ON THE QUANTITATIVE ANALYSIS OF RESONANCE RAMAN SPECTRA: A SYSTEMATIC INVESTIGATION OF HYDROGEN BONDING IN ELECTRONICALLY-EXCITED ACETYLACETONE

Abstract

The \textit{cis}-enol tautomers of simple $\beta$-diketones such as acetylacetone (H$_{3}$C-CO-CH$_{2}$-CO-CH$_{3}$) are ideal target compounds for the investigation of hydrogen bonding and proton transfer, exhibiting a variety of intramolecular processes ({\it 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 $\tilde{B}^{1}\textrm{B}_{2}\ (\pi^*\!\pi)$ potential energy surface of acetylacetone, thereby elucidating the changes in structure and dynamics that accompany $\pi^*\!\leftarrow\!\pi$ electron promotion of the isolated (vapor-phase) species. Data acquired at discrete excitation wavelengths spanning the $\tilde{B}-\tilde{X}$ absorption system ($\lambda_{max}\approx262\,\textrm{nm}$) 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 $\tilde{B}^{1}\textrm{B}_{2}$ manifold. The comprehensive interpretation of all experimental findings was facilitated by {\it 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 ($\tilde{X}^{1}\textrm{A}_{1}$) geometry led to a harmonically-extrapolated $\tilde{B}^{1}\textrm{B}_{2}$ 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.