Synthesis and Characterization of Isoprene-Derived Organosulfates And Other Atmospherically-Relevant Molecules

Varelas, Jonathan Gregory

doi:https://doi.org/10.21985/n2-2np7-gp27

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Synthesis and Characterization of Isoprene-Derived Organosulfates And Other Atmospherically-Relevant Molecules

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Secondary organic aerosol (SOA) particles are a class of highly abundant atmospheric constituents that represent a substantial fraction of carbon within the climate system. A subset of naturally-occurring SOA particles are formed through atmospheric oxidation of biogenic volatile organic compounds (BVOCs), forming oxygenated products of lower volatility that can partition into the condensed phase and subsequently exposed to a variety of aqueous phase transformations. Particulate SOA, and aerosols generally, are thought to have a substantial impact on the radiative budget of the planet, yet their chemical and physical complexity preclude the accurate analysis of these systems and the carbon-based molecules from which they are derived. Adding further convolution is the presence of atmospheric pollutants emitted through anthropogenic activity, such as sulfur dioxide and nitrogen oxides. The interaction between biogenic SOA and man-made pollutants can result in numerous chemical transformations of organic compounds within SOA, one of which is the formation of carbon-based compounds bearing sulfate ester functionality, or organosulfates. Despite the molecular complexity of species within atmospheric aerosols, organosulfates derived from isoprene, the largest non-methane source of atmospheric carbon, has been suggested to be the most dominant and ubiquitous organic compound bearing sulfate ester moieties. However, a lack of authentic chemical standards hampers the accurate study of the formation, properties and atmospheric fate of isoprene-derived organosulfates. In this thesis, we report a synthetic methodology to access organosulfates derived from a highly prevalent oxidation product of isoprene known as isoprene epoxydiol (IEPOX). The development of this method allows for facile access to all possible regiochemical and stereochemical isomers of this family of compounds. Efforts were directed in the preparation of these species as ammonium salts, which are suggested to be relevant to the natural formation of IEPOX-derived organosulfates in the atmosphere. The generation of these compounds also allowed for evaluation of their chemical and physical properties through the use of several analytical methods. Through the measurement of aqueous pH, a general trend of compound acidity was established, correlating strongly with steric congestion of the sulfate ester functionality. The inherent stability of each compound was investigated in both aqueous and acidic media using time-point nuclear magnetic resonance (NMR). It was found that the tertiary isomers of the IEPOX-derived organosulfates exhibited degradation, while the remaining compounds were stable under aqueous and acidic conditions. It is our hope that the preparation of this series of compounds, and our preliminary analysis of their chemical properties, will be a useful avenue of study within the atmospheric and organic synthesis communities. In addition to our efforts concerning IEPOX-derived organosulfates, isotopologues of a-pinene, a highly abundant atmospheric terpene, were synthesized using site-specific deuteration to install deuterium atoms at various positions along the carbon backbone. These labeled species are currently being used in several ongoing domestic and international collaborative efforts to characterize the various atmospheric oxidation pathways of gas phase a-pinene.

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