Hydrogen bonding in the ground and excited states of hexafluoroacetylacetone

The cis-enol forms of beta-diketones are stabilized by strong intramolecular hydrogen bonds that mediate attendant proton-transfer events. Theoretical and experimental investigations on acetylacetone (AA), one of the simplest members of this class, have demonstrated that the proton-transfer process is governed by a double-minimum potential well, where the minima correspond to two equivalent asymmetric (Cs) tautomers separated by a barrier of finite height. Since the intrinsic strength of such bonding motifs can be influenced by chemical substitution, it is expected that the electron-withdrawing fluoromethyl groups in hexafluoroacetylacetone (HFAA) should weaken the hydrogen bond relative to that of AA.;The ground electronic state (X˜1A 1) of hexafluoroacetylacetone has been subjected to synergistic experimental and theoretical investigations designed to resolve controversies surrounding the nature of intramolecular hydrogen bonding for the enol tautomer. Cryogenic (93K) X-ray diffraction studies were conducted on single HFAA crystals grown in situ by means of the zone-melting technique, with the resulting electron density maps affording clear evidence for distinguishable O-H and H···O bonds that span an interoxygen distance of 2.680+/-0.003 A. Such laboratory findings have been corroborated by a variety of quantum-chemical methods including Hartree-Fock (HF), density functional [DFT(B3LYP)], Moller-Plesset perturbation (MPn), and coupled-cluster [CCSD , CCSD(T)] treatments built upon extensive sets of correlation-consistent basis functions. Geometry optimizations performed at the potent CCSD(T)/aug-cc-pVDZ level of theory predict an asymmetric (Cs) equilibrium configuration characterized by an O···O donor-acceptor separation of 2.628 A. Similar analyses of the transition state for proton transfer reveal a symmetric (C2v) structure that presents a potential barrier of 21.29kJ/mol (1779.7 cm-1 ) height. The emerging computational description of HFAA is in reasonable accord with crystallographic measurements and suggests a weakening of hydrogen-bond strength relative to that of the analogous acetylacetone molecule.;Redistribution of charge density upon electronic transition transforms the potential energy surface, thereby exerting a profound influence upon the efficiency of hydron migration. Resonance Raman (RR) spectroscopy has been enlisted to interrogate the excited-state structure and dynamics of the HFAA system. Vapor-phase measurements were performed at discrete excitation wavelengths chosen to span the structureless pi*←pi absorption system (lambda max=266 nm), as well as pre-resonant and non-resonant portions of the spectrum. The acquired data display pronounced differences in intensity patterns for vibrations ascribed to distortions of the chelate ring, reflecting the key structural changes taking place upon electronic excitation. Detailed analyses of RR experimental results have been performed within the time-dependent (correlation-function/propagator) framework of Raman scattering, as guided by high-level ab initio predictions of requisite first-order (gradient) and second-order (Hessian) derivatives for the optically connected electronic manifolds. The emerging picture of B˜1B2 HFAA embodies significant displacement of the "shuttling" hydrogen atom, thereby affirming the incipient motion required for a low-barrier hydrogen bonding (LBHBing) motif.