Mass Spectrometry of the HO₂ Free Radical

Mass spectrometric studies on the production, identification, and determination of thermochemical energies of HO₂ radicals are reported. Reactions found to produce HO₂ radicals, and examined in some detail, were: (1) reaction of H with O₂, (2) reaction of H with H₂O₂, (3) reaction of O with H₂O₂, (4) reaction of OH with H₂O₂, (5) photolysis of H₂O₂, and (6) low‐power electrical discharge in H₂O₂. Of the reactions studied, the low‐power electrical discharge in H₂O₂ provided the most intense and convenient source of HO₂ radicals. Ion—molecule reactions, which are negligible in normal operation of our mass spectrometer, are shown to be a potentially serious source of interference in studies of HO₂ with conventional mass spectrometers.

The ionization potential of HO₂, I(HO)₂, and the appearance potential of HO₂⁺ from H₂O₂, A(HO₂⁺), have been redetermined, and the bond dissociation energies D(H–OOH) and D(H–O₂) have been recalculated. The measured values are: I(HO₂) = 11.53±0.02 ev, A(HO₂⁺) = 15.36±0.05 ev with an estimated absolute accuracy of ±0.1 ev. The derived thermochemical energies are: D₀(H–OOH) = 88.4±2 kcal/mole, D₀(H–O₂) = 45.9±2 kcal/mole, ΔH₀⁰(HO₂) = 5.7±2 kcal/mole for the values at 0°K; and D(H–OOH) = 89.6±2 kcal/mole, D(H–O₂) = 47.1±2 kcal/mole, ΔH₂₉₈⁰ (H–O₂) = 5.0±2 kcal/mole for the corresponding values at 25°C. Possible sources of error are examined and their effect on these values is discussed.