Local structure and molecular motions in imidazolium hydrogen malonate crystal as studied by 2 Hand 13 CNMR

Transcription

1 Hyperfine Interact DOI /s Local structure and molecular motions in imidazolium hydrogen malonate crystal as studied by 2 Hand 13 CNMR 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 Hand 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 2H 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, 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 , Japan M. Mizuno ( ) R. Ohashi CREST, Japan Science and Technology Agency (JST), Honcho, Kawaguchi, Saitama , Japan mizuno@se.kanazawa-u.ac.jp M. Tansho T. Shimizu National Institute for Materials Science, 3-13 Sakura, Tsukuba, Ibaraki , Japan

2 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, we investigated the local structure and molecular motion in imidazolium hydrogen malonate using solid-state 2 Hand 13 C NMR, respectively. We measured 2 H NMR broadline spectra and quadrupole Carr-Purcell-Meiboom-Gill (QCPMG) spectra in the temperature range of K for a sample prepared using imidazole (imidazole-d 3 ) in which hydrogen bonded to carbon was replaced by deuterium. The 2 HNMRQCPMGspectrumismore sensitive to slow reorientational molecular motion than the 2 H NMR broadline spectrum [3, 10]. The relation between these local structures and proton conductivity is discussed. 2 Experimental 2.1 Sample preparation Imidazole and malonic acid were separately dissolved in ethanol and then the solutions were mixed together. Imidazolium hydrogen malonate was obtained by evaporation of the solvent CNMR The 13 C NMR spectrum was measured using a JEOL ECA-500 spectrometer operating at MHz. The high-resolution solid-state NMR spectrum was obtained using magicangle spinning (MAS) and high-power 1 H dipole decoupling (DD). Cross-polarization (CP) was used for signal enhancement. The sample was packed into a 4 mm diameter zirconia rotor. The total suppression of sidebands (TOSS) sequence was used to suppress spinning sidebands. The radio-frequency field strength ν 1 for CP was 45 and 50 khz for 1 Hand 13 C, respectively. The contact time for the CP process was 2 ms. The radio-frequency field strength ν 1 for DD was 50 khz. The MAS rate was set to 5 khz. 13 C chemical shifts were expressed as values relative to tetramethylsilane (TMS) using the ppm line of adamantane as an external reference HNMR 2 H NMR was measured using a JEOL ECA-300 spectrometer at MHz. The 2 HNMR broadline spectra were measured using a quadrupole echo sequence (90 ) x τ (90 ) y τ t acq,whereτand t acq are the interval of echo and acquisition time, respectively. 90 pulse width and τ were 2.5 and 20 μs, respectively. The QCPMG spectra were observed using a sequence (90 ) x τ (90 ) y τ t acq /2 (τ (90 ) y τ t acq ) n. n was 64. τ and t acq were 20 and 100 μs, respectively. 2 H NMR spin-lattice relaxation time (T 1 ) was measured using an inversion recovery method and a saturation recovery method. Simulations of the

3 Local structure and molecular motions in imidazolium hydrogen malonate Fig C{ 1 H}CP/MAS NMR spectrum for imidazolium malonate at 300 K Fig. 2 2 H NMR broadline spectra for imidazolium-d 3 malonate. Broken line is simulation of rigid powder pattern with e 2 qq/h = 170 khz, η = H NMR broadline and QCPMG spectra were performed by homemade Fortran programs using double precision [11]. 3 Results and discussion Figure 1 shows the solid-state 13 C{ 1 H}CP/MAS NMR spectrum for imidazolium hydrogen malonate at 300 K. The peaks at 49 and 174, and 178 ppm are the carbons of the inner portion and carboxyl groups of malonic acid, respectively. The peaks at 119 and 135 ppm are the carbons C2, C3 and C1 of imidazole in the ordered site. Small peaks at 118, 122 and 131 ppm correspond to the carbons of imidazole in the disordered site. There are four kinds of disordered imidazolium ions [1, 6]. The position of the disordered imidazolium ions on an inversion center creates a further two orientations of the imidazolium ion in the disordered site. Thus, the some peaks of the disordered site might overlap with those of the ordered site. Figure 2 shows the temperature dependence of the 2 H NMR broadline spectrum for the powdered sample of imidazolium-d 3 hydrogen malonate. At 223 K, the line shape of the 2 H NMR spectrum indicates a rigid deuterium powder pattern. The quadrupole coupling constant (e 2 qq/h) and asymmetric parameter (η) were estimated from the line shape as 170 khz and 0.05, respectively. Above 263 K, a central sharp peak appeared which shows the existence of imidazole undergoing isotropic rotation. The ratio of the sharp component in the whole spectrum was almost 0.6 % at 313 K. Therefore, the sharp component is predicted to not be imidazole in the regular site but that in the defective crystal.

