Estimated pK_a values for the environmentally relevant C₁ through C₈ perfluorinated sulfonic acid isomers

ABSTRACT

In order to estimate isomer-specific acidity constants (pK_a) for the perfluorinated sulfonic acid (PFSA) environmental contaminants, the parameterization method 6 (PM6) pK_a prediction method was extensively validated against a wide range of carbon oxyacids and related sulfonic/sulfinic acids. Excellent pK_a prediction performance was observed for the carbon oxyacids using the PM6 method, but this approach was found to have a severe positive bias for sulfonic/sulfinic acids. To overcome this obstacle, a correlation was developed between non-adjusted PM6 pK_a values and the corresponding experimentally obtained/estimated acidity constants for a range of representative alkyl, aryl and halogen-substituted sulfonic acids. Application of this correction to the PM6 values allows for extension of this computational method to a new acid functional group. When used to estimate isomer-specific pK_a values for the C₁ through C₈ PFSAs, the modified PM6 approach suggests an adjusted pK_a range from −5.3 to −9.0, indicating that all members of this class of well-known environmental contaminants will be effectively completely dissociated in aquatic systems.

KEYWORDS:

• Acidity constant
• perfluorinated sulfonic acids
• pK_a
• environmental fate

Acknowledgments

The authors thank an anonymous reviewer for suggestions that improved the manuscript.