CHEMPHYSCHEM COMMUNICATIONS DOI: 10.1002/cphc.201402487
Insight into Local Structure and Molecular Dynamics in Organic Solid-State Ionic Conductors Haijin Zhu,[a] Fangfang Chen,[a] Liyu Jin,[b] Luke A. O’Dell,[a] and Maria Forsyth*[a] Elucidating the rate and geometry of molecular dynamics is particularly important for unravelling ion-conduction mechanisms in electrochemical materials. The local molecular motions in the plastic crystal 1-ethyl-1-methylpyrrolidinium tetrafluoroborate ([C2mpyr][BF4]) are studied by a combination of quantum chemical calculations and advanced solid-state nuclear magnetic resonance spectroscopy. For the first time, a restricted puckering motion with a small fluctuation angle of 258 in the pyrrolidinium ring has been observed, even in the lowtemperature phase ( 45 8C). This local molecular motion is deemed to be particularly important for the material to maintain its plasticity, and hence, its ion mobility at low temperatures.
Lithium-ion batteries have gradually taken an important place as the rechargeable battery of choice for portable electronic equipment.[1] The main challenge of this field is to meet the demands of high energy and power densities without compromising safety. Safety and stability concerns associated with organic-solvent-based electrolytes have encouraged the use of alternative materials, such as ionic liquids or their solid-state analogues, organic ionic plastic crystals (OIPCs), as electrolytes. OIPCs represent a unique family of solid electrolyte materials, which have attracted increasing attention as next-generation electrolyte materials for lithium-ion batteries because of their novel physical properties.[2] Plastic crystals are characterized by their mesophases, in which the ions are ordered in the long range, but orientationally and rotationally disordered in the short range (hence, such phases are often termed as “rotator phases”).[3] The physical and electrochemical properties of OIPC materials critically depends on the packing geometry and dynamics of the cation and anion species in the system. At low temperatures, these materials typically exist in a crystalline state and are fully ordered, but as the temperature is increased, local motions are activated and one or more firstorder solid–solid phase transitions occur. Such local motions and structural disorder are highly desirable as they may result
[a] Dr. H. Zhu, Dr. F. Chen, Dr. L. A. O’Dell, Prof. Dr. M. Forsyth Institute for Frontier Materials and ARC Centre of Excellence for Electromaterials Science Deakin University, Geelong, VIC 3216 (Australia) E-mail: [email protected] [b] L. Jin Department of Materials Engineering and ARC Centre of Excellence for Electromaterials Science Monash University, Clayton, VIC 3800 (Australia) Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.201402487.
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
in high ionic diffusivity and thus enhanced conductivity. A typical example of such plastic-crystal ionic compounds is 1-ethyl1-methylpyrrolidinium bis(trifluoromethanesuphonyl)imide. The neat OIPC compound exhibits an ambient-temperature conductivity between 10 7 and 10 9 S cm 1.[2c] Moreover, the addition of a lithium-ion dopant produces a 50-fold increase in this conductivity without changing the phase behavior of the material.[2c] This sharp increase in conductivity is very likely to be due to the fast lithium-ion motion, which is facilitated by the local motions of the OIPC ’matrix’. This coupling effect of the lithium-ion motions to the local molecular mobility of the OIPC matrix has been of great interest for many years.[2b, 3a, 4] Although much effort has been dedicated to understanding the transport behavior of lithium ions in the OIPC matrix in recent years, little progress has been made so far. One of the essential problems is that the motions and symmetry of the OIPC ions can be difficult to characterise and are yet to be fully understood on the molecular level. New insights into these dynamics will allow the rational design of improved OIPC electrolyte materials. Advanced solid-state nuclear magnetic resonance (NMR) methods can allow us to study the rate and amplitude of molecular motions in a site-selective fashion.[5] In this study, we investigate the structure and dynamics of an OIPC material, Nethyl-N-methylpyrrolidinium tetrafluoroborate ([C2mpyr][BF4]). We have used a combination of quantum chemical calculations and solid-state NMR techniques to gain a number of new insights into this system, including: 1) the conformation adopted by the pyrrolidinium ring in the low-temperature phases; 2) the nature of the motions of the cations and the anions in the various solid phases, including the symmetry and amplitude of the motions; and 3) how the solid-phase transitions are related to the structural and mobility changes of the cations and anions. These insights will help to further illuminate the ion-transport mechanisms in this and similar systems. The solid–solid phase transitions of [C2mpyr][BF4] were investigated by differential scanning calorimetry (DSC) and static 1H and 19F solid-state NMR (Figure 1). On heating from 75 up to 100 8C, this material goes through three solid–solid phase transitions at temperatures of 38, 26, and 63 8C. It is worth mentioning that there is another solid–solid phase transition at about 80 8C, corresponding to the transition from the fully ordered (crystalline) phase to the plastic crystal phase.[6] Upon further temperature increases, a decomposition process was observed by thermogravimetric anaylsis (TGA) with an onset temperature of about 275 8C.[6] This material therefore shows a broad plastic crystal temperature window ranging from 80 to 275 8C, and undergoes decomposition prior to melting. These solid–solid transitions show progressive transformation ChemPhysChem 2014, 15, 3720 – 3724
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Figure 1. DSC melting exotherms of [C2mpyr][BF4] and the peak width (full width at half maximum; FWHM) of its static 1H and 19F NMR spectra plotted against temperature. The solid phases are labelled by Roman numerals.
