Spectroscopic detection of theendo-enol form of 2-acetylcyclopentane-1,3-dione

Abstract

According to vibrational spectroscopic data, in the gas phase at 280 °C and in crystals prepared by sublimation, 2-acetylcyclopentane-1,3-dione was established to exist not as trione 1 but as the enol-form 2 with an exo-cyclic C=C bond (v(C=C) = 1593 cm−1) and an intramolecular hydrogen bond. The enol 3 prepared by crystallization from CCl4 is also stabilized by an intramolecular H-bond but has anendo-cyclic C=C bond (v(C=C) = 1545 cm−1). In a trichloroethylene solution, both theexo- andendo-enol forms co-exist, and the percentage of the latter is ∼11 %. The frequencies in the vibrational spectra have been assigned using the normal coordinate calculation of2, 3, and their OD-analogs. The mechanism of tautomer transfer is discussed in terms of PMO theory.