Abstract
AbstractThe gas and solution phase relative thermodynamic stabilities of the 39 linear and branched perfluorooctanoic acid (PFOA) congeners in both their acid and anionic forms were calculated at various levels of density functional (B3LYP, wB97XD, and M062X functionals with the 6-311++G(d,p) basis set) and second order Moller-Plesset perturbation (MP2/6-311++G(d,p)//B3LYP/6-311++G(d,p)) theory using the SMD implicit solvation model. The B3LYP functional does not accurately model the expected trends in thermodynamic stability of PFOA isomers with linear versus branched perfluoroalkyl chains. Calculations obtained with the M062X and MP2 model chemistries suggest these theoretical methods may be more appropriate for relative thermodynamic stability studies on various perfluoroalkyl compounds.
Highlights
The gas and solution phase relative thermodynamic stabilities of the 39 linear and branched perfluorooctanoic acid (PFOA) congeners in both their acid and anionic forms were calculated at various levels of density functional (B3LYP, wB97XD, and M062X functionals with the 6-311++G(d,p) basis set) and second order Moller-Plesset perturbation (MP2/6-311++G(d,p)//B3LYP/6-311++G(d,p)) theory using the SMD implicit solvation model
Analogous to their perfluorooctane sulfonic acid (PFOS) analogs, the gas and solution phase relative thermodynamic stabilities of the 39 PFOA congeners [2] play an important role in assessing whether synthetic conditions for commercial mixtures are under thermodynamic or kinetic control and to allow calculation of various physicochemical properties [3,4,5]
In previous work [3, 5, 6], we reported on comparative semiempirical, density functional (DFT), and second order Moller-Plesset perturbation [MP2] theory studies into the relative thermodynamic stabilities of the 89 PFOS congeners in their neutral and anionic forms
Summary
We extend these efforts towards calculating the gas and solution phase relative thermodynamic stabilities of the 39 linear and branched PFOA congeners. Significant differences were found between the B3LYP/6-311++G(d,p), wB97XD/6-311++G(d,p), M062X/6311++G(d,p), and MP2/6-311++G(d,p)//B3LYP/6-311++G(d,p) predictions for the gas phase relative thermodynamic stabilities of the acid form PFOA congeners (Figure 2). Small differences were found in the ∆∆G◦(g) and relative thermodynamic stability rankings upon aqueous and n-octanol solvation of the acid (Table 3 and Figure 4) and anionic (Table 4 and Figure 5) forms at the M062X/6-311++G(d,p) level of theory. The findings suggest reported [33] thermodynamic stabilities of the 39 PFOA congeners using the B3LYP functional are in significant error, and that more modern functionals and/or higher level approaches are required to accurately model the thermodynamic properties of these compounds using theoretical methods. MP2/6-311++G(d,p)//B3LYP/6-311++G(d,p) ∆∆E◦(g) and thermodynamic stability rank values were taken as the comparative datum
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