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

TLDR

The radical was formed by photolysis of hydrogen peroxide at 2537 Å, photolysis of ozone with hydrogen peroxide at 2537 Å, and photolysis of Cl₂ with hydrogen peroxide at 3500 Å. The study measured HO₂’s absorption spectrum, identifying vibrational frequencies at 1095, 1390, and 3410 cm⁻¹, a UV maximum at 2100 Å, and cross sections of 5×10⁻²⁰ cm²/molecule at 1420 cm⁻¹ and 4.5×10⁻¹⁸ cm²/molecule at 2100 Å, while kinetic analysis revealed second‑order decay with a disproportionation rate constant of 3.6±0.5×10⁻¹² cm³/molecule·s.

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: H–O bond distance 0.96 Å, O–O bond distance 1.3 Å, and 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 HO2+HO2→ H2O2+O2 was found to be 3.6± 0.5× 10−12 cm3/molecule· sec in agreement with a reported value of 3× 10−12 cm3/molecule· sec. The absorption cross section of HO2 at 1420 cm−1, the absorption maximum of the 1395 cm−1 band, is approximately 5× 10−20 cm2/molecule. The absorption cross section at 2100 Å is 4.5× 10−18 cm2/molecule.