Proton Diffusivity in the Protic Ionic Liquid Triethylammonium Triflate

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The Journal of Physical Chemistry

Proton Diffusivity in the Protic Ionic Liquid Triethylammonium Triflate Probed by QENS Tatsiana Burankova,†,‡ Rolf Hempelmann,† Verlaine Fossog,† Jacques Ollivier,¶ Tilo Seydel,¶ and Jan P. Embs∗,‡ †Department of Physical Chemistry, Saarland University, Saarbrücken, 66123, Germany ‡Laboratory for Neutron Scattering and Imaging, Paul Scherrer Institute, Villigen PSI, 5232, Switzerland ¶Institut Laue-Langevin, Grenoble, 38000, France E-mail: [email protected]

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Abstract Quasielastic neutron scattering (QENS) in combination with deuterium labelling allows for studying protonated ”highlighted" species and extracting detailed information about tangled stochastic processes. This approach has been applied to examine proton dynamics in the protic ionic liquid, triethylammonium triflate. The temperature range covered during the experiments (2-440 K) included two melting transitions correspondingly reflected in the global and localized dynamics of the cation. To focus on the dynamics of the acidic proton, QENS-spectra of the sample with the deuterated alkyl side chains were analysed. The remaining hydrogen atom served as a tagged particle for investigating both global long-range motion of the cation and specific dynamics of the proton, and, thus, provided insight into the transport properties of triethylammonium triflate, which is important for designing electrochemical devices. Keywords: protic ionic liquid, quasielastic neutron scattering, diffusion coefficients, phase transition, methyl group rotation, jump diffusion

Introduction In the growing and diverse field of Ionic Liquids (ILs) there is a subset of compounds referred to as Protic Ionic Liquids (PILs), 1 which are produced by combining a Brønsted acid and a Brønsted base. The main advantages of PILs arise from their anhydrous proton conductivity. The proton is transferred from the acid to the base, leading to the presence of protondonor and -acceptor sites and hence of a hydrogen-bond network. Additionally to the high thermal and electrochemical stability, the anhydrous proton conductivity of PILs turned out to be a very beneficial property for fuel cells. 2,3 Therefore, knowledge about ionic transport behaviour in PILs is a requisite for improved design and performance of electrochemical devices. Owing to deep connections between dynamical characteristics of materials, self-diffusion studies are helpful in investigating ionic transport behaviour in PILs. 4 Pulsed field gradient 2 ACS Paragon Plus Environment

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NMR technique (PFG-NMR) has proved to be a convenient tool for measuring the selfdiffusion coefficients of individual ionic species in the time-window of several milliseconds. 5 As a more specific example we will mention a work by Iojoiu, 6 where the impact of water on PILs’ properties and conduction mechanism were investigated by combining conductivity, viscosity and self-diffusion measurements. In the fully dried sample and at low water concentrations all the three species (base, anion and proton) were characterized by similar mobility, whereas the water molecules appeared to be much faster. These observations implied vehicular mechanism of proton transport by the cation. However, at higher temperatures (above 80 ◦ C), the proton migration is presented by a new intermediate value of the diffusion coefficient, so that the interplay of both Grotthuss and vehicular mechanisms can be suggested. There is, however, a well-known discrepancy between conductance values measured directly and evaluated from PFG-NMR diffusion coefficients: 7,8 electrical conductance appears to be lower than expected for fully dissociated ILs. This effect can be even more pronounced in PILs owing to the presence of H-bonding. 9 Migration of non-conducting ion configurations (ion pairs, aggregates) does not contribute to charge-transport and accounts for the observed difference in conductance. Despite the simplicity of this explanation, as it was stressed by Weingärtner, 5 there is still an open question about the dynamics of these ion associations. The possible time scale of the corresponding processes may belong to sub-ps to ps time range, which is not covered by PFG-NMR. On the other hand, this is a typical time range probed by another powerful technique for studying stochastic processes, namely quasielastic neutron scattering (QENS). 10–13 Additionally, QENS allows one to obtain spacial characteristics of observed processes, such as radii of confinement, correlation lengths or jump distances, because typical wavelengths of incident neutrons are of the order of interatomic distances in solids and liquids. Being particularly sensitive to hydrogen atoms, incoherent neutron spectroscopy is beneficial in investigating dynamical processes in hydrogen-rich materials, to which ILs can be ascribed as well, 14–17 and finally could be applied to obtain information on



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proton diffusivity in PILs. 18,19 The objective of our work was to employ QENS and deuterium labelling to explore transport properties in triethylammonium triflate (TEA-TF). This PIL is formed by a proton transfer from trifluoromethanesulfonic acid, which is considered to be one of the strongest acids, to triethylamine. TEA-TF shows promising physicochemical properties for composite proton-conducting materials used as electrolytes in high-temperature proton exchange membrane fuel cells (HT-PEMFC). 6,20,21 Our choice of the studied compound was determined by several reasons. First, there have been recent reports in the literature on its self-diffusion coefficients measured by PFG-NMR. 6,20 As the diffusion coefficients of H+ and the amine are similar and greater than that of the anion, we can consider the vehicular mechanism to be prevailing for the acidic proton transport. Nevertheless, the diffusion coefficient predicted from conductivity measurements were lower, in the same way as for other ILs, indicating a large extent of non-conducting ion associations. Second, a characterisation by means of QENS is well-suited for investigation of the dynamics of the TEA-cation, because it contains 16 hydrogen atoms, whereas the anion does not. To study the cation dynamics of the totally protonated TEA-TF on different time scales both elastic scan measurements and QENS-experiments were performed on the IN10 and IN5 spectrometers at the Institut Laue Langevin (ILL) and FOCUS at the Swiss spallation source SINQ. The temperature range considered during the experiments (2–440 K) encompassed the regions where the sample undergoes several phase transitions with the corresponding changes in the global and localized dynamics of the cation. However, if we want to focus on the acidic proton, but not on the whole cation, the main challenge is the large number of the protons in the ethyl chains of TEA, contributing to the whole QENS signal. In order to observe the dynamics of H+ selectively, we have studied a partially deuterated sample as well, where the hydrogen atoms in the ethyl chains were substituted by the deuterium atoms. In this case the incoherent signal comes from the acidic proton mainly, which serves as a tagged particle and can provide insight into the global motion of the cation and specific dynamics of this proton at the same


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time.

