Ultrafast Photochemistry of Bromoform in Solution Probed in the Deep-UV- to Near-IR Spectral Range Suman K. Pal, Patrick Z. El-Khoury, Andrey S. Mereshchenko and Alexander N. Tarnovsky Department of Chemistry and the Center for Photochemical Sciences Bowling Green State University, Bowling Green, Ohio  

Outline Objectives Previous work on bromoform (CHBr3) Methods: pump-probe and TD-DFT Results Summary

Objectives Environmental importance: produced by microalgae, 200,000 tons per year Atmospheric importance: abundant source of bromine atoms in the earth’s atmosphere causing destruction of the ozone layer Fundamental interest: structure-selective insights into the condensed-phase photochemistry

Previous Work CF2I2 iso-CF2-I-I h=350 nm CF2I2 iso-CF2-I-I Ultrafast time-resolved IR : iso-CF2-I-I observed at 400 fs after excitation 2. CH2Br2 iso-CH2-Br-Br Patrick El-Khoury et al. J. Chem. Phys. 132 (2010) 124501 h=255 nm In cyclohexane: isomer stable In CH3CN: isomer formation in ~8.5 ps. Isomer decays (~100 ps) back to the parent. Couple cluster calculations: < 1 kcal mol-1 energy barrier TS Iso-CH2Br2 CH2Br2 Patrick El-Khoury et al. Chem. Phys. Lett. 493 (2010) 61

Bromoform: Early Gas-Phase Work Excitation at ~200 nm exclusively causes direct two-body decay followed by the C—Br bond dissociation in vibrationally hot radicals CHBr3*  CHBr2·* + Br· CHBr2·*  CHBr·+ Br· Excitation at193 nm: HBr elimination from vibrationally hot CHBr2 radicals CHBr2·*  CBr+ HBr Excitation in the 234-267 nm range: molecular bromine elimination CHBr3*  CHBr + Br2 Simons et al. Trans. Faraday Soc. 57 (1961) 2167 McGivern et al. J. Phys.Chem. A 104 (2002) 10085 Zou et al. J. Phys.Chem. A 108 (2004) 1482 Xu and co-workers J. Chem. Phys. 117 (2002) 2578

Bromoform: Previous Liquid-Phase Work Excitation at ~267 nm in water and water/acetonitrile mixtures causes formation of iso-bromoform, which finally reacts with water to form HBr, CO and HCO. Phillips and co-workers J. Chem. Phys. 120 (2004) 3323; J. Am. Chem. Soc. 126 (2004) 3119 Neat CHBr3 and concentrated ( 0.23 M) cyclohexane solutions upon excitation at 267 nm: iso-bromoform formation: prompt (<0.5 ps) hot iso-bromoform decay: 13 ps lifetime relaxed iso-bromoform: metastable release of Br atoms from hot iso-bromoform: rapid formation of CHBr3-Br complex in concentrated solutions Crim and co-workers J. Phys.Chem. A 114 (2010) 1548

Accessible Spectral Range Ultrafast (100 fs) Laser System We Built: Gap-Free Tunable 208 nm – 22 mm Hurricane (100 fs, 800 nm, 0.8 mJ, 1 KHz) TOPAS/pump TOPAS/probe UV/vis/near-IR pump Tripler 800/266 Continuum probe 0.32-0.78mm 0.23-2.6mm 0.208-22mm 0.266mm Visible probe UV/vis/IR probe (IR: with Dr. K. Glusac) 266 nm pump/probe Accessible Spectral Range

Steady State Absorption of CHBr3 MeCH ACN 255 nm

Transient Absorption Spectra: CHBr3 in MeCH The signal < 0.3 ps is due to solvent At 0.3 ps, the band maxima: ~260, 355, and 395 nm The 355 nm band decays with the build up of the 395 nm band The 395 nm band red-shifts Final 255 and 415 nm bands Pump: 255-nm, <1.5 mJ pulse-1

TD-DFT vertical transition energies of Iso-bromoform Basis set: aug-cc-pvtz a in the gas phase, bin methyleclohexane (PCM), cin acetonitrile (PCM)

TD-DFT vertical transition energies of CHBr2 radical Basis set: aug-cc-pvtz a in the gas phase, bin methyleclohexane (PCM), cin acetonitrile (PCM)

Absorption of iso-CHBr3 Absorption of CHBr2 + iso-CHBr3 Absorption of iso-CHBr3 Br-MCH CT absorption

No Major Bimolecular Chemistry Diffusion-controlled reaction times: 10 mM ~ 10-8 sec 90 mM ~ 10-9 sec No major bimolecular chemistry ! Isomer lifetime: iso-CH2I2, iso-CHI3 ~5 ms Previous assignment of the 415 nm band to CHBr3-Br CT is erroneous

Transient Absorption Spectra: CHBr3 in Acetonitrile Early times: 255 nm band The 500 nm band shows up after 2 ps, 8 ps rise time Long times: the 255 nm band remains The 500 nm band: decays completely, 270 ps lifetime 255-nm Pump: <1.5 mJ pulse-1

CHBr2+ Iso-CHBr3 Iso-CHBr3 CHBr2 Br-ACN CT complex Isomer decays completely

Stabilization of the TS by Polar Solvent 1 TSiso 2 TSiso TSiso 2 2 1 1 Iso-bromoform readily converts back to the parent in acetonitrile

Proposed Scheme

Acknowledgments Tarnovsky Group Prof. Scott Reid (Marquette University) Ohio Supercomputer Center Funding NSF CAREER (Tarnovsky A. N.)

Bromoform in MeCH

MeCH ACN

Proposed Scheme