We investigated the ultrafast photochemical ring-opening in the molecule α-phellandrene by a combination of megaelecronvolt ultrafast electron diffraction and excited state ab initio multiple spawning wavepacket simulations. α- Phellandrene exhibits a number of different conformers which produce different ring-opening photoproducts according to the Woodward-Hoffmann rules. In our study we image the conversion of a specific conformer of α-phellandrene in the Woodward-Hoffmann predicted photoproduct in real time and space.
Thomas Wolf, Jie Yang, David Sanchez, João P. Nunes, Robert Parrish, Xiaozhe Shen, Martin Centurion, Ryan Coffee, James Cryan, Markus Gühr, Kareem Hegazy, Adam Kirrander, Renkai Li, Jennifer Ruddock, Theodore Vecchione, Stephen Weathersby, Peter Weber, Kyle Wilkin, Haiwang Yong, Quiang Zheng, Todd Martinez, Xijie Wang, Michael Minitti
The photoinduced ring opening of 1,3-cyclohexadiene is a prototypical photo-allowed, ultrafast reaction in agreement with the Woodward-Hoffmann rules. Furthermore, it is a model reaction for the biosynthesis of vitamin D. The reaction mechanism involves coupled dynamics of the electrons and nuclei of the molecule in the vicinity of a conical intersection between the excited state and the ground state. It has been intensively investigated by spectroscopic methods, which were primarily sensitive to transient changes in the electronic structure during internal conversion through a conical intersection between the excited state and the ground state. The corresponding structural dynamics have so far only been investigated by a ultrafast x-ray diffraction study. However, spatial resolution down to bond lengths has not been achieved so far. We have investigated the ring opening by gas phase MeV ultrafast electron diffraction with an unprecedented combination of femtosecond temporal and sub Angstrom spatial resolution. The obtained momentum transfer range allows us to follow transient bond length changes by real space transformation of the experimental dataset. Thus, the experimentally observed structural dynamics can be directly compared to quantum molecular dynamics simulations. We, furthermore, can follow the further structural relaxation of the molecule for several hundred femtoseconds beyond relaxation through the conical intersection with the ground state.
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