Experimental Details Sample In this work we have studied two samples of triethylammonium triflate (TEA-TF) (Figure 1). One sample had the totally protonated TEA-cation [(C2 H5 )3 NH], whereas the ethyl chains of the other samples were deuterated ([(C2 D5 )3 NH]). The samples were synthesized and characterized at the Chemistry Department, Saarland University as described previously. 23

Figure 1: Structure of triethylammonium triflate (TEA-TF) Table 1 summarizes the neutron scattering (coherent and incoherent) and absorption cross sections for the cation and for the anion. 24 We see that the incoherent cross section of the TF anion can be neglected. In the case of the totally protonated sample, the incoherent contribution makes up more than 90 % of the total scattering cross section. It allowed us to focus on the single particle dynamics of the TEA cation. Unfortunately, this is not the case for the partially deuterated PIL, TEAD -TF. A substantial amount of the coherent contribution will lead to the presence of correlation effects between ions of the same and opposite sign in the QENS spectra. Nevertheless, from our previous study 25 we could expect that mainly long-range diffusional processes are affected, whereas localized motions can be analysed using the same approach as for the totally protonated samples.



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Table 1: Summary of the neutron cross sections of the studied speciesa System

σscatt [b]

TF TEA TEA-TF TEAD TEAD -TF a

31.33 1357.27 1388.60 241.48 272.81

σabs [b]

σinc [b]

σcoh [b]

1.80 0.01 23.16 1284.83 24.96 1284.84 7.23 111.53 9.03 111.54

31.32 72.45 103.77 129.95 161.27

σinc σscatt

[%]

0.04 94.66 92.53 46.19 40.89

σabs is given for neutrons with the wavelength of 5.75 Å. 1 b = 10−28 m2

DSC Measurements For characterization of the studied ionic liquid, determination of the temperatures of phase transition, DSC measurements have been carried out with a Netzsch DSC 204F1 System. The measurements were performed on heating and cooling with a rate of 5–10 K/min using 20–30 mg samples encapsulated in standard Al crucibles (Figure 2). An argon stream was used during the whole experiment as a protective gas. The DSC traces of TEA-TF show two melting transitions with the temperatures unaffected by the heating rate, whereas both the crystallization and cold crystallization peaks exhibit strong dependence on the temperature history.

QENS Experiment The temperature range between 2 and 320 K was investigated on the IN10 backscattering spectrometers at ILL. The instrument provides a high energy resolution (FWHM ≈ 0.8 µeV) and a moderate wavevector transfer resolution in the Q-range of 0.5 – 2.0 Å−1 , covered by seven detectors. By choosing a suitable combination of the monochromator and the analyser crystals the wavelength of incident neutrons of λ = 6.3 Å was selected. The elastic scan measurements were carried out upon cooling and heating at a rate of 0.50 K/min. QENS measurements were conducted both on IN5 at ILL and FOCUS at SINQ. The temperature range studied at IN5 included all the melting transitions and the temperatures above them, whereas the FOCUS spectra were recorded only at the temperatures above the 6 ACS Paragon Plus Environment

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2.5

cooling, 5 K/min cooling, 10 K/min heating, 5 K/min heating, 10 K/min

2.0

heat flux [mW/mg]

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1.5 1.0 0.5 0.0 −0.5 −1.0 180

200

220

240 260 T [K]

280

300

320

Figure 2: DSC traces of TEA-TF observed on heating and cooling second melting transition. For our experiments we set the wavelengths of incident neutrons to 5.0 Å at IN5 and 5.75 Å at FOCUS. The efficiency of the detectors was calibrated by measuring a vanadium standard. The vanadium spectra were also used as the resolution functions R(Q, E) of the instruments. The applied settings of the spectrometers provided resolution linewidths (FWHM) of 90 µeV and 60 µeV at IN5 and FOCUS, respectively. For further background subtraction empty sample holder runs were performed at different temperatures accordingly. In order to minimize absorption effects, an annular hollow cylindrical sample holder made of aluminium was used in the QENS-experiments. It has an outer diameter of 14.00 mm and an inner diameter of 12.60 mm, the thickness of the walls being equal to 0.25 mm. That yielded the distance between the inner and outer cylinder of 0.20 mm. Such a sample thickness guarantees that neutron beam transmission through the sample exceeds 90 %. Thus,



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the effects of multiple scattering are negligible and unwanted absorption can be considered to be suppressed. Corrections for self-shielding effects were made at the step of data-reduction.

Data Analysis The standard data reduction of the IN5 and IN10 data was performed using the LAMP 26 software package. The DAVE 27 program was used for both the data reduction of the FOCUS spectra and further examination of all the data sets S(Q, E) at separate Q-groups. Where it was necessary to stabilize fitting results, the simultaneous analysis of 2D-scattering maps was carried out directly in the IDL-environment (http://www.exelisvis.com/ProductsServices/ IDL/Language.aspx) using the MPfit procedure. 28 The data analysis included fitting of the QENS spectra of TEA-TF in the liquid phase and fitting of both the QENS spectra and the temperature dependence of elastic intensity in the solid phase. Using a phenomenological approach, QENS spectra of the solid phase can be satisfactorily presented as a combination of a delta-function δ(E) and a Lorentzian function L(E, Γ) 18,29 S(Q, E) = Is (Q) {A0 (Q)δ(E) + [1 − A0 (Q)] L(E, Γ)}

(1)

where Q and E are the scattering vector and the energy transfer, respectively, Γ is the linewidth of the quasielastic contribution (half-width at half maximum, HWHM), Is (Q) is the intensity factor containing the Debye-Waller factor, A0 (Q) stands for the elastic incoherent structure factor (EISF) and provides insight into the geometry of the spatial confinement, in which the localized relaxation of functional groups occurs. 30 Equation 1 can also be applied for the analysis of the elastic scans. Under the condition that the harmonic approximation may be used for the Debye-Waller factor and the resolution function can be presented by a Lorentzian, the dependence of the elastic intensity on temperature reads: 31


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CT 2 ln(Iel ) = − Q + ln 3

A0 (Q) [1 − A0 (Q)] + πΓres π(Γ + Γres )

(2)

where Γres is the HWHM of the resolution function of the instrument, C is the temperature coefficient of the mean squared displacement (hu2 i = CT ). In the liquid phase the delta function is substituted by a narrow Lorentzian curve L(E, Γglob ), which accounts for unrestricted diffusional motion: S(Q, E) = Il (Q) {A0 (Q)L(E, Γglob ) + [1 − A0 (Q)] L(E, Γ + Γglob )}

