Abstract
Coupling between dynamical heterogeneity of ionic liquids and their structural periodicity on different lengthscales can be directly probed by quasielastic neutron scattering with polarization analysis. The technique provides the tools to investigate singleparticle and cooperative ion motions separately and, thus, dynamics of ion associations affecting the net charge transport can be experimentally explored. The focus of this study is the structuredynamic relationship in the protic ionic liquid, triethylammonium triflate, characterized by strong hydrogen bonds between cations and anions. The siteselective deuterium/hydrogenisotope substitution was applied to modulate the relative contributions of different atom groups to the total coherent and incoherent scattering signal. This approach in combination with molecular dynamics simulations allowed us to obtain a sophisticated description of cation selfdiffusion and confined ion pair dynamics from the incoherent spectral component by using the acidic proton as a tagged particle. The coherent contribution of the neutron spectra demonstrated substantial ion association leading to collective ion migration that preserves charge alteration on picosecond time scale, as well as correlation of the localized dynamics occurring between adjacent ions.
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Introduction
Understanding dynamical and structural heterogeneity of ionic liquids (ILs)^{1,2,3} has important implications for explaining and predicting their macroscopic physicochemical properties (viscosity, thermal and electrical conductivity, melting points, etc) relevant for diverse applications^{4,5}. Numerous Xray and neutron diffraction studies as well as computer simulations have already provided a clear picture of the longrange and intermediaterange order in ILs, which distinguishes these novel materials from conventional solvents^{2,3,6}. Dynamics on various time scales have been characterized by many spectroscopic techniques such as different types of NMR^{7,8,9}, dielectric and optical Kerr effect^{10}, and timeresolved fluorescence^{11} spectroscopies. In this perspective, quasielastic neutron scattering (QENS)^{12} is a unique technique, because it can simultaneously probe both structure and dynamics and, hence, enables a direct observation of structuredynamics relationships in ILs. Due to the large neutron incoherent cross section of hydrogen, the method is predominantly sensitive to singleparticle (uncorrelated) motions of protons ubiquitous in organic cations of ILs^{13,14,15,16}. Selective deuterium/hydrogen (D/H) isotope substitution can be applied to intensify coherent scattering^{14,17,18} and to shift the research focus to collective (correlated) processes governed by the charge alteration in ILs^{19,20}.
The full wealth of available information hidden in both incoherent and coherent parts of a QENS spectrum is, however, difficult to extract. The knowledge about the composition of the sample and the corresponding neutron cross section is not sufficient to disentangle collective and single particle processes. The only unambiguous experimental way to overcome this problem is to perform QENS measurements in conjunction with polarization analysis. This approach is getting more common in soft matter research^{21,22,23} and has also appeared to be successful in the case of pyridiniumbased ILs^{22}. The separation of coherent and nuclear spinincoherent scattering clearly demonstrated a collective nature of the longrange diffusion of butylpyridinium cations determined by Coulomb and Van der Waals forces between ions, whereas the localized dynamics of the long alkyl chain turned out to be a true singleparticle process. Changing the balance between the ion interactions by introducing highly directional hydrogen bonds, as in the case of protic ionic liquids (PILs) produced by combining a Brønsted acid and a Brønsted base^{1}, may lead to additional coherence effects in ion dynamics. Therefore, QENS with polarization analysis can be seen as a valuable and informative method for understanding microscopic details of hydrogen bonding and proton conductivity in such systems.
During the last years, several publications on singleparticle dynamics in PILs have appeared in the literature^{14,15,16}. In general, the PIL spectra are interpreted in an analogous way as those of more extensively studied aprotic ILs^{17,18,24,25}, assuming longrange diffusion and various subdiffusive (localized) motions. As acidic protons can hypothetically contribute to anhydrous conductivity of PILs via Grotthus mechanism (proton hopping through the hydrogen bond network), much attention is also explicitly paid to its dynamics. For example, comparing [Im][TFSI] and its deuterated analogue [dIm][TFSI], Hoarfrost et al.^{15} analyzed the temperature dependent efficiency of proton transfer events. In our recent work^{14} we investigated triethylammonium triflate (TEATF, or [NH(C_{2}H_{5})_{3}][SO_{3}CF_{3}]) with the prevailing vehicular mechanism of the acidic proton transport as it has been shown by pulsed field gradient NMR (PFGNMR)^{26,27} and also proved by our computer simulations^{28}. Using D/Hisotope substitution in the ethyl chains of the cation we left the proton attached to the triethylamine unchanged to be used as a tagged particle. The most interesting finding was an additional localized process of this NH proton on the time scale of ~4 ps even in the solid phase, when the global motions of the cations are “frozen”. However, the obtained experimental data were not sufficient to unambiguously determine the nature of this process, first of all, due to the presence of the relatively strong coherent contribution of the partially deuterated sample. Therefore, an experimental separation of coherent and nuclear spinincoherent scattering was required for a more sophisticated analysis.
