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

The 395 nm band red-shifts

Final 255 and 415 nm bands

Pump: 255-nm, <1.5 J pulse-1

250 300 350 400 450 500 550 600 650 700 750 8000.000

0.004

0.008

t0.3ps t0.8ps t1ps

A

250 300 350 400 450 500 550 600 650 700 750 8000.000

0.004

0.008

0.012 t1ps t3ps t10ps t20ps t50psA

250 300 350 400 450 500 550 600 650 700 750 8000.000

0.004

0.008

0.012 t100ps t500ps t1000ps

Wavelength (nm)

A

ISO B3LYP Tv (f)

CAM-B3LYP Tv (f)

M06-2x Tv (f)

S1a

S1b

S1c

651.86 (0.0001) 664.43 (0.0002) 699.44 (0.0001)

519.53 (0.0002) 525.20 (0.0002) 589.55 (0.0002)

503.06 (0.0002) 505.83 (0.0002) 526.80 (0.0004)

S2a

S2b

S2c

600.54 (0.0005) 619.80 (0.0005) 670.01 (0.0001)

480.78 (0.0012) 492.01 (0.0015) 546.57 (0.0000)

463.86 (0.0010) 471.91 (0.0014) 513.02 (0.0235)

S3a

S3b

S3c

388.18 (0.3426) 422.63 (0.4762) 476.87 (0.4071)

374.72 (0.4326) 413.38 (0.5553) 512.44 (0.1625)

366.64 (0.4189) 399.60 (0.5480) 473.41 (0.3482)

S4a

S4b

S4c

308.32 (0.0148) 316.76 (0.0207) 340.88 (0.0163)

277.73 (0.0073) 290.16 (0.0092) 359.03 (0.0000)

273.11 (0.0046) 284.73 (0.0096) 335.40 (0.0080)

S5a

S5b

S5c

289.53 (0.0348) 294.72 (0.0016) 321.24 (0.0039)

267.17 (0.0138) 276.86 (0.0127) 336.26 (0.0001)

269.93 (0.0366) 272.75 (0.0158) 320.96 (0.0197)

S6a

S6b

S6

c

284.19 (0.0070) 284.66 (0.0505) 271.30 (0.0593)

263.80 (0.0403) 259.08 (0.0479) 267.23 (0.2649)

260.99 (0.0211) 262.43 (0.0407) 248.50 (0.1166)

S7a

S7b

S7c

273.68 (0.0066) 269.78 (0.0056) 263.18 (0.0047)

222.63 (0.0412) 229.32 (0.0708) 222.02 (0.0176)

217.81 (0.0341) 223.17 (0.0703) 235.38 (0.1235)

S8a

S8b

S8c

256.85 (0.1354) 257.66 (0.1357) 256.94 (0.1519)

216.13 (0.0138) 215.60 (0.0113) 221.75 (0.0168)

214.11 (0.0131) 212.10 (0.0138) 215.47 (0.0062)

S9a

S9b

S9c

238.16 (0.0769) 243.79 (0.0894) 251.91 (0.0841)

211.09 (0.0106) 207.05 (0.0059) 217.39 (0.5720)

213.06 (0.0193) 209.82 (0.0225) 210.05 (0.1165)

S10a

S10b

S10c

219.26 (0.0074) 215.43 (0.0113) 206.68 (0.0566)

203.84 (0.0817) 205.50 (0.1507) 204.49 (0.1102)

202.01 (0.0249) 202.13 (0.1038) 202.21 (0.1034)

TD-DFT vertical transition energies of Iso-bromoform

a in the gas phase, bin methyleclohexane (PCM), cin acetonitrile (PCM)

Basis set:aug-cc-pvtz

TD-DFT vertical transition energies of CHBr2 radical

Basis set:aug-cc-pvtz

CHBr2? B3LYP

Tv (f) CAM-B3LYP

Tv (f) M06-2x

Tv (f) S1

a

S1b

S1c

422.22 (0.0003) 421.53 (0.0003) 419.37 (0.0002)

376.39 (0.0002) 376.04 (0.0001) 374.76 (0.0001)

404.89 (0.0002) 404.48 (0.0001) 403.53 (0.0001)

S2a

S2b

S2c

283.03 (0.0000) 281.67 (0.0000) 279.43 (0.0000)

263.62 (0.0000) 262.31 (0.0000) 260.25 (0.0000)

268.85 (0.0000) 268.25 (0.0000) 267.82 (0.0000)

S3a

S3b

S3c

270.56 (0.0000) 270.32 (0.0000) 269.34 (0.0000)

252.51 (0.0000) 252.34 (0.0000) 251.70 (0.0000)

265.72 (0.0000) 264.12 (0.0000) 261.94 (0.0000)

S4a

S4b

S4c

267.25 (0.0002) 266.31 (0.0004) 264.50 (0.0005)

251.25 (0.0002) 250.34 (0.0003) 248.55 (0.0003)

254.93 (0.0002) 254.08 (0.0004) 252.01 (0.0004)

S5a

S5b

S5c

261.84 (0.0616) 262.41 (0.0843) 259.92 (0.0802)

