Local OrderâDisorder Transition Driving by Structural Heterogeneity in

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Local Order-Disorder Transition Driving by Structural Heterogeneity in a Benzyl Functionalized Ionic Liquid Luiz F. O. Faria, Vitor Hugo Paschoal, Thamires A. Lima, Fabio Furlan Ferreira, Rafael Sa de Freitas, and Mauro C. C. Ribeiro J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b08829 • Publication Date (Web): 02 Oct 2017 Downloaded from http://pubs.acs.org on October 3, 2017

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The Journal of Physical Chemistry

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Local Order-disorder Transition Driving by Structural

Heterogeneity

in

a

Benzyl

Functionalized Ionic Liquid Luiz F. O. Faria1*, Vitor H. Paschoal1, Thamires A. Lima1, Fabio F. Ferreira2, Rafael S. Freitas3, Mauro C. C. Ribeiro1* 1

Laboratório de Espectroscopia Molecular, Departamento de Química Fundamental, Instituto de Química, Universidade de São Paulo, Av. Prof. Lineu Prestes 748, 05508-000, Brazil 2 Centro de Ciências Naturais e Humanas, Universidade Federal do ABC, Santo André, SP, Brazil 3 Instituto de Física, Universidade de São Paulo, 05314-970 São Paulo, SP, Brazil

Abstract A local order-disorder transition has been disclosed in the thermophysical behavior

of

the

ionic

liquid

1-benzyl-3-methylimidazolium

dicyanamide,

[Bzmim][N(CN)2], and its microscopic nature revealed by spectroscopic techniques. Differential scanning calorimetry and specific heat measurements show a thermal event of small enthalpy variation taking place in the range 250–260 K, which is not due to crystallization or melting. Molecular dynamic simulations and X-ray diffraction measurements have been used to discuss the segregation of domains in the liquid structure of [Bzmim][N(CN)2]. Raman and NMR spectroscopy measurements as a function of temperature indicate that the microscopic origin of the event observed in the calorimetric measurements comes from structural rearrangement involving the benzyl group. The results indicate that the characteristic structural heterogeneity allow for rearrangements within local domains implying the good glass-forming ability for the low viscosity ionic liquid [Bzmim][N(CN)2]. This work sheds light on our understanding of the microscopic origin behind complex thermal behavior of ionic liquids.

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Introduction Ionic liquids, i.e. salts with melting point below 373 K, have been investigated in the last years concerning fundamental issues and several possibilities for technological applications. The concept of task specific ionic liquids, or ionic liquids designed for a determined application, is settled insofar as functionalized ions have been produced. The typical approach is to insert a specific group in the alkyl side chain of cation or anion, e.g. ether, cyano, or hydroxyl group. In particular, ionic liquids functionalized with an aromatic group have been under scrutiny recently.1-9 In an X-ray scattering study, Campetella et al.3 reported nanometric domain segregation in ionic liquids based on the aminoacid anions phenylalanine and homophenylalanine with the choline cation. These authors claimed the occurrence of a medium range order related to stacking of aromatic rings.3 Thus, it seems that a long alkyl chain in the ions is not necessary for developing the characteristic structural heterogeneities of ionic liquids. Serra et al.4 used calorimetry to investigate the thermophysical behavior of ionic liquids containing the 1-benzyl-3-methylimidazolium cation, [Bzmim]+. It has been found that the presence of the benzyl group increases the glass transition and melting temperatures, Tg and Tm, in comparison with analogous ionic liquids having alkyl chain.4 Moreover, the incorporation of the rigid aromatic group in the molecular structure increases the fragility of the glass-forming ionic liquid more than expected only on the basis of Tg increase.1 Liquid structure and intermolecular vibrations of a [Bzmim]+ based ionic liquid have been compared with an equimolar mixture of 1,3-dimethylimidazolium ionic liquid and benzene by femtosecond optical Kerr effect (OKE) spectroscopy and molecular dynamics simulations.5-6 Despite the fact that free motion of the benzene ring in the liquid mixture becomes restricted when covalently tied to the imidazolium ring in


