Experiments and Calculations on Rate Coefficients for Pyrolysis of SO ₂ and the Reaction O + SO at High Temperatures

Rate coefficients for the pyrolysis of SO₂ in Ar in the temperature range 2188-4249 K were determined using a diaphragmless shock tube. The concentration of O atoms was monitored with resonance absorption. Rate coefficients determined in this work show Arrhenius behavior, with k _la(T) = (4.86 ± 1.31) × 10^-9 exp [-(50450 ± 730)/T] cm³ molecule^-1 s^-1; listed errors represent one standard deviation in fitting. These values are consistent with some previous measurements that show a preexponential factor and activation energy greater than other measurements. Theoretical calculations at the G2M(RCC2) level, using geometries optimized with the B3LYP/6-311+G(3df) method, yield energies of transition states and products relative to those of the reactants. Rate coefficients predicted with a microcannonical variational RRKM theory agree well with experimental observations; contributions from electronically excited states of SO₂ are significant. Rate coefficients for the recombination O + SO → SO₂ are predicted to decrease with temperature with k_10a(T) = (4.82 ± 0.05) × 10^-31(T/298)-^2.17±0.03 cm⁶ molecule^-2 s^-1 for the temperature range 298-3000 K. In some experiments, S atoms were monitored with resonance absorption. With detailed chemical modeling, we found that S atoms were mainly produced from the secondary reaction O + SO → S + O₂ rather than from direct pyrolysis of SO₂ or from further pyrolysis of the SO product. Rate coefficients for this secondary reaction, determined to be k_10b(T) = (3.0 ± 0.3) × 10^-11 exp [-(6980 ± 280)/T] cm ³ molecule^-1 s^-1, agree closely with the theoretically predicted value comprising three product-channels via one triplet and two singlet SOO intermediates.