The pyrolysis of 2-, 3-, and 4-methylbenzyl radicals behind shock waves

Abstract

The thermal decomposition of 2-, 3-, and 4-methylbenzyl radicals was studied behind reflected shock waves. α -Bromo- ortho ( meta - and para )-xylenes were used as precursors for a series of experiments with temperatures ranging from 1150 to 1600 K and pressures between 1.5 and 4 bar. Mixtures of 1–5.5 ppm of the precursor diluted in argon were used for the investigations. Initiated by the fast thermal dissociation of the precursor CH 3 C 6 H 4 CH 2 Br→CH 3 C 6 H 4 CH 2 +Br (R3) the methylbenzyl radicals subsequently decomposed. 2-methylbenzyl→ o -CH 2 C 6 H 4 CH 2 +H (R2 o ) 3-methylbenzyl→ m -CH 2 C 6 H 4 CH 2 +H (R2 m ) 4-methylbenzyl→ p -CH 2 C 6 H 4 CH 2 +H (R2 P ) We followed the rate of H-atom formation by using atom resonance absorption spectroscopy (H-ARAS) at 121.6 nm. We report the first directly measured rate coefficients for the decomposition of methylbenzyl radicals: k 2o =5×10 15 exp {−(310±4)kJ mol −1 / RT } s −1 , k 2m =5×10 15 exp{−(340±4) kJ mol −1 / RT } s −1 , and k 2p =5×10 15 exp{−(295±4) kJ mol −1 / RT } s −1 These rate coefficients are about a factor of 4 below the high-pressure limit, and the accuracy is estimated to be 30%. The complete H-atom concentration-time profiles were successfully modeled using a simple mechanism.