OSU 66 th International Symposium on Molecular Spectroscopy

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

Introduction • 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 1: Breslow, R. et al, Journal of American Chemical Society 1987, 109, 7203 -7212 OSU 66 th International Symposium on Molecular Spectroscopy

Background: previous studies… • 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 1: Wittig et al, Abstracts of Papers of the American Chemical Society 1999, 217 U 325 -U 326 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 OSU 66 th International Symposium on Molecular Spectroscopy

Matrix isolation as complementary to ultrafast spectroscopy… • Solenoid activated pulsed nozzle: 1 ms, 5 Hz, … • Applied HV: -600 to -1300 V; 0. 2 to 1 m. A • 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 Br. X (X=H, Cl, Br): rare gas (~1: 300) • Deposition on a KBr window held at 23 K(Ar) and 7 K (Ne) • KBr window cooled at 5 K after deposition • Measure IR(500 -4000 cm-1) and UV-Vis(200 -1100 nm) spectra OSU 66 th International Symposium on Molecular Spectroscopy

Computational … • Geometry optimization: B 3 LYP, M 06 and MP 2 • Vibrational frequencies • Electronic absorption and oscillator strength: TD-DFT (B 3 LYP. CAMB 3 LYP, M 06 and M 06 -2 x) • Aug-cc-p. VTZ and aug –cc-p. VQZ basis sets OSU 66 th International Symposium on Molecular Spectroscopy

Results: structure and binding energy of the complexes… Br-Br 2. 896 2. 921 C-Br-Br 86. 8 82. 9 83. 1 Binding Energy 5. 3 Bond length in Angstrom Binding energy (KCal/mol) calculated at CCSD(T)/aug-cc-p. VTZ // M 06/aug-cc-p. VTZ 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 … 342 nm 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-B 3 LYP/aug-ccp. VTZ level. OSU 66 th International Symposium on Molecular Spectroscopy

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

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

: Results: TD-DFT predictions… Species TD-B 3 LYP/ aug-cc-p. VTZ TD-CAM-B 3 LYP/ aug-cc-p. VTZ TD-M 06 -2 X/ aug-cc-p. VTZ Observed λmax (osc. Strength) CH 3 Br···Br 344. 8 (0. 2546) 335. 5 (0. 2660) 350. 2 (0. 2345) 348. 0 (0. 2396) 342 (0. 2) CH 2 Cl. Br···Br 346. 7 (0. 1599) 346. 2 (0. 2479) 374. 7 (0. 1597) 343. 2 (0. 2181) 333 (0. 2) CH 2 Br 2···Br 355. 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 Br. Cl case… • Four isomers are possible: Br ∙∙∙ Br. CH 2 Cl Br ∙∙∙ Cl. CH 2 Br Cl ∙∙∙ Br. CH 2 Cl Cl ∙∙∙ Cl. CH 2 Br • Stabilization energy of Br ∙∙∙ Br. CH 2 Cl is roughly twice that of Br ∙∙∙ Cl. CH 2 Br • Cl ∙∙∙ Br. CH 2 Cl and Cl ∙∙∙ Br. CH 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 ∙∙∙ Br. CH 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 ECT 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 ! OSU 66 th International Symposium on Molecular Spectroscopy