Gas Phase Thermochemistry for Perfluoroalkyl Carboxylic Acids

Perfluorinated species have become ubiquitous due to their desirable industrial properties including thermal and chemical stability, water resistance, and stain resistance. Despite their utility and widespread use, a number of PFAS have been identified as significant environmental contaminants posing health hazards. Much of the current studies are targeted to understand the fate and distribution of PFAS in water and solid interfaces. In contrast, the current insight into the thermodynamic properties of PFAS is minimal but play a critical role for modeling their reactions. In this study, the thermodynamic energies of perfluoroalkanoic acids (C_nF_2n+1OOH) have been predicted. The correlation consistent Composite Approach (ccCA) and coupled-cluster singles, doubles, with triple excitation (CCSD(T)) methods have been utilized to determine enthalpies of formation and Gibbs free energies including corrections for the conformational ensemble at 298 K. In addition, density fitted and local natural orbital coupled cluster approaches have been used to allow for the evaluation of larger PFAS with the ccCA composite. Thermodynamic predictions made using DFT(B3LYP) have also been evaluated in comparison to ccCA and CCSD(T) energetics due to its widespread usage in computing thermochemical properties.