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Effect of disordered imidazole substructure on proton dynamics in imidazolium malonic acid salt
Author(s) -
Ławniczak Paweł,
Pogorzelec-Glaser Katarzyna,
Pietraszko Adam,
Hilczer Bożena
Publication year - 2021
Publication title -
acta crystallographica section b
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.604
H-Index - 33
ISSN - 2052-5206
DOI - 10.1107/s2052520620014365
Subject(s) - chemistry , hydrogen bond , malonic acid , proton , crystallography , conductivity , malonate , molecule , organic chemistry , physics , quantum mechanics
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.