www.chemphyschem.org times higher than that of phase II to I (4.3 JK 1 mol 1). Generally, the proton line widths are governed by homonuclear dipolar coupling and are sensitive to motions in the magnitude of kHz; whereas the phase transition enthalpies are sensitive to the energy involved in the structural rearrangements. The relative sizes of these changes in line widths, together with the melting enthalpy of the solid transitions may imply that both the changes in the motions and the structural reorientation are relatively minor compare to the transitions of phase IV to III and phase III to II. The coupled dynamics between the cations and anions is further investigated using 13C and 11B NMR. These nuclei exist exclusively in the cations and anions, respectively. Therefore, the chemical shifts and the shape of 13C lines represent the structure and dynamics of cations; whereas the shape of 11B lines reflects the mobility and motional symmetry of the anions. The isotropic chemical shifts in magic angle spinning (MAS) NMR spectra are indicative of the local structural conformation, so any structural change in the crystalline lattice during the solid–solid phase transitions would be expected to result in a change in the peak positions. Figure 2 a shows the 13 C MAS NMR spectra measured at different temperatures. In the 45 8C spectrum, when the material is in solid-phase IV, the C4/C7 and C5/C6 peaks arising from the pyrrolidinium ring
from the ordered phase into more dynamic/disordered phases, leading to higher mobility and plasticity.[2c, 7] The 1H and 19F static NMR spectra of solids usually consist of a broad and featureless peak due to the strong homonuclear dipole–dipole interactions. Since these interactions can be partially averaged by dynamics, the line width of these spectra retains resourceful information about molecular motions[8] and is therefore well suited for studies of local molecular dynamics. In [C2mpyr][BF4], 1H is only present in the cation, and 19F exists only in the anion, which allows us to analyze the motions of cations and anions separately. Figure 1 shows the line widths of the 1H and 19F NMR spectra as functions of the temperature (the 1H and 19F spectra are shown in the Supporting Information, SI, Figure S1). These data indicate significant variations in molecular dynamics for both the cation and anion during each phase transition. During the transition from phase IV to phase III, the 19F NMR line widths narrow from 16 to 9 kHz (a 43 % decrease), whereas the 1H line width shows an even more significant decrease from 38 to 11 kHz (a 71 % drop). For the transition from phase II to I, a relFigure 2. Selected variable temperature (VT) 13C and 11B NMR spectra of [C2mpyr][BF4]. All the spectra were recordatively minor line-narrowing was ed at 11.7 T on a Bruker Avance III spectrometer and a 4 mm double resonance magic angle spinning (MAS) observed for both the 19F and 1H probe. a) 13C 8 kHz MAS NMR spectra. The spectra at 65 and 75 8C were recorded using single-pulse excitation by cross-polarization (CP) to improve the NMR line widths. It is also noted that with a recycle delay of 10 s. The other spectra were recorded signal. Each solid phase is characterized by different 13C chemical shifts. Peaks in the high-temperature (75 8C) the transition enthalpy for phase spectrum were assigned according to the literature,[6] and in the low-temperature ( 45 8C) spectrum based on IV to III and III to II is 52.7 quantum chemical calculations. The carbon sites are labelled corresponding to Figure 3 a. b,c) 11B and 13C NMR JK 1 mol 1, which is about 12 spectra measured from a static sample using single-pulse excitation. 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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CHEMPHYSCHEM COMMUNICATIONS split into two, indicating that they are magnetically non-equivalent. This magnetic inequivalence could be a result of either the conformation of the cation or the asymmetric interionic interaction existing in the solid environment. It is observed that the chemical shift anisotropy (CSA) pattern of the C1 site shows a rather narrow isotropic lineshape at 45 8C (phase IV). This observation suggests that the ethyl group exhibits a fast dynamics in phase IV, which would lead to a motionally averaged (and therefore equivalent) environment for the C4/C7 and C5/C6 sites. Therefore, the observed magnetic inequivalence of C4/C7 and C5/C6 in phase IV is most probably a result of the asymmetric solid environment, rather than the conformation of the cation. This result also suggests that the motion of the whole cation is slow enough in phase IV to resolve this difference. This argument is also supported by the static 13 C NMR spectra shown in Figure 2 c, which show rather broad lines at 73 and 50 8C. During the transition from phase IV ( 45 8C) to phase III ( 38 8C), both the 11B and 13C NMR spectra show a significant line narrowing, and moreover, all the resonance lines in the MAS spectra shift to new positions. Interestingly, although the resonance lines are shifted, the splitting between C4 and C7 is maintained, suggesting that the asymmetric solid environment is preserved in phase III. Even though the molecular dynamics in the cations is increased compared to phase IV, the rotational motion of the whole cation, which could lead to the exchange between C4 and C7 sites, is slow in phase III. Given the simple and symmetric structure of the anions, it is natural to assume that all the anions are tumbling isotropically in space in all the different solid phases, albeit with different motional rates. This assumption is strongly supported by the static 19F and 11B NMR results, which show symmetric Gaussian and/or Lorentzian lines in all phases. Since the temperature range of phase III is quite narrow, some residual NMR signal from phases IV and II is also observed at that temperature, probably because of the temperature gradient in the NMR rotor (estimated to be about 2 8C). In line with the shifts in resonance lines, both the static 11B (Figure 2 b) and 13C (Figure 2 c) NMR spectra show dramatic line narrowing as the temperature increases from 50 (phase IV) to 38 8C (phase III), indicating a sudden increase in the molecular dynamics of both cations and anions. Upon the transition from phase III to phase II, the isotropic chemical shifts of all the carbons change (Figure 2 a). Moreover, the peaks from the C4/C7 and C5/C6 sites merge into single lines. Because both the static 13C and 11B spectra show residual CSA and quadrupolar line broadening, respectively, this must be due to anistropic dynamics (in contrast to isotropic rotations that would average these interactions to zero in solution NMR). This therefore provides strong evidence for rotation of the cation around a certain axis, leading to a fast exchange between the C4/C7 and C5/C6 sites. In contrast, narrow isotropic lines are observed in the static 13C and 11B spectra for phase I, indicating that both cations and anions are tumbling quickly and isotropically in space. To further investigate the geometry and local molecular motions involved in the different solid phases, particularly in low 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemphyschem.org temperature phases, we then considered how the 13C chemical shift (CS) tensors are influenced by the molecular motions. The rigid-limit 13C CS tensors were calculated quantum chemically on several possible interionic [C2 mpyr][BF4] conformers including a gas-phase global energy-minimum structure by density functional theory at the B3LYP/6-31 + G(d) level. The Gaussian 09 software package[11] was used for the calculations. To separately probe the motions of the pyrrolidinium ring and the local reorientation of the CH3 group in the cation, the motionally averaged 13C CSA patterns from the C5 and C3 sites were obtained by means of a two-dimensional 13C SUPER[10] experiment at 45 8C. Figures 3 b and 3 c show a comparison be-
Figure 3. a) Chemical structure of the [C2mpyr][BF4] ion pair. The 13C chemical shift tensor orientations of C3 and C5 are indicated by arrows, as obtained from quantum chemical simulations. The standard convention, dxx, dyy, and dzz, for the three-principle chemical shift values is used to represent the chemical shift anisotropy.[9] For C5, both dxx and dzz are in the plane of C4-C5-C6-C7 of the ring, dxx is parallel to the H-C5-H angle bisector, and dyy is perpendicular to the plane of C4-C5-C6-C7. dzz of C3 points along the C3 N bond, dxx and dyy are in the plane perpendicular to the C3 axis and are much smaller in magnitude compared to dzz. b,c) 13C NMR powder patterns of C5 and C3. The black solid lines represent the experimental patterns obtained from 13C SUPER[10] experiments (11.7 T, 4125 Hz MAS), and the red dotted lines are the fitted simulation results.
tween the calculated and experimental CSA patterns of C5 and C3 (see Figure 3 a). The three principal values of the calculated and experimental CS tensors are presented in Table 1. Interestingly, the experimental powder pattern of C5 (Figure 3 b) displays the lineshape of an almost axially symmetric CS tensor, suggesting either an intrinsically symmetric environment or an asymmetric environment in which the CS tensor is averaged to a symmetric form by local molecular motions. The CS tensors obtained from the calculations represent the rigid limit in which no molecular motion was considered, and therefore, they can provide an answer to this ambiguity. Surprisingly, an excellent agreement was found between the experimental and calculated powder patterns for C5. This result rules out the fast rotation of the pyrrolidinium ring (around the axis indicated in phases III and II in Figure 4) in solid-phase IV because this would cause a change in the CS tensor. ChemPhysChem 2014, 15, 3720 – 3724
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Table 1. Experimental (exptl) and calculated (calcd) CSA parameters.
principal values
line broadening
dxx [ppm] dyy [ppm] dzz [ppm] LB [Hz][c]
C5 Exptl[a] (98 % overlap)
Calcd[b]
C3 Exptl[a] (96 % overlap)
Calcd[b]
48
45.9
76.8
77.7
9.2
9.1
45.2
51.2
2.8
4.4
7.6
0.66
1260
1260
1600
1600
[a] Experimental values were obtained by fitting the experimental 13C powder pattern using the Bruker topspin 3.2 software. The percentages indicate the quality of the fitting. [b] Calculated values were obtained from quantum chemical calculations using the Gaussian 09 software.[11] [c] A Lorentzian line broadening was applied to the experimental CSA pattern to get the best fit, the same line broadening was then applied to the calculated CSA pattern.
namically averaged. Given the orientation of this CS tensor in the molecular frame, a puckering motion of the pyrrolidinium ring can be inferred, in which the nitrogen atom and the covalently bonded methyl and ethyl group swing (asymmetrically) around their equilibrium position, where the dihedral angle between the C7-N-C4 and C4-C5-C6-C7 planes is about 208. We modelled this motion by considering a rapid angular fluctuation (10 kHz) with different angles s. The corresponding motionally averaged 13C powder patterns were calculated using NMR-WEBLAB,[5c, 12] with the best fit obtained for s = 258 (Figure S2). Further quantum chemical analysis shows that this small-angle puckering motion is energetically favorable with DE 1.8 kcal mol 1 (Figure S3). The main findings of this study can therefore be summarized as follows (see also Figure 4): 1. The organic ionic plastic crystal compound [C2mpyr][BF4] shows four solid phases in the temperature range: 80 ~ 100 8C. Static NMR lineshape analysis shows that the isotropic motion of the anions is strongly coupled to the cation dynamics. 2. In (low-temperature) solid-phase IV, the rotation of the whole cation is frozen, whereas a restricted puckering motion with a small fluctuation angle of 258 is observed in the pyrrolidinium ring. 3. Upon phase transition from IV to III, a significant increase in the molecular dynamics of both anions and cations occurs. The librational motion of the pyrrolidinium ring is dominant in phase III, whereas the rotation motion of the whole cation that could lead to the exchange between C4 and C7 sites is slow. The full axial rotation of the cation only begins in phase II (Figure 4). 4. In phase I, the motion of both cations and anions is completely isotropic.
