absorption spectra to these molecules. The absorption spectrum are analyzed by (TD-DFT) at B3LYP/6-311++G(d,p) level of theory in normal hexane (ε =1.88) as solvent, using PCM method, where they are compared with regard to experimental Spectra. For both cis-enol-2 and cis-enol-4 forms of the target compounds, wavelength (l), oscillator strength (), major contributions of the calculated transitions and the theoretical and experimental enol-enol equilibrium constants with the experimental wavelength are given in Table 1. According to figure 1, three experimental bands are observed for these compounds, at about 325-330, 295-300, and 255-265 nm, as strong, shoulder, and weak band, respectively. According to the theoretical results, the strong and shoulder bands at about 325-330 and 295-300 nm (see Fig. 1) have been attributed to the cis-enol-4 and cis-enol-2 forms, respectively, the weak band at about 255-265 nm has been attributed to both cis-enol-2 and cis-enol-4 forms. These equilibrium constants have been obtained theoretically as ratio of oscillator strengths, and experimentally as ratio of absorbance as strong and shoulder bands. Therefore, consistent with the previous report [1-3], both cis-enol-2 and cis-enol-4 forms are present and coexist in the samples.