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J Mol Struct THEOCHEM 869, 2008, 81 82 .pdf


Original filename: J Mol Struct THEOCHEM 869, 2008, 81-82.pdf
Title: Relative gas-phase free energies for the C3 through C8 linear and branched perfluorinated sulfonic acids: Implications for kinetic versus thermodynamic control during synthesis of technical mixtures and predicting congener profile inputs to environmental
Author: "Sierra Rayne; Kaya Forest; Ken J. Friesen"

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Journal of Molecular Structure: THEOCHEM 869 (2008) 81–82

Contents lists available at ScienceDirect

Journal of Molecular Structure: THEOCHEM
journal homepage: www.elsevier.com/locate/theochem

Relative gas-phase free energies for the C3 through C8 linear and branched
perfluorinated sulfonic acids: Implications for kinetic versus thermodynamic
control during synthesis of technical mixtures and predicting congener profile
inputs to environmental systems
Sierra Rayne a,b,*, Kaya Forest c, Ken J. Friesen a,b
a
b
c

Department of Chemistry, The University of Winnipeg, 515 Portage Avenue, Winnipeg, Man., Canada R3B 2E9
The Richardson College for the Environment, The University of Winnipeg, 515 Portage Avenue, Winnipeg, Man., Canada R3B 2E9
Department of Chemistry, Okanagan College, 583 Duncan Avenue West, Penticton, BC, Canada V2A 8E1

a r t i c l e

i n f o

Article history:
Received 8 August 2008
Received in revised form 25 August 2008
Accepted 25 August 2008
Available online 31 August 2008
Keywords:
Perfluorinated sulfonic acids
Free energies
Technical mixtures

a b s t r a c t
Gas-phase optimized geometries, enthalpies, entropies, and free energies of formation at 298 K were
calculated for the 159 C3 through C8 perfluoroalkyl sulfonic acid (PFSA) congeners using the PM6
semi-empirical method. Within each homologue group, linear PFSA congeners are generally less thermodynamically stable than their branched counterparts. The results contrast with the known predominance
of linear isomers in PFSA technical mixtures, suggesting that during the electrochemical synthesis of
PFSAs from straight-chain precursors, full fluorination of the hydrocarbon chain is more rapid than the
kinetics for intramolecular rearrangements of the linear isomer into the more stable branched congeners.
As a result, thermodynamic computational methods will be of little value in predicting likely source input
profiles of PFSAs into environmental systems.
Ó 2008 Elsevier B.V. All rights reserved.

Perfluorinated sulfonic acids (PFSAs) are contaminants detected
worldwide in a range of environmental matrices [1]. Monitoring
and toxicological data to date has focussed almost exclusively on
the straight-chain members of each PFSA homologue, especially
the well-known perfluorooctane sulfonate (PFOS), due to a lack
of both authentic standards for branched congeners and analytical
methods with congener-specific resolution [2,3]. Little is known
about the source input profiles for PFSAs to environmental systems, particularly the expected distribution of congeners with
branched versus linear perfluoroalkyl chains. For this reason, we
sought to compare computationally derived thermodynamic data
for these compounds against the known approximate compositions of technical PFOS mixtures.
Initial geometry optimizations and gas-phase enthalpies, entropies, and free energies of formation at 298 K were calculated for
the 159 C3 through C8 PFSA congeners (Table 1 of supplementary
data) using the PM6 semi-empirical method [4] and thermodynamic functions in MOPAC 2007 [5]. Geometries obtained for the
linear congeners had helical perfluoroalkyl chains (see example
of linear PFOS in Fig. 1 of supplementary data) consistent with

* Corresponding author. Address: Department of Chemistry, The University of
Winnipeg, 515 Portage Avenue, Winnipeg, Man., Canada R3B 2E9. Tel.: +1 204 786
9859; fax: +1 204 775 2114.
E-mail address: s.rayne@uwinnipeg.ca (S. Rayne).
0166-1280/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.theochem.2008.08.025

previous semi-empirical (AM1) [6] and ab initio (B3LYP/3-21G*)
[7] studies on PFOS and ab initio studies on perfluorinated n-alkanes [8] and perfluorooctanesulfonamides [9]. Both thermodynamic data [4] and molecular descriptors (e.g., polarizability,
dipole moment, molecular orbital energies, atomic charges, etc.)
[10] from the PM6 method have recently been validated against
ab initio approaches. Our results show that, within each homologue group, the linear PFSA congeners are generally less thermodynamically stable than their branched counterparts with the
following relative stability rankings for each straight-chain member: C3 n-propyl, 2/2; C4 n-butyl, 4/4; C5 n-pentyl, 8/8; C6 n-hexyl,
16/16; C7 n-heptyl, 36/39; and C8 n-octyl, 72/89.
PFSA technical mixtures are typically obtained via the Simons
Electrochemical Fluorination (ECF) method involving fluorination
of the corresponding n-akyl sulfonic acid precursors in anhydrous
HF [11]. Our data and the known dominance of straight-chain
PFSAs in technical mixtures ( 70%, with the remaining 30% as a
suite of branched isomers) [11,12] suggests the synthetic mechanism involves electrochemical fluorination of a dominantly
straight-chain hydrocarbon starting material (i.e., n-octanesulfonic
acid) proceeding under kinetic limitations for intramolecular rearrangement. Namely, the kinetics for full fluorination of the hydrocarbon chain is more rapid than the kinetics for intramolecular
rearrangements of the linear isomer into the more stable branched
congeners. Thus, complete fluorination is achieved before the

82

S. Rayne et al. / Journal of Molecular Structure: THEOCHEM 869 (2008) 81–82

linear isomer either rearranges into a dominantly branched homologue profile or is substantially ‘‘cracked” to shorter-chain perfluorinated compounds. Only a modest degree of branching is obtained
before fluorination is complete and the process halted.
Because the branched isomers are significantly more thermodynamically stable than the linear congeners (by up to 91 kJ mol 1),
according to our data, if the ECF process was allowed to achieve
thermodynamic equilibrium, the Boltzmann analysis predicts that
negligible quantities («1%) of the linear PFOS would be present in
the final technical mixture. Consequently, it appears that thermodynamic computational methods will be of little value in predicting likely source input profiles of PFSAs into environmental
systems.
Acknowledgements
S.R. thanks the Natural Sciences and Engineering Research
Council (NSERC) of Canada for financial support.
Appendix A. Supplementary data
PM6 calculated gas-phase enthalpies, entropies, and free energies of formation at 298 K for all C3 through C8 PFSA congeners

and illustrations of optimized geometries for four representative
C8 congeners are available as supplementary material. Supplementary data associated with this article can be found, in the online
version, at doi:10.1016/j.theochem.2008.08.025.
References
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