Combining quantum chemical calculations with advanced multi-nuclear solid-state NMR, we have obtained a detailed picture of the dynamics in various phases of the OIPC [C2mpyr][BF4]. For OIPCs, this is the first time that a clear evidence of the restricted puckering motion of the pyrrolidinium ring has been observed in the low-temperature solid phase. This loFigure 4. Model for the cation motions in different solid phases of [C2mpyr][BF4]. calized motion, together with other molecular motions identified in this study, are of particular importance for understanding the mechanical and transport properties of the material. The methods applied in this Figure 3 c shows the experimental and calculated 13C study provide a powerful way to investigate the structure and powder pattern of the methyl group C3. The experimental CS dynamics of plastic crystal materials and their composites. tensor is asymmetric with three significantly different principal values. This result rules out fast rotation of the whole cation around any axis, as such a process occurring on a time scale of Acknowledgements approximately 104 s 1 or faster would result in a motionally averaged, axially symmetric CS tensor. Interestingly, when comProf. Maria Forsyth wishes to acknowledge the financial support paring the experimental CS tensor components to the calculatfrom the Australian Research Council (ARC) through the Australed values (Table 1), it is found that although the principal ian Laureate program funding FL110100013. ARC is also acknowlvalue dxx shows a close match, the two other values dyy and dzz edged for funding Deakin University’s Magnetic Resonance Facilidiffer significantly, suggesting that these components are dy 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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CHEMPHYSCHEM COMMUNICATIONS ty through LIEF grant LE110100141. Dr. Daniel Gunzelmann and Prof. Louis Madsen are thanked for helpful discussions. Keywords: chemical shift anisotropy · molecular dynamics · NMR spectroscopy · plastic crystals · quantum chemical calculations [1] R. Marom, S. F. Amalraj, N. Leifer, D. Jacob, D. Aurbach, J. Mater. Chem. 2011, 21, 9938 – 9954. [2] a) J. M. Pringle, P. C. Howlett, D. R. MacFarlane, M. Forsyth, J. Mater. Chem. 2010, 20, 2056 – 2062; b) D. R. MacFarlane, M. Forsyth, Adv. Mater. 2001, 13, 957 – 966; c) D. R. MacFarlane, J. Huang, M. Forsyth, Nature 1999, 402, 792 – 794. [3] a) L. Jin, K. M. Nairn, C. M. Forsyth, A. J. Seeber, D. R. MacFarlane, P. C. Howlett, M. Forsyth, J. M. Pringle, J. Am. Chem. Soc. 2012, 134, 9688 – 9697; b) H. Ohno, Electrochemical Aspects of Ionic Liquids, Second ed., John Wiley & Sons, New Jersey, 2011. [4] a) J. Adebahr, A. J. Seeber, D. R. MacFarlane, M. Forsyth, J. Phys. Chem. B 2005, 109, 20087 – 20092; b) J. Golding, N. Hamid, D. R. MacFarlane, M. Forsyth, C. Forsyth, C. Collins, J. Huang, Chem. Mater. 2001, 13, 558 – 564. [5] a) Y. Wei, R. Graf, J. C. Sworen, C.-Y. Cheng, C. R. Bowers, K. B. Wagener, H. W. Spiess, Angew. Chem. 2009, 121, 4687 – 4690; Angew. Chem. Int. Ed. 2009, 48, 4617 – 4620; b) M. R. Hansen, T. Schnitzler, W. Pisula, R. Graf, K. Mllen, H. W. Spiess, Angew. Chem. 2009, 121, 4691 – 4695; Angew. Chem. Int. Ed. 2009, 48, 4621 – 4624; Angew. Chem. 2009, 121, 4691 – 4695c) V. Macho, L. Brombacher, H. W. Spiess, Appl. Magn. Reson. 2001, 20, 405 – 432.
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www.chemphyschem.org [6] Y. Shekibi, T. Ruther, J. Huang, A. F. Hollenkamp, Phys. Chem. Chem. Phys. 2012, 14, 4597 – 4604. [7] Y. Shekibi, A. Gray-Weale, D. R. MacFarlane, A. J. Hill, M. Forsyth, J. Phys. Chem. C 2007, 111, 11463 – 11468. [8] a) H. Zhu, H. P. Huinink, P. C. M. M. Magusin, O. C. G. Adan, K. Kopinga, J. Magn. Reson. 2013, 235, 109 – 114; b) K. Schmidt-Rohr, H. W. Spiess, Multidimensional Solid State NMR and Polymers, Academic Press, London 1994; c) H. W. Spiess, Ann. Rev. Mat. Sci. 1991, 21, 131 – 158. [9] H. Sait, I. Ando, A. Ramamoorthy, Prog. Nucl. Magn. Reson. Spectrosc. 2010, 57, 181 – 228. [10] S. F. Liu, J. D. Mao, K. Schmidt-Rohr, J. Magn. Reson. 2002, 155, 15 – 28. [11] Gaussian 09, Revision D.01, M. J. Frisch, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery, Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, . Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski, and D. J. Fox, Gaussian Inc., Wallingford CT 2009. [12] M. R. Hansen, R. Graf, H. W. Spiess, Acc. Chem. Res. 2013, 46, 1996 – 2007. Received: July 7, 2014 Published online on September 18, 2014
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Insight into Local Structure and Molecular Dynamics in Organic Solid-State Ionic Conductors Haijin Zhu,[a] Fangfang Chen,[a] Liyu Jin,[b] Luke A. O’Dell,[a] and Maria Forsyth*[a] Elucidating the rate and geometry of molecular dynamics is particularly important for unravelling ion-conduction mechanisms in electrochemical materials. The local molecular motions in the plastic crystal 1-ethyl-1-methylpyrrolidinium tetrafluoroborate ([C2mpyr][BF4]) are studied by a combination of quantum chemical calculations and advanced solid-state nuclear magnetic resonance spectroscopy. For the first time, a restricted puckering motion with a small fluctuation angle of 258 in the pyrrolidinium ring has been observed, even in the lowtemperature phase ( 45 8C). This local molecular motion is deemed to be particularly important for the material to maintain its plasticity, and hence, its ion mobility at low temperatures.
