Malondialdehyde and acetylacetone. An AM1 study of their molecular structures and keto–enol tautomerism

Abstract

The semiempirical AM1 method has been used for a complete study of the molecular structures of all the possible conformations of malondialdehyde (MDA) and acetylacetone (ACAC) in order to obtain their order of stability and evaluated the hydrogen-bond strength, which here is expected to be stronger than in other organic compounds. Although the calculated lengths and bond angles agree fairly well with the experimental and ab initio values, when available, the degree of non-planarity of the diketo isomers is rather different from those reported in the literature. In particular, the most stable isomer of MDA is the W-trans one (the S-cis isomer shown no minimum) and the most stable isomer of ACAC has the CO groups rotated by ca. 93° in opposite directions. The enol tautomer is predicted to be 24.4 kJ mol^–1(MDA) and 19.2 kJ mol^–1(ACAC) more stable than the diketo tautomer. Although the diketo structures are found to be less polar than the hydrogenbonded ones, the solvation energies, calculated according to the Kirkwood formalism, show that they are the most stabilized on increasing the solvent dielectric constant. The hydrogen-bond strength is evaluated as 37.36 and 32.76 kJ mol^–1 in MDA and ACAC, respectively, in good agreement with the value expected on the grounds of i.r. measurements, but the barrier to be overcome in the proton-transfer process between the two equivalent asymmetrical enol forms appears to be strongly overestimated.