Ff. Fenter et al., STUDY OF THE KINETICS AND EQUILIBRIUM OF THE BENZYL-RADICAL ASSOCIATION REACTION WITH MOLECULAR-OXYGEN, International journal of chemical kinetics, 26(1), 1994, pp. 171-189
The forward rate constant, k(1) and the equilibrium constant, K-p, for
the association reaction of the benzyl radical with oxygen have been
determined. The rate constant k(1) was measured as a function of tempe
rature (between 298 and 398 K) and pressure (at 20 and 760 torr of N-2
by two different technique, argon-lamp flash photolysis and excimer-l
aser flash photolysis, both of which employed UV absorption spectrosco
py (at 253 nm and 305 nm, respectively) to monitor the benzyl radical
concentration. Over the range of conditions studied, we find that the
reaction is independent of pressure and is almost independent of tempe
rature, which is in accord with two early studies of the reaction but
in apparent disagreement with more recent work. For our results in 760
torr of N-2 and for 298 < T/K < 398, a linear least-squares fitting o
f the data yield the expression: k(1) = (7.6 +/- 2.4) x 10(-13) exp[(1
90 +/- 160)K/T] cm(3) molecule(-1) s(-1.) With the flash-photolysis te
chnique, we determined K-p over the temperature range 398-525 K. Exper
imental values were analyzed alone and combined with theoretically det
ermined entropy values of the benzyl and benzylperoxy radicals to dete
rmine the enthalpy of reaction: Delta H(298)degrees = (-91.4 +/- 4) kJ
mol(-1). Previous work on the benzyl radical enthalpy of formation al
lows s to calculate Delta H(f298)degrees (Benzylperoxy) = (117 +/- 6)
kJ mol(-1). In addition, we carried out an RRKM calculation of k(1) us
ing as constraints the thermodynamic information gained by the study o
f K-p. We find that all the studies of the association reaction are in
good agreement once a fall-off effect is taken into account for the m
ost recent work conducted at pressures near 1 torr of helium. (C) 1994
John Wiley & Sons, Inc.