Proton conduction through the nanochannel water in weak-acidic nanoporous crystals [Cu₂(phen)₂(AcO)₂(H₂O)₂][Al(OH)₆Mo₆O₁₈]M⁺(nH₂O) (M⁺ = H⁺, Li⁺, Na⁺, K⁺)

In the weak-acidic nanoporous crystals of [Cu₂(phen)₂(AcO)₂(H₂O)₂][Al(OH)₆Mo₆O₁₈]M⁺(nH₂O) (M⁺ = H⁺, Li⁺, Na⁺, K⁺; M-salt), the microwave conductivity measured by a cavity perturbation technique without electric contacts reveals an activation-type temperature change and isotope effect. The proton conduction through the water network strongly depends on the type and configuration of alkaline-metallic ions intercalated into the hydrophilic nanochannel with a square cross section. The water network with Na⁺ configured close to the framework molecule in Na-salt is soft in comparison with Li- and K-salts, in which Li⁺ and K⁺ situate near the center of nanochannel, and the whole hydrogen bonds are strengthened. From the infrared spectra, an Eigen-type hydrated proton is found to dominate the proton conduction. As for H-salt with the highest proton conductivity and the softest nature among the four salts, a Zundel-type hydrated proton makes also significant contribution for the proton conduction in addition to the Eigen-type hydrated proton.