The current work is a combined experimental and computational study on TEATF. Here we present QENS experiments with polarization analysis on both completely protonated and partially deuterated (TEA_{D}TF, [NH(C_{2}D_{5})_{3}][SO_{3}CF_{3}]) samples in the time window of tens picoseconds. We aim not only to refine the previous results by excluding coherence effects, but also to provide a complete and thorough picture of both singleparticle and collective motions in this model PIL, and in this way to show the relationship between the structure and dynamics. Taking into account the complexity of the system, its structural and dynamical hierarchy, it is essential for interpreting experimental data to use input from the molecular dynamics (MD) simulations^{24}, which cover approximately the same time and distance scale as QENS. The computational support is especially valuable in the case of the collective dynamics, where cationcation, anionanion and cationanion correlated motions give origin to the coherent signal.
Data Analysis
As it follows from the theoretical principles of the polarization analysis^{29,30}, the experimental scattering intensity can be separated into the coherent and nuclear spinincoherent contributions, which are directly related to the corresponding dynamic structure factors, \({S}_{{\rm{coh}}}(Q,E)\) and \({S}_{{\rm{inc}}}(Q,E)\), defined as Fourier transforms of the intermediate scattering functions, \({I}_{{\rm{coh}}}(Q,t)\) and \({I}_{{\rm{inc}}}(Q,t)\):
The next expressions demonstrate the connection between the intermediate scattering functions and the position operators in a system of identical particles:
where N is the total number of scatterers in the system; the angular brackets \(\langle \cdots \rangle \) denote a thermodynamic average. The coherent contribution is determined by the correlation between the positions of different nuclei (i, j) at different times and originates from interference effects, while the incoherent contribution provides information on singleparticle relaxations. In the case of different types of nuclei the intermediate scattering functions have to be averaged with weights depending on their neutron scattering lengths, b_{i}:
These formulas enable calculation of neutron spectra from MD trajectories^{31} and, hence, a direct comparison with the experimental scattering functions.
The model description of cation singleparticle dynamics on a picosecond time scale generally implies a superposition of two relaxation processes^{14,17,18}. The first one is the longrange diffusion, the second one comprises entangled localized motions of side groups (conformational changes of alkyl groups, librations). To resolve the third subpicosecond component reported for ILs^{13,18,25}, the probed dynamical range has to be expanded by decreasing the wavelength of incident neutrons. With the applied experimental setting in the present study the fast relaxation is detected only as a flat background contribution. Because the characteristic linewidths of the confined and longrange processes differ by approximately a factor of ten, the incoherent dynamic structure factor can be presented as a convolution of the independent global, \({S}_{{\rm{glob}}}(Q,E)\), and localized, \({S}_{{\rm{loc}}}(Q,E)\), dynamic structure factors, multiplied by a Debye–Waller factor, \(\exp (\,\,2W)\).
The adequate modeling of the longrange process in ILs is based on the jumpdiffusion model^{32}, where \({S}_{{\rm{glob}}}(Q,E)\) has a Lorentzian shape with the halfwidth \({{\rm{\Gamma }}}_{{\rm{tr}}}\) depending on Q as follows:
where D_{tr} is the selfdiffusion coefficient and \({\tau }_{0}\) is the residence time. It is necessary to mention that, because the time scale of the discussed QENS measurements does not exceed tens of picoseconds, D_{tr} has the meaning of a shorttime diffusion constant. Experiments in a broader time window clearly show that the formalism of the jumpdiffusion model is not strictly valid and the socalled stretched exponential function is required to characterize the longrange process^{13,18,25}. For this reason diffusion coefficients evaluated by other methods such as PFGNMR^{26,27} are usually smaller than those obtained from QENS experiment.
To describe various localized cation motions we applied the Gaussian model^{33}, which considers particles moving inside a confinement with a “soft” boundary.
where D_{loc} stands for the selfdiffusion coefficient of the localized motion and R is the variance of the particle displacement and characterizes the size of the domain, in which the particles are diffusing. Taking into account that the incoherent dynamic structure factor does not contain any crosscorrelation terms and, hence, is an additive function, \({S}_{{\rm{loc}}}(Q,E)\) for the whole cation can be given as a simple sum of the three terms accounting for the motions of “equivalent” hydrogens diffusing in a confinement with the corresponding characteristic radius (R_{H}, R_{1}, and R_{2} for the NH proton, bridging methylene, and end methyl groups, respectively). The contribution of the other elements (C, N, S, O) can be neglected (Table S1 of the Supporting Information). The three terms of \({S}_{{\rm{loc}}}(Q,E)\) have to be weighted with respect to the total number of particles in each group and the incoherent neutron cross sections of H and D.
where the superscripts “prot” and “deut” are used for the TEATF and TEA_{D}TF samples, σ_{H} and σ_{D} are the neutron incoherent cross sections of the hydrogen isotopes H and D, respectively. The diffusion coefficients for the hydrogens of the ethyl groups are considered to be equal to each other (D_{ch}). Although it is obvious that the flexibility of the alkyl groups may result in a distribution of both radii of confinement and diffusion coefficients, as it is suggested, for example, from MD simulations, we had to apply this approximation to ensure the stability of the fit parameters.