(3)

where Il (Q) is the intensity factor for the liquid phase. Instead of using several Lorentzian contributions, it is advantageous to apply analytical models for both localized and long-range motions explicitly. First, this gives a direct access to relevant parameters describing studied systems. Second, the linewidths and the amplitudes of quasielastic contributions have well-defined dependencies on the scattering vector Q. Thus, fitting of a whole 2D-spectrum S(Q, E) can be performed, if E and Q are treated as independent variables at the same time. Considering possible microscopic motions, we used the jump diffusion model 32 proposed by Singwi and Sjölander to describe the unrestricted translational motion. In this case the Γglob (Q) in eq 3 reads:

Γglob (Q) =

h ¯ DQ2 1 + DQ2 τ0

(4)

where D is the diffusion coefficient and τ0 is the residence time. The Gaussian model 33 was applied to describe the localized dynamics of the particles in the confinement with a “soft” boundary: " SG (Q, E) = e

−Q2 σ 2

δ(E) +

∞ X (Q2 σ 2 )n 1 n=1

n!

h ¯ nDloc /σ 2 π (¯ hnDloc /σ 2 )2 + E 2

# (5)

where Dloc stands for the diffusion coefficient of the localized motion and σ characterizes the



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size of the domain, in which the particles are diffusing. Mathematically, σ 2 is the variance of a centred Gaussian variable, which characterizes the displacement of a particle. Localized dynamics of methyl groups at low temperatures can be also envisaged and modelled as rotational diffusion on the surface of a sphere 10,11,34

SR (Q, E) =j02 (QR)δ(E)+ ∞ X k(k + 1) 6τ¯hR 1 2 + (2k + 1)jk (QR) 2 π k=1 k(k + 1) 6τ¯hR + E 2

(6)

where R is the rotational radius, jk (x) is the k -th order spherical Bessel function and τR stands for the rotational relaxation time. It is worth mentioning why the different models were applied for the solid and liquid phase. The analysis of individual Q-groups basically can be performed by using both of them. However, the Gaussian model yielded the fit parameters, which are Q-dependent at lower temperatures. The same was the case for the rotational diffusion model at higher temperatures. To put it differently, the 2D-fitting routine could not be performed with the lowest possible number of free parameters. Therefore, we had to exclude either the Gaussian or rotation diffusion model at lower and higher temperatures, respectively.

Results and Discussion Dynamics in the Solid Phase A time-efficient way to observe the onset or the cessation of dynamic processes in a wide temperature range is to perform elastic scan measurements. The temperature dependence of the elastic intensity obtained for TEA-TF on IN10 on cooling and heating is shown in Figure 3. The abrupt decrease on heating at T =306 K and T =230 K agrees with the phase transition temperatures measured by DSC and is presented in Figure 2. As opposed to the 10 ACS Paragon Plus Environment

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heating regime, the temperatures T =260 K and T =213 K, at which the elastic intensity suddenly increases on cooling, are quite different from those obtained by the DSC-method, because the start of the phase transition in the IL is very sensitive to the cooling rate. To achieve a proper sample thermalisation and satisfactory scattering statistics, the elastic scan measurements could not be carried out at a higher rate than 0.50 K/min, which is much less than a usual experimental setting in a DSC-measurement. The gradual drop of the elastic intensity between 50 and 200 K resembles analogous dependencies of other compounds with end methyl groups, which undergo thermally activated rotation. 34–36 The temperature dependence of the relaxation time is then expressed in terms of Arrhenius’ law, τR = τR0 exp(Ea /RT ) with the activation energy Ea . Combining eq 2 and 6, it is possible to derive a final formula for the quantitative analysis of the drop of the elastically scattered intensity

" ( )# ∞ 2 X (2k + 1)j (QR)Γ CT 2 res k Q +ln 1 − pmob + pmob · j02 (QR) + (7) ln Iel (T, Q) = A− k(k+1)¯ h 3 Γ + res k=1 6τR

pmob is the fraction of mobile hydrogen atoms in the end methyl groups, A is an arbitrary constant arising from possibly different scaling factors of elastic scan curves. The fits were carried out at Q=1.85 Å−1 , as local restricted motions are best probed at larger Q-values. Additionally, the influence of coherent scattering is minimal at Q=1.85 Å−1 in the whole analysed temperature range (Figure 4). The values of the evaluated parameters corroborated the idea about thermally activated methyl group rotations. The fraction of mobile hydrogen atoms pmob turned out to equal 0.58. The number of protons in the end methyl groups of TEA is nine in total. If to assume that only their rotation causes gradual decrease of the elastic intensity in the temperature range of 50 to 215 K, before the first melting transition occurs, then pmob would be equal to 9/16 in the ideal case, which is close to the calculated value. The radius of the sphere is also in a good agreement with the length of the C–H bond



in the methyl group (R=1.16 Å), the activation energy being equal to 9.5 kJ/mol.

1.0

0.8

0.6 I/I0

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0.4

0.2

cooling heating rotational diffusion

0.0 0

50

100

150 T [K]

200

250

300

Figure 3: Temperature dependence of the elastically scattered intensity from TEA-TF measured on cooling and heating at Q=1.85 Å−1 . The dashed line represents the result of the fit to the rotational diffusion model (eq 7) The slope of the curves (Figure 3) between the two melting points is steeper than in the near vicinity of the first phase transition below 225 K. This shows that the localized dynamics becomes considerably faster after the sample undergoes melting. In this temperature range the quasielastic contribution is already intense enough to be observed by spectrometers with much coarser resolution. Therefore we analysed this faster localized process on IN5 with the wavelength of incident neutrons of 5.0 Å. Taking into account the fraction of mobile particles, the low-temperature QENS spectra of TEA-TF were fitted to the following scattering law

SI (Q, E) = Is (Q) · ((1 − pmob )δ(E) + pmob SR (Q, E))


(8)