4 M. Mizuno et al. Fig. 3 2 H NMR QCPMG spectra for imidazolium-d 3 malonate. a Observed spectra; b simulation considering 180 flip of the imidazolium ion; c expansions of a and b Figure 3 shows the temperature dependence of the 2 H NMR QCPMG spectrum. The line width of each peak of this spectrum increased as temperature exceeded 263 K. The line shape of each peak of the spectrum above 263 K can be explained by summing the sharp and broad components. When the imidazolium ion undergoes a 180 flip around the pseudo 2-hold axis, the directions of the principal axes of the electric field gradient at D2 and D3 change and a broadening and decrease in intensity of the peak occur. In contrast, the electric field gradient at D1 is hardly affected by the 180 flip of the imidazolium ion around the pseudo 2-hold axis, since the C1-D1 direction is nearly parallel to the rotational axis. Therefore, no remarkable change appeared in the line width and intensity of the D1 spectrum. The line shape of the spectrum observed above 263 K could be reproduced by summing the sharp component (without the effect of molecular motion) and the broad component accompanying the 180 flip at a ratio of 1 : 2. These results indicate that the line shape of the QCPMG spectrum was dominated by the 180 flip of the imidazolium ion around the pseudo 2-hold axis above 263 K. The rate k 180 for the 180 flip of the imidazolium ion, estimated from the spectral simulation, is shown in Fig. 3c. Figure 4 shows the temperature dependence of k 180 obtained from the 2 H NMR QCPMG spectrum. The activation energy of the 180 flip of the imidazolium ion was estimated to be 58±6 kj/mol from the slope of the log(k 180 ) vs. 1/T plot. The activation energy obtained by proton conductivity, 72 kj/mol, is larger than that of the 180 flip of the imidazolium ion [1]. Figure 5 shows the temperature dependence of 2 HNMRT 1.T 1 of the sharp component in the broadline spectrum was measured by the inversion recovery method using a short repetition time of 1 s. T 1 of the broad component in the broadline spectrum was measured by the saturation recovery method. T 1 decreased as temperature increased. In this temperature range, the relation ω 0 τ>>1 was satisfied. Here, τ and ω 0 are the correlation time of molecular motion and angular resonance frequency of deuterium nuclei, respectively. The correlation time of isotropic rotation (τ iso ) can be represented by τ iso = 4 15 ( ωq ω 0 ) 2 T 1 (1)

5 Local structure and molecular motions in imidazolium hydrogen malonate Fig. 4 Temperature dependence oftherate(k 180 ) of the 180 flip of the imidazolium ion T (K) k 180 (Hz) E a =58± 6 kj/mol / T (K -1 ) Fig. 5 Temperature dependence of 2 H NMR spin-lattice relaxation time T T (K) T 1 (s) / T (K -1 ) where ω q is 3e 2 qq/4. The correlation time of the isotropic rotation of imidazole at 313 K was estimated as τ iso = susingthet 1 value of the sharp component from (1). The relaxation of the broad component of the broadline spectrum is predicted to be caused by the 180 flip of the imidazolium ion. When T 1 is determined by the 180 flip of the imidazolium ion, the correlation time of the 180 flip (τ 180 ) can be represented by τ 180 = 1 ( ) 2 ωq sin 2 (2β) T 1 (2) 5 ω 0 where β is the angle between the rotational axis and the direction of the C-D bond of imidazole that is estimated as 40 for the 180 flip of the imidazolium ion. τ 180 = s was obtained from the T 1 value at 303 K using (2). From the relation of the correlation time and the jumping rate for the two-site jump, k 180 = 1/(2τ 180 ), k 180 is estimated as Hz at 303 K. The k 180 value obtained from the QCPMG spectrum is Hz. Thus, the k 180 values obtained fromt 1 correspond to those obtained from the QCPMG spectrum. Moreover, it was found that both measurements observes a 180 flip of the imidazolium ion.