Finally, the model dynamic incoherent structure factor (Eq. 4 including Eqs 5–7) convoluted with the resolution function of the instrument, R(Q, E), is fitted to the measured scattering intensity:
where bg(Q) is a flat background accounting for faster relaxations unresolved in the accessible experimental time window.
The fits of a pair of the TEATF and TEA_{D}TF spectra were performed in an iterative way at each measured temperature point. In the initial parameter set for the TEATF sample, the contribution of the acidic proton was neglected. This provided the first estimates for the other parameters D_{tr}, D_{ch}, R_{1}, and R_{2}, which were used in the next step of fitting of the TEA_{D}TF spectrum. The procedure was repeated until the difference in the parameters values of two successive iterations was significantly less than the error margins (56 times). Examples of the incoherent spectra fitted with the model scattering function are presented in Fig. S7 of the Supplementary Information.
The number of known analytical models applicable for coherent scattering is significantly smaller than that for the simpler case of incoherent scattering. For example, correlated reorientational motions can be characterized in the systems of noninteracting identical ions/molecules^{34,35}. There are also examples of more general descriptions such as Vineyard’s static approximation^{36} and Sköld’s adhoc ansatz^{37}. However, these approaches cannot be directly and unambiguously transferred to the coherent scattering of ILs, where different types of both intramolecular and intermolecular correlations lead to the appearance of diffraction peaks^{2,3} in the Qrange accessible by QENS and have impact on dynamics^{19,22}. For this reason, in the present work we will use a modelindependent approach assuming that the total coherent dynamic structure factor is a convolution of the correlated longrange (subscript tr) and localized (subscript loc) relaxation processes:
where S(Q) is the coherent structure factor, the other parameters \({{\rm{\Gamma }}}_{{\rm{tr}}}^{{\rm{coh}}}(Q)\), \({{\rm{\Gamma }}}_{{\rm{loc}}}^{{\rm{coh}}}(Q)\) and A(Q) are modulated by S(Q) and do not have an explicit analytical description. Examples of the coherent spectra fitted with the model scattering function are presented in Fig. S8 of the Supplementary Information. The qualitative picture of correlated motions in TEATF provided by Eq. 9 will be interpreted in terms of anionanion, cationanion, and cationcation contributions (Eq. 3) with the help of the MD simulations.
Results and Discussion
Diffraction with Polarisation Analysis
Using the D7 diffuse scattering spectrometer in the diffraction mode we determined the nuclear spinincoherent and coherent parts of the diffraction pattern as presented in Fig. 1. The nuclear spinincoherent scattering dominates the TEATF spectrum, whereas the coherent signal of the partially deuterated TEA_{D}TF is comparable and even stronger than the selfcorrelation contribution in the probed Qrange. Energy redistribution due to highfrequency vibrations (DebyeWaller factor) leads to a gradual decay of the incoherent component with Q. The mean square displacement (msd, \(\langle {u}^{2}\rangle \)) associated with these vibrations can be estimated from the formula \(\exp (\,\,2W)=\exp (\,\,\langle {u}^{2}\rangle {Q}^{2})\). The fast vibrational motions of the ethyl protons significantly change the total msd of TEATF (\(\langle {u}^{2}\rangle \) = 0.12 Å^{2} at T = 320 K) in comparison to TEA_{D}TF (\(\langle {u}^{2}\rangle \) = 0.05 Å^{2} at T = 320 K).
The coherent part of the diffraction spectra exhibits a pattern typical for other ILs^{2,3} with two correlation peaks at about 0.9 Å^{−1} and 1.3–1.8 Å^{−1}. The latter is the socalled adjacency peak^{3}, the position of its maximum shifts with the deuteration of the ethyl chain pointing out to the intramolecular as well as intermolecular origin of the feature. The lowQ peak at 0.9 Å^{−1} is a signature of the unique charge ordering in ILs and referred to as the chargecharge correlation peak. Its position is not influenced by the isotope substitution in the cations. The data are compared with \(S(Q)={I}_{{\rm{coh}}}(Q,0)\) calculated from the MD trajectories using Eq. 3. The experimental and theoretical results show a relatively good agreement, especially for the peak positions, allowing us to use the results of the MD analysis in the interpretation of the experimentally observed dynamics. For example, it is possible to dissect contributions of different atom groups and construct anionanion (anan), cationcation (catcat) and cationanion (catan) subcomponents of S(Q) by adding up corresponding crosssection weighted terms in Eq. 3 (Fig. 2).