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where SR (Q, E) is presented by eq 6. The evaluated parameter values are listed in Table 2. Although the considered process can be also described in terms of rotational diffusion, it is about ten times faster than the previously discussed thermally activated methyl group rotations. In contrast to the lower temperatures, the relaxation time τR remains approximately constant (∼ 4 ps), whereas the radius of the sphere R increases gradually from 1.07 Å to 1.47 Å, indicating that the carbon atoms attached to the central nitrogen atom become more mobile and the whole alkyl chain becomes more flexible. In the same temperature range the QENS-broadening of the partially deuterated TEA is suppressed and very weak to carry out a fitting procedure, giving another experimental indication that the considered process arises from the alkyl-chains. Table 2: Fit results of the QENS spectra of TEA-TF in the temperature range from 250 to 290 K according to eq 8 T, K 250.0 270.0 280.0 290.0

R, Å 1.07±0.05 1.16±0.05 1.13±0.04 1.47±0.05

pmob 0.53±0.03 0.55±0.02 0.55±0.02 0.58±0.02

τR , ps 3.8±0.2 3.9±0.2 3.7±0.2 3.9±0.2

The two successive endothermic phase transitions are ascribed to melting of pure PIL (∼229 K) and water-PIL domains (∼301 K). 37 A trace amount of water is present all the time in IL samples. However, the QENS experiments cannot corroborate this statement. First, the spectra at the temperature higher than that of the first melting transition do not exhibit enhanced unrestricted dynamics of the cation compared to the range below the first melting point. Second, if to compare the integrated intensities within the energy window of [-2.0,2.0] meV, we see that the patterns resemble spectra of a solid characterized by narrow sharp Bragg peaks. When the first melting occurs, their position changes and the redistribution of intensity is observed, but only with the second melting transition all the peaks are smeared and the integrated intensity exhibits wide maxima typical for molten salts in the liquid state (Figure 4).



3.0

T=220 K T=280 K T=320 K

2.5

I(Q) [arb.un]

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2.0

1.5

1.0

0.5 0.5

1.0

˚ −1 ] Q [A

1.5

2.0

Figure 4: Integrated intensity of the partially deuterated sample TEAD -TF measured on IN5 on heating

Dynamics in the Liquid Phase Above the second melting transition the prominent elastic contribution disappears and a quasielastic broadening can be observed. The most surprising fact is that the spectra not only of the protonated but also of the partially deuterated sample (TEAD -TF) are described by two Lorentzian components (eq 3, Figure 5). As only one proton, playing a role of a tagged particle, is present in the structure of the cation, one would expect that one single Lorentzian related to the global diffusion of the cation is sufficient to capture the profile of the spectra. However, it is important to keep in mind that the analysis of the TEAD -TF data is not straightforward as in the case of the totally protonated sample. The reason for this it that


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100

total L(E, Γglob )

I [arb. un.]

80

L(E, Γ + Γglob )

60 40 20 0 100

total L(E, Γglob )

80 I [arb. un.]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


L(E, Γ + Γglob )

60 40 20 0 −1.0

−0.5

0.0 E [meV]

0.5

1.0

Figure 5: QENS spectrum of the completely protonated TEA-TF sample (upper panel) and partially deuterated TEAD -TF (lower panel), measured at 360 K and Q = 1.75 Å−1 (symbols) together with the fit function (blue solid line). The dotted and dashed lines represent Lorentzian functions, of which the solid line is composed.



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the spectra of the partially deuterated IL exhibit a substantial coherent component from both the cation and the anion (Table 1). The incoherent scattering from the 15 deuterium atoms is also comparable with the contribution from one proton. If we assume that the total scattering of the protonated sample SH (Q, E) is dominated by the incoherent scattering from the ethyl-chains, then the incoherent contribution from the 15 deuterium atoms can be excluded from consideration by subtracting as:

SDcorr (Q, E) = SD (Q, E) −

σD SH (Q, E) σH

(9)

where SD (Q, E) is the experimentally obtained dynamical structure factor of the partially deuterated sample, σH and σD are the incoherent cross sections of the hydrogen and the deuterium, respectively. Nevertheless, this procedure did not remove the broader QENScontribution, which means that the N-H proton performs a sort of localized motion on the time scale of the instruments. Subtraction of the incoherent scattering from the deuterated ethyl chains unfortunately does not allow eliminating the coherent scattering as well, but our recent experiment on separation of coherent and incoherent contributions indicated that interference effects are crucial mainly for the slower component, which accounts for the long-range diffusion. 25 For the further analysis of stochastic motions in TEA-TF several groups of “equivalent” hydrogen atoms can be considered (Figure 6): the single N-H proton (RH ), the six proton of the ethyl chains closest to the nitrogen atom (R2 ) and the nine protons of the methyl end–groups (R1 ).


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Figure 6: Structure of the TEA cation and sketch of different possible radii of confinement.

Taking into account the possible influence of the coherent contribution, it was decided not to include any explicit model for the slower process (Γglob ) in the case of TEAD -TF and to choose the Gaussian model (eq 5) for describing the localized dynamics of the single proton. The scattering law used for the fits of the partially deuterated sample reads:

SDcorr (Q, E) = Il (Q) ·

1 Γglob ⊗ SG (Q, E; RH , DH ) π Γ2glob + E 2

(10)

where DH is the diffusion coefficient characterising the localized dynamics of the proton. In the case of the totally protonated sample it was possible to perform the fitting routine including the jump-diffusion model (Eq. 4) explicitly for the long-range diffusion (Γglob ), while the dynamic structure factor for the localized dynamics contains components for each group of the protons:

1 SG (Q, E; RH , DH )+ 16 6 9 + SG (Q, E; R1 , Dloc ) + SG (Q, E; R2 , Dloc ) 16 16

1 Γglob SI (Q, E) = Il (Q) · ⊗ π Γ2glob + E 2

(11)

The fitting routine, however, does not permit to estimate all the variables in eq 11 reliably, as the contribution of the N-H proton is much weaker than that of the other 15 hydrogen 17 ACS Paragon Plus Environment


atoms. To circumvent this, RH and DH were fixed to the values calculated from the TEAD TF spectra. The results of the fits according to eq 10–11 are summarized in Tables 3, 4 and graphically presented in Figures 7, 8, and 9. The linewidths of the Lorentzian curve attributed to the long-range diffusion are depicted in Figure 7 as a function of Q. The linewidth of the protonated sample follows the Q2 law at smaller Q-values, whereas a deviation from this law is observed in the higher Qregion. The linewidth of the partially deuterated sample features so called “de Gennes line narrowing” 11 (modulations of the linewidth at those Q-values, where there are the maxima of the broad lines in the Il (Q) dependence), indicating the presence of a relatively large coherent contribution. The “depth” of this modulation depends both on the temperature and the instrumental resolution function. The coherent contribution determines this behaviour; the shorter the observation time-scale is, the more visible interference effects are in the

