Photolysis of fluorochemicals: Tracking fluorine, use of UV-LEDs, and computational insights

Abstract

The incorporation of fluorinated functional groups in pharmaceuticals and pesticides has been increasing, owing to the modification and improvement of several key physiochemical molecular properties of the molecules. Fluorine-containing pharmaceuticals and agrochemicals eventually make their way to engineered and natural water systems from both point and non-point sources. Photolysis is an important pathway for the transformation of fluorochemicals. Understanding the formation of photoproducts is essential for both designing new benign chemicals and developing better treatment strategies to minimize persistent products and to treat them to produce fluoride as a favorable end-product. The goal of this dissertation is to use 19F-NMR to rapidly screen the formation of fluoro-photoproducts and to understand the effect of different ultraviolet (UV) light systems on the transformation and chemistry of fluorochemicals.Transformation of pesticides with different fluorine motifs was evaluated to quantify the formation of fluorinated products via photolysis in buffered aqueous systems and river water as well as treatment under advanced oxidation process (AOP) and reduction process (ARP) conditions. 19F-NMR chemical shifts and coupling analysis provided information on the formation of products and the structural changes during photolysis. Mass spectrometry results were consistent with the observed 19F-NMR products. These results provide preliminary insights into which functional groups incorporated into pesticides can be mineralized to fluoride under natural and engineered photolysis conditions. The wavelength dependence of photoproduct formation and quantum yields were evaluated for fluorinated pesticides and pharmaceuticals using UV-light emitting diodes (LEDs) with 255, 275, 308, 365, and 405 nm peak wavelengths. Using quantitative 19F-NMR, fluoride, trifluoroacetate, and additional fluorinated byproducts were tracked and quantified. Overall, narrow bandwidth LED photolysis led to a lesser number of fluorinated products as compared to the conventionally used mercury vapor or xenon lamps previously used to simulate sunlight, suggesting LEDs may be a means to limit the number of products formed. Careful selection of wavelengths would be needed to minimize persistent products and accelerate defluorination. Additionally, 19F-NMR is an important tool to evaluate the extent of defluorination. 19F-NMR measurements assist in identifying structures, but this protocol can be further refined by incorporating computations. Density functional theory was used to compute 19F-NMR shifts for parent and product structures for photolysis reactions. Combining experimental-computational 19F-NMR with LC-HRMS improves product identification and quantification for degradation studies for fluorinated compounds. These results have applications for analysis of newly synthesized fluorinated pharmaceuticals and pesticides in development. The results of this dissertation will assist in selecting treatment processes for specific fluorine motifs and in the design of agrochemicals and pharmaceuticals to reduce byproduct formation. Additionally, the results will assist in using UV-LEDs for water treatment, selecting the most efficient wavelengths and informing future design of fluorinated compounds. This work investigating the degradation of fluorinated compounds will assist in the future design of fluorinated chemicals such that persistent and/or toxic byproducts are not formed in the environment.