247.55 (0.0699) 247.96 (0.0936) 245.49 (0.0888)

249.88 (0.0014) 249.15 (0.0023) 247.17 (0.0034)

S6a

S6b

S6c

260.77 (0.0025) 260.35 (0.0037) 259.30 (0.0048)

241.63 (0.0029) 241.19 (0.0044) 240.03 (0.0058)

245.26 (0.0648) 245.40 (0.0925) 242.82 (0.0874)

S7a

S7b

S7c

235.55 (0.0064) 234.72 (0.0082) 233.26 (0.0081)

222.74 (0.0045) 222.03 (0.0057) 220.97 (0.0057)

217.20 (0.0045) 216.29 (0.0060) 215.48 (0.0060)

S8a

S8b

S8c

221.88 (0.0001) 221.13 (0.0003) 219.86 (0.0003)

210.51 (0.0000) 209.83 (0.0001) 208.76 (0.0001)

213.40 (0.0173) 213.50 (0.0236) 212.01 (0.0226)

S9a

S9b

S9c

218.99 (0.0000) 218.18 (0.0000) 216.88 (0.0000)

204.17 (0.0000) 203.65 (0.0252) 202.60 (0.0000)

209.49 (0.0167) 209.36 (0.0193) 208.44 (0.0175)

S10a

S10b

S10c

215.21 (0.0149) 216.57 (0.0231) 215.34 (0.0224)

203.33 (0.0188) 203.59 (0.0000) 202.49 (0.0244)

206.43 (0.0012) 205.73 (0.0000) 205.01 (0.0000)

a in the gas phase, bin methyleclohexane (PCM), cin acetonitrile (PCM)

250 300 350 400 450 500 550 600 650 700 750 8000.000

0.004

0.008

Wavelength (nm)

t1ps CHBr2/TD-DFT

A

250 300 350 400 450 500 550 600 650 700 750 8000.000

0.004

0.008

0.012 t500ps Isomer/TD-DFT

Wavelength (nm)

AAbsorption of CHBr2 + iso-CHBr3

Absorption of iso-CHBr3

Br-MCH CT absorption

250 300 350 400 450 500 550 600 650 700 750 8000.000

0.004

0.008

0.012 CHBr3-Br CTC acc.

to Shoute and Neta t500ps, iso-CHBr

3

Wavelength (nm)

A

Isomer lifetime: iso-CH2I2, iso-CHI3 ~5 s

Previous assignment of the 415 nm band to CHBr3-Br CT iserroneous

250 300 350 400 450 500 550 600 650 700 750 8000.0

0.5

1.0 50 ps spectra

A

, nor

m.

Wavelength /nm

500 mM 90 mM 10 mM

Diffusion-controlled reaction times:10 mM ~ 10-8 sec90 mM ~ 10-9 sec

No major bimolecular chemistry !

No Major Bimolecular Chemistry

Transient Absorption Spectra: CHBr3 in Acetonitrile

Early times: 255 nm band

The 500 nm band shows up after 2 ps,8 ps rise time

255-nm Pump: <1.5 J pulse-1

250 300 350 400 450 500 550 600 650 700 750 8000.000

0.002

0.004

0.006

t0.5ps t2ps t6ps t20ps

A

250 300 350 400 450 500 550 600 650 700 750 8000.000

0.002

0.004

0.006

Wavelength / nm

t50ps t200ps t500ps t1000ps

A

Long times:the 255 nm band remains

The 500 nm band:decays completely,270 ps lifetime

250 300 350 400 450 500 550 600 650 700 750 8000.000

0.002

0.004

0.006

Wavelength / nm

t100ps Isomer/TD-DFT

A

250 300 350 400 450 500 550 600 650 700 750 8000.000

0.002

0.004

Wavelength / nm

t1000ps CHBr2/TD-DFT

A

CHBr2+Iso-CHBr3

Iso-CHBr3

Br-ACN CT complex

CHBr2

Isomer decays completely

Iso-bromoform readily converts back to the parent in acetonitrile

Stabilization of the TS by Polar Solvent

1 TSiso 2

1

2TSisoTSiso

1

2

Proposed Scheme

C

Br

HBr

Br

CH

Br

Br

Br

CH

Br

Br

Br

CH

Br

Br

Br

255 nm

255 nm

285 nm/355 nm

285 nm/355 nm255 nm

400 - 500 nm

ACN

MeCH Slow Decay

ps

ps

Acknowledgments

• Tarnovsky Group• Prof. Scott Reid (Marquette University)• Ohio Supercomputer Center

• Funding NSF CAREER (Tarnovsky A. N.)

Bromoform in MeCH

0 50 100 150 2000.0

0.5

1.0

1.5

Abs

orba

nce

/Cel

l pat

thle

ngth

Concentration / mM

MeCH

ACN

Proposed Scheme

C

Br

HBr

Br

CH

Br

Br

Br

CH

Br

Br

Br

CH

Br

Br

Br

255 nm

255 nm

285 nm/355 nm

285 nm/355 nm255 nm

400 - 500 nm

ACN

MeCH Slow Decay

ps

ps