All the subcomponents exhibit intense peaks or antipeaks at 0.9 Å^{−1}. This picture is typical for systems of two species distributed with equal periodicity^{3}. The adjacency correlation peak at 1.3–1.8 Å^{−1} of the neutron spectrum is mainly formed by the cation crosscorrelation functions in both TEATF and TEA_{D}TF. The main difference is that the carbon skeleton contributes largely to the total S(Q) of the protonated sample, whereas the deuterium atoms are responsible for the dominating part of S(Q) in TEA_{D}TF.
Single Particle Dynamics
Longrange translation, localized conformational and librational motions of the ethyl chains as well as the restricted dynamics of the acidic proton have been previously described on a picosecond time scale in TEATF^{14}. The interpretation of the results has been, however, based on the assumption that any interference effects of the inter and intramolecular processes relax fast enough and do not distort the incoherent signal originating from the localized ion motions, as it was previously observed for a pyridiniumbased IL^{22}. Experimental separation of coherent and nuclear spinincoherent scattering allows us to estimate the limits of validity of this approach for TEATF.
As can be seen from Eq. 7 comparison between the completely protonated and paritally deuterated samples enables evaluation of a more complete set of parameters. The corresponding temperature dependencies are presented in Fig. 3. The longrange process of TEATF and TEA_{D}TF is characterized by the same value of the selfdiffusion coefficient, D_{tr}, suggesting that the isotope effect is minimal for the transport properties in the liquid state^{14}. The obtained values of D_{tr} are in excellent agreement with the previously published data measured on the FOCUS timeofflight spectrometer at SINQ, Switzerland with a similar resolution function^{14}. The temperature dependence of D_{tr} follows the Arrhenius law \(D={D}_{0}\,\exp \,(\,\,{E}_{{\rm{A}}}/RT)\) with the activation energy \({E}_{{\rm{A}}}=15.4\pm 0.5\) kJ/mol. The existing quantitative and qualitative difference with the PFGNMR results^{26,27} is the consequence of the significantly shorter time scale probed by QENS.
Various entangled localized motions of the ethyl chains are the source of the quasielastic component characterized by the parameters R_{1}, R_{2}, and D_{ch}. After removing the coherent contribution, the diffusion coefficients turn out to be approximately twice as fast as the corresponding values obtained for the total, unseparated spectra on FOCUS^{14}. Broader linewidths and consequently faster localized dynamics were also observed for the “pure” incoherent contribution of the aprotic pyridiniumbased IL^{22}. However, in the present case the effect is more pronounced indicating a greater impact of correlated dynamics of the three ethyl chains in the triethylammonium cation.
The restricted dynamics of the NH proton discussed in our previous work^{14} are also present in the refined incoherent contribution of TEA_{D}TF, corroborating the earlier observations. The absolute values for D_{H} and R_{H} have yet changed significantly after the subtraction of the coherent component. To explain the nature of this process we addressed the DFT computations and MD simulations^{28}. A close contact between the triethylammonium cation and the triflate anion in the gasphase equilibrium corresponds to a strong hydrogen bond, the potential energy surface for the NH proton exhibiting a single minimum. Thus, it is not highly probable that the observed localized process is related to a direct proton exchange between the cation and anion. In this regard, the MD simulations can offer some insights, because the NH incoherent scattering function can be directly compared with those of the other atom groups. It should be mentioned, however, that the MD relaxation processes appear to be significantly slower as compared to the experiment. This a major drawback of nonpolarizable force fields^{28,38,39}. Moreover, while a sum of several exponents is a good approximation for the QENS intermediate scattering functions, the corresponding MD curves are decidedly “stretched”, requiring, for example, KohlrauschWilliamsWatts (KWW) functions for modeling. Under these conditions only qualitative comparison between the MD simulations and the QENS data is possible.
Figure 4 reveals a remarkable similarity between the intermediate scattering functions of the NH proton, the nitrogen atom of the triethylammonium cation and the sulfur atom of the triflate anion during the first tens of picoseconds (the corresponding paircorrelation functions remain almost constant). This timerange is roughly equivalent to the experimental one including the discussed localized dynamics of the acidic proton. It means that the fast spatially restricted component of the TEA_{D}TF incoherent spectrum may reflect the localized dynamics of the anioncation pair. According to the MD simulation, this correlated motion of the cationanion pair persists over a longer time period than the lifetime of an individual hydrogen bond between them, as can be seen from a faster decay of the oxygen intermediate scattering function.
