A KINETIC AND MECHANISTIC STUDY OF THE SELF-REACTION AND REACTION WITH HO2 OF THE BENZYLPEROXY RADICAL

The kinetics and mechanism of the reactions C6H5CH2O2 + C6H5CH2O2 --> 2C6H5CH2O + O2 (3a), C6H5CH2O2 + C6H5CH2O2 --> C6H5CHO + C6H5CH2OH + O 2 (3b), and C6H5CH2O2 + HO2 --> C6H5CH2OOH + O2 (4) have been investig ated using two complementary techniques: flash photolysis/UV absorptio n for kinetic measurements and continuous photolysis/FTIR spectroscopy for end-product analyses and branching ratio determinations. The reac tion of chlorine atoms with toluene was found to yield benzyl radicals exclusively and was used to generate benzylperoxy radicals in excess oxygen. During this study, relative reaction rate constants of chlorin e atoms with compounds related to those involved in the reaction mecha nism have been measured at room temperature: k(Cl+toluene) = (6.1 +/- 0.2) X 10(-11), k(Cl+benzaldehyde) = (9.6 +/- 0.4) X 10(-11), k(Cl+ben zyl chloride) = (9.7 +/- 0.6) X 10(-12), k(Cl+benzyl alcohol) = (9.3 /- 0.5) X 10(-11), k(Cl+benzene) < 5 X 10(-16), all in units of cm3 mo lecule-1 s-1. The products identified following the self-reaction 3 we re benzaldehyde, benzyl alcohol, and benzyl hydroperoxide. The latter is the product of the reaction Of C6H5CH2O2 with HO2. The yield of pro ducts allowed us to determine the branching ratio alpha = k3a/k3 = 0.4 . The UV absorption spectrum of the benzylperoxy radical was determine d from 220 to 300 nm. It was similar to those of alkylperoxy radicals, with a maximum cross section at 245 nm of 6.8 X 10(-18) cm2 molecule. -1 Kinetic data were obtained from the detailed simulation of experime ntal decay traces recorded at 250 nm over the temperature range 273-45 0 K. The resulting rate expressions are k3 = (2.75 +/- 0.15) X 10(-14) exp[(1680 +/- 140) K/T] cm3 molecule-1 s-1 and k4 = (3.75 +/- 0.32) X 10(-13) exp[(980 +/- 230)K/T) cm3 molecule-1 s-1 (errors = 1sigma). T he UV absorption traces in the flash-photolysis kinetic study were wel l accounted for by the identified products in the FTIR study, thus pro viding good confidence in the results. However, about 20% of the produ cts have remained unidentified. Some uncertainties persist in the reac tion mechanism leading us to assign a fairly large uncertainty of abou t 50% to the rate constants k3 and k4 over the whole temperature range . This work shows that the aromatic substituent does not provide any s pecificity in the reactivity of peroxy radicals and confirms that larg e radicals tend to react faster with HO2 than generally assumed in cur rent atmospheric models.