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Effect of disordered imidazole substructure on proton dynamics in imidazolium malonic acid salt

The influence of a disorder in cation substructure on proton conductivity of imidazolium malonate (Im-MAL) is studied. Imidazolium in salts with dicarboxylic acids have been found to have a well ordered hydrogen-bond network and only in Im-MAL [Pogorzelec-Glaser et al. (2006). Mater. Sci.-Pol. (2006), 24, 245–252] were two types of cation observed: ordered Im-I and disordered Im-II. Im-I is involved in hydrogen bonds with malonic acid molecules, whereas Im-II is disordered between two symmetrically equivalent positions with occupancy of 0.5. NMR studies by Mizuno et al. [Hyperfine Interact. (2015), 230, 95–100] showed an 180° flip of ordered Im-I and calculated contribution of Im-I flipping to proton conductivity of Im-MAL. Ławniczak et al. [Solid State Ionics (2017), 306, 25] reported that temperature variation of the proton conductivity by impedance spectroscopy yielded the conductivity value higher than that calculated by Mizuno for Im-I. Moreover these detailed structure studies at 240 K and 280 K excluded any phase transition. Repeated X-ray studies from 14 K to 360 K show a continuous increase in anisotropic displacement factors. The half-occupied hydrogen bonds linking the Im-II nitro­gen atoms with hydroxyl oxygen atoms may be considered as electric dipoles and the interbond proton transfer as dipolar switching. It assumed here a coherent switching at low temperatures and a decrease of the coupling at higher temperatures with the disappearance at cross-over temperature at 318 K. The possible proton pathway in the crystal structure is determined and the contribution of the proton dynamics of Im-II to phonon-assisted proton diffusion in the ordered substructure is estimated.