UC Irvine

Nitroaromatics are a class of organic molecules that have historically shown great utility in agricultural, pharmaceutical, and explosives industries. Though their use in pesticides has been phased out in the United States, studies of atmospheric aerosol have shown that a significant amount of these molecules still persist in the environment. A subclass of these molecules called nitrophenols have been shown to form naturally through the combustion of plant material. This process, called biomass burning, can produce individual nitrophenol concentrations that reach as high as 0.1 micrograms per cubic meter. Since these molecules are also often strong light absorbers, they often are responsible for a significant portion of visibility degradation by atmospheric aerosol. The goal of this thesis is to investigate the unique optical properties of nitroaromatics and use these results to better understand the processing of nitroaromatics in the atmosphere. Nitrophenols can exist as a handful of isomers, but the most common forms in ambient samples are 2-nitrophenol and 4-nitrophenol. The latter, 4-nitrophenol, is heavily used in the textile industry even today, and there is a large pool of work being done to find efficient ways to remove this molecule from industrial waste waters. The second chapter of this dissertation evaluates the effects of different solvents, namely water and 2-propanol, on the photochemistry of 2-nitrophenol, 4-nitrophenol, 2,4-dinitrophenol, and 2,4,6-trinitrophenol. UV/Vis absorption is used to track changes in nitrophenol concentrations. Electronic structure calculations provide insight into the energetics of states suspected of being involved in photochemical reaction pathways. Of all of the nitrophenols evaluated in this work, solvent has the strongest effect on the electronic properties of 2,4-dinitrophenol. The third chapter of this dissertation expands on the role of the surrounding matrix, switching from liquid solvents toward solid organic glasses, on the photochemistry of 2,4-dinitrophenol and 4-nitrocatechol. A sugar-alcohol called isomalt is proposed as a laboratory-based surrogate for aging in secondary organic aerosol. Photochemical degradation is measured inside of the isomalt glass using a custom-built UV/Vis spectrometer setup. The efficiency of photodegradation of 2,4-dinitrophenol and 4-nitrocatechol is compared among solid and liquid matrices. The photochemical reaction rates of these compounds appear to scale with the viscosity of the encapsulating matrix but are still competitive when encapsulated in a glass. Further inquiry into the importance of triplet states in 4-nitrocatechol photochemistry is explored in the fourth chapter of this dissertation. Ultrafast transient absorption spectroscopy is used to track which excited states are populated after excitation with a 340 nm laser pulse. Once again, the observations in water and 2-propanol are compared, revealing differences in excited state lifetimes of 4-nitrocatechol in each solvent. The roles of different excited states toward photodegradation are discussed. An excited-state proton transfer pathway appears to be the main outlet for excited 4-nitrocatechol in both solvents. In both the fourth and fifth chapters, the utility of quantum chemical calculations toward the estimation of intersystem crossing rates is also evaluated. For 4-nitrocatechol, these calculations are compared toward the described experiments. The fifth chapter shifts the focus to a simpler molecule, nitrobenzene, to try to minimize the likelihood of chemical processes interfering with interpretation of the ultrafast experimental data. Results from nitrobenzene calculations are compared to extensive work reported in the literature. The sixth chapter of this dissertation attempts to unravel mysteries behind a strong chromophore that is observed in the secondary organic aerosol made from the reaction of indole and the nitrate radical. Previous works suggest that it is a nitroindole, of which there exist seven possible isomers, and could not confirm which. The identity of the isomer is revealed in this work through a combination of chromatography and UV/Vis absorption spectroscopy. Further characterization is done with aerosol mass spectrometry, comparing the mass spectra of indole/NO3 aerosol to those of aerosolized nitroindole standards. There are strong indications that nitroindole can reliably be observed with aerosol mass spectrometry, and that this molecule has previously been observed but unidentified in field samples. This dissertation provides a detailed analysis of the photochemistry of nitrophenols and other nitroaromatic systems. Isomeric differences are evaluated for both nitrophenols and nitroindoles. The breadth of these analyses, ranging from solutions to solid organic glasses to aerosol, culminates in a deeper understanding of the atmospheric fate of this class of molecules.