| Abstract
| - The complete basis set method CBS-QB3 has been used to study the thermochemistry and kinetics of theesters ethyl propanoate (EP) and methyl butanoate (MB) to evaluate initiation reactions and intermediateproducts from unimolecular decomposition reactions. Using isodesmic and isogeitonic equations and atomizationenergies, we have estimated chemically accurate enthalpies of formation and bond dissociation energies forthe esters and species derived from them. In addition it is shown that controversial literature values may beresolved by adopting, for the acetate radical, CH3C(O)Ȯ,(298.15K) = −197.8 kJ mol-1 and for thetrans-hydrocarboxyl radical, Ċ(O)OH, −181.6 ± 2.9 kJ mol-1. For EP, the lowest energy decompositionpath encounters an energy barrier of ∼210 kJ mol-1 (∼50 kcal mol-1), which proceeds through a six-memberedring transition state (retro-ene reaction) via transfer of the primary methyl H atom from the ethyl group to thecarbonyl oxygen, while cleaving the carbon−ether oxygen to form ethene and propanoic acid. On the otherhand, the lowest energy path for MB has a barrier of ∼285 kJ mol-1, producing ethene. Other routes leadingto the formation of aldehydes, alcohols, ketene, and propene are also discussed. Most of these intramolecularhydrogen transfers have energy barriers lower than that needed for homolytic bond fission (the lowest ofwhich is 353 kJ mol-1 for the Cα−Cβ bond in MB). Propene formation is a much higher energy demandingprocess, 402 kJ mol-1, and it should be competitive with some C−C, C−O, and C−H bond cleavage processes.
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