Collective Dynamics
Although the difference between the H and D neutron coherent cross sections is less significant than that between the incoherent ones, the isotope substitution changes the sensitivity of the coherent QENS to different crosscorrelation contributions (Fig. 2). Thus, \({S}_{{\rm{coh}}}(Q,E)\) of TEA_{D}TF is mainly affected by cationcation inter and intramolecular correlated motions, while the coherent spectrum of TEATF contains all components. The influence of collective cationanion dynamics is most pronounced at the chargecharge diffraction peak (Q = 0.9 Å^{−1}).
In general two quasielastic contributions are required to describe the coherent spectra of both samples (Eq. 9, Fig. S8). The slower relaxation process is related to the longrange ion transport as can be seen from Fig. 5, where the linewidths \({{\rm{\Gamma }}}_{{\rm{tr}}}^{{\rm{coh}}}(Q)\) are compared with the Qdependence calculated from the jumpdiffusion model (Eq. 5) for the incoherent spectrum. The typical narrowing of the quasielastic lines at the diffraction correlation peaks, referred to as de Gennes narrowing^{40}, can be seen for both TEATF and TEA_{D}TF. This effect literally means that there exist longlived local arrangements of ions diffusing collectively on the picosecond time scale. The strength of the line modulation is comparable for both samples within experimental errors suggesting that the catcat, anan, and catan crosscorrelation diffusional components are characterized by similar or close relaxation times.
In contrast to the previously studied case of the pyridiniumbased IL^{22} \({S}_{{\rm{coh}}}(Q,E)\) of TEATF exhibits the second broader component, which allowed us to characterize the loss of coherence through localized dynamics. Fast intramolecular motions determine the shape of the QENS spectra in the energy transfer range of ~[−1, 1] meV (Fig. 6) and the observed effect strongly depends on the type of dominating correlations (catcat, catan, anan), which can be highlighted by means of deuterium labeling. The dynamics of the ethyl chains (carbon and deuterium atoms) in TEA_{D}TF are mainly observed at the adjacency peak (1.3–1.8 Å^{−1}). In this Qrange it is, therefore, possible to formally apply the Gaussian model as in the case of the incoherent spectra (Eq. 6). The estimates of the effective diffusion coefficient are very close to the D_{ch}values (Fig. 3a) and the characteristic confinement size is of order R_{1} (Fig. 3c). The quasielastic linewidths of the totally protonated sample are narrower due to the catan and anan contributions, which slow down the average relaxation time. Thus, the coherence in local ion arrangements may be maintained despite fast stochastic intramolecular motions. This effect may arise from the stronger interaction between ions due to the hydrogen bond in the PIL, as well as it may depend on the ion size and shape. For example, in regard to internal dynamics the structures of the bis(trifluoromethylsulfonyl)imide anion and 1butylpyridinum cation allow more degrees of freedom. As a result, the coherent contribution of this aprotic IL did not feature the second broader component^{22} seen in the present case. The chargecharge correlation peak at Q = 0.9 Å^{−1} corresponds to the length scale longer than that of the adjacency peak. It inevitably leads to a faster decay of the negative catan crosscorrelation terms due to localized internal motions and, consequently, to a significantly broader QENS spectrum in the energy transfer range of 1 meV of TEATF as compared to TEA_{D}TF (Fig. 6a). Moreover, the observed difference may also originate from overall faster anion relaxation, because mainly the anan component forms the chargecharge correlation peak of TEATF (Fig. 2).
Conclusion
In summary, an extensive understanding of picosecond dynamics of the model protic ionic liquid TEATF has been achieved by means of a synergistic approach combining QENS with the polarization analysis and MD simulations. The experimental separation of coherent and nuclear spinincoherent scattering permits a sophisticated description of collective and singleparticle processes, while the MD analysis provides the tools to disentangle crosscorrelation terms between selected groups of atoms.
Longrange diffusion as well as spatially restricted dynamics of the ethyl chains and the acidic proton have been characterized for the refined nuclear spinincoherent spectra. Although the subtraction of the coherent contribution leads to significant changes in the estimates of the diffusion coefficients for localized dynamics, the qualitative picture of molecular motions seen with QENS on the picosecond time scale remains the same as has been previously inferred from the total dynamic structure factor. The enhanced localized dynamics of the acidic proton have been observed as well. Based on the analysis of the MD trajectories we assume that this process reflects the spatially restricted dynamics of cationanion pairs in the liquid state.
