EXPERIMENTAL AND THEORETICAL-STUDIES OF THE C2H5+O-2 REACTION-KINETICS

The thermal reaction between C2H5 and O2, which yields both C2H5O2 and C2H4 + HO2, has been studied both experimentally and theoretically. The experiments were conducted in a heatable tubular reactor coupled to a photoionization mass spectrometer. C2H5 was produced by homogeneous 193- or 248-nm photolysis of suitable precursors. C2H5 decay and C2H4 growth profiles were recorded in time-resolved experiments. The measured reaction parameters include second-order rate constants for the loss of C2H5 (296-850 K) and the product branching ratio, [C2H4]/([C2H5O2] + [C2H4]) (296-723 K). Buffer gas (primarily He) pressures from 0.5 to 15 Torr were used. The theoretical study involved modeling the kinetics of the reaction with a coupled mechanism, one in which the reactive routes both begin by the reversible formation of a chemically activated C2H5O2* adduct. Collisional stabilization yields C2H5O2 while rearrangement via a cyclic transition state leads to the formation of C2H4 + HO2. Reactivation of C2H5O2 was provided for. All the rate constants of the mechanism were calculated by using RRKM theory. Adjustable parameters that describe unknown features of the potential energy surface and the interaction of the buffer gas with the metastable adduct were required. This combined theoretical-experimental description of the C2H5 + O2 reaction quantitatively accounts for the behavior of this reaction over the full range of conditions that have been used in the current and all prior laboratory studies of the kinetics in this reaction. In particular, the mechanism change that occurs near 600 K is accurately described. The C2H5 + O2 reaction kinetics has been cast in terms of a four-step mechanism which can easily be incorporated into combustion models. Analytical expressions for the rate constants of these four steps have been calculated by using the results of this study and are reported. © 1990 American Chemical Society.