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[Bzmim]+, the results indicated similar local structure and formation of π-stacked benzene-cation complexes in both cases. It is worth mentioning, however, that the ionic liquid made of [Bzmim]+ and the bis(trifluoromethylsulfononyl)imide anion, [NTf2]-, is a glass forming liquid.1, 4, 8, 10 whereas the equimolar mixture of benzene and the ionic liquid 1-ethyl-3-methylimidazolium, [emim]+, with the [NTf2]- anion forms an inclusion crystal with a congruent melting temperature.11 This inclusion crystal is maintained mainly by π-π interactions between the benzene and imidazolium ring, and interactions between the acidic hydrogen atoms of imidazolium ring and the [NTf2]- anion with π electrons of benzene.11 The microscopic nature of different phase behavior of pure [Bzmim][NTf2] and an equimolar mixture of [emim][NTf2] and benzene remains elusive. In this work, we investigated the [Bzmim]+ ionic liquid having dicyanamide, [N(CN)2]-, as counter ion (see Figure 1). Different relative orientations between the two aromatic rings is expected for the [Bzmim]+ structure in the liquid phase. In contrast to the [NTf2]- anion, which acquires different conformations in the liquid phase, the [N(CN)2]- anion is an almost rigid species. The [Bzmim][N(CN)2] is an ionic liquid of relatively low viscosity (η = 102 mPa.s at 298.15 K),2,

10

but nevertheless is a good

glass former. Indeed, the molecular structure of the [Bzmim]+ cation is analogous to well-known molecular glass-forming liquids composed by aromatic rings, for instance, salol (phenyl salicylate).12 We found along thermophysical characterization of [Bzmim][N(CN)2] that besides the glass transition there is another thermal event at temperature higher than Tg. This thermal event is not characterized as crystallization or melting involving a very small heat change and depends on the rate of temperature change. It is the aim of this work to provide evidences from different spectroscopic techniques (X-ray diffraction, Raman, and NMR) that [Bzmim][N(CN)2] remains



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amorphous along all of temperature range investigated and that the microscopic nature of this thermal event is related to local rearrangements of the benzyl group. Molecular dynamics (MD) simulations have been also performed in order to characterize the liquid structure and segregation of domains in [Bzmim][N(CN)2] supporting the understanding of microscopic origin obtained from experiments.

Figure 1. Structure of 1-benzyl-3-methylimidazolium, [Bzmim]+, and dicyanamide, [N(CN)2]-.

Experimental and Computational Details Ionic liquid sample. The ionic liquid [Bzmim][N(CN)2] sample (> 98 % purity) was purchased from Iolitec and used without further purification, except by drying process under high vacuum (below 10-5 mbar) for 48 h before analyses.

Differential scanning calorimetry. The differential scanning calorimeter model Q500 (TA Instruments) under a dynamic N2 atmosphere (50 ml min-1) was used in the analyses. Approximately 10 mg of samples was hermetically sealed in an aluminum pan. The samples were heated to 323 K to remove crystal nuclei eventually present in


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the liquid phase. The heating and cooling rates were 1 or 10 K min-1. Duplicate measurements were performed to confirm the results.

Specific heat. Specific heat measurements were performed using a Quantum Design Dynacool system. It was employed a standard semi-adiabatic heat pulse technique that determines sample specific heat by measuring the thermal response to a change in heating conditions of a sample in high vacuum (0.01 µbar). It was used the synthetic sapphire sample platform and the addendum heat capacity was measured separately and subtracted.

