Raman Spectroscopic Study of Temperature and Pressure Effects on

Article pubs.acs.org/JPCB

Raman Spectroscopic Study of Temperature and Pressure Effects on the Ionic Liquid Propylammonium Nitrate Luiz F. O. Faria, Tatiana C. Penna, and Mauro C. C. Ribeiro* Laboratório de Espectroscopia Molecular, Instituto de Química, Universidade de São Paulo, CP 26077, CEP 05513-970, São Paulo, SP, Brazil ABSTRACT: Raman spectroscopy has been used to decipher structural rearrangements in the protic ionic liquid propylammonium nitrate, [C3H7NH3][NO3], as a function of temperature (180−420 K) at atmospheric pressure and as a function of pressure (0.1 MPa−2.0 GPa) at room temperature. Spectral modifications of the Raman bands belonging to the anion and cation normal modes indicate structural changes occurring in both the polar and nonpolar nanoscale domains of [C3H7NH3][NO3]. The crystalline phase of [C3H7NH3][NO3] at low temperature has cations in the anti conformation and undertakes a transition with increasing temperature to a phase with cations mostly in the gauche conformation. The distorted network of hydrogen bonds gives a distribution of local environments around the anions that remains in the normal liquid phase at high temperature. The sample under high pressure might become microscopically heterogeneous, allowing for micro-Raman imaging of different ordered phases of [C3H7NH3][NO3] in a diamond anvil cell. the cations: A given [NO3]− anion is not engaged in three equally strong hydrogen bonds; instead, one of the hydrogen bonds is significantly weaker than the other two. Such asymmetry in hydrogen-bond interactions was also observed by Zahn et al.12 in ab initio molecular dynamics simulations of [CH3NH3][NO3]; by Bodo et al.13 in quantum chemistry calculations of ionic clusters of [CH3NH3][NO3], [C2H5NH3][NO3], and [C3H7NH3][NO3]; and by Gontrani et al.14 in classical molecular dynamics simulations performed with a three-body model for [C2H5NH3][NO3]. Neutron diffraction data of several protic ionic liquids with ions containing different numbers of acceptor and donor sites were recently compared by Hayes et al.,15 who showed that [C2H5NH3][NO3] and [C3H7NH3][NO3] have relatively long and nonlinear hydrogen bonds. Fumino et al.16−18 identified low-frequency intermolecular vibrational modes associated with anion−cation hydrogen bonds in the far-infrared spectra of alkylammonium nitrates. In the high-frequency range of intramolecular vibrational modes, Raman spectroscopy has provided insight into changes in the local structure following liquid−solid and solid−solid transitions of [CH3NH3][NO3]7 and [C2H5NH3][NO3].8 In this work, we use Raman spectroscopy to reveal rearrangements of the local structure in propylammonium nitrate, [C3H7NH3][NO3], at low temperatures or high pressures. Several high-pressure vibrational spectroscopy studies have been reported mainly for ionic liquids based on

I. INTRODUCTION Protic ionic liquids are now considered an interesting class of ionic liquids appropriate for recent applications, such as solvents and catalysts in organic synthesis reactions and electrolytes in proton-conducting fuel cells.1 Concerning the equilibrium structure of the liquid phase, the dominant structural motif in both protic and nonprotic ionic liquids is charge ordering resulting from Coulombic interactions. Furthermore, nanoscale structural heterogeneity due to the segregation of polar and nonpolar domains, which is now considered a fundamental concept in nonprotic ionic liquids,2 also occurs in protic ionic liquids.3−5 On the other hand, anion−cation hydrogen bonding plays a more important role in the equilibrium structure of protic, rather than nonprotic, ionic liquids. Relatively strong hydrogen bonds in protic ionic liquids add directional local interactions and an extended threedimensional network as other structural motifs, with important implications for ionic dynamics and phase transitions to glassy or crystalline phases with the occurrence of polymorphism.1,6−10 In the past several years, alkylammonium nitrates have become the most investigated protic ionic liquids. These ionic liquids exhibit segregation of the nonpolar domains, in which van der Waals interactions dominate, even for the case of cations with relatively short alkyl chains.3−6,11 The polar domains have strong Coulombic interactions between the [NO3]− anions and the nitrogen-atom end of the cations in a distorted network driven by hydrogen bonds. X-ray measurements and quantum chemistry calculations performed by Bodo et al.7 for methylammonium nitrate, [CH 3NH3][NO3], revealed that the anions are asymmetrically coordinated to © 2013 American Chemical Society