0.04

T = 330 K T = 350 K T = 370 K T = 390 K

0.03 Γglob [meV]

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0.02

0.01

0.00 0.0

0.4

0.8 ˚ −1 ] Q [A

1.2

1.6

Figure 7: Comparison of the fit results for the linewidth of the slow diffusional process obtained for the completely protonated (dashed lines) and partially deuterated samples (dots) on FOCUS. Owing to the coherent contribution the long-range translational process in TEAD -TF is slower at those Q-values which correspond to the location of the maxima in S(Q). 18 ACS Paragon Plus Environment

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D [×10−10 m2 /s]

FOCUS IN5 10

D DH Dloc

1 2.2

2.4

2.6

2.8 1000/T [K−1 ]

3.0

3.2

Figure 8: Arrhenius plots of the diffusion coefficients describing the long-range diffusion and the localized dynamics of the TEA-cation. The open symbols denote the parameters evaluated from the FOCUS data; the solid symbols represent the IN5 results. The dashed and solid lines are the fits according to eq 12 1.2

1.0

0.8 ˚ R [A]

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0.6

0.4

0.2

0.0

RH R1 R2

FOCUS IN5 320

360

400

440

T [K]

Figure 9: Temperature dependencies of the confinement dimension for the three proton groups of the TEA cation. The open symbols denote the parameters evaluated from the FOCUS data; the solid symbols represent the IN5 results. The solid lines are guides to the eye. 19 ACS Paragon Plus Environment


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Table 3: Temperature dependence of the diffusion parameters in the liquid state (Diffusion coefficients for the localized and long-range motion and residence time) long-range process

T [K]

−10

D · 10

2

[m /s]

τ0 [ps]

310 330 350 370 390

1.43±0.01 2.16±0.02 2.91±0.03 4.00±0.04 5.10±0.05

21.8±0.4 11.3±0.2 10.8±0.2 10.2±0.1 9.1±0.1

320 360 400 440

1.76±0.02 4.30±0.13 8.13±0.11 12.53±0.11

12.0±0.6 10.9±0.6 8.8±0.6 6.3±0.3

localized motions −10

DH · 10 [m2 /s] FOCUS 2.79±0.02 3.69±0.02 5.29±0.02 6.48±0.02 7.26±0.03 IN5 5.31±0.05 7.55±0.06 10.23±0.07 13.15±0.10

Dloc · 10−10 [m2 /s] 7.65±0.03 10.93±0.06 12.02±0.06 13.41±0.04 14.93±0.04 10.18±0.03 12.19±0.03 16.65±0.03 20.73±0.05

long-range diffusion of the TEA cation. For some PILs the diffusivity of the N-H proton is considerably facilitated in comparison with that of the cation through the Grotthus mechanism (proton hopping through the hydrogen bond network). For example, the interplay of the vehicular and Grotthus mechanisms has been studied for [Im][TFSI] experimentally by means of QENS and PFG-NMR. 19 In this work the linewidths of the QENS spectra were analysed in terms of the jump diffusion model (eq 4), the residence time τ0 serving as a measure of the time between proton transfer events. Using these techniques it was demonstrated that the proton hopping is temperature dependent and becomes less efficient with increase in temperature, because the number of hydrogen bonds and their lifetime decrease. However, in the case of the TEAD cation we could not proceed in the similar way. The residence time determines deviation from the Q2 -law at high Q-values, where the presence of the coherent contribution is significant for the partially deuterated sample. Moreover, the “apparent” diffusion coefficient of the N-H proton would be even less than for the whole cation because of the coherent contribution. Although we do not observe any enhanced proton dynamics in the slower process, the spectra of TEAD -TF exhibit a broader Lorentzian contribution and, hence, point out to the


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Table 4: Temperature dependence of the radii of confinement T [K]

RH [Å]

310 330 350 370 390

0.340±0.005 0.386±0.005 0.444±0.005 0.485±0.006 0.533±0.007

320 360 400 440

0.403±0.002 0.492±0.002 0.567±0.003 0.620±0.003

R1 [Å] FOCUS 0.784±0.003 0.954±0.004 1.016±0.006 1.061±0.007 1.125±0.009 IN5 0.822±0.002 0.865±0.003 0.877±0.008 0.876±0.002

R2 [Å] 0.000±0.000 0.338±0.003 0.414±0.003 0.481±0.003 0.537±0.003 0.384±0.001 0.529±0.007 0.699±0.006 0.876±0.002

existence of an additional faster process of the N-H proton. So, this hydrogen atom is not tightly connected to the core of the cation and has additional degrees of freedom as compared to the translational motion of the cation as a whole. The origin of this broad component in the spectra of TEA-TF will be discussed later. The Gaussian model (eq 5, 10) applied for the analysis of the broader QENS-component of the TEAD -TF spectra yielded the results displayed in Figures 8–9. For illustrative purposes the fit parameters of the long-range diffusion and the localized dynamics of the ethyl chains are plotted on the graphs as well. The diffusion coefficients depend on the instrumental resolution function, owing to two reasons that may come into interplay. First, very narrow Lorentzian contributions cannot be reliably resolved if the resolution function is much broader, or vice versa broader contributions cannot be fitted if the available dynamical range is too narrow. Although the linewidths of the IN5 and FOCUS resolution functions are not so different, we could see this effect at smaller Q-values, especially for the partially deuterated sample. The presence of substantial coherent contribution compels us to evaluate the linewithds of the slow process for every Q-group individually, increasing the inaccuracy in determination of the faster component at the same time. Second, the dynamics of ILs proved to be decidedly heterogeneous; the relaxation processes in ILs have in principle non-Debye behaviour. By altering the linewidth of the instrumental resolution function one adjusts the



