Post on 08-Jan-2018
description
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