The thermal unimolecular decomposition rate constants of ethoxy radicals

Abstract

We experimentally determined complete falloff curves of the rate constant for the unimolecular decomposition of ethoxy radicals. Two different techniques, laser flash photolysis and fast flow reactor were used both coupled to a detection of C₂H₅O radicals by laser induced fluorescence. Experiments were performed at total pressures between 0.001 and 60 bar of helium and in the temperature range of 391–471 K. Under these conditions the β-C–C scission (1a) CH₃CH₂O+M→CH₂O+CH₃+M is the dominating decomposition channel. From a complete analysis of the experimental falloff curves the low and the high pressure limiting rate constants of k_1a,0=[He] 3.3×10^-8 exp(-58.5 kJ mol^-1/RT) cm³ s^-1 and k_1a,∞=1.1×10¹³ exp(-70.3 kJ mol^-1/RT) s^-1 were extracted. We estimate an uncertainty for the absolute values of these rate constants of ±30%. Preexponential factor and activation energy are significantly lower than previous estimations. The rate constants are discussed in terms of statistical unimolecular rate theory. Excellent agreement between the experimental and the statistically calculated rate constants has been found. BAC-MP4, QCISD(T), or higher level of theory provide a reliable picture of the energy and the structure of the transition state of this radical bond dissociation reaction. On the same theoretical basis we predict the high pressure limiting rate constant for the β-C–H scission (1b) CH₃CH₂O+M→CH₃CHO+H+M of k_1b,∞=1.3×10¹³ exp(-84 kJ mol^-1/RT) s^-1. Atmospheric implications are discussed.