The coherent QENS spectra have provided evidence of highly correlated picosecond motions in TEATF. The longrange diffusion can be considered as a collective process of ion associations. Their characteristic size is at least as large as the chargecharge periodicity of the PIL structure. Owing to the strong interaction between adjacent ions, the localized dynamics also turn out to be partially of collective nature, but motion coherence becomes gradually less significant at larger distances. Correlated longrange diffusion of ions on the picosecond time scale appears to be a common feature of both aprotic and protic ILs, whereas the nature of spatially restricted dynamics strongly depends on the ion structure and interaction between the ions. A complex interplay of singleparticle and collective motions underlies the dynamical heterogeneity of ILs and accounts for their time scaledependent transport characteristics.
Materials and Methods
Samples
The sample of TEATF ([NH(C_{2}H_{5})_{3}][SO_{3}CF_{3}]) and its partially deuterated analogue TEA_{D}TF ([NH(C_{2}D_{5})_{3}][SO_{3}CF_{3}]) were synthesized and characterized at the Department of Physical Chemistry, Saarland University (Sample Synthesis and NMR Characterization of the Supporting Information). The neutron scattering (coherent and incoherent) and absorption cross sections for the cation and for the anion are summarized in Table S1 of the Supporting Information.
QENS experiment
QENS experiments^{41} in conjunction with polarization analysis were conducted on the D7 diffuse scattering spectrometer^{30} with the wavelength of incident neutrons of 5.7 Å covering the Qvalues in the range of 0.3–2.0 Å^{−1}. The measurements were performed both in the diffraction and timeofflight modes. The polarization efficiency of the instrument was determined by measuring an amorphous quartz standard. The efficiency of the detectors was calibrated by measuring a vanadium standard. The frequency of the Fermi chopper used in the timeofflight mode was equal to 145 Hz, providing the resolution function of 98 μeV (FWHM)^{42}. The low temperature incoherent TEATF spectra (T = 10 K) were used for the estimation of the linewidth of the instrument resolution. In order to minimize absorption and multiple scattering effects, an annular hollow cylindrical sample holder made of aluminum was used. The distance between the inner and outer cylinder was equal to 0.20 mm. Such a sample thickness guaranteed that neutron beam transmission through the sample exceeded 90%.
The standard data reduction of the D7 spectra was performed in the LAMP software package^{43}. The raw data were corrected for empty cell, cryostat and timeindependent (ambient neutrons/electronic noise) background contributions, sample geometry dependent selfattenuation and detector efficiency, converted to energy scale and finally binned into several Qgroups with ΔQ = 0.1 Å^{−1} to ensure adequate data statistics. After the separation of the coherent and nuclear spinincoherent scattering^{22,30}, simultaneous fitting in the (E, Q)domain was performed in a program module^{44} based on the MPfit procedure^{45}.
MD analysis
The details of the MD simulation are presented in our previous publication^{28}. The comparison of the MD trajectories with respect to the neutron scattering experiment was carried out using the nMoldyn/MDANSE software^{31}. In particular, weighted incoherent and coherent intermediate scattering functions were calculated for all the particles in the simulation as well as for selected groups of atoms (bridging methylene groups, terminal methyl groups, NH proton etc). The weights of the terms are defined from the neutron scattering lengths and proportional to \({b}_{\alpha ,{\rm{inc}}}^{2}\) and \({b}_{\alpha ,{\rm{coh}}}{b}_{\beta ,{\rm{coh}}}\) for the incoherent and coherent contributions, respectively.
Data Availability
The datasets generated and analysed during the current study are available from the corresponding author on request.
References
Greaves, T. L. & Drummond, C. J. Protic ionic liquids: Evolving structure–property relationships and expanding applications. Chem. Rev. 115, 11379–11448 (2015).
Hayes, R., Warr, G. G. & Atkin, R. Structure and nanostructure in ionic liquids. Chem. Rev. 115, 6357–6426 (2015).
Araque, J. C., Hettige, J. J. & Margulis, C. J. Modern room temperature ionic liquids, a simple guide to understanding their structure and how it may relate to dynamics. J. Phys. Chem. B 119, 12727–12740 (2015).
Shen, Y. et al. Protic ionic liquids with fluorous anions: Physicochemical properties and selfassembly nanostructure. Phys. Chem. Chem. Phys. 14, 7981–7992 (2012).
Murphy, T., Varela, L. M., Webber, G. B., Warr, G. G. & Atkin, R. Nanostructure–thermal conductivity relationships in protic ionic liquids. J. Phys. Chem. B 118, 12017–12024 (2014).
Castner, E. W. & Wishart, J. F. Spotlight on ionic liquids. J. Chem. Phys. 132, 120901 (2010).
Weingärtner, H. NMR studies of ionic liquids: Structure and dynamics. Curr. Opin. Colloid Interface Sci. 18, 183–189 (2013).
