15min:

S. A. BROADBENT, S. M. WILSON AND P. H. VACCARO, Department of Chemistry, Yale University, P.O. Box 208107, New Haven, CT 06520; B. R. JOHNSON, Department of Chemistry, Rice University, P.O. Box 1892, Houston, TX 77251.

Acetylacetone (AA), one of the simplest -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 experiment and theory have demonstrated conclusively that the X¹\textrmA₁ ground electronic state exhibits an asymmetrical equilibrium geometry with a potential barrier of finite height separating two equivalent conformers of C_s symmetry. In contrast, ab initio calculations have suggested that the electronically excited B¹\textrmB₂ ( ^* ) manifold supports a symmetric (C_2v) 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 \textrmnm, essentially coincident with the peak of the ^* 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 O distance. Of special note is the 1620 -2800 \textrmcm^-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.