Gas-phase reaction of the OH-benzene adduct with O-2: reversibility and secondary formation of HO2

The reaction of OH radicals with benzene and consecutive reactions of benze ne-OH adducts with O-2 were studied in the gas phase in N-2-O-2 mixtures at atmospheric pressure and room temperature. OH was produced by pulsed 248 n m photolysis of H2O2. Time-resolved detection of both OH and benzene-OH add ucts was performed by continuous-wave (cw) UV-laser long-path absorption at around 308 nm. The reaction: OH+benzene --> products [reaction (1)] was no t affected by the presence of O-2. Rate constants k(1)=(1.10 +/- 0.07)x10(- 12) cm(3) s(-1) and (1.06 +/- 0.07)x10(-12) cm(3) s(-1) were obtained in N- 2 and O-2, respectively. In N-2 addition of NO2 did not change k(1), from w hich an upper limit of 5% is derived for formation of H atoms in reaction ( 1). An absorption cross-section of sigma(308 nm)=(5.8 +/- 1.5)x10(-18) cm(2 ) and a self-reaction rate constant of (3.4 +/- 1.7)x10(-11) cm(3) s(-1) we re determined for the benzene-OH adduct. Upper limits of 5x10(-15) cm(3) s( -1), 1x10(-14) cm(3) s(-1) and 5x10(-14) cm(3) s(-1) were obtained for reac tions of the adduct with benzene, H2O2 and NO, respectively. The adduct kin etics in the presence of O-2 is consistent with the reversible formation of a peroxy radical: adduct+O-2<----> adduct-O-2 [reaction (2a/-2a)]. An equi librium constant of K-2a=(2.7 +/- 0.4)x10(-19) cm(3) was determined and a r ate constant of k(2a)=(2 +/- 1)x10(-15) cm(3) s(-1) was roughly estimated. The effective adduct loss from the equilibrium can be explained by (i) an a dditional irreversible reaction of the adduct with O-2 with a rate constant of (2.1 +/- 0.2)x10(-16) cm(3) s(-1), or (ii) a unimolecular reaction of t he peroxy radical, with a rate constant of (7.6 +/- 0.8)x10(2) s(-1). For a reaction of the peroxy radical with O-2 an upper limit of 1x10(-17) cm(3) s(-1) is estimated. Addition of NO reveals formation of HO2 in the presence of O-2 by recovering OH via HO2+NO. Applying numerical methods, reaction m odels were tested to describe the observed complex kinetics of OH. The data are consistent with rapid HO2 formation following a peroxy radical+NO reac tion with a rate constant of (1.1 +/- 0.4)x10(-11) cm(3) s(-1). Extrapolati on of HO2 formation rates to [NO]=0 points at a second source of HO2 not pr eceded by any RO2+NO reaction.