Kinetics of the C6H5 + O2 Reaction at Low Temperatures

The kinetics of the C₆H₅+O₂ reaction has been studied with the cavity-ring-down (CRD) method by monitoring the rate of C₆H₅O₂ radical formation at 496.4 nm. The second-order rate constants measured under the conditions, 20 Torr ≤ P ≤80 Torr (Ar), 297 K ≤ T≤473 K, were found to be pressure-independent, with a small negative activation of 0.32 kcal/mol. A least-squares analysis of a dozen sets of data obtained by two different kinetic evaluation methods gives rise to ko₂= 10-11.00±0.08exp[(+161 ± 66)/T] cm³/molecules where the errors represent one-standard deviations, evaluated with the weighting factors w_i = (ki/σi)². Under the conditions employed in the present work, the C₆H₅ + O₂ reaction takes place primarily by the addition-stabilization process C₆H₅ + O₂ ⟷ C₆H₅O₂ (+M) ΒC₆H₅O. A search carried out at 575.4 nm for the production of C₆H₅O, which could be conveniently and sensitively detected by the CRD method, failed to detect the radical. This finding is fully consistent with the apparent large activation energy for the formation of C6H5O from the C₆H₅ + O₂ reaction (6 kcal/mol) recently reported by Frank and co-workers (ref 30) using the atomic resonance absorption-shock tube method at T < 900 K.