Jonathan S. Feenstra

Ultrafast Electron Diffraction Reveals Dark Structures of the Biological Chromophore Indole

With UED, as detailed in the experimental section, we are able to determine both ground- and excited-state structures and obtain the temporal behavior for excitation of indole at 267 nm. For the ground-state structure (see Scheme SI1 in the Supporting Information), the experimental and the theoretical molecular scattering function, sM(s), together with the radial distributions, f(r), are shown in Figure 1. The refined structural parameters are listed in Table SI1 in the Supporting Information, together with values obtained from density functional theory (DFT) calculations. The satisfactory agreement between experiment and theory gives the refined structural parameters, which were found to have discrepancies at most within 0.007 A and 0.18° for bond lengths and angles, respectively.

Excited state molecular structures and reactions directly determined by ultrafast electron diffraction

In this communication, we report on the use of ultrafast electron diffraction to determine structural dynamics of excited states and reaction products of isolated aromatic carbonyls, acetophenone and benzaldehyde. For a 266 nm excitation, a bifurcation of pathways is structurally resolved, one leading to the formation of the triplet state (quinoid structure) and another to chemical products: for benzaldehyde the products are benzene and carbon monoxide (hydrogen migration and bond rupture) while those for acetophenone are the benzoyl and methyl radicals (bond rupture). The refined structures are compared with those predicted by theory. These dark structures and their radiationless transitions define the reduced energy landscape for complex reactions.

Ultrafast electron diffraction of transient cyclopentadienyl radical: A dynamic pseudorotary structure

Ultrafast electron diffraction (UED) is applied here in the study of the reaction of cylcopentadienyl cobalt dicarbonyl (CpCo(CO)_2) which proceeds to give product structures. These structures were probed by picosecond electron pulses. The major product of the fragmentation was found to be the cyclopentadienyl radical. The dynamic nature of the radical was best represented by a pentagonal molecular structure having D_(5h) symmetry with elevated mean amplitudes of vibration. Comparisons between theory and experiment are presented. The structure is that of the transition state between the compressed (dienylic) and the elongated (allylic) conformations but with longer bond distances, reflecting the dynamics of the pseudorotary surface.