1. 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

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

3. 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

4. Previous Work CF2I2 iso-CF2-I-I
h=350 nm
CF2I iso-CF2-I-I
Ultrafast time-resolved IR : iso-CF2-I-I observed at 400 fs after excitation
CH2Br iso-CH2-Br-Br
Patrick El-Khoury et al. J. Chem. Phys (2010)
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 (2010) 61

5. 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 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

6. 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

7. 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 mm mm mm
0.266mm
Visible probe
UV/vis/IR probe
(IR: with Dr. K. Glusac)
266 nm pump/probe
Accessible Spectral Range

8. Steady State Absorption of CHBr3
MeCH
ACN
255 nm

9. 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

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

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

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

13. 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

14. 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

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

16. Stabilization of the TS by Polar Solvent1
TSiso 2
TSiso
TSiso 2 2 1 1
Iso-bromoform readily converts back to the parent in acetonitrile

17. Proposed Scheme

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

19. Bromoform in MeCH

20. MeCH
ACN

21. Proposed Scheme