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

The thermal decomposition of toluene-d5 (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 ≈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-d5 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,k1=6.91×1013exp(-40180/T)(1469–1714K)k2=5.99×1015exp(-50060/T)(1469–1714K)The experimental results were also used to obtain rate constants for total toluene-d5 decomposition. Arrhenius analysis gives in units of s−1,kTotal=2.21×1014exp(-41760/T)(1469–1714K)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 ≈0.9 at lower-T (<1600 K) 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.