Infrared Spectroscopic and DFT Vibrational Mode Study of Perfluoro(2-ethoxyethane) Sulfonic Acid (PES), a Model Nafion Side-Chain Molecule

Abstract

We have used attenuated total reflection infrared (ATR-IR) spectroscopy to study the model compound perfluoro(2-ethoxyethane) sulfonic acid (PES) and the spectral changes induced by humidity variations to improve understanding of the IR spectrum of Nafion. This work was supported by density functional theory (DFT) calculations of the PES molecule and ion complexes to confirm assignments and determine local modes that contributed to specific absorptions in the IR spectrum. The work illustrates the difficulties of interpreting the spectrum of Nafion with several mixed modes being present. However, the loss of degeneracy in the -SO3- asymmetric stretching mode is clearly observed in difference spectra, and the use of DFT calculations provides an insight into changes induced by the variation in humidity.

Infrared Spectroscopy of Anionic, Cationic, and Zwitterionic Surfactants

“…It is also important to observe that these sulfonate groups present higher frequency values ranging from 1158 to 1183 cm −1 , similar to those found for the asymmetric stretching. Considering the interaction of the sulfonate group with the hydronium ion, H 3 O + , DFT calculations presented a split of 18 cm −1 for the ν sym (SO 2 ) mode, which is in good agreement with a split of 10 cm −1 in experimental conditions [61].…”

“…Furthermore, the frequencies of these shoulders for these molecules range from 1324 to 1367 cm −1 , which are considerable higher than those found for the HPS molecule. Even higher frequencies for the ν as (SO 2 ) mode are also assigned for sulfonyl or sulfonic functional groups, which range from 1407 to 1456 cm −1 [60][61][62].…”

Operando Raman Micro-Spectroscopy of Polymer Electrolyte Fuel Cells

“…These group modes are consequent of a dissociated Nafion exchange site (sulfonate form) with C 3V local symmetry. 22,48,[51][52][53][54][55] C 1 normal mode eigenvector animation snapshots.-The dehydrated Nafion spectrum (blue) features bands at 1414 cm −1 and 910 cm −1 . These are group modes that have motional participation of the exchange group in the sulfonic acid form (C 1 local symmetry).…”

Vibrational Spectroscopy for the Characterization of PEM Fuel Cell Membrane Materials

“…Near 1235 and 1300 cm À1 , hydration of the ÀSO À 3 groups is likely a factor influencing the changes [9,10,20,21,28]. The coupling of atomic motions of hydrophilic functional groups into C-F vibrations, as discussed by Warren and McQuillan [51], may also play a role.…”

“…Traditional assignments for the main Nafion polymer bands are summarized in [51]. The work combined infrared spectral measurements and density functional theory (DFT) vibrational mode analysis.…”