Quantum–Chemical Studies on the Superposition of Absorption Bands in the Carbonyl Region of the IR Spectra of Acetylacetone

Using the MO LCAO SCF method in the PM3 approximation, we have investigated the influence of various internal motions of the enol molecule of acetylacetone on the harmonic vibrational frequencies of the C=O and C=C groups. The results of the investigation are discussed with reference to the available literature data on vibrational spectra of acetylacetone and its substituted derivatives. They suggest that the nature of the wide intense IR band 1620 cm−1 in the spectra of acetylacetone vapors responsible for the absorption of its enol double bonds and its temperature dependence are determined by the tunnel mechanism of enol proton motion in the H–bond channel, the probability of whose realization is controlled by the internal rotation of methyl substituents. © Plenum Publishing Corporation 2001.