Kinetics of C6H5 Radical Reactions with Toluene and Xylenes by Cavity Ringdown Spectrometry

Abstract

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 (ref 13) gave the expression k(C7H8) = (4.15 × 10-3)T4.5 exp(800/T) cm3/(mol s) for the temperature range 300−1450 K.