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 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 〈ΔE〉down determinations was apparent and fits the expression 〈ΔE〉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 〈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 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); k10 = 6.63 × 109T4.99 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 × 1010T1.037 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.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry