Comparative DFT Study of Octanoic Acid and Perfluorooctane Sulfonate (PFOS): Insights into Charge Localization and Stability of Forever Molecules*

Perfluorinated compounds such as perflouro-octane sulfonate acid (PFOS) are among the most persistent environmental pollutants, earning the title “forever molecules” due to their highly resistant nature to biological and chemical degradation. This remarkable stability is largely attributed to the strong carbon-fluorine bonds.[1] Recent experiments have shown that low-temperature photocatalytic reactions leading to charge transfer to the forever molecules can result in breakage of the molecule.[2] In this study, we performed a comparative density function theory (DFT) study to understand the difference between the electronic behaviour of PFOS and Octanoic acid (PHOA) – a non-fluorinated complex of Perflouro-octanoic acid (PFOA) created by replacing all the fluorine atoms with hydrogen atoms. We mainly worked on two different methods to find out the weakest site of a complex.

Firstly, we calculated the binding energies of each hydrogen atom attached in the neutral octanoic acid to find out the weakest C-H and C-F bonds. This would be our reference values to be compared with our methods. For the first method, we added an electron to the neutral system to form the anionic complex. This was done to calculate the charge difference in the anion system relative to the neutral system. Our results reveal that the excess electron localizes near the site associated with the weakest C-H and C-F bonds we had found earlier. This approach is in line with the Hubbard U corrected DFT (DFT+U) methodology, where the value of U is fundamentally linked to how the energy of the system changes with charge difference. Although we did not employ DFT+U in this study, the observed charge difference provides insights into where such corrections might be most impactful. 

We took the study a step ahead by calculating the vibrational frequencies on each of the C-H and C-F bonds to see if the weakest site also exhibits lower frequency. The second step was done using the concept of Einstein oscillators. We did this by treating each individual atom in the complex as an individual oscillator and calculated its frequencies. The result from this analysis was in line with what we had found from our charge transfer method.