A photoionization study of the hydroperoxyl radical, HO2, and hydrogen peroxide, H2O2

Hydrogen peroxide and the hydroperoxyl radical have been studied by photoio nization mass spectrometry. The radical has been produced in situ from H2O2 by hydrogen abstraction with fluorine atoms produced in a microwave discha rge. The adiabatic ionization energy of the hydroperoxyl radical has been m easured as IE(HO2) = 11.352 +/- 0.007 eV, while that of hydrogen peroxide h as been determined to be IE(H2O2) = 10.631 +/- 0.007 eV. The OH+ and HO2+ f ragments from H2O2 were also measured and their threshold behavior was anal yzed in terms of model functions. The 0 K appearance energy of the HO2+ fra gment was determined to be AE(0)(HO2+/H2O2) = 15.112 +/- 0.035 eV. From the se values, the O-H bond dissociation energy in hydrogen peroxide of D-0(H-O OH) = 86.7 +/- 0.8 kcal mol(-1) (87.9 +/- 0.8 kcal mol(-1) at 298 K) was de termined, yielding the enthalpy of formation of the hydroperoxyl radical of Delta H-f0(-)(HO2) = 4.0 +/- 0.8 kcal mol(-1) (3.3 +/- 0.8 kcal mol(-1) at 298 K), and also implying the proton affinity of oxygen of PA(O-2) = 100.6 +/- 0.8 kcal mol(-1) and the enthalpy of deprotonation of hydrogen peroxid e of Delta H-acid (-)(298)(H2O2) = 376.4 +/- 0.8 kcal mol(-1). (C) 1998 Els evier Science B.V. All rights reserved.