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