Theoretical investigation of solvent effect on the keto–enol tautomerization of pentane-2,4-dione and a comparison between experimental data and theoretical calculations

Abstract

The equilibrium constants of stable keto and enol forms of pentane-2,4-dione (known as acetylacetone) are estimated, using the second-order Møller−Plesset (MP2), density functional theory (B3LYP and M06-2X), composite methods (G4, G3, G3B3, CBS-QB3, and G3MP2B3), double-hybrid density functional theory (B2PLYP), and long-range corrected (LC) hybrid functional (ωB97X-D). These methods are integrated with the PCM, CPCM, and SMD models to elucidate the effect of solvent on thermodynamic parameters. The reported measured enol contents in the solutions and gas phase are utilized to benchmark the predictions of different quantum mechanical methods for the keto–enol equilibrium in acetylacetone. In this study, we calculated the enol content in 16 acetylacetone solutions and in the gas phase. Among the applied methods, the MP2 level and the B3LYP level underestimate and overestimate, respectively, the enol content of acetylacetone in the gas phase and solutions. The G3B3 and G3MP2B3 levels give reasonable agreement with the measured data. The best results obtained by calculations at the B2PLYP/6-31+G(2d,p) and CBS-QB3 levels, with mean absolute errors (MAE) relative to experiments of 2.30 and 5.45 and root mean square deviation (RMSD) errors of 0.78 and 1.66, respectively. According to our calculations, one enol and two keto forms (Ket1 and Ket2) coexist in polar solutions. The effect of solvent was more pronounced on the structure and stability of the Ket2 tautomer than others. The strength of the intramolecular hydrogen bond in the enol form of acetylacetone is almost independent of the solvent polarity.