1. High Temperature Dissociation of o-Benzyne and C6H5F
Robert S. Tranter and Patrick T. Lynch
Chemical Sciences and Engineering Division,
Argonne National Laboratory
Xueliang Yang
Department of Chemical and Biomolecular Engineering,
North Carolina State University
7th International Conference on Chemical Kinetics, MIT, July 10th-14th 2011

2. Introduction
ortho-Benzyne : o-C6H4 -- Few but important investigations
Identified as a reaction pathway in benzene reactions to C4H2 and C2H2 (Wang et al. 2000)
C6H5 → C4H3 + C2H2 → H + C4H2 + C2H2
C6H5 → H + o-C6H4 → H + C4H2 + C2H2
Product analysis (TOFMS, FTIR) and high level calculations (Zhang et al. 2007)
C2H2 adds to C6H5 and o-C6H4 forming precursors for soot. (Friedrichs et al. 2009, Flash Pyrolysis- MS)
o-C6H4 recombination itself important at low temp (Tranter et al. 2010, ST- LS-TOFMS)
o-C6H4 also eliminates H atoms to c-C6H3 + H (Xu et al. 2007, H-ARAS in ST)
References:
Wang, H., Laskin, A., Moriarty, N., Frenklach, M. (2000) Proc. Combust. Inst., 28,
Zhang. X., Maccarone, A., Nimlos, M., Kato, S., Bierbaum, V., Ellison, G., Ruscic, B., Simmonet, A., Allen, W., Schaefer, H. (2007) J. Chem Phys. ,126,
Friedrichs, G., Goos, E., Gripp, J. Nicken, H. Schönborn, J., Vogel, H., Temps, F. (2009) Z. Phys. Chemie, 223, 387.
Tranter, R., Klippenstein, S. Harding, L., Giri, B., Yang, X., Kiefer, J. (2010) J. Phys. Chem. A, 114, 8240.
Xu, C., Braun-Unkhoff, M. Naumann, C., Frank, P. (2007) Proc. Combust. Inst., 31,
7th ICCK, MIT, July 10th - 14th 2011

3. Introduction (cont.)
Need to extend kinetic information to higher temperature
Laser Schlieren Densitometry (reaction rates) and TOF-MS (product analysis) in DFST
Use fluorobenzene (C6H5F) as precursor
Fluorobenzene pyrolysis also interesting in its own right:
Should dissociate primarily by HF elimination as source of high temperature o-C6H4 (Lee et al. 2006, Flash Photolysis)
Experiments suggest four center transition state
Approximately 4% branching ratio to H elimination in flash photolysis
Unclear if these results extend to thermal excitation
Compare this with primarily I elimination from Iodobenzene (Tranter et al. 2010)
Lee, S., Wu, C., Yang, S, Lee, Y. (2006) J. Chem Phys. 125,
7th ICCK, MIT, July 10th - 14th 2011 3

4. Diaphragmless Shock Tube
Driver section
Driven section
Expansion
Tank
Reflectron TOF-MS
Ion Source
Laser Schlieren
Reagent mixtures: 1% C6H5F / Kr (LS) , 2% C6H5F / 3% Ar / Ne (TOF-MS)
Reaction temperature: LS: K
TOF-MS: 1500 – 2100 K
Reaction pressure: LS: 60 ± 3 and 121 ± 4 Torr
TOF-MS: 420 ±30 Torr
7th ICCK, MIT, July 10th - 14th 2011

5. Laser Schlieren Densitometry
Measure the time resolved angular deflection,
Rotatable mirror
HeNe Laser
Split photodiode
The net density gradient of the reaction system
Recall importance of initial density gradient since it allows assignment of k1
Note endothermic reactions have positive density gradients and exothermic reactions have negative density gradients
r: rate of the reaction
Hr: heat of reaction
N: change of the mole number
C6H5F → o-C6H4+HF
ΔHr = 70 kcal/mol
7th ICCK, MIT, July 10th - 14th 2011

6. Raw LS data
7th ICCK, MIT, July 10th - 14th 2011

7. 1% C6H5F/Kr, T2=2699 K, P2=59 Torr density gradient
Well established codes and correlations for finding t0 (within μs)
Low pressure (<60 Torr) experiments have incubation delay
7th ICCK, MIT, July 10th - 14th 2011

8. Product Analysis – TOF-MS
2% C6H5F / 3% Ar / 95% Ne
T5 = 2106 K, P5 = 446 Torr
HF+
Ne+
C6H2+
30 kHz sampling, electron impact ionization
Scale to inert Ar internal standard
Sampling indicated:
no F+, no C-F scission
no C6H4F+, small contribution of C-H scission channel
C4H2+ and C2H2+ – Retro Diels Adler of o-C6H4
C6H2+ produced early
C6H5F+
C4H2+
Ar+
C2H2+
7th ICCK, MIT, July 10th - 14th 2011