Received: July 6, 2013 Revised: August 13, 2013 Published: August 26, 2013 10905

dx.doi.org/10.1021/jp4066778 | J. Phys. Chem. B 2013, 117, 10905−10912

The Journal of Physical Chemistry B

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1-alkyl-3-methylimidazolium cations,19−25 but to the best of our knowledge, this is the first study of a protic ionic liquid under high pressure. Raman spectroscopy has been instrumental in showing that a distribution of conformers of cations are allowed in the liquid phase and that, eventually, a particular conformer might be preferred upon crystallization, so that the polymorphism is related to conformational changes (see the review of ref 26). In two recent studies of the effects of temperature and pressure on the crystallization of the same ionic liquid, 1butyl-3-methylimidazolium hexafluorophosphate, Russina et al.22 concluded that conformational changes of the butyl chain imply structural rearrangement in the polar domains, whereas Saouane et al.24 concluded that conformational changes taking place in nonpolar domains are rather independent of structural changes in the polar domains. In this work, an interesting feature of the Raman spectrum of [C3H7NH3][NO3] is that the Raman band corresponding to the totally symmetric stretching mode of the anion, νs(NO3), is a convenient probe of modifications in the local environment around the anions, whereas Raman bands in the 800−900 cm−1 range can be used as a signature of the conformation of the alkyl chain. It is shown that [C3H7NH3][NO3] exhibits several local structural arrangements in both the polar and nonpolar domains as functions of temperature or pressure. Furthermore, nuclei that are randomly generated under high pressure in the broad distribution of local environments of the distorted hydrogen-bond network might have their growth frustrated toward long-range-ordered structures. We also observed that a sample of [C3H7NH3][NO3] in a high-pressure diamond anvil cell becomes microscopically heterogeneous, with different Raman spectra in different parts of the sample.

Raman spectra as a function of pressure at room temperature were obtained in a Renishaw Raman imaging microscope (inVia) with a Leica microscope and a CCD detector using a laser line at 632.8 nm (He−Ne laser, Renishaw 7N1753) focused into the sample by a 20× Leica objective. The spectral resolution was 2.0 cm−1 in all pressure-dependent measurements. The low-frequency range, 20 < ω < 250 cm−1, of the Raman spectrum of [C3H7NH3][NO3] under pressure was obtained with the Jobin-Yvon T64000 spectrometer with a coupled microscope. High pressure was achieved with a diamond anvil cell from EasyLab Technologies Ltd. (model Diacell LeverDAC Maxi) having a diamond culet size of 500 μm. A Boehler microDriller (EasyLab) was used to drill a 250μm hole in a stainless steel gasket (10-mm diameter, 250-μm thickness) preindented to ∼150 μm. Pressure calibration was done by the usual method of measuring the shift of the fluorescence line of ruby.27 Density functional theory (DFT) calculations were performed with the Gaussian 03 package28 to obtain vibrational frequencies of ionic clusters of [C3H7NH3][NO3]. The calculations considered Becke’s three-parameter hybrid exchange functional and the Lee−Yang−Parr correlation functional (B3LYP)29,30 with the 6-31++G(d,p) basis set. We first performed a structural optimization by classical molecular dynamics simulation and then proceeded with the optimization using DFT, because the number of local minima of the potential energy surface becomes very large when the cluster size increases. We found no imaginary vibrational frequencies after DFT optimization of the cluster. The anti and gauche conformers of the CCCN dihedral angle of the [C3H7NH3]+ cation were optimized. Quantum chemistry calculation of a single ionic pair of alkylammonium nitrate is unstable toward formation of the corresponding amine and nitric acid.7,13 The vibrational frequencies were calculated for clusters of n = 2, 3, and 4 ionic pairs, with the [C3H7NH3]+ cations in the anti conformation.

