Density Functional Theory Calculations Decipher Complex Reaction Pathways of 6:2 Fluorotelomer Sulfonate to Perfluoroalkyl Carboxylates Initiated by Hydroxyl Radical

6:2 Fluorotelomer sulfonate (6:2 FTSA) is a ubiquitous environmental contaminant belonging to the family of per- and polyfluoroalkyl substances. Previous studies showed that hydroxyl radical ((OH)-O-center dot) efficiently transforms 6:2 FTSA into perfluoroalkyl carboxylates (PFCAs) of different chain lengths (C2-C7), yet the reaction mechanisms were not elucidated. This study used density functional theory (DFT) calculations to map the entire reaction path of 6:2 FTSA initiated by (OH)-O-center dot and experimentally verified the theoretical results. Optimal reaction pathways were obtained by comparing the rate constants calculated from the transition-state theory. We found that 6:2 FTSA was first transformed to C7 PFCA and C6F13 center dot; C6F13 center dot was then further reacted to C2-C6 PFCAs. The parallel addition of (OH)-O-center dot and O-2 to CnF2n+1 center dot was essential to producing C2-C6 PFCAs. The critical step is the generation of alkoxyl radicals, which withdraw electrons from the adjacent C-C groups to result in chain cleavage. The validity of the calculated optimal reaction pathways was further confirmed by the consistency with our experimental data in the aspects of O-2 involvement, identified intermediates, and the final PFCA profile. This study provides valuable insight into the transformation of polyfluoroalkyl substances containing aliphatic carbons in (OH)-O-center dot-based oxidation processes.

Automated summary

In this study, the reaction path of 6:2 FTSA initiated by (OH)-O-center dot was mapped using density functional theory (DFT) calculations and experimentally verified. It was found that the parallel addition of (OH)-O-center dot and O-2 to CnF2n+1 center dot was essential to producing C2-C6 PFCAs. The generation of alkoxyl radicals and chain cleavage were identified as critical steps in the transformation process.