9. Core Reaction Mechanism
C6H5F
ΔHr = + 70
ΔHr = + 118
ΔHr = + 119
ΔHr = + 70
HF +
o-C6H4
c-C6H3 +HF
C6H4F + H
+ C6H5F
+C6H5
ΔHr = + 56
ΔHr = + 119
Weight of arrows are conceptual, heavier arrows are more important pathways, not a complete flux analysis.
Estimation of branching ratio from literature
C6H5 is not a problem, we have it taken care of
ΔHr units: kcal/mol
ΔHr = + 46
C4H2+ C2H2
H+ c-C6H3
C6H2+H
Sub-mechanisms (86 reactions, 29 species)
C6H5 and o-C6H4 recombination
C2H2 (high T)
Tranter, R., Klippenstein, S. Harding, L., Giri, B., Yang, X., Kiefer, J. (2010). J. Phys. Chem. A, 114, 8240.
Kiefer J., Sidhu, S., Kern, R., Xie, K., Chen, H., Harding, L. (1992) Combust. Sci. Tech., 82, 101.
7th ICCK, MIT, July 10th - 14th 2011

10. Simulation Results and sensitivity to C6H5F decomposition rate (k1+k2)
C6H5F→o-C6H4 + HF
ΔHr = +70 kcal/mol
(4) o-C6H4 → C4H2 + C2H2
ΔHr = +56 kcal/mol
Experiment
Simulations
Preferred
(k1+k2)*1.3
(k1+k2)*0.7
7th ICCK, MIT, July 10th - 14th 2011

11. Branching ratio of C6H5F decomposition
C6H5F→o-C6H4 + HF
ΔHr = +70 kcal/mol
(2) C6H5F→C6H4F + H
ΔHr = +118 kcal/mol
BR = k1/(k1+k2)
Experiment
Simulations
Preferred (BR =7%)
BR = 25%
BR = 0
7th ICCK, MIT, July 10th - 14th 2011

12. (5) o-C6H4 + C6H5F → C6H5 + c-C6H3 + HF
ΔHr = + 119
Experiment
Simulations
With reaction 5
Without reaction 5
k5 draws out in time, the hump from secondary dissociation.
7th ICCK, MIT, July 10th - 14th 2011

13. C6H5F → o-C6H4 + HF: 93 +/- 2% of C6H5F dissociation
RRKM calc details: E0 = 93.5 kcal/mol
TS freq, etc from: Wu, C., Wu, Y., Lee, Y. (2004) J. Chem. Phys. 121, (B3LYP / 6-311G(d,p) level)
7th ICCK, MIT, July 10th - 14th 2011

14. o-C6H4 → C4H2 + C2H2:
T dependent branch of o-C6H4 dissociation, follows Xu et al.
RRKM calc details: E0= 89.3 kcal/mol
TS freq, etc from: Zhang. X., Maccarone, A., Nimlos, M., Kato, S., Bierbaum, V., Ellison, G., Ruscic, B., Simmonet, A., Allen, W., Schaefer, H. (2007) J. Chem. Phys., 126,
(CCSD(T)/cc-PVTZ level)
Moskaleva, L., Madden, L., Lin, M. (1999) Phys. Chem. Chem. Phys.,
7th ICCK, MIT, July 10th - 14th 2011

15. o-C6H4 → c-C6H3 + H vs. C2H2 + C4H2 Branching ratio:
Ratio of c-C6H3 + H to total o-C6H4 dissociation
7th ICCK, MIT, July 10th - 14th 2011

16. o-C6H4 + C6H5F → C6H5 + c-C6H3 + HF:
Fit equation:
7th ICCK, MIT, July 10th - 14th 2011

17. Summary
High temperature o-C6H4 dissociation rates in the falloff region were measured using LS, and an RRKM model was developed:
Wang et al. extrapolations about 1/3x those measured
Moskaleva et al. calculations about 2x those measured
High temperature C6H5F dissociation rates also measured and RRKM model developed:
C6H5F dissociates primarily by HF elimination (~93% BR) in line with photo-dissociation studies.
o-C6H4 attacking C6H5F found to be an important reaction in this system and first experimental estimates of this reaction rate determined.
7th ICCK, MIT, July 10th - 14th 2011

18. Acknowledgments Christopher J. Annesley, Argonne National Laboratory
Professor John Kiefer, University of Illinois at Chicago
This work was supported by the Office of Basic Energy Sciences,
Division of Chemical Sciences, Geosciences, and Biosciences,
U. S. Department of Energy, under contract DE-AC02-06CH11357
7th ICCK, MIT, July 10th - 14th 2011