Direct determination of hydrogen-bonded structures in resonant and tautomeric reactions using ultrafast electron diffraction

We elucidate the keto-enol tautomeric equilibrium in acetylacetone, the structure of both keto and enol forms, and the nature of the intramolecular O-H⋯O HB in enolic acetylacetone using our ultrafast electron diffraction apparatus, thereby shedding new light on the nature of the hydrogen bond in resonant tautomeric structures. The enolic structure exhibits some π-resonance delocalization; however, this delocalization is not strong enough to give a symmetric skeletal geometry. The long O⋯O distance in the refined structure renders the homonuclear O-H⋯O hydrogen bond in acetylacetone localized and asymmetric. Copyright © 2004 American Chemical Society.