Local structure and molecular motions in imidazolium hydrogen malonate crystal as studied by 2 H and 13 C NMR
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Local structure and molecular motions in imidazolium hydrogen malonate crystal as studied by 2 H and 13 C NMR M. Mizuno · M. Chizuwa · T. Umiyama · Y. Kumagai · T. Miyatou · R. Ohashi · T. Ida · M. Tansho · T. Shimizu
© Springer International Publishing Switzerland 2014
Abstract The local structure and molecular motion of the imidazolium hydrogen malonate crystal were investigated using solid-state 2 H and 13 C NMR. The imidazolium ion undergoes isotropic rotation, which is correlated with a defect in the crystal, as observed by 2 H NMR broadline spectra above 263 K. A 180◦ flip of the imidazolium ion in the regular site was observed from 2 H NMR quadrupole Carr-Purcell-Meiboom-Gill (QCPMG) spectra. The Grotthuss mechanism was accompanied by a 180◦ flip of the imidazolium ion in regular sites. Moreover, the proton transfer associated with the imidazolium ion of the defective crystal is important for proton conductivity of the imidazolium hydrogen malonate crystal. Keywords Hydrogen bond · Proton conduction · Quadrupole interaction · 2 H NMR
1 Introduction Imidazolium salts of dicarboxylic acids show relatively high proton conductivity [1, 2]. Imidazole and dicarboxylic acid are connected by a hydrogen bond and form a two-dimensional network in the crystal of these salts. Proton conductivity is dominated by continuous proton transfer in the hydrogen network between imidazolium ions and the carboxyl group, and the Proceedings of the 5th Joint International Conference on Hyperfine Interactions and International Symposium on Nuclear Quadrupole Interactions (HFI/NQI 2014), Canberra, Australia, 21-26 September 2014 M. Mizuno · M. Chizuwa · T. Umiyama · Y. Kumagai · T. Miyatou · R. Ohashi · T. Ida Department of Chemistry, Graduate School of Natural Science & Technology, Kanazawa University, Kakuma, Kanazawa, Ishikawa 920-1192, Japan M. Mizuno () · R. Ohashi CREST, Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan e-mail: [email protected] M. Tansho · T. Shimizu National Institute for Materials Science, 3-13 Sakura, Tsukuba, Ibaraki 305-0003, Japan
M. Mizuno et al.
reorientational motions of the imidazolium ion such as a 180◦ flip and rotational vibration are predicted to play an important role [3–5]. In these crystals, only imidazolium hydrogen malonate has both an ordered and a disordered imidazole ring [1, 6]. The ratio of the ordered and disordered sites of the imidazolium ion is 2:1. Although proton conductivity of imidazolium hydrogen malonate exceeds 10−3 S/m around room temperature [1], the relation between proton conductivity and local structure in the imidazolium hydrogen malonate crystal has not yet been clarified. A high-resolution solid-state 13 C NMR spectrum exhibits the local structure of a crystal. A 2 H NMR static spectrum is affected by the quadrupole interaction and its line shape is very sensitive to the mode and rate of molecular motion, since the electric gradient at the deuteron site is averaged by molecular motion [7–9]. In the present study
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