The thermal unimolecular decomposition rate constants of ethoxy radicals

Abstract

We experimentally determined complete fallo † curves of the rate constant for the unimolecular decomposition of ethoxy radicals. Two di †erent techniques, laser Ñash photolysis and fast Ñow reactor were used both coupled to a detection of radicals by laser induced Ñuorescence. Experiments were performed at total C 2 H 5 Op ressures between 0.001 and 60 bar of helium and in the temperature range of 391È471 K. Under these conditions the b-CÈC scission (1a)is the dominating decompositionFrom a complete analysis of the experimental fallo † curves the low and the high pressure limiting rate constants of 3.3 ] 10~8 exp([58.5 kJ mol~1/RT ) cm3 s~1 and exp([70.3 kJ k 1a,0 \ [He] k 1a,= \ 1.1 ] 1013 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 signiÐcantly 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 b-CÈH scission (1b) of exp([84 kJ CH 3 CH 2 O~] M ] CH 3 CHO ] H~] M k 1b,= \ 1.3 ] 1013 mol~1/RT ) s~1. Atmospheric implications are discussed.

Ab Initio Study on the Thermal Decomposition of CH3CF2O Radical

“…However, based on an analogy with the decomposition of other oxy-radicals for C-C bond scission, our calculated value comes out to be of the same order of magnitude. 27 Our modeling calculation also results the A-factor for C-C bond scission to be 1.3 × 10 13 s -1 which is also in good agreement with the value of 1.1 × 10 13 s -1 for CH3CH2O obtained by Caralp et al 28 Similarly, the rate constant for F-elimination via reaction 2 involving TS2 as the transition state is calculated to be 4. .…”

Rate coefficients and mechanisms of the reaction of cl‐atoms with a series of unsaturated hydrocarbons under atmospheric conditions

“…The available thermodynamic and kinetic data ( [22] and this work) for the unimolecular decomposition of some of the simpler ␤-chlorinated alkoxy radicals are collected in Table II. The decompositions of the ␤-chlorinated species are characterized by somewhat lower H values (3-5 kcal mol −1 ) than for the corresponding unsubstituted radical decompositions [54,56,57], which leads to only a modest reduction in the corresponding activation energies (0-2 kcal mol −1 ). It has been shown [48,58] that the activation energy (E a ) for a series of alkoxy radical decompositions involving a common leaving group is related to the where IP refers to the ionization potential of the (common) leaving group in eV, and E a and H d are in kcal mol −1 .…”

Photodissociation Dynamics of the Ethoxy Radical (C2H5O)

“…Schlegel found the most facile path for the OH + C 2 H 4 reaction be the addition of OH to C 2 H 4 to form CH 2 CH 2 -OH, followed by isomerization to ethoxy radical over a 1.2-eV barrier and dissociation to CH 3 + CH 2 O. On the other hand, isomerization does not appear to be important in the thermal 19 or chemically activated 18 decomposition of ethoxy itself, as the two observed channels, CH 3 + CH 2 O and H + CH 2 CHO, result from simple bond fission.…”

“…Absolute rate constants for thermal decomposition of ethoxy have been measured recently. 19 However, the excited state photochemistry of the ethoxy radical is largely unknown. It is expected to be considerably more complex than in the methoxy radical, for which electronic excitation leads primarily to CH 3 + O products, 20 because ethoxy has many more low-lying dissociation channels: the four reactant and product channels listed above plus C 2 H 3 + H 2 O and CH 3 CO + H 2 .…”