Overtone-Induced Degradation of Perfluorinated Alcohols in the Atmosphere

Abstract

Perfluorinated alcohols (PFOHs) are thermally unstable and degrade via loss of HF, ultimately forming perfluorocarboxylic acids. Experiments and calculations of the high activation barrier for the decomposition of CF₃OH suggest that the reaction occurs exclusively heterogeneously, perhaps with the involvement of water. Here, we use density functional theory calculations to show that PFOHs may readily complex with water and are expected to be present as a few percent of the total PFOH concentration under ambient atmospheric conditions. The presence of water lowers the HF elimination barrier sufficiently that excitation to low-lying O−H vibrational overtone levels (v_OH ≥ 3) may cause reaction. Photolysis rate constants for CF₃OH·H₂O and CF₃CF₂OH·H₂O were estimated to be 6.1 × 10^-8 and 5.6 × 10^-8 s^-1, respectively. PFOH−water complexes should undergo degradation much faster than the corresponding gas-phase unimolecular decomposition of PFOHs, which requires excitation into the v_OH = 5 or 6 vibrational level. Overtone-driven gas-phase reactions of PFOH−water complexes could be moderately competitive with heterogeneous reactions with water in dry locations. Overtone-induced degradation of PFOHs is likely a modest atmospheric source of PFCAs to the environment.