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
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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
o Environmental importance:produced by microalgae, 200,000 tons per year
o Atmospheric importance: abundant source of bromine atoms in the
earth’s atmosphere causing destruction of the ozone layer
o Fundamental interest:structure-selective insights into the
condensed-phase photochemistry
1. 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
Previous Workh=350 nm
h=255 nm
40 60 80 100 120
0
10
20
30
40
50
60
70
Gas Phase PCM (cyclohexane) PCM (acetonitrile)
Rel
ativ
e E
nerg
y (k
cal m
ol-1)
C-Br-Br Angle (Degrees)
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
Patrick El-Khoury et al. Chem. Phys. Lett. 493 (2010) 61
CH2Br2
Iso-CH2Br2
TS
Patrick El-Khoury et al. J. Chem. Phys. 132 (2010) 124501
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) 2167McGivern et al. J. Phys.Chem. A 104 (2002) 10085Zou 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
Ultrafast (100 fs) Laser System We Built: Gap-Free Tunable 208 nm – 22 m
Accessible Spectral Range
Hurricane(100 fs, 800 nm, 0.8 mJ, 1 KHz)
TOPAS/pump
TOPAS/probe
UV/vis/near-IR pump
Tripler 800/266
Continuum probe0.32-0.78m
0.23-2.6m
0.208-22m
0.266m
Visible probe
UV/vis/IR probe(IR: with Dr. K. Glusac)
266 nm pump/probe
Steady State Absorption of CHBr3
200 220 240 260 280 3000
500
1000
1500
2000
2500
/
M-1cm
-1
Wavelength / nm
255 nm
ACN
MeCH
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