X-ray diffraction. XRD data were collected at room temperature using a STADI-P powder difractometer (Stoe®, Darmstadt, Germany) in transmission geometry by using a MoKα1 (λ = 0.7093 Å) wavelength selected by a curved Ge (111) crystal, with tube voltage of 50 kV and current of 40 mA. The sample was loaded in a 0.5-mm special glass capillary and the diffracted intensities were by a silicon microstrip detector, Mythen 1 K (Dectris®, Baden, Switzerland). XRD measurements were carried out keeping the detector at a fixed position covering a range of 18.84◦ during 36000 s. Temperature-dependent XRD measurements performed at the W09A-XDS beamline of the Brazilian Synchrotron Light Laboratory (LNLS, Campinas, SP, Brazil) are showed as supplementary information and all experimental description is presented therein.

Raman spectroscopy. Raman spectra were recorded with a Jobin-Yvon T64000 triple monochromator spectrometer equipped with CCD using the 647.1 nm line of a mixed argon-krypton Coherent laser. The spectra were obtained in the 180° scattering geometry with no polarization selection of the scattered radiation. Spectral resolution



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was kept at 2.0 cm-1. Temperature control (±0.1 K) was achieved with an OptistatDN cryostat (Oxford Instruments) in which the sample is contained in a small glass tube. Raman spectra were obtained following different cooling protocols.

Nuclear Magnetic Resonance. 1H-NMR spectra as function of temperature were collected using a Bruker 500 MHz AIII spectrometer. The ionic liquid sample was transferred to a 2.5 mm NMR capillary tube and the H1 peak at 3.86 ppm, as observed for

an

analogous

ionic

liquid

([emim][N(CN)2],

[emim]+

=

1-ethyl-3-

mehtylimidazolium) referenced to the internal standard TMS, was used as reference.

Molecular dynamics simulations. MD simulations were performed using the LAMMPS package13 with 800 ionic pairs. A random initial configuration with low density was generated using Packmol.14 This initial configuration was submitted to a 1 ns run in the NVT ensemble. Afterwards, the size of the simulation box was allowed to vary in the NPT ensemble at 400 K for another 2.0 ns and then cooled in steps of 10 K until 300 K. After this protocol, two runs in NPT and NVT ensembles of 10 ns were performed for further equilibration. Finally, a production run of 20 ns was done in the NVT ensemble. The timestep was 0.5 fs and the Nosé-Hoover barostat-thermostat was used with relaxation constants equal to 100 fs. The force field parameters for [N(CN)2]were taken from Dhungana et al.15 The parametrization for [Bzmim]+ was done along the lines of CL&P force field16 with dihedral parameters adjusted by Xue et al.6 Charges were derived from quantum chemistry calculations at MP2/aug-cc-pVDZ level through fitting using the CHelpG scheme as implemented in Gaussian version 09.17 The Lennard-Jones parameters were used from the OPLS-AA force field.18 The charges were averaged accordingly to the atomic types showed in Figure S1 of the


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supplementary information. The charges and and Lennard-Jones parameters are given in Table S1. Intramolecular parameters for the imidazolium ring were taken from the AMBER 94.19 This new force field for [Bzmim]+ resulted in an excellent agreement between experimental10 and simulated densities of [Bzmim][N(CN)2] at 300 K, 1.15 and 1.154 g mL-1, respectively. Furthermore, the matching between the X-ray weighted static structure factor, SX(Q), calculated from the MD simulation and the experimental XRD pattern reinforce the appropriateness of the force field. The total SX(Q) and partial

Sαβ(Q) were calculated directly from its definition as previously presented.20 All other structural analysis were performed with the TRAVIS program21 using 2000 configurations sampled over the 20.0 ns production run.

Quantum chemistry calculations. Optimized molecular structures and vibrational frequencies for isolated [Bzmim]+ were obtained by MP2/aug-cc-pVDZ level of theory using the Gaussian09 package.17 Two different conformers of [Bzmim]+ were optimized with no imaginary vibrational frequencies, so that these geometries are minima of potential surface.