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“exposition time”, during which the information about particle motions is acquired. 38 It is evident that one Lorentzian (or single exponential decay) for the description, for example, of the long-range diffusion is only an approximation valid for a certain accessible time-scale. That is why, the intermediate scattering function is quite often fitted with the stretched exponential function in the (Q, t)-domain. 39,40 The restricted motion of the N-H proton is found to be mostly affected by the above mentioned factor, while the IN5 and FOCUS diffusion coefficients characterising the confined motions of the alkyl chains are pretty similar. In the latter case the resolution effect becomes apparent in the temperature dependence of the confinement size (Figure 9). The difference in the radii of the inner and outer hydrogen atoms is less for the IN5 data than for FOCUS; R1 and R2 even tend to one value at elevated temperatures. The interplay of several localized motions occurring on close but different time scales (chain librations, torsional rotations) may lead to this sort of result. If the “exposition” is less than the mean time between distinct conformational states of the chains then the proton is detected around more or less the same position; while the observation time, which is greater than the relaxation time of torsional rotations, would allow capturing the proton dynamics over a larger volume. The temperature dependence of all the diffusion coefficients can be modelled by Arrhenius’ equation

D = D0 exp{−Ea /RT }

(12)

where Ea is the activation energy, R=8.31 J/(mol·K) denotes the gas constant (Table 5, Figure 8). It is worth mentioning that the diffusion coefficients of TEA-TF measured by PFG-NMR follow Vogel-Fulcher-Tamman equation. 20 The absolute values of the QENS and PFG-NMR diffusion coefficients are also different. This can be explained by the different time scales, on which the two experimental methods operate. As it was demonstrated by numerous molecular dynamics simulations, 41 the gradient of mean square displacement (MSD) curves changes with observation time, causing different regimes of dynamics. With QENS one gets 22 ACS Paragon Plus Environment

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the short-time (on the picosecond time scale) diffusion coefficient, whereas PFG-NMR probes the long-time diffusion coefficient on the millisecond time scale. Table 5: Best fit parameters of the temperature dependence of the diffusion coefficients characterising the TEA dynamics to the Arrhenius equation (eq 12) FOCUS long-range diffusion (D) restricted motion of the alkyl chains (Dloc ) restricted diffusion of the N-H proton (DH ) IN5 long-range diffusion (D) restricted motion of the alkyl chains (Dloc ) restricted diffusion of the N-H proton (DH )

D0 · 10−8 , m2 /s Ea , kJ/mol 7.28±0.15 1.81±0.02 3.25±0.04

16.04±0.06 8.04±0.03 12.18±0.04

24.3±0.5 1.45±0.01 1.50±0.03

19.18±0.07 7.20±0.02 8.90±0.05

There remains an open question about the origin of the dynamical process related to the second broad contribution in the spectra of the partially deuterated sample. Several explanations are possible. It seems reasonable that owing to the chain librations and recoil effect the centre of the cation performs a sort of localized motion. But there are some facts that do not corroborate this assumption. First, the diffusion coefficients Dloc and DH are different. The DH values lie even closer to D, which describes the long-range diffusion. Second, although the radii of confinement are similar to the radii of the inner protons belonging to the ethyl chains at room temperature, their temperature dependence is decidedly different. Moreover, at T =310 K near to the phase transition the alkyl chains are partially “frozen”, which is manifested by R2 =0.0 Å (Table 4), whereas the localized process can still be observed for the N-H proton. The second plausible explanation would be that this spatially restricted process and the QENS-broadening associated with it is just a remainder of the global diffusion, which cannot be satisfactorily presented by a single Lorentzian curve and is not visible for the totally protonated sample because of the other quasielastic contributions. This argument can be disproved as well. The trace of the discussed broadening is detectable even at the temperatures lower than the melting point, when the cation is immobile as a whole. On the other hand, dynamics of hydrogen bonds and ion pairs, proton transfer from 23 ACS Paragon Plus Environment


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the cation to the anion and vice versa remain possible explanations, as their time and length scale match nicely those of the discussed localized process. The evaluated parameters could be also compared with the recently reported results of quantum chemistry calculations 42–44 and molecular dynamic simulations 44 for the family of the studied PIL. These works proved that interactions in PILs have strong directionality in comparison with similar aprotic liquids due to hydrogen bonds. For the family of alkylammonium salts with the triflate anion it was shown that the most stable and energetically favourable geometry is when the anion has contact with the N-H bond. The interatomic distances between the key particles (the oxygen atom of TF, the nitrogen atom of TEA and the N-H proton) in the most energetically favourable configuration are R(O-H)=0.97 Å (the proton belongs to the acid); R(N-H)=1.06 Å and R(O-H)=1.59 Å (the proton belongs to the base). 43,44 The characteristic size of the confinement with a soft boundary is about 2 0.45 Å (T=350 K); the correlation time can be evaluated as τ0 = RH /DH ≈4 ps. This

estimation affirms the assumption that the observed localized process might be connected with the proton exchange between the ions and formation of ion pairs. Moreover, there are other possible ways of reversible proton transfer. These are proton exchange between the molecules of triflic acid and the triflate anions, and also between TEA cations and neutral tertiary amines, if the basicity of the latter is increased by surrounding anions. 45

Conclusions Both the low temperature dynamics and the dynamics of TEA-TF in the liquid state have been studied and summarized in this work. The measurements performed on a backscattering spectrometer showed the presence of thermally activated methyl group rotation at temperatures below the first melting point. The temperature range between the two endothermic phase transitions was investigated by means of the QENS-method. This region was also characterized by localized, but much faster motions of the end methyl groups, while the core of the TEA cation remained “frozen”. 24 ACS Paragon Plus Environment

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The information of interest for electrochemical applications of PILs is related to the transport properties of the N-H proton and their impact on the anhydrous conductivity. Therefore, the dynamical behaviour of the totally protonated and partially deuterated samples have been compared for the temperatures higher than the second melting phase transition. Deuterium labelling allowed us to highlight the key particle and to focus on the motions of the tagged N-H proton. It is necessary to emphasize that the application of the QENS technique shifted the examined time range to the picosecond time scale inaccessible by PFGNMR, which has been used for studying TEA-TF so far. The long-range diffusion, as well as the localized process was observed for both the totally protonated and partially deuterated samples. The spectra profiles were satisfactorily captured by the scattering law that included the jump-diffusion model and the Gaussian model for the separate groups of the hydrogen atoms in the cation. The most important finding in this context was the enhanced spatially restricted dynamics of the N-H proton detected in the spectra of the partially deuterated sample TEAD -TF and characterized by a relaxation time of ≈4 ps. This information eludes observation by the PFG-NMR method, which allows measuring only long-time diffusion coefficients and provides an evidence of the vehicular mechanism on the corresponding time scale. On the picosecond time scale, explored by QENS, the NH proton is mobile relative to the core of the cation. Interference effects, however, influence the slower diffusional component and are especially prominent in the case of TEAD -TF. As both the anion and the cation are sources of the coherent contribution, which is comparable with the incoherent one, for a more sophisticated analysis experimental separation of the two components would be advantageous.

