Unraveling Hidden Infrared Spectral Signatures in PFAS Thermal Degradation with Two-Dimensional Correlation Spectroscopy

Per- and polyfluoroalkyl substances (PFAS) are difficult to degrade into more innocuous chemical species, which makes investigating their molecular characteristics at increased temperatures crucial for effective thermal PFAS remediation strategies. This Letter reports on the first development and implementation of infrared (IR) two-dimensional correlation spectroscopy (2D-COS) to elucidate hidden vibrational modes of gas-phase perfluorooctanoic acid (PFOA, C₈F₁₅O₂H) and the potassium salt of perfluorooctanesulfonic acid (K-PFOS, C₈F₁₇SO₃HK) undergoing thermal degradation (22-700 °C) in both N₂ and air environments. Carbon-fluorine moieties unnoticed in conventional one-dimensional (1D) IR spectra were revealed via 2D-COS. Their spectral features were cross-correlated with other observed PFAS vibrations to distinguish peaks originating from the initial parent molecule and those of the degradation byproducts. Rotational-vibrational (rovibrational) spectra were observed for K-PFOS in air, a result that may be leveraged to extract fundamental structural properties of gas-phase PFAS. Overall, we observed distinct carbon-fluorine vibrational modes in PFOA and K-PFOS, providing unique mechanistic insights, such as bond scission at different carbon locants. The methodology and findings reported herein are critical to illustrating how different molecular configurations affect degradation pathways, offering a deeper understanding of PFAS behavior under thermal conditions.