Reflected shock tube studies of high-temperature rate constants for OH + C2H2 and OH + C2H4

Abstract

The reflected shock tube technique with multi-pass absorption spectrometric detection of OH-radicals at 308 nm (corresponding to a total path length of ∼4.9 m) has been used to study the reactions, OH + C₂H₂ → products (1) and OH + C₂H₄ → C₂H₃ + H₂O (2). The present optical configuration gives a S/N ratio of ∼1 at ∼0.5−1.0 × 10¹² radicals cm⁻³. Hence, kinetics experiments could be performed at [OH]₀ = ∼4–20 ppm thereby minimizing secondary reactions. OH was produced rapidly from the dissociations of either CH₃OH or NH₂OH (hydroxylamine). A mechanism was then used to obtain profile fits that agreed with the experiment to within <±5%. The derived Arrhenius expressions, in units of cm³ molecule⁻¹ s⁻¹ are: k₁ = (1.03 ± 0.24) × 10⁻¹⁰ exp(−7212 ± 417 K/T) for 1509–2362 K and k₂ = (10.2 ± 5.8) × 10⁻¹⁰ exp(−7411 ± 871 K/T) for 1463–1931 K. The present study is the first ever direct measurement for reaction (1) at temperatures >1275 K while the present results extend the temperature range for (2) by ∼700 K. These values are compared with earlier determinations and with recent theoretical calculations. The calculations agree with the present data for both reactions to within ±10% over the entire T-range.