Trends in organic peroxide (ROOR) formation in the reactions of C1–C4 alkyl peroxy radicals (RO₂) in gas

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>i.e.</i>, 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 past decade, observations have suggested that this channel could be an important source of condensable compounds and, ultimately, new aerosol particles in Earth's atmosphere. However, experimental evidence for specific RO₂ reactions is scarce. In this work, the formation of ROOR in the self- and cross-reactions of eight RO₂ (CH₃O₂, ¹³CH₃O₂, CD₃O₂, C₂H₅O₂, 1- and iso-C₃H₇O₂, 1- and <i>tert</i>-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, <i>γ</i>, for the ROOR channel of the self-reaction was quantified relative to these other channels. The results allowed for the first time to identify some trends in <i>γ</i> with respect to the RO₂ structure: <i>γ</i> decreases with increasing RO₂ chain length for the linear/primary radicals, ranging from (14.1 ± 7)% for CH₃O₂ to (1.1 ± 0.5)% for 1-C₄H₉O₂, while branched radicals exhibit much higher <i>γ</i> than their linear counterparts, with <i>γ</i> = (17.2 ± 8.6)% for iso-C₃H₇O₂ and (46.6 ± 23.2)% for <i>tert</i>-C₄H₉O₂. The formation of ROOR products from RO₂ reactions in the atmosphere should thus be strongly dependent on the RO₂ structure.