Symmetry-based IR group modes as dynamic probes of Nafion ion exchange site structure.

The categorization of Nafion side-chain IR group modes by exchange site local symmetry enables correlation of exchange site structure to state-of-hydration and ion exchange. During dehydration the Nafion-[H] exchange site transitions from the sulfonate to sulfonic acid form, corresponding to C 3V and C 1 symmetry, respectively: C 3V,HF (1061 cm −1 ) and C 3V, LF (969 cm −1 ) bands transition to C 1,HF (1414 cm −1 ) and C 1,LF (910 cm −1 ) bands. Metal ions (M) with ΔH hyd below 550 kJ/mol alter the exchange site at all states-of-hydration. They bind with C 3V symmetry and exhibit high orbital overlap with the sulfonate sulfur and oxygen atoms. The Nafion-[H] C 3V,LF (primarily sulfonate) vanishes at full dehydration with retention of the C 3V,HF (primarily ether link). Theoretical Nafion-[M] C 3V,LF bands (e.g., 940 cm −1 for Li + ) are not experimentally observed. Hydration waters of ions with ΔH hyd > 1800 kJ/mol cannot be displaced by sulfonate oxygens. Thus, the Nafion-[H] C 3V,LF persists at all states of hydration. [ABSTRACT FROM AUTHOR]