1. Pulsed-jet discharge matrix isolation and computational study of Bromine atom complexes: Br---BrXCH 2 (X=H,Cl,Br) OSU 66 th International Symposium on Molecular Spectroscopy By Aimable Kalume (Marquette University) Columbus, June 20 th -24 th, 2011

2. OSU 66 th International Symposium on Molecular Spectroscopy Halogen radicals are known to initiate many atmospheric reactions These reactions involve weakly bound pre-reactive donor acceptor complexes (halogen atom as donor) These complexes affect both the rate and selectivity of reactions. 1 Introduction 1: Breslow, R. et al, Journal of American Chemical Society 1987, 109, 7203-7212

3. OSU 66 th International Symposium on Molecular Spectroscopy Background: previous studies… 1: Wittig et al, Abstracts of Papers of the American Chemical Society 1999, 217 U325-U326 2: Bondybey et al, Journal of Chemical Physics 2000, 112, 3803-3811 3: Chateauneuf, J. E., Journal of Organic Chemistry 1999, 64, 1054-1055 4: Neta et al, Journal of Physical Chemistry 1993, 97, 9120-9123 Wittig and co-workers detected Cl···HCl complex, by photodissociation of HCl dimer. 1 Bondybey and co-workers detected Br···HBr in solid Ne. 2 Chateauneuf has shown the formation of a weakly bound “Cl-solvent” charge transfer complex. 3 Neta and co-workers studied Br complexes with various halocarbons in solution, using pulsed radiolysis. 4

4. OSU 66 th International Symposium on Molecular Spectroscopy Matrix isolation as complementary to ultrafast spectroscopy… Solenoid activated pulsed nozzle: 1ms, 5Hz, … Applied HV: -600 to -1300V; 0.2 to 1 mA The dependence of the absorption intensity on discharge current varies dramatically for different species, which provides a convenient diagnostic for identification.

5. OSU 66 th International Symposium on Molecular Spectroscopy Experimental set-up… Mixture of precursor and Argon or Neon gases: CH 2 BrX (X=H, Cl, Br): rare gas (~1:300) Deposition on a KBr window held at 23K(Ar) and 7K (Ne) KBr window cooled at 5K after deposition Measure IR(500-4000 cm -1 ) and UV-Vis(200-1100 nm) spectra

6. OSU 66 th International Symposium on Molecular Spectroscopy Computational … Geometry optimization: B3LYP, M06 and MP2 Vibrational frequencies Electronic absorption and oscillator strength: TD-DFT (B3LYP. CAM-B3LYP, M06 and M06-2x) Aug-cc-pVTZ and aug –cc-pVQZ basis sets

7. OSU 66 th International Symposium on Molecular Spectroscopy Results: structure and binding energy of the complexes… Br-Br C-Br-Br Binding Energy 2.921 83.1 5.3 2.896 86.8 5.3 2.926 82.9 5.3 Bond length in Angstrom Binding energy (KCal/mol) calculated at CCSD(T)/aug-cc-pVTZ // M06/aug-cc-pVTZ

8. OSU 66 th International Symposium on Molecular Spectroscopy Results: IR spectra of CH 3 Br···Br in Ar Infrared spectrum of a CH 3 Br:Ar (1:220) matrix following pulsed deposition on a KBr window at 23 K and subsequent cooling to 5 K, with (red) and without (black) discharge

9. OSU 66 th International Symposium on Molecular Spectroscopy Results: IR and UV-Vis spectra of CH 3 Br···Br in Ar … Comparison with the predicted IR spectrum of the CH 3 Br···Br complex UV-Visible spectrum of with the predicted spectrum at the TD CAM-B3LYP/aug-cc- pVTZ level. 342nm

10. OSU 66 th International Symposium on Molecular Spectroscopy Photochemistry: excitation of the CT at 355nm … Br···BrCH 2 X + hν CT → Br – ···BrCH 2 X + → [Br···BrCH 2 X]* → Br + BrCH 2 X k BET

11. OSU 66 th International Symposium on Molecular Spectroscopy Photochemistry: excitation of the CT at 355nm … k BET

12. OSU 66 th International Symposium on Molecular Spectroscopy Results: TD-DFT predictions… SpeciesTD-B3LYP/ aug-cc-pVTZ TD-CAM-B3LYP/ aug-cc-pVTZ TD-M06 aug-cc-pVTZ TD-M06-2X/ aug-cc-pVTZ Observed λmax (osc. Strength) CH 3 Br···Br344.8 (0.2546)335.5 (0.2660)350.2 (0.2345)348.0 (0.2396)342 (0.2) CH 2 ClBr···Br346.7 (0.1599)346.2 (0.2479)374.7 (0.1597)343.2 (0.2181)333 (0.2) CH 2 Br 2 ···Br355.6 (0.2025)356.6 (0.2370)353.4 (0.1509)353.1 (0.2172)351 (0.2) :

13. OSU 66 th International Symposium on Molecular Spectroscopy Complexity of the CH 2 BrCl case… Four isomers are possible: Br ∙∙∙ BrCH 2 Cl Br ∙∙∙ ClCH 2 Br Cl ∙∙∙ BrCH 2 Cl Cl ∙∙∙ ClCH 2 Br Stabilization energy of Br ∙∙∙ BrCH 2 Cl is roughly twice that of Br ∙∙∙ ClCH 2 Br Cl ∙∙∙ BrCH 2 Cl and Cl ∙∙∙ BrCH 2 Br results from C-Cl bond breaking Both radicals are experimentally identified in a ratio of CH 2 Br/CH 2 Cl ~ 1/3 Theoretically, Br ∙∙∙ BrCH 2 Cl is the global minimum. Experimentally, it is hard to distinguish these complexes, since they have similar IR and UV/vis spectra.

14. OSU 66 th International Symposium on Molecular Spectroscopy Comparison with other Mulliken donors … Mulliken Correlation: The E CT should scale linearly with the ionization energy of the donor.

15. OSU 66 th International Symposium on Molecular Spectroscopy Conclusion Use of pulsed jet discharge matrix isolation technique to generate CH 2 XBr---Br (X=H, Cl, Br) complexes. The characterization of the complexes by spectroscopy (IR and Electronic) was supported by high level calculations. The examination of the photochemistry of the complexes has shown a rapid back-electron transfer. The CT band energies of these complexes were compared with that of Br with other donors and showed a reasonable linear Mulliken Correlation.

16. OSU 66 th International Symposium on Molecular Spectroscopy Acknowledgments Lab Dr. Scott Reid Dr. Lisa George Funding National Science Foundation ACS-Petroleum Research Fund Wehr Foundation (Marquette University) Thank you !