Electronic Structures and Spectra of the Keto and Enol Forms of Acetylacetone

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Abstract
Near and vacuum UV absorption spectra of acetylacetone were measured in the vapor phase and in several organic solvents at various temperatures. From the temperature dependence of the spectra, the equilibrium constant and enthalpy change between the keto and enol forms of acetylacetone were determined to be 25 at 20 °C and −4.3 kcal/mol, respectively, in the vapor phase. Rydberg excitation bands were observed at 47170 and 53300 cm−1 for the enol form, and ≈52000 cm−1 for the keto form. The estimated values of the quantum defect indicate that the Rydberg orbitals are 3s and 3p. A modified CNDO-CI method was applied to the valence-shell transitions. A weak band observed at ≈34000 cm−1 for the keto form was assigned to the first n-π* transition, and relatively strong bands observed at 38020 and 56800 cm−1 for the enol form, to the first and second π-π* transitions, respectively. Special attention was paid to the charge-transfer (CT) character pertinent to the intramolecular hydrogen bond in the excited states of the enol form. Configuration analysis of the wave functions revealed us that the first π-π* band observed at 38020 cm−1 has the CT character.

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