Yaghini, N., GomezGonzalez, V., Varela, L. M. & Martinelli, A. Structural origin of proton mobility in a protic ionic liquid/imidazole mixture: Insights from computational and experimental results. Phys. Chem. Chem. Phys. 18, 23195–23206 (2016).
Khudozhitkov, A. E. et al. Dynamical heterogeneities in ionic liquids as revealed from deuteron NMR. Chem. Commun. 54, 3098–3101 (2018).
Hunger, J. & Buchner, R. Femto to Nanosecond Dynamics in Ionic Liquids: From Single Molecules to Collective Motions, (Springer International Publishing, Cham, 53–71 2016).
Pandey, S., Baker, S. N., Pandey, S. & Baker, G. A. Fluorescent probe studies of polarity and solvation within room temperature ionic liquids: A review. J. Fluoresc. 22, 1313–1343 (2012).
Bée, M. Quasielastic Neutron Scattering, Principles and Applications in Solid State Chemistry, Biology and Materials Science. (Adam Hilger, Bristol, 1988).
Russina, O. & Triolo, A. Chapter 4  Ionic liquids and neutron scattering. In FernandezAlonso, F. & Price, D. L. (eds) Neutron Scattering  Applications in Biology, Chemistry, and Materials Science, vol. 49 of Experimental Methods in the Physical Sciences, 213–278 (Academic Press, 2017).
Burankova, T. et al. Proton diffusivity in the protic ionic liquid triethylammonium triflate probed by quasielastic neutron scattering. J. Phys. Chem. B 119, 10643–10651 (2015).
Hoarfrost, M. L., Tyagi, M., Segalman, R. A. & Reimer, J. A. Proton hopping and longrange transport in the protic ionic liquid [Im][TFSI], probed by pulsedfield gradient NMR and quasielastic neutron scattering. J. Phys. Chem. B 116, 8201–8209 (2012).
Mamontov, E., Luo, H. & Dai, S. Proton dynamics in N,N,N’,N’tetramethylguanidinium bis(perfluoroethylsulfonyl)imide protic ionic liquid probed by quasielastic neutron scattering. J. Phys. Chem. B 113, 159–169 (2009).
Embs, J. P., Burankova, T., Reichert, E. & Hempelmann, R. Cation dynamics in the pyridinium based ionic liquid 1nbutylpyridinium bis((trifluoromethyl)sulfonyl) as seen by quasielastic neutron scattering. J. Phys. Chem. B 116, 13265–13271 (2012).
Burankova, T., Simeoni, G., Hempelmann, R., Mora Cardozo, J. F. & Embs, J. P. Dynamic heterogeneity and flexibility of the alkyl chain in pyridiniumbased ionic liquids. J. Phys. Chem. B 121, 240–249 (2017).
Yamamuro, O., Yamada, T., Kofu, M., Nakakoshi, M. & Nagao, M. Hierarchical structure and dynamics of an ionic liquid 1octyl3methylimidazolium chloride. J. Chem. Phys. 135, 054508 (2011).
Kofu, M. et al. Heterogeneous slow dynamics of imidazoliumbased ionic liquids studied by neutron spin echo. J. Phys. Chem. B 117, 2773–2781 (2013).
Gaspar, A. M. et al. Using polarization analysis to separate the coherent and incoherent scattering from protein samples. Biochimica et Biophysica Acta (BBA)  Proteins and Proteomics 1804, 76–82 (2010).
Burankova, T., Hempelmann, R., Wildes, A. & Embs, J. P. Collective ion diffusion and localized single particle dynamics in pyridiniumbased ionic liquids. J. Phys. Chem. B 118, 14452–14460 (2014).
Paternò, G. M., Stewart, J. R., Wildes, A., Cacialli, F. & Sakai, V. G. Neutron polarisation analysis of polymer:fullerene blends for organic photovoltaics. Polymer 105, 407–413 (2016).
Aoun, B. et al. Translational and reorientational dynamics of an imidazoliumbased ionic liquid. J. Phys. Chem. Lett. 1, 2503–2507 (2010).
Ferdeghini, F. et al. Nanostructuration of ionic liquids: impact on the cation mobility. a multiscale study. Nanoscale 9, 1901–1908 (2017).
Iojoiu, C. et al. PILsbased Nafion membranes: a route to hightemperature PEFMCs dedicated to electric and hybrid vehicles. Polym. Adv. Technol. 19, 1406–1414 (2008).
Iojoiu, C. et al. Ionic liquids and their hosting by polymers for HTPEMFC membranes. Fuel Cells 10, 778–789 (2010).
