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

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, 1545-1555. 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, 044312. 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, 231-239. 7th ICCK, MIT, July 10th - 14th 2011

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, 144301. 7th ICCK, MIT, July 10th - 14th 2011 3

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: 2000-2700 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

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

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

1% C6H5F/Kr, T2=2699 K, P2=59 Torr density gradient Well established codes and correlations for finding t0 (within 0.1-0.2 μs) Low pressure (<60 Torr) experiments have incubation delay 7th ICCK, MIT, July 10th - 14th 2011

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

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

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

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

(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

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, 8792. (B3LYP / 6-311G(d,p) level) 7th ICCK, MIT, July 10th - 14th 2011

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, 044312. (CCSD(T)/cc-PVTZ level) Moskaleva, L., Madden, L., Lin, M. (1999) Phys. Chem. Chem. Phys., 1. 3967-3972. 7th ICCK, MIT, July 10th - 14th 2011

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

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

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

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