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

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,

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, : Chateauneuf, J. E., Journal of Organic Chemistry 1999, 64, : Neta et al, Journal of Physical Chemistry 1993, 97, 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

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.

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( cm -1 ) and UV-Vis( nm) spectra

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

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

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

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

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

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

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) :

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.

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.

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.

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 !