Determining the Geometry and Magnetic Parameters of Fluorinated Radicals by Simulation of Powder ESR Spectra and DFT Calculations:  The Case of the Radical RCF₂CF₂^• in Nafion Perfluorinated Ionomers

Abstract

The ESR spectrum of the chain-end radical RCF2CF2* detected in Nafion perfluorinated membranes exposed to the photo-Fenton reagent was accurately simulated by an automatic fitting procedure, using as input the hyperfine coupling tensors of the two F alpha and two F beta nuclei as well as the corresponding directions of the principal values from density functional theory (DFT) calculations. An accurate fit was obtained only for different orientations of the hyperfine coupling tensors for the two F alpha nuclei, indicating a nonplanar structure about the C alpha radical center. The fitted isotropic hyperfine splittings for the two F beta nuclei in the Nafion radical, 24.9 and 27.5 G, are significantly larger than those for the chain-end radical in Teflon (15 G), implying different radical conformations in the two systems. The excellent fit indicated that the geometry and electronic structure of free radicals can be obtained not only from single-crystal ESR spectroscopy, but also, in certain cases, from powder spectra, by combination with data from DFT calculations. The optimized structures obtained by DFT calculations for the CF3CF2CF2CF2* or CF3OCF2CF2* radicals as models provided additional support for the pyramidal structure determined from the spectral fit. Comparison and analysis of calculated and fitted values for the hyperfine splittings of the two F beta nuclei suggested that the radical detected by ESR in Nafion is ROCF2CF2*, which originates from attack of oxygen radicals on the Nafion side chain. The combination of spectrum fitting and DFT is considered important in terms of understanding the hyperfine splittings from 19F nuclei and the different conformations of fluorinated chain-end-type radicals RCF2CF2* in different systems, and also for elucidating the mechanism of Nafion fragmentation when exposed to oxygen radicals in fuel cell conditions.

Fragmentation of Fluorinated Model Compounds Exposed to Oxygen Radicals: Spin Trapping ESR Experiments and Implications for the Behaviour of Proton Exchange Membranes Used in Fuel Cells

“…Our experiments suggest that sulphonate groups can also be attacked by Á OH radicals, possibly with different reaction rates, as was suggested in a recent paper by Zhou et al [89]. The observation of two radical adducts in the MC that mimics the Nafion side chain, PFEESA, may suggest that Nafion fragmentation can also take place in the side chain, both at the sulphonic groups and near the ether bond, as proposed in our previous papers [16,25]. The H 2 O 2 used here (38 mM) has a profound influence on the ultimately trapped species observed in our experiments.…”

“…Based on the different F b hfs in Teflon and in Nafion, we have suggested that the radical in Nafion may be formed by degradation initiated near or at the side chain [16]. Additional support for this assignment has emerged recently, on the basis of fitting of the ESR spectrum and DFT calculations [25].…”

Automatic fitting procedures for EPR spectra of disordered systems: Matrix diagonalization and perturbation methods applied to fluorocarbon radicals

“…Accordingly, the fitting of hyperfine coupling tensors had to be done using numerical derivatives in the latter case; the fitting of the g-tensor and the line-width using analytical formulae (6) and (7) proceeded significantly faster. F 2 • radical [20] makes it difficult to obtain precise coupling values when the line-width is of comparable magnitude. A rather trivial condition to obtain accurate parameters by fitting is therefore that a change of the parameters should affect the shape of the spectrum.…”

Electron spin resonance

“…It is proposed that the radical detected by ESR in Nafion is of type ROCF 2 CF 2 d , which is formed by the action of oxygen radicals on the Nafion side chain. 119 Isotropic and anisotropic ESR spectra were recorded for the radical anions of hexafluorocyclobutene (c-C 4 F 6 À ), octafluorocyclopentene (c-C 5 F 8 À ) and perfluoro-2-butene (CF 3 CFQCFCF 3…”