Temperature dependence of rate coefficients of reactions of NO₂ with CH₃S and C₂H₅S

Rate coefficients of reactions (1) CH₃S + NO₂ and (2) C₂H₅S + NO₂ were determined using laser photolysis and laser-induced fluorescence. CH₃S and C₂H₅S radicals were generated on photolysis of CH₃SSCH₃ and C₂H₅SSC₂H₅, respectively, with a KrF excimer laser at 248 nm. Their concentrations were monitored via fluorescence excited by emission from a dye laser at 371.4 nm (for CH₃S) or 410.3 nm (for C₂H₅S) pumped with a pulsed XeCl excimer laser at 308 nm. Our measurements show that k₁ (297 K) = (1.01 ± 0.15) × 10^-10 cm³ molecule^-1 s^-1 is similar to a value reported by Balla et al. and approximately 50% greater than other previously reported values; listed errors represent 95% confidence limits. Reaction 1 has a negative activation energy: k₁ = (4.3 ± 1.3) × 10^-11 exp[(240 ± 100)/T] cm³ molecule^-1 s^-1 for T = 222-420 K. Reaction 2 has a rate coefficient similar to reaction 1 at room temperature: k₂ (296 K) = (1.05 ± 0.16) × 10^-10 cm³ molecule^-1 s^-1 but has a small positive activation energy with k₂ = (2.4 ± 0.7) × 10^-10 exp[-(210 ± 80)/T] cm³ molecule^-1 s^-1 for T = 223-402 K. The temperature dependence of k₂ is determined for the first time. Reactions of NO₂ with HS, CH₃S, and C₂H₅S are compared.