WEAK COLLISION EFFECTS IN THE REACTION C2H5-REVERSIBLE-ARROW-C2H4+H

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 [DELTAE]down for the exponential down energy-loss 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 [DELTAE]down determinations was apparent and fits the expression [DELTAE]down = 0.255T1.0(+/-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 collision effects in helium down to 285 K. Values for [DELTAE]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 simulation 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); k1(0) = 6.63 X 10(9)T-4.99 exp(-20,130 K/T) cm3 molecule-1 s-1. Prior published experiments on both the forward and reverse reactions (C2H5 + (M) double-line arrow pointing left and right 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 200-1 100 K, k1infinity = 1.11 x 10(10)T1.037 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.