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

Please use this identifier to cite or link to this item: http://hdl.handle.net/1811/21498

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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
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.
URI: http://hdl.handle.net/1811/21498
Other Identifiers: 2004-WG-06
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