Photoinduced Electron Transfer in a Prototypical Mulliken Donor-Acceptor Complex: C 2 H 4 ∙∙∙ Br 2 OSU 66 th International Symposium on Molecular Spectroscopy By Aimable Kalume (Marquette University) Columbus, June 20 th -24 th, 2011

OSU 66 th International Symposium on Molecular Spectroscopy Pre-reactive intermediate for electrophilic bromination, a textbook organic reaction Prototype for halogen bonding interactions, important in many fields from protein-ligand interactions to crystal engineering The mechanistic details of olefinic and aromatic electrophilic addition reactions are still debated; however, recent experiments indicate the importance of a single electron transfer step that is preceded by complex formation Studies of the charge transfer photochemistry of the isolated complex is a means to study this in detail Legon and co-workers have studied this and related complexes in the gas-phase using microwave spectroscopy Introduction: Why C 2 H 4 ∙∙∙ Br 2 ???

OSU 66 th International Symposium on Molecular Spectroscopy Background: Photolysis mechanism of EDB 1 1: Kalume, A.; George, L.; El-Khoury, P. Z.; Tarnovsky, A.; Reid, S. A. J. Phys. Chem. A 2010, 114, 9919–9926

OSU 66 th International Symposium on Molecular Spectroscopy Background: Observation of the C 2 H 4 ---Br 2 complex 1:Kalume, A.; George, L.; El-Khoury, P. Z.; Tarnovsky, A.; Reid, S. A. J. Phys. Chem. A 2010, 114, 9919–9926 Photolysis of matrix isolated 1,2-dibromoethane produces the complex as a primary product

OSU 66 th International Symposium on Molecular Spectroscopy Background: Observation of the C 2 H 4 ---Br 2 complex 1:Kalume, A.; George, L.; El-Khoury, P. Z.; Tarnovsky, A.; Reid, S. A. J. Phys. Chem. A 2010, 114, 9919–9926 The yield of the complex is increased upon annealing The position of the charge-transfer band is in excellent agreement with theory Experiment: λ max = 237 nm, f = 0.5 TDCAM-B3LYP/aug-cc-pVQZ: λ max = 232 nm, f = 0.52

OSU 66 th International Symposium on Molecular Spectroscopy Experimental set up … mbar To DP Sample/ RG KBr window Cold Head 4 K Pulsed deposition Pulsed deposition Dual nozzle late mixing source

OSU 66 th International Symposium on Molecular Spectroscopy Results: IR spectra… 1: L. George, L. Wittmann, A. Kalume and S. Reid, J. Phys. Chem. Lett , 1, C 2 H 4 /Ar Br 2 /Ar C 2 H 4 /Ar Br 2 /Ar C 2 H 4 /Ar Br 2 /Ar

OSU 66 th International Symposium on Molecular Spectroscopy Previous study… 1 1:Maier, G.; Senger, S. Bromine Complexes of Ethylene and Cyclopropene. Matrix-Ir-Spectroscopic Identification, Photochemical Reactions, Ab Initio Studies. Liebigs Ann./Recl. 1997, 317–326. Excitation at λ > 300 nm, presumed to access the charge transfer band, led to formation of 1,2-dibromoethane, with a 1:1 ratio of anti to gauche conformers This is consistent with a radical addition mechanism

OSU 66 th International Symposium on Molecular Spectroscopy Why a radical mechanism? 1: L. George, L. Wittmann, A. Kalume and S. Reid, J. Phys. Chem. Lett , 1, Our work shows that excitation at λ > 300 nm does not access the charge transfer band, but rather the Br 2 chromophore The dominance of the radical mechanism is then understood

OSU 66 th International Symposium on Molecular Spectroscopy Photochemistry: CT excitation at 266 nm… 1: L. George, L. Wittmann, A. Kalume and S. Reid, J. Phys. Chem. Lett , 1, Following 266 nm excitation of the charge transfer band, only the anti-conformer of the photoproduct is observed.

OSU 66 th International Symposium on Molecular Spectroscopy Photochemistry: electron transfer shown… 1: L. George, L. Wittmann, A. Kalume and S. Reid, J. Phys. Chem. Lett. 2010, 1,

OSU 66 th International Symposium on Molecular Spectroscopy Conclusion 1: L. George, L. Wittmann, A. Kalume and S. Reid, J. Phys. Chem. Lett. 2010, 1, The prototypical Mulliken donor-acceptor complex C 2 H Br 2 was observed as a photoproduct of matrix- isolated 1,2-dibromoethane and the UV/Visible spectrum of the complex was measured for the first time The position and intensity of the charge transfer band is in excellent agreement with theory A late-mixing strategy was developed to isolate the complex Excitation into the charge transfer band leads to electron transfer and the anti-conformer of the reaction product. In contrast, excitation at longer wavelengths proceeds via a radical mechanism.

OSU 66 th International Symposium on Molecular Spectroscopy Acknowledgments Lab Dr. Scott Reid Dr. Lisa George Funding NSF ACS-PRF Wehr Foundation (Marquette University) Thank you !