Study of the kinetics and equilibrium of the benzyl-radical association reaction with molecular oxygen

The forward rate constant, k₁, and the equilibrium constant, K_p, for the association reaction of the benzyl radical with oxygen have been determined. The rate constant k₁ was measured as a function of temperature (between 298 and 398 K) and pressure (at 20 and 760 torr of N₂) by two different techniques, argon-lamp flash photolysis and excimer-laser flash photolysis, both of which employed UV absorption spectroscopy (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 temperature, 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 N₂ and for 298 < T/K < 398, a linear least-squares fitting of the data yield the expression: k₁ = (7.6 ± 2.4) × 10^?13 exp[(190 ± 160) K / T ] cm³ molecule^?1 s^?1. With the flash-photolysis technique, we determined K_p over the temperature range 398–525 K. Experimental values were analyzed alone and combined with theoretically determined entropy values of the benzyl and benzylperoxy radicals to determine the enthalpy of reaction: ΔH = (?91.4 ± 4) kJ mol^?1. Previous work on the benzyl radical enthalpy of formation allows us to calculate ΔH°_{f 298} (Benzylperoxy) = (117 ± 6) kJ mol^?1. In addition, we carried out an RRKM calculation of k₁ using as constraints the thermodynamic information gained by the study of 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 most recent work conducted at pressures near 1 torr of helium. © 1994 John Wiley & Sons, Inc.