Unimolecular Decomposition of β-Hydroxyethylperoxy Radicals in the HO•-Initiated Oxidation of Ethene:  A Theoretical Study

In an attempt to clarify the fate of ethene in the cleaner regions of the troposphere, where the NO x concentrations are low, the energetical viability of four possible pathways for the unimolecular decomposition of β-hydroxyethylperoxy radical ( 1 ) have been investigated from the theoretical point of view. The direct hydrogen atom transfer from β-hydroxyethyl radical to triplet O 2 giving either vinyl alcohol or oxirane has also been investigated. Each pathway has been characterized by means of density functional theory (B3LYP) and quantum-mechanical (CCSD(T)) calculations with basis sets ranging in quality from the 6-31G(d,p) to the 6-311+G(3df,2p). The 1,5-migration of the hydroxy H-atom in 1 yielding β-hydroperoxyethoxy radical followed by C−C bond cleavage leading to a hydrogen-bonded [ • CH 2 OOH···CH 2 O] complex (CX3) is found to be the energetically preferred pathway. At the RCCSD(T) level of theory, the 0 K energy of the rate-determining transition structure of this mechanism (TS5) is found to lie 3.6 kcal mol -1 below the sum of the 0 K energies of the β-hydroxyethyl radical and triplet O 2 . Furthermore, it is shown that CX3 can evolve into a three-component hydrogen-bonded [CH 2 O··· • OH···CH 2 O] complex (CX4) with a 0 K activation energy of only 0.4 kcal mol -1 . Assuming that a significant fraction of the β-hydroxyethylperoxy radicals produced in the addition of triplet O 2 to β-hydroxyethyl radical decomposes before collisional stabilization, one might expect the HO • − initiated oxidation of one molecule of ethene to yield two molecules of formaldehyde.