A shock tube investigation of H atom production from the thermal dissociation of ortho-benzyne radicals

The thermal dissociation of ortho-benzyne (o-C6H4) has been studied behind reflected shock waves under very isolated conditions. In the shock tube experiments 1,2-diiodobenzene was employed as a thermal source for the o-C6H4 radical. For different series of experiments the temperature ranged from 1600 to 2400 K at pressures between 1.4 and 2 bar. Very low initial concentrations of the radical precursor, 0.5-4 ppm diluted in argon, were used. In situ atomic resonance absorption measurement of iodine atoms formed during the thermal dissociation of the radical precursor molecule provides for a precise determination of the initial 1,2-diiodobenzene concentration. ARAS (atomic resonance absorption spectroscopy) was also used to record the absorption profiles of hydrogen atoms obtained during the pyrolysis of ortho-benzyne. From the measured H atom absorption profiles and by taking into account recent results from literature, it is shown that the thermal dissociation of ortho-benzyne occurs via two pathways: besides the molecular route: o-C6H4 -> C4H2 + C2H2 (R1a) the second product channel R1b leading to direct H atom elimination had to be included o-C6H4 -> c-C6H3 + H (R1b) Due to lack of experimental data, the rate expression for R I a was estimated by taking the activation energy of 347 kJ/mol from literature. Depending on k(1a) a rate expression k(1b) was deduced from modeling H atom formation, with a value for the activation energy of about 419 kJ/mol. The reaction rates of both channels are coupled by the branching ratio alpha = k(1b)/(k(1a) + k(1b)), with alpha = 2.36E - 04 center dot T - 0.3. It was found that for a wide variation of k(1a) (factors 0.1-20), k(1b) had also be adjusted with the same factors to match the H atom formation, thus the branching ratio is independent of the value of k(1a) in the investigated temperature range. (C) 2006 The Combustion Institute. Published by Elsevier Inc. All rights reserved.