Weak collision effects in the reaction ethyl radical .dblarw. ethene + hydrogen

Abstract

The unimolecular decomposition of C[sub 2]H[sub 5] in helium has been investigated near the low-pressure limit (T = 876-1094 K; P =0.8-14.3 Torr). Rate constants (k[sub 1]) 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 [l angle][Delta]E[r angle][sub down] for the exponential down energyloss probability were obtained for each experiment performed. The microcanonical rate constants, k[sub 1](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 [l angle][Delta]E[r angle][sub down] determinations was apparent and fits the expression [l angle][Delta]E[r angle][sub down] = 0.255T[sup 1.0([+-]0.1)] cm[sup [minus]1]. It is shown that this expression(derived from experiments conducted between 876 and 1094 K) provides a reasonable representation of observed weak collision effects in helium down to 285K. Values for [l angle][Delta]E[r angle][sub down]for C[sub 2]H[sub 5] decomposition in other bath gases were obtained by reexamining published data on the fall-off of the C[sub 2]H[sub 5] decomposition inmore » helium (200-1100 K); k[sub 1][sup 0]=6.63 [times] 10[sup 9]T[sup [minus]4.99] exp([minus]20,130 K/T) cm[sup 3]molecule[sup [minus]1]s[sup [minus]1]. Prior published experiments on both the forward and reverse reactions (C[sub 2]H[sub 5] +(M) [leftrightarrow] C[sub 2]H[sub 4] + H + (M)) in the fall-off region were reevaluated expression for the high-pressure limit rate constant for the temperature range 200-1100 K,k[sub 1][sup [infinity]] = 1.11 [times] 10[sup 10]T[sup 1.037] exp(-18,504/T) s[sup [minus]1]s. 49 refs., 12 figs., 5 tabs.« less