Extensive studies of secondary organic aerosol (SOA) formation have identified isoprene epoxydiol (IEPOX) intermediates as key species in the formation of isoprene-derived SOA. Recent work has suggested that isoprene-derived dimers and oligomers may constitute a significant fraction of SOA, but a mechanism for the formation of such abundant SOA components has yet to be established. The potential for dimer formation from the nucleophilic addition of 2-methyltetrol to trans-β-IEPOX was assessed through a series of model epoxide−nucleophile experiments using nuclear magnetic resonance (NMR) spectroscopy. These experiments helped establish a rigorous understanding of structural, stereochemical, and NMR chemical shift trends, which were used along with nucleophilic strength calculations to interpret the results of the trans-β-IEPOX + 2-methyltetrol reaction and evaluate its relevance in the atmosphere. A preference for less sterically hindered nucleophiles was observed in all model systems. In all addition products, a significant increase in NMR chemical shift was observed directly adjacent to the epoxide−nucleophile linkage, with smaller decreases in chemical shift at all other sites. A partial NMR assignment of a single trans-β-IEPOX + 2-methyltetrol nucleophilic addition product was obtained, but nucleophilic strength calculations suggest that 2-methyltetrol is a poor nucleophile. Therefore, this reaction is unlikely to significantly contribute to dimer and oligomer formation on SOA. Nevertheless, the structural and stereochemical considerations, NMR assignments, and NMR chemical shift trends reported here will prove useful in future attempts to synthesize dimer and oligomer analytical standards.
Laboratory and field measurements have demonstrated that 2-methyl glyceric acid (2-MG) is the base component of a wide range of chemical species found in methacrolein-derived secondary organic aerosol (SOA). In order to explore the recently proposed hypothesis that a lactone oxidation intermediate is the origin of 2-MG and its derivatives in SOA, nuclear magnetic resonance techniques were used to study kinetics and reaction products of the aqueous phase reactions of a model lactone, β-propiolactone (BPL). BPL was found to react with a lifetime of 4-10 h (depending on solution conditions) via a general acid catalyzed mechanism, which suggests that lactones similar to BPL are reactive on an atmospherically relevant time scale. BPL was also shown to form a variety of nucleophilic addition products (organosulfates and nitrates and oligomers) similar to the 2-MG-based species observed in previous experiments involving the photooxidation and SOA processing of methacrolein. While many of the BPL reaction products could be rationalized via an epoxide-like nucleophilic addition mechanism, evidence for ester-like nucleophilic addition was suggested through the observation of inorganic ion-catalyzed oligomer formation. The formation of oligomers was found to depend strongly on the proportion of organic acid nucleophile present in its deprotonated form. Therefore, due to the nature of the general acid catalysis and importance of deprotonated acids for efficient BPL oligomerization, it is suggested that oligomerization from lactone intermediates will be more efficient at higher SOA pH values. This result may help explain why overall isoprene-derived SOA formation has been observed to be largely pH-independent. Overall, the results strongly support the previous conclusion that a lactone intermediate is responsible for the formation of 2-MG-related species found in methacrolein-derived SOA.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.