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Excited state molecular structures and reactions directly determined by ultrafast electron diffraction
Jonathan S Feenstra1, Sang Tae Park, Ahmed H Zewail
1Laboratory for Molecular Science, Arthur Amos Noyes Laboratory for Chemical Physics, California Institute of Technology, Pasadena, CA 91125, USA.
The Journal of Chemical Physics
|December 27, 2005
Summary
Ultrafast electron diffraction reveals how acetophenone and benzaldehyde molecules split. Pathways lead to triplet states or chemical products like benzene and radicals.
Area of Science:
- Physical Chemistry
- Chemical Dynamics
- Spectroscopy
Background:
- Aromatic carbonyl compounds like acetophenone and benzaldehyde are crucial in photochemistry.
- Understanding their excited-state dynamics and reaction pathways is key to controlling chemical reactions.
- Previous studies lacked detailed structural information on transient intermediates and products.
Purpose of the Study:
- To investigate the structural dynamics of excited states and reaction products of isolated acetophenone and benzaldehyde.
- To resolve the bifurcation of photochemical pathways upon excitation.
- To compare experimentally determined structures with theoretical predictions.
Main Methods:
- Ultrafast electron diffraction (UED) was employed to probe structural changes on femtosecond to picosecond timescales.
- A 266 nm laser pulse was used to excite the aromatic carbonyl molecules.
- Time-resolved structural analysis was performed to capture transient species and final products.
Main Results:
- The study structurally resolved a bifurcation in photochemical pathways for both molecules upon 266 nm excitation.
- For benzaldehyde, one pathway led to a triplet state (quinoid structure), while the other yielded benzene and carbon monoxide via hydrogen migration and bond rupture.
- For acetophenone, pathways led to a triplet state or to benzoyl and methyl radicals through bond rupture.
Conclusions:
- The refined structures of excited states and reaction products were determined and compared with theoretical models.
- The identified transient structures and their radiationless transitions are crucial for understanding the reduced energy landscape of complex photochemical reactions.
- Ultrafast electron diffraction provides unprecedented insight into the ultrafast structural dynamics of aromatic carbonyl photochemistry.