Active Thermochemical Tables:  Accurate Enthalpy of Formation of Hydroperoxyl Radical, HO2

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 2006-06, Vol.110 (21), p.6592-6601
• Main Authors: Ruscic, Branko, Pinzon, Reinhardt E, Morton, Melita L, Srinivasan, Nanda K, Su, Meng-Chih, Sutherland, James W, Michael, Joe V
• Format: Article
• Language: English
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp056311j

Through the use of the Active Thermochemical Tables approach, the best currently available enthalpy of formation of HO2 has been obtained as Δf H 298°(HO2) = 2.94 ± 0.06 kcal mol-1 (3.64 ± 0.06 kcal mol-1 at 0 K). The related enthalpy of formation of the positive ion, HO2 +, within the stationary electron convention is Δf H 298°(HO2 +) = 264.71 ± 0.14 kcal mol-1 (265.41 ± 0.14 kcal mol-1 at 0 K), while that for the negative ion, HO2 - (within the same convention), is Δf H 298°(HO2 -) = −21.86 ± 0.11 kcal mol-1 (−21.22 ± 0.11 kcal 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 Δacid G 298°(H2O2) = 369.08 ± 0.11 kcal mol-1, with the corresponding enthalpy of deprotonation of H2O2 of Δacid H 298°(H2O2) = 376.27 ± 0.11 kcal mol-1 (375.02 ± 0.11 kcal mol-1 at 0 K). In addition, a further improved enthalpy of formation of OH is briefly outlined, Δf H 298°(OH) = 8.93 ± 0.03 kcal mol-1 (8.87 ± 0.03 kcal mol-1 at 0 K), together with new and more accurate enthalpies of formation of NO, Δf H 298°(NO) = 21.76 ± 0.02 kcal mol-1 (21.64 ± 0.02 kcal mol-1 at 0 K) and NO2, Δf H 298°(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, Δf H 298°(H2O2) = −32.45 ± 0.04 kcal mol-1 (−31.01 ± 0.04 kcal mol-1 at 0 K). The new thermochemistry of HO2, together with other arguments given in the present work, suggests that 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 experimentally explored in the companion paper of Srinivasan et al. (next paper in this issue).