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The reactions of SO 3 with H, O, and OH radicals have been investigated by ab initio calculations. For the SO 3 + H reaction (1), the lowest energy pathway involves initial formation of HSO 3 and rearrangement to HOSO 2 , followed by dissociation to OH + SO 2 . The reaction is fast, with k 1 ) 8.4 × 10 9 T 1.22 exp(-13.9 kJ mol -1 /RT) cm 3 mol -1 s -1 (700-2000 K). The SO 3 + O f SO 2 + O 2 reaction (2) may proceed on both the triplet and singlet surfaces, but due to a high barrier the reaction is predicted to be slow. The rate constant can be described as k 2 ) 2.8 × 10 4 T 2.57 exp(-122.3 kJ mol -1 /RT) cm 3 mol -1 s -1 for T > 1000 K. The SO 3 + OH reaction to form SO 2 + HO 2 (3) proceeds by direct abstraction but is comparatively slow, with k 3 ) 4.8 × 10 4 T 2.46 exp(-114.1 kJ mol -1 /RT) cm 3 mol -1 s -1 (800-2000 K). The revised rate constants and detailed reaction mechanism are consistent with experimental data from batch reactors, flow reactors, and laminar flames on oxidation of SO 2 to SO 3 . The SO 3 + O reaction is found to be insignificant during most conditions of interest; even in lean flames, SO 3 + H is the major consumption reaction for SO 3 .

The thermal dissociation of SO 3 has been studied for the first time in the 1000-1400 K range. The experiments were conducted in a laminar flow reactor at atmospheric pressure, with nitrogen as the bath gas. On the basis of the flow reactor data, a rate constant for SO 3 + N 2 f SO 2 + O + N 2 (R1b) of 5.7 × 10 17 exp(-40000/T) cm 3 /(mol s) is derived for the temperature range 1273-1348 K. The estimated uncertainty is a factor of 2. The rate constant corresponds to a value of the reverse reaction of k 1 ≈ 1.8 × 10 15 cm 6 mol -2 s -1 . The reaction is in the falloff region under the investigated conditions. The temperature and pressure dependence of SO 2 + O (+N 2 ) was estimated from the extrapolation of low temperature results for the reaction, together with an estimated broadening parameter and the high-pressure limit determined recently by Naidoo, Goumri, and Marshall (Proc. Combust. Inst. 2005, 30, 1219-1225. The theoretical rate constant is in good agreement with the experimental results. The improved accuracy in k 1 allows a reassessment of the rate constant for SO 3 + O f SO 2 + O 2 (R2) based on the data of Smith, Tseregounis, and Wang (Int. J. Chem. Kinet. 1982, 14, 679-697), who conducted experiments on a low-pressure CO/O 2 /Ar flame doped with SO 2 . At the location in the flame where the net SO 3 formation rate is zero,A value of 6.9 × 10 10 cm 3 mol -1 s -1 is obtained for k 2 at 1269 K with an uncertainty a factor of 3. A recommended rate constant k 2 ) 7.8 × 10 11 exp(-3065/T) cm 3 mol -1 s -1 is consistent with other flame results as well as the present flow reactor data.

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