Per- and polyfluorinated alkyl substances (PFASs) are robust “forever” chemicals that have become global environmental contaminants due to their inability to degrade using traditional techniques. This has led to the regulation of two legacy PFASs in the United States: perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS). A central property of all PFASs is their preference to adopt helical conformations, described by the rotation of CF<sub>2</sub> units around the carbon backbone. This may shield the backbone from degradation and cause significant implications regarding their environmental impact. Though most PFASs in environmental matrices are present at lower concentrations, ongoing degradation techniques involve concentrated PFAS samples. One characterization method that requires concentrated samples is 19-Fluorine NMR, which is currently used to probe the structures of PFASs and their degradation products. In this work, we developed a protocol for computing 19-F NMR chemical shifts of cationic, anionic and zwitterionic PFASs with different chain lengths and functional groups. This protocol helps establish accurate methods in replicating experimental chemical shifts, to determine how helicity impacts the structures of these PFASs.

Helicity in perfluorinated molecules: Computational method development for predicting 19F NMR chemical shifts and connection to structure

ACS Fall 2021

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