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 (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 degrees around the CC-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-enol --> diketone 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-enol --> diketone) conversion is 245 kJ (.) mol(-1), that of the reverse (diketone --> omega-enol) conversion 222kJ (.) mol(-1); thus, an almost symmetric barrier which is not thermally accessible, is defined. The energy barrier for the sickle-enol --> diketone conversion is considerably lower (187 kJ (.) mol(-1)), to access the sickle form from the more stable omega form, a rotation is needed (energy barrier: 88 kJ (.) mol(-1)). The HF-calculated barriers are 1.3-1.4 times higher than those obtained with the BLYP method.