Results and Discussion A. Thermophysical behavior Figure 2 shows DSC curves of [Bzmim][N(CN)2] at different cooling and heating rates. The only event observed in the DSC curve recorded at 10 K min-1 is the glass transition at Tg = 214 K. However, DSC curves measured along a slower rate of 1 K min-1 exhibit another event within the 250–260 K range. This thermal event involves



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a very small value of enthalpy variation and it has been observed either as exothermic (∆H = 0.002 kJ mol-1) or endothermic (∆H = 0.031 kJ mol-1) peak in the heating DSC curve. These findings already suggest that it must be related to any kind of tiny process of structural ordering or disordering that is not assigned to crystallization or melting because no any previous or subsequent event is observed in the DSC curve. Furthermore, it is worth remembering that usual ∆H of crystallization or melting of ionic liquids is much higher, for instance, melting of [Bzmim][PF6] involves ∆H ~ 25 kJ mol-1.4

-1

10 K min

Tg = 214 K

Heat flow - exo up

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

-1

1 K min

Tg = 210 K

T = 253 K -1 ∆Η = 0.031 kJ mol

-1

1 K min

Tg = 210 K

T = 256 K -1 ∆Η = 0.002 kJ mol

200 220 240 260 280 300 320

T/K Figure 2. DSC scans of [Bzmim][N(CN)2] using distinct cooling and heating rates: 10 K min-1 (upper panel) and 1 K min-1 (middle and lower panels).

We confirmed the occurrence of this thermal event above Tg by measuring the specific heat under constant pressure (Cp) of [Bzmim][N(CN)2] within the 100–300 K


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range. Figure 3 shows the Cp(T) curves along cooling and heating measurements. The jump of Cp at the glass transition temperature amounts to ∆Cp ≈ 115 J K-1 mol-1, which is within the same order as values previously obtained from DSC measurements of [Bzmim][N(CN)2], 222

10

and 170

2

J K-1 mol-1. The inset in Figure 3 highlights the

250–270 K range of the cooling and heating Cp(T) curves. It is clear from the Cp(T) data the small changing of slope in the heating curve at ~258 K. The slight decrease of Cp after this temperature suggests that an exothermic process related to some kind of structural organization is taking place in the supercooled liquid [Bzmim][N(CN)2].

350 330

Cp / J mol-1 K-1

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

300 250

328

T = 258 K

326 250

255

260

265

270

T = 215 K g

-1

-1

∆Cp = 115 J mol K

200 -1

0.6 K min -1 0.05 K min

150 100

100

150

200

250

300

T/K Figure 3. Specific heat under constant pressure of [Bzmim][N(CN)2] obtained during cooling (black symbols) and heating (red symbols) measurements. An enlarged view of the blue squared region of the main figure is displayed in the inset.

B. Liquid structure Before discussing the Raman and NMR data that unravel the microscopic origin of the event found by calorimetric measurements in the 250–260 K range, insights on the liquid structure of [Bzmim][N(CN)2] are needed. It is worth noting first that XRD



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patterns of [Bzmim][N(CN)2] as a function of temperature indeed do not show Bragg peaks that would indicate crystalline phases. Thus, an amorphous phase is maintained along all of the temperature range investigated in this work (see Figure S2). Classical MD simulations have been considered a powerful tool for investigating structure and dynamics of ionic liquids, in particular the segregation of polar and nonpolar domains of nanometric scale within the bulk of the liquid.22-23 A recent XRD study showed that there is nanodomain segregation in ionic liquids having aromatic aminoacid anions as indicated by a low wavevector peak in the diffraction pattern at Q ~ 0.4 Å-1.3 In order to check whether this low-Q peak is also found in the diffraction pattern of [Bzmim][N(CN)2], we performed XRD experiment and calculated by MD simulation the X-ray weighted structure factor, SX(Q). Figure 4 compares experiment and simulation, and the fair agreement between them indicates the appropriateness of the force field used in the MD simulation for discussing the liquid structure.

SX(Q)

2

Calculated

1 0

-1

Experimental

Intensity

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

0.5

1.0

1.5

2.0

2.5

3.0

-1

Q/Å

Figure 4. Comparison between calculated structure factor, SX(Q), and experimental XRD pattern of [Bzmim][N(CN)2]. The red dashed lines are guides to the eye for the peak positions.


