KINETICS AND MECHANISM OF THE DISPROPORTIONATION OF HO2 IN THE GAS-PHASE

The kinetics of the reaction HO2 + HO2 → H2O2 + O2 (1) have been investigated over the pressure range 3-760 torr and the temperature range 273-339 K by using molecular modulation/UV absorption spectrometry to monitor HO2. HO2 was produced by reaction with O2 of H or CHO generated by photolysis of Cl2 in the presence of either H2 or HCHO. k1 was invariant with pressure above 25 torr and its temperature dependence is given by k1 = (3.8 ± 1.4) × 10-14 exp((1250 ± 200)/T) cm3 molecule-1 s-1 (error = 2 sd). The corresponding value of the absorption cross section for HO2 at the monitoring wavelength (220 nm) was 3.45 × 10-18 cm2. At pressures ≤10 torr, k1 decreased with falling pressure and its temperature dependence was reduced. The presence of water vapor increased the effective value of k1, in agreement with other recent work. A mechanism is proposed for the overall reaction involving an intermediate H2O4 molecule, which offers a plausible explanation of the observed effects of temperature and pressure. © 1979 American Chemical Society.