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 C3V and C1 symmetry, respectively: C3V,HF (1061 cm−1) and C3V, LF (969 cm−1) bands transition to C1,HF (1414 cm−1) and C1,LF (910 cm−1) bands. Metal ions (M) with ΔHhyd below 550 kJ/mol alter the exchange site at all states-of-hydration. They bind with C3V symmetry and exhibit high orbital overlap with the sulfonate sulfur and oxygen atoms. The Nafion-[H] C3V,LF (primarily sulfonate) vanishes at full dehydration with retention of the C3V,HF (primarily ether link). Theoretical Nafion-[M] C3V,LF bands (e.g., 940 cm−1 for Li+) are not experimentally observed. Hydration waters of ions with ΔHhyd > 1800 kJ/mol cannot be displaced by sulfonate oxygens. Thus, the Nafion-[H] C3V,LF persists at all states of hydration.