Trends in Organic Peroxide (ROOR) Formation in the Reactions of C1-C4 Alkyl Peroxy Radicals (RO2) in the gas

Abstract

Organic peroxy radicals (RO2) are important intermediates in aerobic systems such as Earth’s atmosphere. The existence of a channel producing dialkyl peroxides (ROOR) in their self- and cross-reactions (i.e. between the same or different radicals) has long been debated and considered a theoretical “key problem in the atmospheric chemistry of peroxy radicals”. Over the last decade, observations have suggested that this channel could be an important source of condensable compounds and, ultimately, new aerosol particles in Earth’s atmosphere. But experimental evidence for specific RO2 reactions remain scarce. In this work, the formation of ROOR in the self- and cross reactions of eight different RO2, CH3O2, 13CH3O2, CD3O2, C2H5O2, 1- and iso-C3H7O2, 1- and tert-C4H9O2, could be observed by modifying the ionisation conditions on a proton transfer mass spectrometer. The ROOR formation channel was confirmed to be in competition with the other product channels rather than precede them. For six of the RO2 studied the branching ratio, γ, for the ROOR channel of the self-reaction was thus quantified relatively to these other channels. The results allow for the first time to determine some trends for γ with the RO2 structure: a decrease with the RO2 chain length for the linear/primary radicals, from (14.1 ± 7) % for CH3O2 to (1.1 ± 0.5) % for 1-C4H9O2, while the branching ratios for the branched radicals are much larger then for their linear counterparts, with γ = (17.2 ± 8.6) % for iso-C3H7O2 and (46.6 ± 23.2) % for tert-C4H9O2. The formation of ROOR products from RO2 reactions in the atmosphere should thus be highly dependent on the RO2 structure.