Knowledge Bank

Acetylacetone (AA), one of the simplest $\beta$-diketones, exhibits a strong intramolecular hydrogen bond that stabilizes the enol tautomer of the isolated (gas-phase) species and mediates the attendant proton- transfer process. Both $experimenta$ and $theoryb$ have demonstrated conclusively that the $X~1A1$ ground electronic state exhibits an asymmetrical equilibrium geometry with a potential barrier of finite height separating two equivalent conformers of $Cs$ symmetry. In contrast, ab initio $calculationsc$ have suggested that the electronically excited $B~1B2(\pi \ast \pi )$ manifold supports a symmetric $(C2v$) minimum energy configuration which has the shuttling hydron located midway between the oxygen atom centers. This assertion, with its prediction of a low-barrier hydrogen-bonding motif, has been investigated experimentally by means of Resonance Raman Spectroscopy. Excitation at 266 nm, essentially coincident with the peak of the $\pi \ast \leftarrow \pi$ transition, results in Raman profiles dominated by intense spectral features that stem from vibrational modes involving substantial distortion of the chelate ring, including marked displacement of the $O\dots O$ distance. Of special note is the $1620-2800cm-1$ region, which is not expected to contain any fundamental transitions, yet exhibits rich structure that has been assigned to overtone and combination bands. All of these data are consistent with a large change in molecular geometry upon electronic excitation. Resonance Raman spectra of deuterated derivatives and structural analogues of AA afford an additional means for unraveling the observed excited-state behavior. Ongoing extensions of these studies will be discussed, as well as efforts toward theoretical analysis based on the time-dependent formalism for Raman scattering.