II. EXPERIMENTAL AND COMPUTATIONAL DETAILS The ionic liquid propylammonium nitrate, [C3H7NH3][NO3], was purchased from Iolitec and used without further purification. It was dried under a high vacuum (below 10−5 mbar) for several days, and Karl Fischer analysis indicated that water content in the ionic liquid was reduced below 30 ppm. Thermophysical characterization of [C3H7NH3][NO3] was performed with a differential scanning calorimeter (model DSC 2910, TA Instruments) using aluminum pans, hermetically sealed using a sample encapsulating press, with an inert atmosphere (argon). Liquid nitrogen was used as the coolant. The sample was first heated above room temperature to 320 K to remove crystal nuclei eventually present in the liquid phase. The sample was slowly cooled at a rate of 20 K min−1 to 180 K and then reheated at 5 K min−1. Raman spectra as a function of temperature at atmospheric pressure were recorded with a Jobin-Yvon T64000 triplemonochromator spectrometer equipped with a charge-coupled device (CCD). The spectra were excited with the 647.1-nm line of a mixed argon−krypton Coherent laser, and the spectral resolution was 2.0 cm−1. Raman spectra as a function of temperature were obtained in the usual 90° scattering geometry with no selection of polarization of the scattered radiation. Temperature control was achieved with an Optistat DN cryostat (Oxford Instruments). The ionic liquid was rapidly cooled to 180 K at a rate of 20 K min−1, and sample crystallization was observed by visual inspection. The sample was then reheated in a stepwise manner, allowing for 30 min of equilibration each 10 K, until spectral changes due to a solid− solid transition at 220 K were observed.

III. RESULTS AND DISCUSSION III.A. Temperature Effects. The differential scanning calorimetry (DSC) scan of [C3H7NH3][NO3] in Figure 1 shows crystallization of the sample at 209 K and a solid−solid phase transition starting at T ≈ 220 K during heating of the crystal. The endothermic peak observed at 242 K in Figure 1 suggests that partial melting takes place before the melting of [C3H7NH3][NO3] at 271 K. The complex pattern of the DSC scan of [C3H7NH3][NO3], with broad peaks or shoulders on

Figure 1. DSC scan of [C3H7NH3][NO3]. 10906



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similar to that of the liquid phase (Tm = 381 K). Even though the high-temperature crystals of [C2H5NH3][NO3] and [C3H7NH3][NO3] are certainly more disordered than the low-temperature crystals, for these two systems, there are clear differences between the Raman spectra of the crystal I phase in the 220−280 K range and the liquid phase. A distinctive feature of the ionic liquid [C3H7NH3][NO3] in comparison to previous results for [CH3NH3][NO3]7 and [C2H5NH3][NO3]8 is the important modification of the Raman spectrum in the 800−900 cm−1 range for different phases of [C3H7NH3][NO3]. The DFT calculations discussed below indicate that bands at 828 and 868 cm−1 are signatures of the gauche and anti conformers, respectively, of the [C3H7NH3]+ cation. The left panel of Figure 2 shows that the liquid phase contains a distribution of cations in both conformations, crystal II contains cations in the anti conformation, and crystal I contains cations mostly in the gauche conformation with a small population of the anti conformation. In the polar domains, the temperature-dependent rearrangements of the anion−cation structures are essentially the same for both [C2H5NH3][NO3] and [C3H7NH3][NO3]. The relatively broad band of the νs(NO3) mode in the liquid phase (green line in the right panel of Figure 2) has a highfrequency shoulder, suggesting that a distribution of environments experienced by anions remains after melting. Figure 3

the main peaks, has been observed in DSC studies of others ionic liquids (see the review in ref 31). Recently, Imanari et al.32 showed that the complexity of the thermal behavior of ionic liquids links phase transitions to conformational changes of the cations. In the case of [C3H7NH3][NO3], the temperaturedependent Raman spectra discussed below show that structural rearrangements following phase transitions occur in both the polar and nonpolar domains. Figure 2 shows Raman spectra of [C3H7NH3][NO3] in the ranges 700−900 and 1000−1100 cm−1 at different temper-

Figure 2. Raman spectra of [C3H7NH3][NO3] as a function of temperature at atmospheric pressure. The intensities of the Raman spectra were normalized by the most intense band in each panel.