Lithium-ion batteries have gradually taken an important place as the rechargeable battery of choice for portable electronic equipment.[1] The main challenge of this field is to meet the demands of high energy and power densities without compromising safety. Safety and stability concerns associated with organic-solvent-based electrolytes have encouraged the use of alternative materials, such as ionic liquids or their solid-state analogues, organic ionic plastic crystals (OIPCs), as electrolytes. OIPCs represent a unique family of solid electrolyte materials, which have attracted increasing attention as next-generation electrolyte materials for lithium-ion batteries because of their novel physical properties.[2] Plastic crystals are characterized by their mesophases, in which the ions are ordered in the long range, but orientationally and rotationally disordered in the short range (hence, such phases are often termed as “rotator phases”).[3] The physical and electrochemical properties of OIPC materials critically depends on the packing geometry and dynamics of the cation and anion species in the system. At low temperatures, these materials typically exist in a crystalline state and are fully ordered, but as the temperature is increased, local motions are activated and one or more firstorder solid–solid phase transitions occur. Such local motions and structural disorder are highly desirable as they may result
[a] Dr. H. Zhu, Dr. F. Chen, Dr. L. A. O’Dell, Prof. Dr. M. Forsyth Institute for Frontier Materials and ARC Centre of Excellence for Electromaterials Science Deakin University, Geelong, VIC 3216 (Australia) E-mail: [email protected] [b] L. Jin Department of Materials Engineering and ARC Centre of Excellence for Electromaterials Science Monash University, Clayton, VIC 3800 (Australia) Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.201402487.
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
in high ionic diffusivity and thus enhanced conductivity. A typical example of such plastic-crystal ionic compounds is 1-ethyl1-methylpyrrolidinium bis(trifluoromethanesuphonyl)imide. The neat OIPC compound exhibits an ambient-temperature conductivity between 10 7 and 10 9 S cm 1.[2c] Moreover, the addition of a lithium-ion dopant produces a 50-fold increase in this conductivity without changing the phase behavior of the material.[2c] This sharp increase in conductivity is very likely to be due to the fast lithium-ion motion, which is facilitated by the local motions of the OIPC ’matrix’. This coupling effect of the lithium-ion motions to the local molecular mobility of the OIPC matrix has been of great interest for many years.[2b, 3a, 4] Although much effort has been dedicated to understanding the transport behavior of lithium ions in the OIPC matrix in recent years, little progress has been made so far. One of the essential problems is that the motions and symmetry of the OIPC ions can be difficult to characterise and are yet to be fully understood on the molecular level. New insights into these dynamics will allow the rational design of improved OIPC electrolyte materials. Advanced solid-state nuclear magnetic resonance (NMR) methods can allow us to study the rate and amplitude of molecular motions in a site-selective fashion.[5] In this study, we investigate the structure and dynamics of an OIPC material, Nethyl-N-methylpyrrolidinium tetrafluoroborate ([C2mpyr][BF4]). We have used a combination of quantum chemical calculations and solid-state NMR techniques to gain a number of new insights into this system, including: 1) the conformation adopted by the pyrrolidinium ring in the low-temperature phases; 2) the nature of the motions of the cations and the anions in the various solid phases, including the symmetry and amplitude of the motions; and 3) how the solid-phase transitions are related to the structural and mobility changes of the cations and anions. These insights will help to further illuminate the ion-transport mechanisms in this and similar systems. The solid–solid phase transitions of [C2mpyr][BF4] were investigated by differential scanning calorimetry (DSC) and static 1H and 19F solid-state NMR (Figure 1). On heating from 75 up to 100 8C, this material goes through three solid–solid phase transitions at temperatures of 38, 26, and 63 8C. It is worth mentioning that there is another solid–solid phase transition at about 80 8C, corresponding to the transition from the fully ordered (crystalline) phase to the plastic crystal phase.[6] Upon further temperature increases, a decomposition process was observed by thermogravimetric anaylsis (TGA) with an onset temperature of about 275 8C.[6] This material therefore shows a broad plastic crystal temperature window ranging from 80 to 275 8C, and undergoes decomposition prior to melting. These solid–solid transitions show progressive transformation ChemPhysChem 2014, 15, 3720 – 3724
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Figure 1. DSC melting exotherms of [C2mpyr][BF4] and the peak width (full width at half maximum; FWHM) of its static 1H and 19F NMR spectra plotted against temperature. The solid phases are labelled by Roman numerals.
