Theoretical studies on the pK~a~ values of perfluoroalkyl carboxylic acids: Non-helical conformation acidity constants for n-perfluorooctanoic acid (n-PFOA)

Abstract

A molecular mechanics force field method (MMFF94) systematic rotor conformational search was conducted on the well known environmental contaminant n-perfluorooctanoic acid (n-PFOA). The MMFF94 conformational search yielded 2915 separate low- through high-energy conformers of n-PFOA. Ranking of these structures gave the 94 lowest MMFF94 energy conformations that were subjected to subsequent density functional theory (DFT) computational investigations at the B3LYP/6-311++G(d,p) level of theory. Application of a thermodynamic cycle approach, coupled with aqueous and gas phase DFT calculations on the molecular and anionic forms for each of the 94 lowest energy n-PFOA conformers, gave estimated acidity constants (pK~a~ values) for a final converged set of twelve n-PFOA conformers, including the global minimum helical perfluoroalkyl chain geometry and eleven non-helical higher energy conformations. Collectively, the twelve lowest energy conformers appear to completely encompass the global conformational space for n-PFOA, indicating no higher energy conformations likely remain that could appreciably influence the observed composite aqueous pK~a~ of this compound. The estimated pK~a~ values of all eleven non-helical n-PFOA conformers were within 0.1 pK~a~ units of the helical conformer pK~a~. The findings support previous predictions and recent experimental evidence for an aqueous phase n-PFOA monomeric pK~a~ of about zero.