Ab initio study of the keto-enol equilibrium of malonaldehyde

The mechanism of the keto-enol tautomerism of malonaldehyde was studied by ab initio methods using 6-21G** and 6-311G** basis functions at the HF leve l. Two separate mechanisms were examined: through-space proton transfer in the omega-shaped form and through-space proton transfer in a sickle-shaped form obtained from the omega form by rotation. The transition state structu re of the omega form is non-planar, whereas that of the sickle form is plan ar. The sickle form is connected with a 2(nd) Order saddle, indicating that there should exist a lower energy barrier, i.e. that the through-bond mech anism may be preferred. The calculated energy barriers of keto-enol tautome rism for the sickle form is twice as high as those for the omega form.