www.chemphyschem.org times higher than that of phase II to I (4.3 JK 1 mol 1). Generally, the proton line widths are governed by homonuclear dipolar coupling and are sensitive to motions in the magnitude of kHz; whereas the phase transition enthalpies are sensitive to the energy involved in the structural rearrangements. The relative sizes of these changes in line widths, together with the melting enthalpy of the solid transitions may imply that both the changes in the motions and the structural reorientation are relatively minor compare to the transitions of phase IV to III and phase III to II. The coupled dynamics between the cations and anions is further investigated using 13C and 11B NMR. These nuclei exist exclusively in the cations and anions, respectively. Therefore, the chemical shifts and the shape of 13C lines represent the structure and dynamics of cations; whereas the shape of 11B lines reflects the mobility and motional symmetry of the anions. The isotropic chemical shifts in magic angle spinning (MAS) NMR spectra are indicative of the local structural conformation, so any structural change in the crystalline lattice during the solid–solid phase transitions would be expected to result in a change in the peak positions. Figure 2 a shows the 13 C MAS NMR spectra measured at different temperatures. In the 45 8C spectrum, when the material is in solid-phase IV, the C4/C7 and C5/C6 peaks arising from the pyrrolidinium ring
from the ordered phase into more dynamic/disordered phases, leading to higher mobility and plasticity.[2c, 7] The 1H and 19F static NMR spectra of solids usually consist of a broad and featureless peak due to the strong homonuclear dipole–dipole interactions. Since these interactions can be partially averaged by dynamics, the line width of these spectra retains resourceful information about molecular motions[8] and is therefore well suited for studies of local molecular dynamics. In [C2mpyr][BF4], 1H is only present in the cation, and 19F exists only in the anion, which allows us to analyze the motions of cations and anions separately. Figure 1 shows the line widths of the 1H and 19F NMR spectra as functions of the temperature (the 1H and 19F spectra are shown in the Supporting Information, SI, Figure S1). These data indicate significant variations in molecular dynamics for both the cation and anion during each phase transition. During the transition from phase IV to phase III, the 19F NMR line widths narrow from 16 to 9 kHz (a 43 % decrease), whereas the 1H line width shows an even more significant decrease from 38 to 11 kHz (a 71 % drop). For the transition from phase II to I, a relFigure 2. Selected variable temperature (VT) 13C and 11B NMR spectra of [C2mpyr][BF4]. All the spectra were recordatively minor line-narrowing was ed at 11.7 T on a Bruker Avance III spectrometer and a 4 mm double resonance magic angle spinning (MAS) observed for both the 19F and 1H probe. a) 13C 8 kHz MAS NMR spectra. The spectra at 65 and 75 8C were recorded using single-pulse excitation by cross-polarization (CP) to improve the NMR line widths. It is also noted that with a recycle delay of 10 s. The other spectra were recorded signal. Each solid phase is characterized by different 13C chemical shifts. Peaks in the high-temperature (75 8C) the transition enthalpy for phase spectrum were assigned according to the literature,[6] and in the low-temperature ( 45 8C) spectrum based on IV to III and III to II is 52.7 quantum chemical calculations. The carbon sites are labelled corresponding to Figure 3 a. b,c) 11B and 13C NMR JK 1 mol 1, which is about 12 spectra measured from a static sample using single-pulse excitation. 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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CHEMPHYSCHEM COMMUNICATIONS split into two, indicating that they are magnetically non-equivalent. This magnetic inequivalence could be a result of either the conformation of the cation or the asymmetric interionic interaction existing in the solid environment. It is observed that the chemical shift anisotropy (CSA) pattern of the C1 site shows a rather narrow isotropic lineshape at 45 8C (phase IV). This observation suggests that the ethyl group exhibits a fast dynamics in phase IV, which would lead to a motionally averaged (and therefore equivalent) environment for the C4/C7 and C5/C6 sites. Therefore, the observed magnetic inequivalence of C4/C7 and C5/C6 in phase IV is most probably a result of the asymmetric solid environment, rather than the conformation of the cation. This result also suggests that the motion of the whole cation is slow enough in phase IV to resolve this difference. This argument is also supported by the static 13 C NMR spectra shown in Figure 2 c, which show rather broad lines at 73 and 50 8C. During the transition from phase IV ( 45 8C) to phase III ( 38 8C), both the 11B and 13C NMR spectra show a significant line narrowing, and moreover, all the resonance lines in the MAS spectra shift to new positions. Interestingly, although the resonance lines are shifted, the splitting between C4 and C7 is maintained, suggesting that the asymmetric solid environment is preserved in phase III. Even though the molecular dynamics in the cations is increased compared to phase IV, the rotational motion of the whole cation, which could lead to the exchange between C4 and C7 sites, is slow in phase III. Given the simple and symmetric structure of the anions, it is natural to assume that all the anions are tumbling isotropically in space in all the different solid phases, albeit with different motional rates. This assumption is strongly supported by the static 19F and 11B NMR results, which show symmetric Gaussian and/or Lorentzian lines in all phases. Since the temperature range of phase III is quite narrow, some residual NMR signal from phases IV and II is also observed at that temperature, probably because of the temperature gradient in the NMR rotor (estimated to be about 2 8C). In line with the shifts in resonance lines, both the static 11B (Figure 2 b) and 13C (Figure 2 c) NMR spectra show dramatic line narrowing as the temperature increases from 50 (phase IV) to 38 8C (phase III), indicating a sudden increase in the molecular dynamics of both cations and anions. Upon the transition from phase III to phase II, the isotropic chemical shifts of all the carbons change (Figure 2 a). Moreover, the peaks from the C4/C7 and C5/C6 sites merge into single lines. Because both the static 13C and 11B spectra show residual CSA and quadrupolar line broadening, respectively, this must be due to anistropic dynamics (in contrast to isotropic rotations that would average these interactions to zero in solution NMR). This therefore provides strong evidence for rotation of the cation around a certain axis, leading to a fast exchange between the C4/C7 and C5/C6 sites. In contrast, narrow isotropic lines are observed in the static 13C and 11B spectra for phase I, indicating that both cations and anions are tumbling quickly and isotropically in space. To further investigate the geometry and local molecular motions involved in the different solid phases, particularly in low 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemphyschem.org temperature phases, we then considered how the 13C chemical shift (CS) tensors are influenced by the molecular motions. The rigid-limit 13C CS tensors were calculated quantum chemically on several possible interionic [C2 mpyr][BF4] conformers including a gas-phase global energy-minimum structure by density functional theory at the B3LYP/6-31 + G(d) level. The Gaussian 09 software package[11] was used for the calculations. To separately probe the motions of the pyrrolidinium ring and the local reorientation of the CH3 group in the cation, the motionally averaged 13C CSA patterns from the C5 and C3 sites were obtained by means of a two-dimensional 13C SUPER[10] experiment at 45 8C. Figures 3 b and 3 c show a comparison be-
Figure 3. a) Chemical structure of the [C2mpyr][BF4] ion pair. The 13C chemical shift tensor orientations of C3 and C5 are indicated by arrows, as obtained from quantum chemical simulations. The standard convention, dxx, dyy, and dzz, for the three-principle chemical shift values is used to represent the chemical shift anisotropy.[9] For C5, both dxx and dzz are in the plane of C4-C5-C6-C7 of the ring, dxx is parallel to the H-C5-H angle bisector, and dyy is perpendicular to the plane of C4-C5-C6-C7. dzz of C3 points along the C3 N bond, dxx and dyy are in the plane perpendicular to the C3 axis and are much smaller in magnitude compared to dzz. b,c) 13C NMR powder patterns of C5 and C3. The black solid lines represent the experimental patterns obtained from 13C SUPER[10] experiments (11.7 T, 4125 Hz MAS), and the red dotted lines are the fitted simulation results.
tween the calculated and experimental CSA patterns of C5 and C3 (see Figure 3 a). The three principal values of the calculated and experimental CS tensors are presented in Table 1. Interestingly, the experimental powder pattern of C5 (Figure 3 b) displays the lineshape of an almost axially symmetric CS tensor, suggesting either an intrinsically symmetric environment or an asymmetric environment in which the CS tensor is averaged to a symmetric form by local molecular motions. The CS tensors obtained from the calculations represent the rigid limit in which no molecular motion was considered, and therefore, they can provide an answer to this ambiguity. Surprisingly, an excellent agreement was found between the experimental and calculated powder patterns for C5. This result rules out the fast rotation of the pyrrolidinium ring (around the axis indicated in phases III and II in Figure 4) in solid-phase IV because this would cause a change in the CS tensor. ChemPhysChem 2014, 15, 3720 – 3724
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Table 1. Experimental (exptl) and calculated (calcd) CSA parameters.
principal values
line broadening
dxx [ppm] dyy [ppm] dzz [ppm] LB [Hz][c]
C5 Exptl[a] (98 % overlap)
Calcd[b]
C3 Exptl[a] (96 % overlap)
Calcd[b]
48
45.9
76.8
77.7
9.2
9.1
45.2
51.2
2.8
4.4
7.6
0.66
1260
1260
1600
1600
[a] Experimental values were obtained by fitting the experimental 13C powder pattern using the Bruker topspin 3.2 software. The percentages indicate the quality of the fitting. [b] Calculated values were obtained from quantum chemical calculations using the Gaussian 09 software.[11] [c] A Lorentzian line broadening was applied to the experimental CSA pattern to get the best fit, the same line broadening was then applied to the calculated CSA pattern.
namically averaged. Given the orientation of this CS tensor in the molecular frame, a puckering motion of the pyrrolidinium ring can be inferred, in which the nitrogen atom and the covalently bonded methyl and ethyl group swing (asymmetrically) around their equilibrium position, where the dihedral angle between the C7-N-C4 and C4-C5-C6-C7 planes is about 208. We modelled this motion by considering a rapid angular fluctuation (10 kHz) with different angles s. The corresponding motionally averaged 13C powder patterns were calculated using NMR-WEBLAB,[5c, 12] with the best fit obtained for s = 258 (Figure S2). Further quantum chemical analysis shows that this small-angle puckering motion is energetically favorable with DE 1.8 kcal mol 1 (Figure S3). The main findings of this study can therefore be summarized as follows (see also Figure 4): 1. The organic ionic plastic crystal compound [C2mpyr][BF4] shows four solid phases in the temperature range: 80 ~ 100 8C. Static NMR lineshape analysis shows that the isotropic motion of the anions is strongly coupled to the cation dynamics. 2. In (low-temperature) solid-phase IV, the rotation of the whole cation is frozen, whereas a restricted puckering motion with a small fluctuation angle of 258 is observed in the pyrrolidinium ring. 3. Upon phase transition from IV to III, a significant increase in the molecular dynamics of both anions and cations occurs. The librational motion of the pyrrolidinium ring is dominant in phase III, whereas the rotation motion of the whole cation that could lead to the exchange between C4 and C7 sites is slow. The full axial rotation of the cation only begins in phase II (Figure 4). 4. In phase I, the motion of both cations and anions is completely isotropic.