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The absence of a low-Q peak in the static structure factor is not a definitive evidence of lacking domain segregation in [Bzmim][N(CN)2]. It has been shown by MD simulations that cancellations between correlations and anti-correlations of many partial

Sαβ(Q) for species α and β may prevent some peaks of being observed in the total S(Q).22,

24

Figure 5 shows some Sαβ(Q) calculated for group of atoms of relevant

moieties of [Bzmim][N(CN)2]: benzyl-benzyl SBz_Bz(Q), anion-anion San_an(Q), imidazolium-imidazolium Sim_im(Q), anion-imidazolium San_im(Q), and anion-benzyl

San_Bz(Q). The peaks at 0.9 and 1.5 Å-1 concern, respectively, charge-charge and adjacencies correlations,22, 24 so that the first is more intense in San_an(Q) and Sim_im(Q) and the latter is more intense in San_im(Q). Notwithstanding, the SBz_Bz(Q) presents a higher intensity peak at lower wavevectors. The most intense feature in SBz_Bz(Q) and

San_im(Q) is the low-Q peak in the 0.5–0.7 Å-1 range, which indicates intermediate range order correlation. Accordingly, San_Bz(Q) exhibits anti-correlation peak proper to polarnonpolar alternation. Therefore, the presence of benzyl group favors formation of domain segregation in liquid [Bzmim][N(CN)2]. The finding of low-Q peak in XRD pattern of ionic liquid having aromatic aminoacid anion3 is most probably due to the fact that heavy atoms with more electrons, i.e. oxygen atoms, imply higher X-ray contrast between domains.



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12 1.5 1.0 0.5 0.0

Bz-Bz

0.4 0.2 0.0

Sαβ(Q)

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

anion-anion

1.5 1.0 0.5 0.0

im-im

1.0

anion-im

0.5 0.0 0.0 -0.3 -0.6 -0.9

anion-Bz 0.5

1.0

1.5

2.0

2.5

-1

Q/Å

Figure 5. From top to bottom: calculated partial Sαβ(Q) for benzyl-benzyl, anion-anion, imidazolium-imidazolium,

anion-imidazolium,

and

anion-benzyl

moieties

of

[Bzmim][N(CN)2]. The dashed red lines are guide to the eye for the peak positions.

The local organization in the ionic liquid is better visualized by the spatial distribution function (SDF) showing probability maps of occurrence for a group of atoms around a central ion. Figure 6 shows SDFs for anion, benzyl, and imidazolium ring around the [Bzmim]+ cation. The preferred location of [N(CN)2]- is close to acidic hydrogen atoms of the imidazolium ring (Fig. 6.a), so that this SDF pattern is similar to the one found in [emim][N(CN)2].15 Imidazolium rings within the second shell around [Bzmim]+ distribute mainly above and below the central imidazolium ring indicating cation-cation stacking (Fig. 6.c) in line with previous finding for [Bzmim][NTf2].6 The benzyl group has higher distribution above and below imidazolium and benzyl rings (Fig. 6.b). Thus, benzyl-benzyl stacking plays a significant role in segregation of domains in liquid [Bzmim][N(CN)2]. The combination of all of these SDFs shown in


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Fig. 6.d provides a clear view of excluding positions of anion and benzyl group around the central cation.

Figure 6. Spatial Distribution Function (SDF) around [Bzmim]+ (isosurface value indicated in parentheses): (a) anion (red, 5.5); (b) benzyl (blue, 3.3); (c) imidazolium (green, 3.4); (c) the combination of all of the distributions shown together, where the green area is overlapped with the blue one.

