The proton dynamics of imidazole methylphosphonate: an example of cooperative ionic conductivity
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Imidazole methylphosphonate models the hydrogen bonding and dynamics of a potential anhydrous polymer electrolyte. Understanding the behavior of anhydrous electrolytes is crucial to creating polymer materials with better performance in a fuel cell environment. This model salt exhibits ionic conductivity in the solid-state and the method of ion conduction in the solid-state differs depending on the choice of anion and cation pairs. Previous investigation of a sulfonate analogue suggests fast ring dynamics, which contributes to the ionic conductivity. However, (13)C CODEX NMR shows that rotation of the imidazole ring is somewhat slower in the methlyphosphonate compound, with a timescale for the two-site ring flip of 31 +/- 9 ms at ambient temperature. (31)P CODEX and variable temperature (1)H MAS NMR spectra confirm that ionic conductivity is facilitated by dynamics at the bifurcated hydrogen bonds between anions, with a timescale of 57 +/- 4 ms at ambient temperature for rotation of the phosphonate about the C(3v) axis. Increasing temperature introduces thermal motion and promotes the rotation of the imidazole ring together with the rotation of the phosphonate group. This leads to a cooperative mechanism of ion conduction between imidazole and the methylphosphonate at higher temperatures, which was unseen in a previous study of the benzimidazole methylphosphonate analogue.
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