Dissociation of ortho-benzyne radicals in the high temperature fall-off regime

The decomposition of ortho-benzyne radicals (o-C6H4), generated from the dissociation of a new precursor, fluorobenzene (C6H5F), has been investigated in a diaphragmless shock tube in a combined laser schlieren densitometry, LS, (P2 = 30 ± 2, 59 ± 3, 121 ± 5 Torr, 2050 < T2 < 2980)/time-of-flight mass spectrometry, TOF-MS (P5 = 1150 ± 200 Torr, 2300 < T5 < 2800 K) study. The LS density gradient profiles were simulated, and excellent agreement was found between simulations and experimental profiles. Rate coefficients for C6H5F → o-C6H4 + HF, o-C6H4 → C4H2 + C2H2, and o-C6H4 → C6H3 + H were obtained. Good agreement with Xu et al. [Proc. Combust. Inst., 31(2007) 231-239] with respect to the o-benzyne dissociation branching ratio was found. However a strong pressure dependence was also observed in o-benzyne dissociation which was not seen by Xu et al. For o-C6H4 → C4H2 + C2H2: k2a,120Torr = (1.2 ± 0.4) × 1063T−14.27 exp(−52,710/T) k2a,60Torr = (1.4 ± 0.5) × 1060T−13.595 exp(−50,538/T), k2a,30Torr = (4.0 ± 1.3) × 1057T−13.015 exp(−48,628/T) s−1. The inclusion of high temperature o-benzyne abstraction reactions is necessary to simulate the reacting system, and we report estimates of the rate of o-C6H4 + C6H5F.