Another relevant issue of [Bzmim][N(CN)2] liquid structure for this work is the conformational freedom of the [Bzmim]+ cation. Xue et al.6 reported that the energy barrier for conformational change of [Bzmim]+ in [Bzmim][NTf2] is very small, ~ 2 kJ mol-1. We found indeed all of dihedral angles between benzyl and imidazolium rings along the MD simulation of liquid [Bzmim][N(CN)2] indicating mixture of conformers proper to the low energy barrier. On the other hand, in quantum chemistry MP2 calculations of the isolated [Bzmim]+, two optimized conformers are obtained with dihedrals 0o and 153o, respectively, eclipsed and anti-conformers shown in Figure 7.



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The energy difference between these conformers, 3.7 kJ mol-1, is close to the energy barrier obtained for bulk phase.6

Figure 7. The optimized [Bzmim]+ conformers eclipsed (bottom) and anti (top) obtained by MP2 calculation of an isolated cation. The atoms forming the dihedral angle between benzyl and imidazolium rings are indicated by the red line.

C. Temperature dependent local order-disorder transition The spectroscopic results discussed in this section reveal the consequences of the characteristic features of the liquid structure of [Bzmim][N(CN)2] on its thermophysical behavior seen in the calorimetric measurements. These include the small thermal event observed at 250–260 K being related to rearrangements within domains defined by the benzyl groups and the resulting glass forming ability despite of [Bzmim][N(CN)2] being a liquid of relatively low viscosity. Figure 8 shows different regions of the Raman spectrum of [Bzmim][N(CN)2] in liquid and glassy phases. The low frequency range, ω < 150 cm-1, which probes the


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intermolecular dynamics, is clearly different for liquid and glass. The absence of any sharp Raman band in the low frequency range of lattice vibrations is another indication that [Bzmim][N(CN)2] does not crystallize upon cooling. Rotational and translational molecular dynamics gives in the liquid phase Raman spectrum the strong quasi-elastic scattering, whose intensity is reduced in the glassy phase spectrum leading to the boson peak at ∼20 cm-1 characteristic of Raman spectra of glasses.25 The glassy phase spectrum also exhibits the broad band assigned to ring librations from both the imidazolium and benzyl moieties. Previous OKE spectroscopy studies of [Bzmim]+ based ionic liquids considered two functions to fit the libration band of the experimental spectra, 40 and 90 cm-1, respectively, for benzyl and imidazolium librations.6-7

Raman Intensity

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

liquid - 298 K glass - 170 K

νs(Bz)

νs(CN)

Quasielastic scattering Boson peak Ring librations

* 50

100

150

200

250 1000 1008 wavenumber / cm-1

2160 2190 2220

Figure 8. Raman spectra of [Bzmim][N(CN)2] in liquid (black lines) and glassy (red lines) phases. The asterisk in the left panel indicates the intramolecular band used to normalize the low frequency region. The blue arrows in the middle and right panel indicate the opposite frequency shift of the benzene ring breathing mode, νs(Bz), and the symmetric stretching mode of cyano groups of the anion, νs(CN).

One advantage of the high frequency range of the Raman spectrum of [Bzmim][N(CN)2] is the presence of two sharp bands acting as vibrational probes of



