Pyrolysis of C6D5CH3: Rate constants and branching ratios in the high-temperature thermal decomposition of toluene

The thermal decomposition of toluene-d(5) (C6D5CH3) has been studied at high temperatures with the reflected shock-tube technique using H-and D-atom ARAS. The experiments were performed at high-T (1469-1859 K) at nominal pressures approximate to 0.25-1.50 atm. The present study utilizing the ultra-sensitive H-atom ARAS technique has provided a direct measurement of the branching ratio in the two-channel high-temperature thermal decomposition of toluene-d(5) giving (1) C6D5CH2 + H and (2) C6D5 + CH3. Fall-off is observed in both decomposition channels at high-T (> 1700 K), but the lower-T rate constants can be well represented by simple Arrhenius expressions in units of s(-1) as, k(1) = 6.91 x 10(13) exp(-40180/T) (1469-1714 K) k(2) = 5.99 x 10(15) exp(-50060/T) (1469-1714 K) The experimental results were also used to obtain rate constants for total toluene-d(5) decomposition. Arrhenius analysis gives in units of s(-1), k(Total) 2.21 x 10(14) exp(41760/T) (1469-1714 K) The isotope effect is minimal and the present results therefore represent direct measurements of branching ratios and rate constants for toluene decomposition. The branching ratios to benzyl + H vary from approximate to 0.9 at lower-T (< 1600 K) to approximate to 0.75 at higher-T (> 1700 K). The excellent agreement between the present experiments and the theoretical predictions by Klippenstein et al. lead us to conclude that the high-T thermal decomposition of toluene is now well-characterized. (C) 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved.