Does solvent density play a role in the keto–enol tautomerism of acetylacetone?

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

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.

Electronic Interactions in Iminophosphorane Superbase Complexes with Carbon Dioxide

Iminophosphoranes or phosphazenes are an important class of compounds with increasing use in synthetic organic chemistry as neutral organic superbases exhibiting low nucleophilicity. Their electronic structure and therefore their properties strongly depend on substitution, but there have been very few theoretical studies devoted to this topic, and more specifically to the formation of electron donor-acceptor complexes of iminophosphoranes with electrophiles. In this work, we have investigated the interaction with carbon dioxide at different ab initio levels. Carbon dioxide usually behaves as a Lewis acid and the reaction with iminiphosphoranes has been described as a nonconventional aza-Wittig process leading to isocyanates. The reaction can be conducted in supercritical CO₂ conditions (carbon dioxide acts as both solvent and reactant), which is a promising strategy in the context of green chemistry. Our calculations have been carried out at the CCSD(T)/aug-cc-pVTZ//MP2/aug-cc-pVTZ level for model systems and at the M06-2X/6-611+G(d,p) level for a larger species used in experiments. The electronic interactions and the interaction energies are analyzed and discussed in detail using the natural bond orbital method. Proton affinities and gas-phase basicities are provided as well.

Modeling Solvation in Supercritical CO2

In recent decades, a microscopic understanding of solute-solvent intermolecular interactions has been key to advances in technologies based on supercritical carbon dioxide. In many cases, computational work has provided the impetus for new discoveries, shedding new light on important concepts such as the local structure around the solute in the supercritical medium, the influence of the peculiar properties of the latter on the molecular behavior of dissolved substances and, importantly, CO₂ -philicity. In this Review, the theoretical work that has been relevant to these developments is surveyed and, by presenting some crucial open questions, the possible routes to achieving further progress based on the interplay between theory and experiments is discussed.