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different domains of the liquid. The Raman band at 2192 cm-1 is assigned to the symmetric stretching mode of the cyano groups of [N(CN)2]-, νs(CN), and the band at 1003 cm-1 is assigned to the breathing mode of the benzene ring, νs(Bz). (The latter assignment is supported by the atomic displacements shown in Figure S3 for the vibrational mode at 1001 cm-1 obtained by MP2 calculations of the isolated [Bzmim]+ cation in the two optimized conformers). It is clear from Figure 8 the opposite directions of frequency shift for νs(Bz) and νs(CN) when the liquid and glassy phases spectra are compared. The shift of νs(Bz) mode when the liquid is cooled is not a trivial effect and it suggests the probe oscillator experiencing attractive interactions. It is of interest to compare the temperature dependence of the νs(Bz) and νs(CN) modes along the 250-260 K range as it provides evidence of structural change in the same temperature range in which the calorimetric data of Figures 2 and 3 showed a tiny thermal event. Figure 9 shows the temperature dependence of νs(Bz) and νs(CN) vibrational frequencies along two sequences of measurements using different cooling rates. The vibrational frequency shift of the νs(Bz) mode depends on the cooling rate, and it changes the slope in the temperature range indicated by the blue area, which corresponds to the thermal effect observed above Tg in the calorimetric data. Accordingly, the change of slope in the νs(Bz) vibrational frequency is more pronounced when the cooling rate is lower. This finding of cooling rate dependence for the νs(Bz) mode contrasts to the νs(CN) mode. The νs(CN) vibrational frequency shifts with temperature continuously with no signature of any change of slope at 250-260 K, so that it can be assigned to a simple effect of increasing density when cooling. Therefore, Raman spectroscopy provides evidence that a subtle structural organization, which is dependent on the rate of temperature variation, is taking place in the liquid at 250-260 K involving the benzyl moiety.


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1003.6

νs(Bz)

1003.2

wavenumber / cm-1

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

1002.8

νs(CN)

2195

2194

2193

2192

1 K min-1 0.4 K min-1

180

210

240

270

300

T/K Figure 9. Vibrational frequency shift as function of temperature of the Raman bands assigned to νs(Bz) and νs(CN) modes of [Bzmim][N(CN)2]. The black and red symbols indicate two measurements along distinct cooling rates, respectively, 1 and 0.4 K min-1. The blue colored area indicates the temperature range in which the small thermal event was observed in the calorimetric experiments shown in Figures 2 and 3.

NMR spectroscopy gives further evidence that the microscopic origin of the event at 250-260 K is related to the benzyl moiety. Figure S4 shows 1H-NMR spectra of [Bzmim][N(CN)2] at 298 K and 258 K, the latter being the lowest temperature achieved in the NMR measurements. Unfortunately, the peaks broaden as temperature decreases and the signal from hydrogen atoms of benzyl (H6/H7/H8) and imidazolium ring (H4/H5) merged into a single broad peak. On the other hand, signals of hydrogen atoms of the methylene group (H3) and the most acidic hydrogen atom of the imidazolium ring (H2) are well separated. Figure 10 shows the temperature dependence along



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cooling and heating of the relative shifts of H3 and H2 atoms using as reference the methyl group (H1). Distinct trends are observed when the temperature is below 260 K: the relative shift of H3 changes the slope, while the relative shift of H2 continues increasing. The H3 shift of methylene group is related to changes involving the benzyl group, so that NMR data corroborates the conclusion drawn from Raman data as the structural rearrangement at 250-260 K involves the benzyl moiety.

H6

H2 H3

%∆(δH2) / ppm

1.5

H1

H7 H3 N

1.0

H8

H6

N

H5

H1 H1

H5

H7

0.5 0.0

-0.5

%∆(δH3) / ppm

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

0.2 K min-1 0.3 K min-1

0.6

0.3

0.0 260

270

280

290

300

T/K Figure 10. 1H-NMR peak shifts of H2 (top panel) and H3 (bottom panel) in [Bzmim][N(CN)2] using as reference the H1 peak of the methyl group. The inset shows the [Bzmim]+ structure with hydrogen atoms numbering. Data obtained along cooling and heating processes are shown by black and red symbols, respectively. The blue colored area indicates the temperature range in which the small thermal event was observed in the calorimetric experiments shown in Figs. 2 and 3.


