Computational approaches may underestimate pK_a values of longer-chain perfluorinated carboxylic acids: Implications for assessing environmental and biological effects

摘要

Acidity constants were calculated using the semiempirical PM6 pK_a estimation method for all C_2 through C_9 perfluoroalkyl car-boxylate (PFCA) congeners and the straight-chain C_(10) through C_(13) isomers. According to the PM6 estimates, the linear congeners within each PFCA homologue group have the highest pK_a values by up to 6 units depending on the degree of branching in the perfiuoroalkyl chain. In general, the higher the degree of branching in the perfiuoroalkyl chain within a homologue group, the lower the estimated pK_a value. When the branching is closest to the terminal carboxylate group, the effect on the calculated pK_a is greatest. Although the PM6 calculated pK_a values agree well with previously reported estimates for selected linear PFCA congeners using the SPARC and COSMOtherm approaches, all computational approaches only show good agreement with reported experimental values for short chain PFCAs (C_2 through C_5). Increasing divergences are observed between calculated and experimental results by up to several pK_a units as the perfiuoroalkyl chain length increases beyond C_5. The findings demonstrate a need for additional experimental pK_a measurements for an expanded set of both linear and branched PFCA congeners to confirm previous experimental reports that are potentially in error, and upon which to calibrate existing computational methods and environmental, toxicological, and waste treatment method models.