Knowledge Bank

Excitation into the lowest $n\pi \ast$ band of acetylacetone (I) results in extensive $\Pi$ electron redistribution accompanied by a tendency for the labile proton to seek a central position between the two oxygens. As this displacement is so closely coupled to distortions of the $C-C$ and $C-O$ bonds, the band shape of the electronic transition is broad and structureless. The use of model compounds has enabled us to determine the fraction of the band’s half width which is due to chelation. We have also observed the spectral results of breaking the internal hydrogen bond by lattice induced distortions in the low temperature phase of solid cyclohexane. The internal hydrogen bond is weakened by excitation, as evidenced by the UV spectra of externally hydrogen bonded configurations following photolysis in rigid alcoholic glasses. The barrier for proton transfer in acetylacetone-$f3(II)$ is larger than in I. We have observed structured $n\pi \ast$ absorption for this compound $(at77\circ K$ and $4.2\circ K$ ), accompanied by a large temperature effect on the half band width.