Experimental and theoretical investigations of rate coefficients of the reaction S(³P) + O₂ in the temperature range 298-878 K

Rate coefficients of the reaction S + O₂ with Ar under 50 Torr in the temperature range 298-878 K were determined with the laser photolysis technique. S atoms were generated by photolysis of OCS with a KrF excimer laser at 248 nm; their concentration was monitored via resonance fluorescence excited by atomic emission of S produced from microwave-discharged SO₂. Our measurements show that k(298 K) = (1.92±0.29)×10^-12 cm³ molecule^-1 s^-1, in satisfactory agreement with previous reports. New data determined for 505-878 K show non-Arrhenius behavior; combining our results with data reported at high temperatures, we derive an expression k(T) = (9.02±0.27) ×10^-19T ^2.11=0.15 exp[(730±120)/T] cm³ molecule ^-1 s^-1 for 298≤T≤3460 K. 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 multichannel RRKM calculations agree satisfactorily with experimental observations; the reaction channel via SOO(¹A′) dominates at T<500K, whereas channels involving formation of SOO(³A″) followed by isomerization to SO ₂ before dissociation, and formation of SOO(¹A″) followed by direct dissociation, become important at high temperatures, accounting for the observed rapid increase in rate coefficient.