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 pi-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.

Supporting Online Material S1. Refined ground-state structures of pyridine, picoline and lutidine. Distances are in angstroms and angles are in degrees. S2. Refined transient structures of hot ground-state lutidine and ring-opened picoline. Distances are in angstroms and angles are in degrees.

Nitro compounds release NO, NO2, and other species, but neither the structures during the reactions nor the time scales are known. Ultrafast electron diffraction (UED) allowed the study of the NO release from nitrobenzene, and the molecular pathways and the structures of the transient species were identified. It was observed, in contrast to previous inferences, that nitric oxide and phenoxyl radicals are formed dominantly and that the time scale of formation is 8.8+/-2.2 ps. The structure of the phenoxyl radical was determined for the first time, and found to be quinoid-like. The mechanism proposed involves a repulsive triplet state, following intramolecular rearrangement. This efficient generation of NO may have important implications for the control of by-products in drug delivery and other applications.

The photophysics and photochemistry of molecules with complex electronic structures, such as aromatic carbonyls, involve dark structures of radiationless processes. With ultrafast electron diffraction ͑UED͒ of isolated molecular beams it is possible to determine these transient structures, and in this contribution we examine the nature of structural dynamics in two systems, benzaldehyde and acetophenone. Both molecules are seen to undergo a bifurcation upon excitation ͑S 2 ͒. Following femtosecond conversion to S 1 , the bifurcation leads to the formation of molecular dissociation products, benzene and carbon monoxide for benzaldehyde, and benzoyl and methyl radicals for acetophenone, as well as intersystem crossing to the triplet state in both cases. The structure of the triplet state was determined to be "quinoidlike" of * character with the excitation being localized in the phenyl ring. For the chemical channels, the product structures were also determined. The difference in photochemistry between the two species is discussed with respect to the change in large amplitude motion caused by the added methyl group in acetophenone. This discussion is also expanded to compare these results with the prototypical aliphatic carbonyl compounds, acetaldehyde and acetone. From these studies of structural dynamics, experimental and theoretical, we provide a landscape picture for, and the structures involved in, the radiationless pathways which determine the fate of molecules following excitation. For completeness, the UED methodology and the theoretical framework for structure determination are described in this full account of an earlier communication ͓J. S. Feenstra et al., J. Chem. Phys. 123, 221104 ͑2005͔͒.

Ultrafast electron diffraction is reported for the elimination reaction of acetylacetone. For this multipath reaction, upon 266 nm excitation, it is revealed that the major reaction channel is fragmentation from the β carbon, producing OH and 3-penten-2-on-4-yl radicals, with a time constant of 247 ( 34 ps. We determine the structures involved and the changes in bond lengths. These structure-dynamics correlations in different states provide a mechanistic description of the complex reaction landscape.