Weak Collision Effects in the Reaction C2H5 C2H4 + H

Article abstract of DOI:10.1021/j100106a012

The unimolecular decomposition of C2H5 in helium has been investigated near the low-pressure limit (T = 876 - 1094 K; P = 0.8 - 14.3 Torr).Rate constants (k1) have been determined as a function of temperature and pressure in the indicated ranges in time-resolved experiments.The reaction was isolated for quantitative study in a heated tubular reactor coupled to a photoionization mass spectrometer.Weak collision effects (fall-off behavior) were analyzed using a master equation analysis.Values of (ΔE)_down for the exponential down energyloss probability were obtained for each experiment performed.The microcanonical rate constants, k1(E), needed to solve the master equation were obtained from a transition state model for the reaction which is described.The temperature dependence of these (ΔE)_down determination was apparent and fits the expression (ΔE)_down = 0.255T^1.0(+/-0.1) cm^-1.It is shown that this expression (derived from experiments conducted between 876 and 1094 K) provides a reasonable representation of observed weak collison effects in helium down to 285 K.Values for (ΔE)_down for C2H5 decomposition in other bath gases were obtained by reexamining published data on the fall-off of the C2H5 unimolecular rate constant in N2, SF6, and C2H6.The experimental results and data simulations were used to obtain a parametrized expression for k1(T,M), the low-pressure limit rate constant for C2H5 decomposition in helium (200 - 1100 K); k1⁰ = 6.63 x 10⁹T^-4.99 exp(-20,130 K/T) cm³ molecule^-1 s^-1.Prior published experiments on both the forward and reverse reactions (C2H5 + (M) <*> C2H4 + H + (M)) in the fall-off region were reevaluated and used in conjunction with an RRKM model of the transition state to obtain a new recommended expression for the high-pressure limit rate constant for the temperature range 200 - 1100 K, k1^<*> = 1.11 x 10¹⁰T^1.037 exp(-18,504/T)s^-1.Parametrization of the density and temperature dependence of k1 in helium according to the modified Hinshelwood expression introduced by Gilbert et al. is provided.

Full text of DOI:10.1021/j100106a012

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