6 M. Mizuno et al. Here, we confirm the relation between the 180 flip of the imidazolium ion and the proton conductivity of the imidazolium hydrogen malonate crystal. The proton conductivity σ is represented by assuming one-dimensional proton diffusion [12]: σ = nq2 λ 2 (3) 2τ 180 k B T where n, q, andλ are the number density of the charge carriers (NH protons), their charge, and the mean distance of proton movement by the reorientation of the imidazolium ion, respectively. k B and T are the Boltzmann constant and temperature, respectively. The proton conductivity σwas estimated using τ 180 obtained by the 2 HNMRQCPMGspectrum and T 1. The nearest neighbor distance of imidazolium ions was used for λ [6]. σ was estimated as S/m from τ 180 at 303 K using (3). This value is smaller than the observed value ( S/m) by one digit. Although the 180 flip of the imidazolium ion plays an important role in proton conduction, other factors contribute to proton conduction other than the Grotthuss mechanism associated with the 180 flip of the imidazolium ion. The imidazolium ion undergoing fast isotropic rotation in the defective crystal might affect proton conduction of the imidazolium hydrogen malonate crystal. 4Conclusion The imidazolium ion undergoes fast isotropic rotation in the crystal defect observed above 263 K. The imidazolium ion in regular sites undergoes a 180 flip and its activation energy is 58 kj/mol. The observed proton conductivity of imidazolium hydrogen malonate could not be explained using only the Grotthuss mechanism accompanied with the 180 flip of the imidazolium ion in regular sites. The imidazolium ion in the defective crystal is predicted to contribute to proton conduction. Acknowledgments This work was supported a Grant-in-Aid for Scientific Research (No , ) from the Ministry of Education, Culture, Sports, Science and Technology, Government of Japan. References 1. Pogorzelec-Glaser, K., Garbarczyk, J., Pawlaczyk, Cz., Markiewicz, E.: Mat. Sci. Pol. 24, (2006) 2. Pogorzelec-Glaser, K., Pawlaczyk, Cz., Pietraszko, A., Markiewicz, E.: J. Power Sources 173, (2007) 3. Umiyama, T., Ohashi, R., Ida, T., Mizuno, M.: Chem. Lett. 42, (2013) 4. Rachocki, A., Pogorzelec-Glaser, K., Tritt-Goe, J.: App. Magn. Reson 34, (2008) 5. Rachocki, A., Pogorzelec-Glaser, K., Pietraszko, A., Tritt-Goe, J.: J. Phys. Condens. Matter 20, (2008) 6. Callear, S.K., Hursthouse, M.B., Threlfall, T.L.: Cryst. Eng. Comm. 12, (2010) 7. Vold, R.R.: Nuclear magnetic resonance probes of molecular dynamics. In: Tycko, R. (ed.), pp Kluwer Academic Publishers, Norwell, MA (1994) 8. Vold, R.R., Vold, R.L.: Advances in magnetic and pptical resonance. In: Warren, W.S. (ed.), vol. 16, pp Academic Press Inc, San Diego (1991) 9. Schmidt-Rohr, K., Spiess, H.W.: In: Multidimensional Solid-State NMR and Polymers. Academic Press, London (1994) 10. Larsen, F.H., Jakobsen, H.J., Ellis, P.D., Nielsen, N.C.: Chem. Phys. Lett. 292, (1998) 11. Araya, T., Niwa, A., Mizuno, M., Endo, K.: Chem. Phys. 344, (2008) 12. Hickman, B.S., Mascal, M., Titman, J.J., Wood, I.G.: J. Am. Chem. Soc. 121, (1999)