V. Delchev, H. Mikosch, G. Nikolov:
"The Keto-Enol Equilibrium of Pentane-2,4-dione Studied by ab initio Methods";
Monatshefte für Chemie, 132 (2001), 3; S. 339 - 348.

Kurzfassung englisch:
The mechanism of the keto-enol interconversion of pentane-2,4-dione (trivial name: acetylacetone, acac) was examined at the restricted Hartree-Fock (HF) level and the DFT correlation functional BLYP method using the 6-311G** basis, both included in the program GAUSSIAN 98. Two initial enol forms are considered: the omega and sickle forms, related by a rotation of 180° around the OC*CC bond. The study is restricted to the through-space transfer of the hydroxyl proton to C(2). The two geometry-optimized enol forms are planar; the geometry optimization of the diketone forms leads to the same non-planar structure, regardless of the starting enol geometry. The transition state of the through-space omega-enolMdiketone conversion has also a non-planar structure, indicating that the hydroxyl proton moves outside of the CCC plane. The BLYP-calculated energy barrier of the forward (omega-enolMdiketone) conversion is 245 kJ·molm1, that of the reverse (diketoneMomega-enol) conversion 222 kJ·molm1; thus, an almost symmetric barrier, which is not thermally accessible, is defined. The energy barrier for the sickle-enolMdiketone conversion is considerably lower (187 kJ·molm1), to access the sickle form from the more stable omega form, a rotation is needed (energy barrier: 88 kJ·molm1). The HF-calculated barriers are 1.3-1.4 times higher than those obtained with the BLYP method.

ab initio Calculations; Pentane-2 · 4-dione; Keto-enol equilibrium; Transition states

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