Reaction of OH Radicals with C ₂ H ₄ : Kinetics, Products, Temperature, and Pressure Dependence

The kinetics and products of the reaction of OH radicals with ethylene were studied using a low-pressure discharge-flow reactor combined with modulated molecular beam mass spectrometry. The total rate constant of the reaction ( k 1 ) was determined as a function of pressure (0.4–20 Torr of helium) and temperature (240–1000 K). The title reaction was found to proceed through two channels: adduct forming and H atom abstraction. For the addition channel, the high- and low-pressure limit rate coefficient were extracted from a global fit of the fall off curves observed at different temperatures (present low pressure and available in the literature high-pressure data for the reaction rate constant) with the two-parameter expression k = k 0 k ∞ [ M ] k 0 [ M ] + k ∞ × 0.6 ( 1 + ( log ( k 0 [ M ] k ∞ ) ) 2 ) − 1: k 0 = 4.6 × 10 –29 (T/298) −3.9 cm 6 molecule –2 s –1, k ∞ = 8.0 × 10 –12 (T/298) −1.0 cm 3 molecule –1 s –1 in the temperature range 240–470 K. Moreover, this parametrization was found to reasonably reproduce existing measurements of k 1 with N 2 bath gas down to 69 K. The hydrogen atom abstraction channel was found to be the only important reaction pathway at T > 700 K. The rate constant of this reaction channel was determined in the temperature range 375–1000 K, as being equal to the overall rate constant at T > 700 K, and through the measurements of the yield of the reaction product, C 2 H 3 radical, at T = 375–690 K: k 1b = (1.43 ± 0.10) × 10 –14 ( T /298) 3.96 ± 0.09 cm 3 molecule –1 s –1 . This expression was found to describe well earlier shock tube measurements at high temperatures (up to 1930 K) and can be recommended for use in the temperature range 375–1930 K. The rate constants measured in this study for both addition and abstraction channels are in good agreement with available theoretical calculations.