Combining quantum chemical calculations with advanced multi-nuclear solid-state NMR, we have obtained a detailed picture of the dynamics in various phases of the OIPC [C2mpyr][BF4]. For OIPCs, this is the first time that a clear evidence of the restricted puckering motion of the pyrrolidinium ring has been observed in the low-temperature solid phase. This loFigure 4. Model for the cation motions in different solid phases of [C2mpyr][BF4]. calized motion, together with other molecular motions identified in this study, are of particular importance for understanding the mechanical and transport properties of the material. The methods applied in this Figure 3 c shows the experimental and calculated 13C study provide a powerful way to investigate the structure and powder pattern of the methyl group C3. The experimental CS dynamics of plastic crystal materials and their composites. tensor is asymmetric with three significantly different principal values. This result rules out fast rotation of the whole cation around any axis, as such a process occurring on a time scale of Acknowledgements approximately 104 s 1 or faster would result in a motionally averaged, axially symmetric CS tensor. Interestingly, when comProf. Maria Forsyth wishes to acknowledge the financial support paring the experimental CS tensor components to the calculatfrom the Australian Research Council (ARC) through the Australed values (Table 1), it is found that although the principal ian Laureate program funding FL110100013. ARC is also acknowlvalue dxx shows a close match, the two other values dyy and dzz edged for funding Deakin University’s Magnetic Resonance Facilidiffer significantly, suggesting that these components are dy 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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CHEMPHYSCHEM COMMUNICATIONS ty through LIEF grant LE110100141. Dr. Daniel Gunzelmann and Prof. Louis Madsen are thanked for helpful discussions. Keywords: chemical shift anisotropy · molecular dynamics · NMR spectroscopy · plastic crystals · quantum chemical calculations [1] R. Marom, S. F. Amalraj, N. Leifer, D. Jacob, D. Aurbach, J. Mater. Chem. 2011, 21, 9938 – 9954. [2] a) J. M. Pringle, P. C. Howlett, D. R. MacFarlane, M. Forsyth, J. Mater. Chem. 2010, 20, 2056 – 2062; b) D. R. MacFarlane, M. Forsyth, Adv. Mater. 2001, 13, 957 – 966; c) D. R. MacFarlane, J. Huang, M. Forsyth, Nature 1999, 402, 792 – 794. [3] a) L. Jin, K. M. Nairn, C. M. Forsyth, A. J. Seeber, D. R. MacFarlane, P. C. Howlett, M. Forsyth, J. M. Pringle, J. Am. Chem. Soc. 2012, 134, 9688 – 9697; b) H. Ohno, Electrochemical Aspects of Ionic Liquids, Second ed., John Wiley & Sons, New Jersey, 2011. [4] a) J. Adebahr, A. J. Seeber, D. R. MacFarlane, M. Forsyth, J. Phys. Chem. B 2005, 109, 20087 – 20092; b) J. Golding, N. Hamid, D. R. MacFarlane, M. Forsyth, C. Forsyth, C. Collins, J. Huang, Chem. Mater. 2001, 13, 558 – 564. [5] a) Y. Wei, R. Graf, J. C. Sworen, C.-Y. Cheng, C. R. Bowers, K. B. Wagener, H. W. Spiess, Angew. Chem. 2009, 121, 4687 – 4690; Angew. Chem. Int. Ed. 2009, 48, 4617 – 4620; b) M. R. Hansen, T. Schnitzler, W. Pisula, R. Graf, K. Mllen, H. W. Spiess, Angew. Chem. 2009, 121, 4691 – 4695; Angew. Chem. Int. Ed. 2009, 48, 4621 – 4624; Angew. Chem. 2009, 121, 4691 – 4695c) V. Macho, L. Brombacher, H. W. Spiess, Appl. Magn. Reson. 2001, 20, 405 – 432.
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www.chemphyschem.org [6] Y. Shekibi, T. Ruther, J. Huang, A. F. Hollenkamp, Phys. Chem. Chem. Phys. 2012, 14, 4597 – 4604. [7] Y. Shekibi, A. Gray-Weale, D. R. MacFarlane, A. J. Hill, M. Forsyth, J. Phys. Chem. C 2007, 111, 11463 – 11468. [8] a) H. Zhu, H. P. Huinink, P. C. M. M. Magusin, O. C. G. Adan, K. Kopinga, J. Magn. Reson. 2013, 235, 109 – 114; b) K. Schmidt-Rohr, H. W. Spiess, Multidimensional Solid State NMR and Polymers, Academic Press, London 1994; c) H. W. Spiess, Ann. Rev. Mat. Sci. 1991, 21, 131 – 158. [9] H. Sait, I. Ando, A. Ramamoorthy, Prog. Nucl. Magn. Reson. Spectrosc. 2010, 57, 181 – 228. [10] S. F. Liu, J. D. Mao, K. Schmidt-Rohr, J. Magn. Reson. 2002, 155, 15 – 28. [11] Gaussian 09, Revision D.01, M. J. Frisch, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery, Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, . Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski, and D. J. Fox, Gaussian Inc., Wallingford CT 2009. [12] M. R. Hansen, R. Graf, H. W. Spiess, Acc. Chem. Res. 2013, 46, 1996 – 2007. Received: July 7, 2014 Published online on September 18, 2014
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