Mora Cardozo, J. F., Burankova, T., Embs, J. P., Benedetto, A. & Ballone, P. Density functional computations and molecular dynamics simulations of the triethylammonium triflate protic ionic liquid. J. Phys. Chem. B 121, 11410–11423 (2017).
Moon, R. M., Riste, T. & Koehler, W. C. Polarization analysis of thermalneutron scattering. Phys. Rev. 181, 920–931 (1969).
Stewart, J. R. et al. Disordered materials studied using neutron polarization analysis on the multidetector spectrometer, D7. J. Appl. Crystallogr. 42, 69–84 (2009).
Goret, G., Aoun, B. & Pellegrini, E. MDANSE: An interactive analysis environment for molecular dynamics simulations. J. Chem. Inf. Model. 57, 1–5 (2017).
Singwi, K. S. & Sjölander, A. Diffusive motions in water and cold neutron scattering. Phys. Rev. 119, 863–871 (1960).
Volino, F., Perrin, J.C. & Lyonnard, S. Gaussian model for localized translational motion: Application to incoherent neutron scattering. J. Phys. Chem. B 110, 11217–11223 (2006).
Neumann, D. A. et al. Coherent quasielastic neutron scattering study of the rotational dynamics of C60 in the orientationally disordered phase. Phys. Rev. Lett. 67, 3808–3811 (1991).
Wilmer, D. et al. Anion reorientation in an ion conducting plastic crystal  coherent quasielastic neutron scattering from sodium orthophosphate. Physica B 266, 60–68 (1999).
Vineyard, G. H. Scattering of slow neutrons by a liquid. Phys. Rev. 110, 999–1010 (1958).
Sköld, K., Rowe, J. M., Ostrowski, G. & Randolph, P. D. Coherent and incoherentscattering laws of liquid argon. Phys. Rev. A: At., Mol., Opt. Phys. 6, 1107–1131 (1972).
Sunda, A. P., Mondal, A. & Balasubramanian, S. Atomistic simulations of ammoniumbased protic ionic liquids: Steric effects on structure, low frequency vibrational modes and electrical conductivity. Phys. Chem. Chem. Phys. 17, 4625–4633 (2015).
Nasrabadi, A. T. & Gelb, L. D. Structural and transport properties of tertiary ammonium triflate ionic liquids: A molecular dynamics study. J. Phys. Chem. B 121, 1908–1921 (2017).
Gennes, P. D. Liquid dynamics and inelastic scattering of neutrons. Physica 25, 825–839 (1959).
Embs, J. P., Mora Cardozo, J. F. & Wildes, A. QENS on the protic ionic liquid triethylammonium triflate, https://doi.org/10.5291/ILLDATA.603436 (2015).
Fennell, T., ManginThro, L., Mutka, H., Nilsen, G. & Wildes, A. Wavevector and energy resolution of the polarized diffuse scattering spectrometer D7. Nucl. Instrum. Methods Phys. Res., Sect. A 857, 24–30 (2017).
Richard, D., Ferrand, M. & Kearley, G. J. Analysis and visualisation of neutronscattering data. J. Neutron Res. 4, 33–39 (1996).
Burankova, T. Dynamics and Structure of Ionic Liquids by Means of Neutron Scattering. Ph.D. thesis, http://scidok.sulb.unisaarland.de/volltexte/2014/5817/pdf/Burankova_thesis_final.pdf (Saarland University, 2014).
Markwardt, C. B. Nonlinear least squares fitting in IDL with MPFIT. ADASS XVIII 411, 251–254, http://arxiv.org/pdf/0902.2850v1 (2009).
Acknowledgements
This work was financially supported by the Swiss National Science Foundation (SNF) through the project “Influence of Hbonding on dynamical properties of Ionic Liquids” (Project Number 200021_157175). The beamtime on D7 at Institute LaueLangevin is greatfully acknowledged. The authors thank Prof. Dr. R. Hempelmann for longlasting collaboration and support, Dr. P. Ballone for performing MD simulations, fruitful discussions and valuable comments on the manuscript.
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J.P.E. conceived the experiment, supervised the project. Neutron scattering experiments, data reduction were performed by J.F.M.C., A.W. and J.P.E., T.B. analyzed the neutron data and performed the comparison with the MD output. J.F.M.C. carried out MD simulations. D.R. performed synthesis and characterization of the samples. All authors discussed and interpreted the data. The manuscript was written by T.B. All authors discussed and commented on the manuscript.
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Burankova, T., Mora Cardozo, J.F., Rauber, D. et al. Linking Structure to Dynamics in Protic Ionic Liquids: A Neutron Scattering Study of Correlated and SingleParticle Motions. Sci Rep 8, 16400 (2018). https://doi.org/10.1038/s4159801834481w
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DOI: https://doi.org/10.1038/s4159801834481w
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