15min:

S. A. BROADBENT, C. CHATTERJEE 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.

The cis -enol tautomers of simple -diketones such as acetylacetone (H₃C-CO-CH₂-CO-CH₃) are ideal target compounds for the investigation of hydrogen bonding and proton transfer, exhibiting a variety of intramolecular processes ( e.g. , low-barrier hydrogen bonding) that have been predicted to play pivotal roles in the behavior of substantially larger complexes. Resonance Raman (RR) spectroscopy has been used to probe the electronically-excited B¹\textrmB₂ ( ^*\! ) potential energy surface of acetylacetone, thereby elucidating the changes in structure and dynamics that accompany ^*\! \! electron promotion of the isolated (vapor-phase) species. Data acquired at discrete excitation wavelengths spanning the B-X absorption system ( _max~262 \textrmnm) displayed pronounced differences in intensity patterns. The selective activity of overtone and combination bands involving displacement of the H--chelated ring indicated a low-barrier, hydrogen-bonding motif for the B¹\textrmB₂ manifold. The comprehensive interpretation of all experimental findings was facilitated by ab initio geometry optimizations and force-field calculations performed for the pertinent electronic states at substantial levels of coupled-cluster theory (CCSD and EOM--CCSD with augmented correlation-consistent basis sets). The Hessian matrix and gradient vector computed for the electroncially-excited surface at the fully-relaxed ground-state (X¹\textrmA₁) geometry led to a harmonically-extrapolated B¹\textrmB₂ equilibrium structure that bears evidence for the low-barrier hydrogen bonding phenomenon. The vibrational results emerging from this ``vertical Hessian'' treatment were employed as initial parameters for a least-squares regression procedure designed to simulate observed RR spectra by means of a time-dependent propagator formalism that incorporated effects arising from the Duschinsky rotation of normal coordinates and the non-Condon character of transition moments. Quantitative information extracted from these analyses will be discussed, with particular emphasis directed towards unraveling the unimolecular dynamics of acetylacetone.