Formation of highly oxidized multifunctional compounds: autoxidation of peroxy radicals formed in the ozonolysis of alkenes – deduced from structure–product relationships

摘要

It has been postulated that secondary organic particulate matter plays apivotal role in the early growth of newly formed particles in forest areas.The recently detected class of extremely low volatile organic compounds(ELVOC) provides the missing organic vapors and possibly contributes asignificant fraction to atmospheric SOA (secondary organic aerosol). The sequential rearrangement ofperoxy radicals and subsequent O addition results in ELVOC which arehighly oxidized multifunctional molecules (HOM). Key for efficiency of suchHOM in early particle growth is that their formation is induced by oneattack of the oxidant (here O), followed by an autoxidation processinvolving molecular oxygen. Similar mechanisms were recently observed andpredicted by quantum mechanical calculations e.g., for isoprene. To assessthe atmospheric importance and therewith the potential generality, it iscrucial to understand the formation pathway of HOM.To elucidate the formation path of HOM as well as necessary and sufficientstructural prerequisites of their formation we studied homologous series ofcycloalkenes in comparison to two monoterpenes. We were able to directlyobserve highly oxidized multifunctional peroxy radicals with 8 or 10 O atomsby an Atmospheric Pressure interface High Resolution Time of Flight MassSpectrometer (APi-TOF-MS) equipped with a NO-chemical ionization (CI) source.In the case of O acting as an oxidant, the starting peroxy radical is formedon the so-called vinylhydroperoxide path. HOM peroxy radicals and theirtermination reactions with other peroxy radicals, including dimerization,allowed for analyzing the observed mass spectra and narrowing down the likelyformation path. As consequence, we propose that HOM are multifunctionalpercarboxylic acids, with carbonyl, hydroperoxy, or hydroxy groups arisingfrom the termination steps. We figured that aldehyde groups facilitate theinitial rearrangement steps. In simple molecules like cycloalkenes,autoxidation was limited to both terminal C atoms and two further C atoms inthe respective α positions. In more complex molecules containingtertiary H atoms or small, constrained rings, even higher oxidation degreeswere possible, either by simple H shift of the tertiary H atom or byinitialization of complex ring-opening reactions.