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Summing up, the liquid structure of [Bzmim][N(CN)2] is characterized by domain segregation driven by stacking interactions between benzyl groups in nonpolar domains and coulombic interactions between imidazolium ring and [N(CN)2]- in polar domains. Thus, it is not always necessary the presence of a long chain in 1-alkyl-3methylimidazolium cations to generate structural heterogeneities.3 On the other hand, in comparison with an alkyl side chain, the benzyl is a bulky and quite anisotropic group that promotes steric hindrance. This feature of the structural organization of [Bzmim][N(CN)2] may result in a local order-disorder transition involving the benzyl moieties that can be manifest as a thermal event of very small ∆H around 250-260 K. These structural rearrangements in specific liquid regions give either a slight exothermic event of more efficient packing and interaction in nonpolar domains or a slight endothermic event of partial “melting” of this local organization. Previous studies concerning ionic liquid phase transitions showed that structural rearrangements in polar and non-polar domains might be independent of each other.26-29 In the case of [Bzmim][N(CN)2], however, crystallization has been not observed. The many conformations available for the two aromatic rings of [Bzmim]+ difficult efficient ion packing needed for crystallization. As a consequence, [Bzmim][N(CN)2] is a good glass forming liquid. The ionic liquid [Bzmim][N(CN)2] should be compared with analogous fragile molecular glass formers containing aromatic rings with rotational freedom between them. For example, α-phenyl-o-cresol and salol exhibit Tg close to the value for [Bzmim][N(CN)2], ca. 220 K.12, 30 However, these molecular liquids can crystallize or vitrify depending on the cooling rate.12, 31 Baran et al.12 discussed the relation between nucleation of metastable nuclei and glass transition in salol. These authors argued that nuclei of dimensions in the range of critical radius of nucleation appear and disappear



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continuously, until a glassy state is formed at Tg composed of crystal nuclei surrounded by an amorphous matrix.12 It has been found in many liquids that these crystal nuclei start to be formed at T ~ 1.2Tg. This temperature is also close to the crossover temperature when the temperature dependence of relaxation time or viscosity changes for a non-Arrhenius behavior.12, 32-33 Interestingly, the local order-disorder transition of [Bzmim][N(CN)2] discussed in this work is also observed close to 1.2Tg. It would be interesting for future studies whether the thermal event and local organization at T ~ 1.2Tg might imply any signature in the temperature dependence of transport coefficients or rheological properties of the ionic liquid [Bzmim][N(CN)2].

Conclusion Liquid structure and its consequence on thermal behavior of a benzyl functionalized ionic liquid has been studied using experiments and simulation. The calorimetric measurements indicated glass formation in all experiments, but another thermal event dependent on the rate of temperature variation may be observed at T ~ 1.2Tg. The enthalpy variation associated to this event is very small and X-ray diffraction data also exclude the occurrence of crystallization of [Bzmim][N(CN)2]. The MD simulations showed that the imidazolium ring interacts mainly with the anion in polar domains whereas stacking of benzyl moieties occurs in nonpolar domains. Proper to the presence of distinct probes for polar and nonpolar domains, Raman and NMR spectroscopies were useful to reveal that the nature of the thermal event found in the supercooled liquid should be assigned to some local organization involving benzyl moieties of [Bzmim]+ cations. This local order-disorder transition is an interesting consequence of the structural heterogeneity in [Bzmim][N(CN)2]. The findings of this


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work suggest that the well-known nanometric domain segregation might promote local order-disorder transition in supercooled ionic liquids, even though an associated thermal event can be very small precluding of being easily seen in usual calorimetric measurements.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Force field intermolecular parameters for [Bzmim]+ cation; temperature dependent X-ray diffraction data of [Bzmim][N(CN)2]; atomic displacements of benzyl ring breathing mode from MP2 optimized [Bzmim]+ conformers; 1H-NMR spectra at 298 K and 258 K to [Bzmim][N(CN)2].

Acknowledgment

The authors are indebted to FAPESP (Grant nos. 2015/05803-0, 2014/15049-8, and 2012/13119-3) and CNPq for financial support (402289/2013-7), and LNLS (Brazilian Synchrotron Light Laboratory) for synchrotron X-ray diffraction experiments in the W09A-XDS beamline (proposal XDS 20160087).

Author information

Corresponding author *E-mail address: [email protected] (L.F.O.F.); [email protected] (M.C.C.R.)



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Notes The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. No competing financial interests have been declared.

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