AN INVESTIGATION OF EXCITED-STATE STRUCTURE AND DYNAMICS IN ACETYLACETONE THROUGH USE OF RESONANCE RAMAN SPECTROSCOPY

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 Title: AN INVESTIGATION OF EXCITED-STATE STRUCTURE AND DYNAMICS IN ACETYLACETONE THROUGH USE OF RESONANCE RAMAN SPECTROSCOPY Creators: Broadbent, S. A.; Wilson, S. M.; Vaccaro, P. H.; Johnson, Bruce R. Issue Date: 2004 Publisher: Ohio State University Abstract: 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 $experiment^{a}$ and $theory^{b}$ have demonstrated conclusively that the $\tilde{X}^{1} A_{1}$ 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 $calculations^{c}$ have suggested that the electronically excited $\tilde{B}^{1}B_{2} (\pi^{\ast}\pi)$ 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 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 $O\ldots O$ distance. Of special note is the $1620 - 2800 cm^{-1}$ 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. Description: $^{a}$R. Boese, M. Y. Antipin, D. Blaser, and K. A. Lyssenko, J. Phys. Chem. B 102, 1584 (1998). $^{b}$V. V. Sliznev, S. B. Lapshina, and G. V. Girichev, J. Struct. Chem. 43, 47 (2002). $^{c}$H. P. Upadhyaya, A. Kumar, and P. D. Naik, J. Chem. Phys. 110, 11850 (2003). Author Institution: Department of Chemistry, Yale University; Department of Chemistry, Rice University URI: http://hdl.handle.net/1811/21498 Other Identifiers: 2004-WG-06