Shock tube studies using a novel multipass absorption Cell: Rate constant results for OH + H ₂ and OH + C ₂H ₆

The reflected shock tube technique with a novel multipass absorption spectrometric detection method has been used to study two OH-radical reactions: OH + H ₂ → H ₂O + H (1), and OH + C ₂H ₆ → H ₂O + C ₂H ₅ (2). Reaction 1 was chosen in order to test the method against earlier rate constant determinations. The measurements on both reactions were performed in single-shot experiments with low initial concentrations of hydroxyl radicals, [OH] ₀ = (5 - 15) × 10 ¹² molecule cm ^-3, which allowed reliable isolation of the elementary reactions. The measured rate constants are: k ₁ = 5.44 × 10 ^-11 exp(-3220 K/T) (832-1359 K), and k ₂ = 1.10 × 10 ^-10 exp(-2537 K/T) (822-1367 K), both in cm ³ molecule ^-1 s ^-1. The present study extends the experimental T-range for reaction 2. This new work and earlier lower-T studies have been combined to give a new evaluation, k ₂ - 2.68 × 10 ^-18 (T/K) ^2.224 exp(-373 K/T) cm ³ molecule ^-1 s ^-1. An ab initio potential energy surface was additionally used to derive a theoretical expression for this reaction.