photoinduced electron transfer in a prototypical mulliken donor-acceptor complex: c 2 h 4 ∙∙∙...
TRANSCRIPT
![Page 1: Photoinduced Electron Transfer in a Prototypical Mulliken Donor-Acceptor Complex: C 2 H 4 ∙∙∙ Br 2 OSU 66 th International Symposium on Molecular Spectroscopy](https://reader035.vdocuments.us/reader035/viewer/2022062314/56649f1f5503460f94c3732c/html5/thumbnails/1.jpg)
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
![Page 2: Photoinduced Electron Transfer in a Prototypical Mulliken Donor-Acceptor Complex: C 2 H 4 ∙∙∙ Br 2 OSU 66 th International Symposium on Molecular Spectroscopy](https://reader035.vdocuments.us/reader035/viewer/2022062314/56649f1f5503460f94c3732c/html5/thumbnails/2.jpg)
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 ???
![Page 3: Photoinduced Electron Transfer in a Prototypical Mulliken Donor-Acceptor Complex: C 2 H 4 ∙∙∙ Br 2 OSU 66 th International Symposium on Molecular Spectroscopy](https://reader035.vdocuments.us/reader035/viewer/2022062314/56649f1f5503460f94c3732c/html5/thumbnails/3.jpg)
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
![Page 4: Photoinduced Electron Transfer in a Prototypical Mulliken Donor-Acceptor Complex: C 2 H 4 ∙∙∙ Br 2 OSU 66 th International Symposium on Molecular Spectroscopy](https://reader035.vdocuments.us/reader035/viewer/2022062314/56649f1f5503460f94c3732c/html5/thumbnails/4.jpg)
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
![Page 5: Photoinduced Electron Transfer in a Prototypical Mulliken Donor-Acceptor Complex: C 2 H 4 ∙∙∙ Br 2 OSU 66 th International Symposium on Molecular Spectroscopy](https://reader035.vdocuments.us/reader035/viewer/2022062314/56649f1f5503460f94c3732c/html5/thumbnails/5.jpg)
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
![Page 6: Photoinduced Electron Transfer in a Prototypical Mulliken Donor-Acceptor Complex: C 2 H 4 ∙∙∙ Br 2 OSU 66 th International Symposium on Molecular Spectroscopy](https://reader035.vdocuments.us/reader035/viewer/2022062314/56649f1f5503460f94c3732c/html5/thumbnails/6.jpg)
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
![Page 7: Photoinduced Electron Transfer in a Prototypical Mulliken Donor-Acceptor Complex: C 2 H 4 ∙∙∙ Br 2 OSU 66 th International Symposium on Molecular Spectroscopy](https://reader035.vdocuments.us/reader035/viewer/2022062314/56649f1f5503460f94c3732c/html5/thumbnails/7.jpg)
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
![Page 8: Photoinduced Electron Transfer in a Prototypical Mulliken Donor-Acceptor Complex: C 2 H 4 ∙∙∙ Br 2 OSU 66 th International Symposium on Molecular Spectroscopy](https://reader035.vdocuments.us/reader035/viewer/2022062314/56649f1f5503460f94c3732c/html5/thumbnails/8.jpg)
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
![Page 9: Photoinduced Electron Transfer in a Prototypical Mulliken Donor-Acceptor Complex: C 2 H 4 ∙∙∙ Br 2 OSU 66 th International Symposium on Molecular Spectroscopy](https://reader035.vdocuments.us/reader035/viewer/2022062314/56649f1f5503460f94c3732c/html5/thumbnails/9.jpg)
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
![Page 10: Photoinduced Electron Transfer in a Prototypical Mulliken Donor-Acceptor Complex: C 2 H 4 ∙∙∙ Br 2 OSU 66 th International Symposium on Molecular Spectroscopy](https://reader035.vdocuments.us/reader035/viewer/2022062314/56649f1f5503460f94c3732c/html5/thumbnails/10.jpg)
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.
![Page 11: Photoinduced Electron Transfer in a Prototypical Mulliken Donor-Acceptor Complex: C 2 H 4 ∙∙∙ Br 2 OSU 66 th International Symposium on Molecular Spectroscopy](https://reader035.vdocuments.us/reader035/viewer/2022062314/56649f1f5503460f94c3732c/html5/thumbnails/11.jpg)
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
![Page 12: Photoinduced Electron Transfer in a Prototypical Mulliken Donor-Acceptor Complex: C 2 H 4 ∙∙∙ Br 2 OSU 66 th International Symposium on Molecular Spectroscopy](https://reader035.vdocuments.us/reader035/viewer/2022062314/56649f1f5503460f94c3732c/html5/thumbnails/12.jpg)
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.
![Page 13: Photoinduced Electron Transfer in a Prototypical Mulliken Donor-Acceptor Complex: C 2 H 4 ∙∙∙ Br 2 OSU 66 th International Symposium on Molecular Spectroscopy](https://reader035.vdocuments.us/reader035/viewer/2022062314/56649f1f5503460f94c3732c/html5/thumbnails/13.jpg)
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 !