| Abstract
| - Through the use of the Active Thermochemical Tables approach, the best currently available enthalpy offormation of HO2 has been obtained as ΔfH298°(HO2) = 2.94 ± 0.06 kcal mol-1 (3.64 ± 0.06 kcal mol-1 at0 K). The related enthalpy of formation of the positive ion, HO2+, within the stationary electron conventionis ΔfH298°(HO2+) = 264.71 ± 0.14 kcal mol-1 (265.41 ± 0.14 kcal mol-1 at 0 K), while that for the negativeion, HO2- (within the same convention), is ΔfH298°(HO2-) = −21.86 ± 0.11 kcal mol-1 (−21.22 ± 0.11kcal mol-1 at 0 K). The related proton affinity of molecular oxygen is PA298(O2) = 100.98 ± 0.14 kcal mol-1(99.81 ± 0.14 kcal mol-1 at 0 K), while the gas-phase acidity of H2O2 is ΔacidG298°(H2O2) = 369.08 ± 0.11kcal mol-1, with the corresponding enthalpy of deprotonation of H2O2 of ΔacidH298°(H2O2) = 376.27 ± 0.11kcal mol-1 (375.02 ± 0.11 kcal mol-1 at 0 K). In addition, a further improved enthalpy of formation of OHis briefly outlined, ΔfH298°(OH) = 8.93 ± 0.03 kcal mol-1 (8.87 ± 0.03 kcal mol-1 at 0 K), together withnew and more accurate enthalpies of formation of NO, ΔfH298°(NO) = 21.76 ± 0.02 kcal mol-1 (21.64 ±0.02 kcal mol-1 at 0 K) and NO2, ΔfH298°(NO2) = 8.12 ± 0.02 kcal mol-1 (8.79 ± 0.02 kcal mol-1 at 0 K),as well as H2O2 in the gas phase, ΔfH298°(H2O2) = −32.45 ± 0.04 kcal mol-1 (−31.01 ± 0.04 kcal mol-1 at0 K). The new thermochemistry of HO2, together with other arguments given in the present work, suggeststhat the previous equilibrium constant for NO + HO2 → OH + NO2 was underestimated by a factor of ∼2,implicating that the OH + NO2 rate was overestimated by the same factor. This point is experimentallyexplored in the companion paper of Srinivasan et al. (next paper in this issue).
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