Mass Spectrometry of the HO2 Free Radical

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Abstract
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.