HIGH-TEMPERATURE REACTIONS OF BENZYL RADICALS

The thermal decomposition of benzyl radicals has been investigated behind reflected shock waves by monitoring the formation of hydrogen atoms. The results reported here have been obtained by using a shock tube in conjunction with atomic resonance absorption spectrometry (ARAS). The temperatures ranged between 1450 and 1700 K at total pressures around 2 bar. The test gas mixtures consisted of argon with relative concentrations of a few ppm benzyliodide, C7H7I. From the measurements of the temporal H-atom concentrations, in connection with thermochemical restraints, it was deduced that the benzyl decay is preceeded by isomerization of benzyl. Due to the low initial concentration of the radical precursor, it was possible to evaluate a first order rate coefficient for the initiation reaction: R 1a C7H7 --> benzyl isomer k1a = 10(15.53 +/- 0.3) exp(-(42900 +/- 2000) K/T) s-1 for M congruent-to 1.5 . 10(-5) mol cm-3. The heat of formation of the benzyl radical is discussed. A value of DELTA-H-f,298(0) = 215.5 kJ mol-1 fits best to the experimental results.