Global Master Equation Analysis of Rate Data for the Reaction C2H4 + H ⇄ C2H5: ΔfH0⊖C2H5

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While forward and reverse rate constants are frequently used to determine enthalpies of reaction and formation, this process is more difficult for pressure-dependent association/dissociation reactions, especially since the forward and reverse reactions are usually studied at very different temperatures. The problems can be overcome by using a data-fitting procedure based on a master equation model. This approach has been applied to existing experimental pressure-dependent forward and reverse rate coefficients for the reaction C 2 H 4 + H ⇄ C 2 H 5 ( k 1, k –1 ) using the MESMER code to determine Δ f H 0 ⊖ C 2 H 5 from the enthalpy of the reaction. New measurements of k 1, k –1 were included in analysis. They are based on laser flash photolysis with direct observation of H atom time profiles by vacuum ultraviolet laser-induced fluorescence under conditions where the approach to equilibrium could be observed. Measurements were made over the temperature range 798–828 K and with [He] from 2.33 to 7.21 × 10 18 molecule cm –3 . These data were then combined with a wide range of existing experimental data with helium as the bath gas (112 measurements of k 1 and k –1, covering the temperature range 285–1094 K, and [He] = 7.1 × 10 15 –1.9 × 10 19 molecule cm –3 ) and fitted using the master equation solver MESMER. The required vibrational frequencies and rotational constants of the system were obtained from ab initio calculations, and the activation threshold for association (Δ E thresh ), enthalpy of reaction (Δ r H 0 ⊖ ), imaginary frequency (υ imag ), and helium energy-transfer parameters (⟨Δ E ⟩ d,298 ( T /298) n ) were optimized. The resulting parameters (errors are 2σ) are Δ E thresh = 11.43 ± 0.34 kJ mol –1, Δ r H 0 ⊖ = −145.34 ± 0.60 kJ mol –1, υ imag = 730 ± 130 cm –1, ⟨Δ E ⟩ d,298 = 54.2 ± 7.6 cm –1, and n = 1.17 ± 0.12. A value of Δ f H 298.15 ⊖ (C 2 H 5 ) = 120.49 ± 0.57 kJ mol –1 is obtained by combining Δ r H 0 ⊖ with standard enthalpies of formation for H and C 2 H 4 and making the appropriate temperature corrections. The dependence of these parameters on how the internal rotor and CH 2 inversion modes are treated has been explored. The experimental data for other bath gases have been analyzed, and data sets compatible with the potential energy surface parameters determined above have been identified. The parameters are virtually identical but with slightly smaller error limits. Parameterization of k 1, k –1 using the Troe formalization has been used to investigate competition between ethyl decomposition and reaction with oxygen under combustion conditions.