The Effect of Alcohol and Carbonyl Functional Groups on the Competition between Unimolecular Decomposition and Isomerization in C[subscript 4] and C[subscript 5] Alkoxy Radicals

Abstract

Presented here are computed rates for the thermal unimolecular decomposition of a variety of alkoxy radicals with four- and five-carbon length backbones. Three classes of molecules are examined: alkoxy radicals with saturated hydrocarbon backbones, those with alcohol functional groups, and those with carbonyl functional groups. The chosen species represent many of those found during the combustion of fossil fuels as well as bio-derived alternatives. Density functional theory calculations were benchmarked against higher level coupled cluster calculations and used to explore the potential energy surfaces of these systems. Transition state theory was used to calculate high-pressure limit rate coefficients of all radical intermediates in the regimes relevant to atmospheric chemistry and combustion. We show that the assumption that alkoxy radicals quickly decompose via β-scission to aldehydes and other radicals is not valid for some of the alkoxy radicals investigated in this work. We further illustrate how intra-H migrations in larger alkoxy radicals with carbonyl and alcohol functional groups can dominate unimolecular decomposition under combustion and atmospheric relevant conditions. Finally, we discuss why carbonyl groups can increase or decrease intra-H migration barriers depending on their location relative to the transferring H-atom.