Photoinduced Electron Transfer in a PrototypicalMulliken Donor-Acceptor Complex: C2H4 ∙∙∙ Br2

OSU 66th International Symposium on Molecular Spectroscopy

By Aimable Kalume (Marquette University)

Columbus, June 20th -24th , 2011

OSU 66th 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 C2H4 ∙∙∙ Br2 ???

OSU 66th 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 66th International Symposium on Molecular Spectroscopy

Background: Observation of the C2H4---Br2 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 66th International Symposium on Molecular Spectroscopy

Background: Observation of the C2H4---Br2 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: λmax = 237 nm, f = 0.5 TDCAM-B3LYP/aug-cc-pVQZ: λmax = 232 nm, f = 0.52

OSU 66th International Symposium on Molecular Spectroscopy

Experimental set up …

10-6 mbar

To DP

Sample/RG

KBr window

Cold Head 4 K

Pulsed deposition

• Pulsed deposition • Dual nozzle late mixing source

OSU 66th International Symposium on Molecular Spectroscopy

Results: IR spectra…

1: L. George, L. Wittmann, A. Kalume and S. Reid, J. Phys. Chem. Lett.. 2010, 1, 2618 - 2621

C2H4/ArBr2/Ar

C2H4/ArBr2/Ar

C2H4/ArBr2/Ar

OSU 66th 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 66th International Symposium on Molecular Spectroscopy

Why a radical mechanism?

1: L. George, L. Wittmann, A. Kalume and S. Reid, J. Phys. Chem. Lett.. 2010, 1, 2618 - 2621

• Our work shows that excitation at λ > 300 nm does not access the charge transfer band, but rather the Br2 chromophore• The dominance of the radical mechanism is then understood

OSU 66th 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.. 2010, 1, 2618 - 2621

Following 266 nm excitation of the charge transfer band, only the anti-conformer of the photoproduct is observed.

OSU 66th 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, 2618 - 2621

OSU 66th International Symposium on Molecular Spectroscopy

Conclusion

1: L. George, L. Wittmann, A. Kalume and S. Reid, J. Phys. Chem. Lett. 2010, 1, 2618 - 2621

• The prototypical Mulliken donor-acceptor complex C2H4---Br2 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 66th International Symposium on Molecular Spectroscopy

Acknowledgments

Lab• Dr. Scott Reid• Dr. Lisa George

Funding

• NSF• ACS-PRF• Wehr Foundation (Marquette University)

Thank you !