Acknowledgement Financial support by the German Research Foundation (DFG) within the Scientific Priority Program SPP 1191 Ionic Liquids, is gratefully acknowledged (project no. HE2403/8-3). The



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authors acknowledge the Institute Laue-Langevin (ILL) for beam time on IN10 and IN5.

Notes and References (1) Greaves, T. L.; Drummond, C. J. Protic Ionic Liquids: Properties and Applications. Chem. Rev. 2008, 108, 206–237. (2) Nakamoto, H.; Watanabe, M. Bronsted Acid-Base Ionic Liquids for Fuel Cell Electrolytes. Chem. Commun. 2007, 24, 2539–2541. (3) MacFarlane, D. R.; Tachikawa, N.; Forsyth, M.; Pringle, J. M.; Howlett, P. C.; Elliott, G. D.; Davis, J. H.; Watanabe, M.; Simon, P.; Angell, C. A. Energy Applications of Ionic Liquids. Energy Environ. Sci. 2014, 7, 232–250. (4) Bin, M. A.; Susan, H.; Noda, A.; Watanabe, M. Electrochemical Aspects of Ionic Liquids; John Wiley & Sons, Inc., 2011; pp 65–85. (5) Weingärtner, H. NMR Studies of Ionic Liquids: Structure and Dynamics. Curr. Opin. Colloid Interface Sci. 2013, 18, 183–189. (6) Iojoiu, C.; Hana, M.; Y.Molmeret,; M.Martinez,; Cointeaux, L.; Kissi, N. E.; Teles, J.; Leprêtre, J.-C.; Judeinstein, P.; Sanchez, J.-Y. Ionic Liquids and Their Hosting by Polymers for HT-PEMFC Membranes. Fuel Cells 2010, 10, 778–789. (7) MacFarlane, D. R.; Forsyth, M.; Izgorodina, E. I.; Abbott, A. P.; Annat, G.; Fraser, K. On the Concept of Ionicity in Ionic Liquids. Phys. Chem. Chem. Phys. 2009, 11, 4962– 4967. (8) Ueno, K.; Tokuda, H.; Watanabe, M. Ionicity in Ionic Liquids: Correlation with Ionic Structure and Physicochemical Properties. Phys. Chem. Chem. Phys. 2010, 12, 1649– 1658.


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Page 27 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(9) Miran, M. S.; Kinoshita, H.; Yasuda, T.; Susan, M. A. B. H.; Watanabe, M. Hydrogen Bonds in Protic Ionic Liquids and Their Correlation with Physicochemical Properties. Chem. Commun. 2011, 47, 12676–12678. (10) Bée, M. Quasielastic Neutron Scattering, Principles and Applications in Solid State Chemistry, Biology and Materials Science; Adam Hilger, Bristol, 1988. (11) Hempelmann, R. Quasielastic Neutron Scattering and Solid State Diffusion; Oxford series on neutron scattering in condensed matter; Clarendon Press, Oxford, 2000. (12) Fitter, J., Gutberlet, T., Katsaras, J., Eds. Neutron Scattering in Biology; Biological and Medical Physics, Biomedical Engineering; Springer, 2006. (13) Sakai, V. G.; Arbe, A. Quasielastic Neutron Scattering in Soft Matter. Curr. Opin. Colloid Interface Sci. 2009, 14, 381–390. (14) Mamontov, E.; Baker, G. A.; Luo, H.; Dai, S. Microscopic Diffusion Dynamics of Silver Complex-Based Room-Temperature Ionic Liquids Probed by Quasielastic Neutron Scattering. ChemPhysChem 2011, 12, 944–950. (15) Aoun, B.; González, M. A.; Russina, M.; Price, D. L.; Saboungi, M.-L. Dynamics of Butyl- and Hexyl-Methylimidazolium Bromide Ionic Liquids. J. Phys. Soc. Jpn. 2013, 82, SA002. (16) Chathoth, S. M.; Mamontov, E.; Fulvio, P. F.; Wang, X.; Baker, G. A.; Dai, S.; Wesolowski, D. J. An Unusual Slowdown of Fast Diffusion in a Room Temperature Ionic Liquid Confined in Mesoporous Carbon. Europhys. Lett. 2013, 102, 16004. (17) Burankova, T.; Reichert, E.; Fossog, V.; Hempelmann, R.; Embs, J. P. The Dynamics of Cations in Pyridinium-Based Ionic Liquids by Means of Quasielastic- and Inelastic Neutron Scattering. J. Mol. Liq. 2014, 192, 199–207.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(18) Mamontov,

E.;

Luo,

H.;

Dai,

S.

Proton

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Dynamics

in

N,N,N’,N’-

Tetramethylguanidinium Bis(perfluoroethylsulfonyl)imide Protic Ionic Liquid Probed by Quasielastic Neutron Scattering. J. Phys. Chem. B 2009, 113, 159–169. (19) Hoarfrost, M. L.; Tyagi, M.; Segalman, R. A.; Reimer, J. A. Proton Hopping and Long-Range Transport in the Protic Ionic Liquid [Im][TFSI], Probed by Pulsed-Field Gradient NMR and Quasi-Elastic Neutron Scattering. J. Phys. Chem. B 2012, 116, 8201–8209. (20) Martinez, M.; Molmeret, Y.; Cointeaux, L.; Iojoiu, C.; Lepretre, J.-C.; Kissi, N. E.; Judeinstein, P.; Sanchez, J.-Y. Proton-Conducting Ionic Liquid-Based Proton Exchange Membrane Fuel Cell Membranes: The Key Role of Ionomer-Ionic Liquid Interaction. J. Power Sources 2010, 195, 5829–5839. (21) Langevin, D.; Nguyen, Q. T.; Marais, S.; Karademir, S.; Sanchez, J.-Y.; Iojoiu, C.; Martinez, M.; Mercier, R.; Judeinstein, P.; Chappey, C. High-Temperature IonicConducting Material: Advanced Structure and Improved Performance. J. Phys. Chem. C 2013, 117, 15552–15561. (22) Fumino, K.; Fossog, V.; Wittler, K.; Hempelmann, R.; Ludwig, R. Dissecting AnionCation Interaction Energies in Protic Ionic Liquids. Angew. Chem. Int. Ed. 2013, 52, 2368–2372. (23) Ref. 22, Supplementary Material. (24) Sears, V. F. Neutron Scattering Lengths and Cross Sections. Neutron News 1992, 3, 26–37. (25) Burankova, T.; Hempelmann, R.; Wildes, A.; Embs, J. P. Collective Ion Diffusion and Localized Single Particle Dynamics in Pyridinium-Based Ionic Liquids. J. Phys. Chem. B 2014, 118, 14452–14460.


