Kinetics of C₆H₅ radical reactions with toluene and xylenes by cavity ringdown spectrometry

The kinetics for the metathetical reactions of phenyl radical with toluene and xylenes have been studied experimentally and theoretically. The absolute bimolecular rate constants for the reactions of C₆H₅ with toluenes (C₇H₈ and C₇D₈) and xylenes (three C₈H₁₀ isomers) were measured by cavity ringdown spectrometry at temperatures between 295 and 483 K. For the reaction with toluene, a strong isotope effect was observed, whereas for xylene reactions no structural preference was noticed among the three isomers. The weighted least-squares analysis of each reaction gave rise to the following rate constant expressions in units of cm³/(mol s): k(C₇H₈) = (2.08 ± 0.11) × 10¹¹ exp[-(1027 ± 35)/T]; k(C₇D₈) = (2.27 ± 0.43) × 10¹¹ exp[-(1340 ± 64)/T]; k(C₈H₁₀) = (1.48 ± 0.11) × 10¹¹ exp[-(526 ± 27)/T]. Additionally, we have carried out hybrid density functional theory (B3LYP) calculations for the reactions of C₇H₈ and C₇D₈ using the 6-31G-(d,p) basis set. The predicted rate constants using the conventional transition state theory with the calculated vibrational frequencies and moments of inertia fit well to the experimental results with only minor adjustments in the calculated reaction barriers. Combination of our low-temperature C₇H₈ kinetic data with those obtained at high temperatures in shock waves gave the expression k(C₇H₈) = (4.15 × 10^-3)T^4.5 exp(800/T) cm³/(mol s) for the temperature range 300-1450 K.