Spectra and Kinetics of the Hydroperoxyl Free Radical in the Gas Phase

Abstract

The absorption spectrum of the hydroperoxyl radical (HO₂) has been obtained by the molecular‐modulation technique. The radical was formed by the photolysis of hydrogen peroxide at 2537 Å, by the photolysis of ozone in the presence of hydrogen peroxide at 2537 Å, and by the photolysis of Cl₂ in the presence of hydrogen peroxide at 3500 Å. The vibrational frequencies of HO₂ have been observed to be 1095, 1390, and 3410 cm⁻¹. Details of the vibrational spectrum are consistent with the molecular geometry: $\mathrm{H–O}$ bond distance 0.96 Å, $\mathrm{O–O}$ bond distance 1.3 Å, and $\mathrm{H–O–O}$ angle approximately 108°. The absorption spectrum of HO₂ in the ultraviolet has a maximum at 2100 Å. Kinetic analysis of the modulated absorption signals shows that the HO₂ radical decays by a process second order in HO₂ concentration. The rate constant for the disproportionation reaction ${\mathrm{HO}}_2+{\mathrm{HO}}_2\to {\mathrm{H}}_2{\mathrm{O}}_2+{\mathrm{O}}_2$ was found to be ${\text{3.6± 0.5× 10}}^{\text{−12}}\hspace{0.17em}{\mathrm{cm}}^3/\mathrm{molecule}·\sec$ in agreement with a reported value of ${\text{3× 10}}^{\text{−12}}\hspace{0.17em}{\mathrm{cm}}^3/\mathrm{molecule}·\sec$ . The absorption cross section of HO₂ at 1420 cm⁻¹, the absorption maximum of the 1395 cm⁻¹ band, is approximately ${\text{5× 10}}^{\text{−20}}\hspace{0.17em}{\mathrm{cm}}^2/\mathrm{molecule}$ . The absorption cross section at 2100 Å is ${\text{4.5× 10}}^{\text{−18}}\hspace{0.17em}{\mathrm{cm}}^2/\mathrm{molecule}$ .