A new look at reactions of 2-butoxy and 1-butoxy radicals in the presence of oxygen.

Under atmospheric conditions, experiments show that 2-butoxy radicals in the presence of oxygen yield acetaldehyde and butanone such that the concentration ratio [acetaldehyde] [O2]/[butanone] shows a linear dependence on oxygen concentration [O2]. [Zabel et al., Phys. Chem. Chem. Phys., 2002, 4, 2579; Cox et al.Phys. Chem. Chem. Phys, 2005, 7, 3702.] A similar oxygen dependence was found by Cox et al. in the reactions of 1-butoxy radicals. These experiments imply a unimolecular rate constant that apparently depends on oxygen pressure. Previously this has been explained by postulating the presence of a small amount of excited radicals assumed to undergo 'prompt' dissociation. We propose an alternative interpretation based on the solution of the time-dependent master equation that does not require the putative presence of excited radicals. We do this by allowing the time to run until the so-called long-time steady-state, and then show that the oxygen dependence arises quite naturally if the transport matrix takes into account that in addition to the principal reaction channel (dissociation into acetaldehyde in the case of 2-butoxy and isomerization in the case of 1-butoxy), there is a small "leak" of alkoxy radicals due to the reaction with oxygen, and provided the calculation takes into account that in the cited experiments there is a continuous incoming flux of butoxy radicals.