Electronic structure and proton transfer in ground-state hexafluoroacetylacetone.

The ground electronic state (X(1)A(1)) of hexafluoroacetylacetone (HFAA) 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(1)-H and H...O(2) 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)], Møller-Plesset perturbation (MPn), and coupled cluster [CCSD, CCSD(T)] treatments built upon extensive sets of correlation-consistent basis functions. Geometry optimizations performed at the CCSD(T)/aug-cc-pVDZ level of theory predict an asymmetric (C(s)) 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 (C(2v)) structure that presents a potential barrier of 21.29 kJ/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.