The keto-enol equilibrium of pentane-2,4-dione studied by ab initio methods

The mechanism of the keto-enol interconversion of pentane-2,4-dione (trivia l name: acetylacetone, acac) was examined at the restricted Hartree-Fock (H F) level and the DFT correlation functional BLYP method using the 6-311G(** ) basis, both included in the program GAUSSIAN 98 Two initial enol forms ar e considered: the omega and sickle forms, related by a rotation of 180 degr ees around the CC-CC bond. The study is restricted to the through-space tra nsfer 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 s ame non-planar structure, regardless of the starting enol geometry. The tra nsition state of the through-space omega-enol --> diketone conversion has a lso a non-planar structure, indicating that the hydroxyl proton moves outsi de of the CCC plane. The BLYP-calculated energy barrier of the forward (ome ga-enol --> diketone) conversion is 245 kJ (.) mol(-1), that of the reverse (diketone --> omega-enol) conversion 222kJ (.) mol(-1); thus, an almost sy mmetric barrier which is not thermally accessible, is defined. The energy b arrier for the sickle-enol --> diketone conversion is considerably lower (1 87 kJ (.) mol(-1)), to access the sickle form from the more stable omega fo rm, a rotation is needed (energy barrier: 88 kJ (.) mol(-1)). The HF-calcul ated barriers are 1.3-1.4 times higher than those obtained with the BLYP me thod.