Page 29 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(26) Richard, D.; Ferrand, M.; Kearley, G. J.; Bradley, A. D. The Lamp Book. 2013; http: //www.ill.eu/?id=2024. (27) Azuah, R. T.; Kneller, L. R.; Qiu, Y.; Tregenna-Piggott, P. L. W.; Brown, C. M.; Copley, J. R. D.; Dimeo, R. M. DAVE: A Comprehensive Software Suite for the Reduction, Visualization, and Analysis of Low Energy Neutron Spectroscopic Data. Journal of Research of NIST 2009, 114, 341–358. (28) Markwardt, C. B. Non-Linear Least Squares Fitting in IDL with MPFIT. Astronomical Data Analysis Software and Systems XVIII 2009, 411, 251–254. (29) Aoun, B.; González, M. A.; Ollivier, J.; Russina, M.; Izaola, Z.; Price, D. L.; Saboungi, M.-L. Translational and Reorientational Dynamics of an Imidazolium-Based Ionic Liquid. J. Phys. Chem. Lett. 2010, 1, 2503–2507. (30) Lechner, R. E. Neutrons in Soft Matter ; John Wiley & Sons, Inc., 2011; pp 203–268. (31) Lechner, R. E.; Bleif, H. J.; Dachs, H.; Marx, R.; Stahn, M.; Anderson, I. TwoDimensional Proton Diffusion in CsH3 O2 . Solid State Ion. 1991, 46, 25–32. (32) Singwi, K. S.; Sjölander, A. Diffusive Motions in Water and Cold Neutron Scattering. Phys. Rev. 1960, 119, 863–871. (33) Volino, F.; Perrin, J.-C.; Lyonnard, S. Gaussian Model for Localized Translational Motion: Application to Incoherent Neutron Scattering. J. Phys. Chem. B 2006, 110, 11217–11223. (34) Frick, B.; Fetters, L. J. Methyl Group Dynamics in Glassy Polyisoprene: A Neutron Backscattering Investigation. Macromolecules 1994, 27, 974–980. (35) Zorn, R.; Frick, B.; Fetters, L. J. Quasielastic Neutron Scattering Study of the Methyl Group Dynamics in Polyisoprene. J. Chem. Phys 2002, 116, 845–853.



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(36) Bordallo, H. N.; Argyriou, D. N.; Barthés, M.; Kalceff, W.; Rols, S.; Herwig, K. W.; Fehr, C.; Juranyi, F.; Seydel, T. Hydrogen in N-Methylacetamide: Positions and Dynamics of the Hydrogen Atoms Using Neutron Scattering. J. Phys. Chem. B 2007, 111, 7725–7734. (37) Noto, V. D.; Negro, E.; Sanchez, J.-Y.; Iojoiu, C. Structure-Relaxation Interplay of a New Nanostructured Membrane Based on Tetraethylammonium Trifluoromethanesulfonate Ionic Liquid and Neutralized Nafion 117 for High-Temperature Fuel Cells. J. Am. Chem. Soc. 2010, 132, 2183–2195. (38) Unruh, T.; Smuda, C.; Busch, S.; Neuhaus, J.; Petry, W. Diffusive Motions in Liquid Medium-Chain n-Alkanes as Seen by Quasielastic Time-of-Flight Neutron Spectroscopy. J. Chem. Phys 2008, 129, 121106. (39) Triolo, A.; Russina, O.; Arrighi, V.; Juranyi, F.; Janssen, S.; Gordon, C. M. Quasielastic Neutron Scattering Characterization of the Relaxation Processes in a Room Temperature Ionic Liquid. J. Chem. Phys 2003, 119, 8549–8557. (40) Sarangi, S. S.; Zhao, W.; Müller-Plathe, F.; Balasubramanian, S. Correlation between Dynamic Heterogeneity and Local Structure in a Room-Temperature Ionic Liquid: A Molecular Dynamics Study of [bmim][PF6 ]. ChemPhysChem 2010, 11, 2001–2010. (41) Habasaki, J.; Ngai, K. L. Heterogeneous Dynamics of Ionic Liquids from Molecular Dynamics Simulations. J. Chem. Phys 2008, 129, 194501. (42) Mori, K.; Hashimoto, S.; Yuzuri, T.; Sakakibara, K. Structural and Spectroscopic Characteristics of a Proton-Conductive Ionic Liquid Diethylmethylammonium Trifluoromethanesulfonate [dema][TfOH]. Bull. Chem. Soc. Jpn. 2010, 83, 328–334. (43) Mori, K.; Kobayashi, T.; Sakakibara, K.; Ueda, K. Experimental and Theoretical Investigation of Proton Exchange Reaction between Protic Ionic Liquid Diethylmethylammonium Trifluoromethanesulfonate and H2 O. Chem. Phys. Lett. 2012, 552, 58–63. 30 ACS Paragon Plus Environment

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(44) Tsuzuki, S.; Shinoda, W.; Miran, M. S.; Kinoshita, H.; Yasuda, T.; Watanabe, M. Interactions in Ion Pairs of Protic Ionic Liquids: Comparison with Aprotic Ionic Liquids. J. Chem. Phys 2013, 139, 174504. (45) Kumar, M.; Venkatnathan, A. Mechanism of Proton Transport in Ionic-Liquid-Doped Perfluorosulfonic Acid Membranes. J. Phys. Chem. B 2013, 117, 14449–14456.



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+

[(C₂H₅)₃NH]

R2

RH

R1 +

[(C₂D₅)₃N H]


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Proton Diffusivity in the Protic Ionic Liquid Triethylammonium Triflate

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