Synthesis Enabled Investigations into the Acidity and Stability of Atmospherically-relevant Isoprene-derived Organosulfates

Atmospheric organosulfates are a class of compounds present in secondary organic aerosols that are thought to serve as a marker of anthropogenic pollution. Organosulfates derived from isoprene epoxydiol (IEPOX) have been shown to be the most abundant and ubiquitous class of these compounds, but a lack of authentic standards that account for regiochemical and stereochemical derivatives has made the study of these compounds challenging. Herein we present a synthetic protocol to access the suite of the eight IEPOX-derived organosulfates that utilizes prenol as a common starting material and affords each compound as an ammonium salt. Our method generates either syn or anti stereochemical isomers with complete control over sulfate-ester regiochemistry. We present an evaluation of the inherent acidities of each compound by measuring aqueous pH of organosulfate solutions. The syn and anti tertiary organosulfate isomers demonstrated the lowest acidity (2.45 and 2.33, respectively) compared to the primary and secondary regioisomers of the suite. We also present preliminary stability data for each organosulfate compound utilizing aqueous time-point NMR spectroscopy. Primary and secondary organosulfates demonstrated no spectral change under both neutral and acidic conditions. The syn and anti tertiary organosulfates showed decomposition in approximately 30 days under aqueous conditions, and 10 and 12 days respectively under acidic conditions. The results presented provide new insights into the physical properties and atmospheric fate of IEPOX-derived organosulfates that should be valuable for climate modeling and future atmospheric studies.