Experiments and calculations on rate coefficients for pyrolysis of SO2 and the reaction O plus 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 0 atoms was monitored with resonance absorption. Rate coefficients determined in this work show Arrhenius behavior, with k(1a)(T) = (4.86 +/- 1.31) x 10(-9) exp [-(50450+/-730)/T] cm(3) 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) x 10(-31) (T/298)(-2.17+/-0.03) cm(6) 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 + O-2 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)(7) = (3.0 +/- 0.3) x 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.