Pressure dependence of phenylperoxy radical formation in the reaction of phenyl radical with molecular oxygen

Pressure dependence of the phenylperoxy radical (C 6 H 5 O 2 ) formation in the reaction of phenyl radical (C 6 H 5 ) with molecular oxygen (O 2 ) has been investigated with the cavity ring‐down method by detecting the C 6 H 5 O 2 at 504 nm. The rate constant of the C 6 H 5 + O 2 reaction was determined to be k = (1.2 ± 0.1) × 10 −11 cm 3 molecule −1 s −1 by the least‐square analysis of several sets of the C 6 H 5 O 2 rise profiles at 30 Torr and at 296 K. This value is almost identical to those previously obtained at 40 Torr (Yu and Lin, J Am Chem Soc 1994, 98, 9571–9576) and 760 Torr (Tonokura et al., J Phys Chem A 2002, 106, 5908–5917), indicating that the rate constant has no pressure dependence in the pressure range of 30–760 Torr. The signal intensity ratio of C 6 H 5 O 2 to C 6 H 5 was obtained in the pressure range of 5–120 Torr. Varying the third body carrier gas (N 2 ) had a significant effect on the production of the C 6 H 5 O 2 . To validate the k determination by C 6 H 5 O 2 rise detection and determine the branching ratio of C 6 H 5 O 2 in the C 6 H 5 + O 2 reaction system, theoretical calculations were also performed. The rate constant obtained with Rice–Ramsperger–Kassel–Marcus/master equation calculations is in good agreement with that measured in this experiment. The branching ratio forming C 6 H 5 O 2 obtained by the theoretical calculation also exhibited pressure dependence, which is consistent with the experimental results. The branching ratio increases with increasing pressure and reaches approximately 0.85 at 120 Torr. © 2011 Wiley Periodicals, Inc. Int J Chem Kinet 44: 41–50, 2012