Estudo teórico do equilíbrio tautomérico e conformacional de B-dicetonas

Abstract

Beta-diketones are a class of organic compounds characterized through the presence of two carbonyls separated by a carbon unit. Historically, the study of their structures provided the first experimental data on the phenomenon called keto-enol tautomerism, in addition to the particular equilibrium between different enol forms being a source of data for the study of low-energy barrier proton transfer processes. In these works, in general, thermodynamic analyzes are based on nuclear magnetic resonance (NMR) spectroscopy and the evaluation of the constants of keto-enol and enol-enol equilibria are performed from theoretical studies using computational chemistry tools, analyzing minimum structures of each equilibrium. Few theoretical works on beta-diketone systems estimate the energy barriers of tautomerism and its relationship with the substituent and the polarity of the medium, and these few do so using methods of high computational cost with large basis sets, and very often without a study of vibrational and electronic absorption spectroscopies. With the motivation to contribute in this research area, a theoretical study of the conformational, keto-enol and enol-enol equilibria for the following series of B-diketones was carried out in this work: acetylacetone (Acac), benzoylacetone (Bzac), dibenzoylacetone (Dbm), trifluordibenzoylacetone (Btfa), trifluoroacetylacetone (Tfac) and Haxafluoroacetylacetate (Hfac), where low cost computational methods were employed, namely the semi-empirical AM1 method, the density functional theory method with the B3lyp functional and the ab initio Hartree-Fock (HF) method, the last two using the 6-31G basis. The results according to the used methods showed agreement with experimental data from the literature in respect to the ketone tautomers having greater conformational rotational freedom on the ketone ring against to the enolic tautomers. In respect to the keto-enol equilibrium, the influence of the substituent groups on energy barriers is observed, in the case of the studied compounds, the phenyl group (Ph) reducing the barrier height, and the trifluoromethyl group (CF3) increasing it. It is also noted that the transition states (TS) of the keto-enol equilibrium behave according to Hammond's postulate, having their energy destabilized when it has a structure similar to the keto form, which has greater energy than the enol, and stabilized energy, when it resembles the enol tautomer, in general more stable. In the enol-enol equilibrium, the barrier has the same behavior as in the keto-enol case, being influenced by the substituents. Using theoretical vibrational analysis, Gibbs barrier estimation, structural prediction, population analysis of molecular orbitals and prediction of ultraviolet absorption spectra, it was verified that the transition state presents a structure stable enough to provide the establishment of an ultrafast equilibrium between the reagent and product minima. The beta-diketones that have phenyl groups allow to characterize this equilibrium more consistently. To the hydrogen centralization is ascribed a larger ionic or covalent character, a greater TS barrier being found when covalent and smaller when ionic. The Ph substituting group reinforces the ionic character, while the CH3 and CF3 groups the covalent character.