Direct observation of the rate of H-atom formation in the thermal decomposition of ortho-, meta-, and para-xylene behind shock waves between 1300 and 1800 K

The rate of H-atom formation in the thermal decomposition of o-, m-, and p-xylene was studied behind reflected shock waves between 1300 and 1800 K and at pressures of 1.6 to 4 bar by using atomic resonance absorption spectroscopy at 121.6 nm. For the thermal decomposition of p-xylene, m-xylene, and o-xylene p-CH(3)C(6)H(4)CH(3) --> p-CH(3)C(6)H(4)CH(2) + H (R1) m-CH(3)C(6)H(4)CH(3) --> m-CH(3)C(6)H(4)CH(2) + H (R2) o-CH(3)C(6)H(4)CH(3) --> o-CH(3)C(6)H(4)CH(2) + H (R3) the following rate constants were determined: k(1) = 1 x 10(16) exp(-380 kJ mol(-1) /RT) s(-1), k(2) = 1 x 10(16) exp(-382 kJ mol(-1)/RT) s(-1). and k(3) = 1 x 10(16) exp(-378 kJ mol(-1)/RT) s(-1). The rate constants are all close to the high-pressure limit, and the accuracy was estimated to be 30%. The H-atom-concentration time profiles were modeled using a simple mechanism in which the parallel C-C bond split, and secondary reactions have been taken into account. We found that within our experimental conditions, the C-C bond scission contributes about 15% to the overall decomposition rate constant of the xylenes.