Abstract
AbstractA systematic and comprehensive semiempirical, Hartree-Fock (HF) ab initio, and B3LYP density functional theory (DFT) study was conducted on the relative thermodynamic properties of various linear and branched perfluorinated and perhydrogenated alkyl compounds. The semiempirical AM1, PM3, and PM6 methods all consistently and accurately predict that branched alkyl compounds will generally be more thermodynamically stable than their linear counterparts. In contrast, HF and B3LYP calculations with the 6-31G(d,p), 6-31++G(d,p), and 6-311++G(d,p) basis sets predict that linear isomers will be more stable than branched analogs. These different linear versus branched perfluoroalkyl/perhydroalkyl thermodynamic property trends between semiempirical and ab initio/DFT methods were evident in both gas and aqueous phase calculations. Comparison of experimentally determined thermodynamic properties for several classes of linear and branched alkanes and alcohols with values calculated at the PM6 and B3LYP/6-311++G(d,p) levels of theory supported the well known findings that such DFT and HF approaches incorrectly predict branched alkyl compounds will be less thermodynamically stable than linear isomers. Calculations at the MP2/6-311++G(d,p)//B3LYP/6-311++G(d,p) and M05-2X/6-311++G(d,p) levels of theory on a representative subset of the linear and branched perfluorinated compounds supported the thermodynamic conclusions from the PM6 method. Strong agreement between PM6 estimated thermodynamic properties and available experimental data supports use of this computational method for accurately calculating the well established higher thermodynamic stability of branched alkyl compounds. Branched perfluoroalkyl compounds are thus expected to be more thermodynamically stable than their linear analogs.
Highlights
Perfluoroalkyl sulfonic acids (PFSAs) are widely used in commercial activities and products [1] and have become globally distributed contaminants over the past several decades with a range of toxicological issues.[2]
Whereas the semiempirical perfluoromethyl substituent fluorines was 2.6613 Å (PM6) method generally predicts a decrease in gas phase thermodynamic stability with increasing linearity of the perfluoroalkyl chain,[10] density functional theory (DFT) calculations at the B3LYP/6-31++G(d,p) level have suggested that the linear n-PFOS isomer is more thermodynamically stable than its monomethyl branched counterparts [12,13]
Gas phase calculations for the linear perfluorooctane sulfonic acid (n-PFOS; C8 PFSA 89) and its six monomethyl branched isomers (1- through 6-CF3-PFOS; C8 PFSAs 83 through 88) (Fig. 1) using the semiempirical AM1, PM3, and PM6 methods and Hartree-Fock (HF) ab initio and B3LYP density functional theory (DFT) calculations using the 6-31G(d,p), 6-31G++(d,p), and 6311G++(d,p) basis sets indicate computational method dependent relative Gibbs free energy thermodynamic stability rankings for these compounds (Table 1)
Summary
Perfluoroalkyl sulfonic acids (PFSAs) are widely used in commercial activities and products [1] and have become globally distributed contaminants over the past several decades with a range of toxicological issues.[2]. Gas phase calculations for the linear perfluorooctane sulfonic acid (n-PFOS; C8 PFSA 89) and its six monomethyl branched isomers (1- through 6-CF3-PFOS; C8 PFSAs 83 through 88) (Fig. 1) using the semiempirical AM1, PM3, and PM6 methods and Hartree-Fock (HF) ab initio and B3LYP density functional theory (DFT) calculations using the 6-31G(d,p), 6-31G++(d,p), and 6311G++(d,p) basis sets indicate computational method dependent relative Gibbs free energy thermodynamic stability rankings for these compounds (Table 1).
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