atures. The differences between the Raman spectra of the lowtemperature crystalline phase at 180 K (crystal II) and the phase observed after heating the crystal to 220 K (crystal I) are clear from Figure 2. The Raman bands corresponding to the anion normal modes δ(NO3) and νs(NO3), at 717 and 1041 cm−1, respectively, split into different numbers of bands in crystals I and II, and they broaden in the liquid phase. Henderson et al.8 observed a similar modification of the anion Raman bands following the solid−solid phase transition at 220 K and melting at 280 K of [C2H5NH3][NO3]. The split of a degenerate normal mode, such as δ(NO3), is common depending on the symmetry of the crystalline site, but the split of the totally symmetric mode νs(NO3) has been assigned to the fact that the crystalline structure determined by X-ray diffraction of [C2H5NH3][NO3] is an asymmetric unit containing two anions and two cations.8 The occurrence of two νs(NO3) bands in the Raman spectrum of the hightemperature crystal I is reasonable in light of the two nonequivalent [NO3]− anions in unit cell of [C2H5NH3][NO3]. The complex pattern of the Raman spectrum in the range of the νs(NO3) normal mode for crystal II is related to the fact that the [NO3]− anion has three different N−O distances in the low-temperature crystalline phase or the fact that less intense cation Raman bands overlap this spectral range.8 Nevertheless, the Raman spectra shown in Figure 2 for [C3H7NH3][NO3] strongly suggest that the local environment around the anions changes in a manner similar to that found previously for [C2H5NH3][NO3], independently of different lengths of the alkyl chain. The effect of a shorter alkyl chain is seen in the Raman spectra reported by Bodo et al.7 for [CH3NH3][NO3]: The crystal II−crystal I transition of [CH3NH3][NO3] takes place at much higher temperature, 351 K, and the Raman spectrum of crystal I is already very

Figure 3. νs(NO3) Raman band of the ionic liquid [C3H7NH3][NO3] at (top) 293 and (bottom) 420 K. Black circles and red lines represent the experimental spectra and the spectra fit with two Lorentzian components (shown as blue lines), respectively.

shows that the main band at 1042 cm−1 becomes more symmetrical at high temperatures because the relative intensity of the shoulder at 1049 cm−1 decreases. It is well-known that the vibrational frequencies of the nitrate normal modes are strongly dependent on the strength of the anion−cation interactions in high-temperature molten alkali nitrates (see the review in ref 33). It is interesting to compare the main band at 1042 cm−1 and the shoulder at 1049 cm−1 observed in the Raman spectrum of liquid [C3H7NH3][NO3] with the values for alkali nitrates. Janz and James34 showed that, in the series of molten alkali nitrates CsNO3, RbNO3, KNO3, NaNO3, and LiNO3, the νs(NO3) vibrational frequency increases in the order 1043, 1046, 1048, 1053, and 1067 cm−1, respectively. In aqueous solution, the νs(NO3) vibrational frequency is 1050 cm−1.34 In the case of [C3H7NH3][NO3], the finding that the 10907



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contour of the Raman band of the νs(NO3) normal mode changes with temperature gives further support for the occurrence of a corresponding temperature-dependent distribution of local structures around the anions in the normal liquid phase. Quantum chemistry methods have been instrumental in deciphering the structures of ionic clusters in alkylammonium nitrates ionic liquids.7,13 The calculation of the [NO3]− vibrational frequencies is not feasible if the DFT calculation is performed for a single ionic pair because proton transfer takes place with the formation of propylamine and nitric acid. We performed DFT calculations for two conformers, gauche and anti, of the CCCN dihedral angle of the [C3H7NH3]+ cation to address the vibrational normal modes in the 800−900 cm−1 range, and we performed DFT calculations with increasing numbers of ionic pairs, n = 2, 3, and 4, with the cations in only the anti conformation. In line with previous calculations by Bodo et al.,7,13 we found better correspondence between the theoretical and experimental Raman spectra as the number of ionic pairs increased. Similar calculations have been discussed at length in refs 7 and 13, so we keep the discussion brief to those aspects directly related to this work that have been not addressed in these previous studies. The DFT calculations showed that an intense band is expected in the 800−900 cm−1 range whose actual position depends whether the cation is in the anti or gauche conformation. Figure 4 shows the relatively complex pattern

Figure 5. Vibrational frequencies of the (top) δ(NO3) and (bottom) νs(NO3) normal modes calculated by DFT for clusters of [C3H7NH3][NO3] containing n = 2, 3, and 4 ionic pairs. The labels A−D for the n = 4 cluster in the bottom panel correspond to the four [NO3]− anions in the configuration of Figure 6.

calculations confirm that significantly different frequencies are expected from a distribution of different anion−cation local structures. Figure 6 shows the optimized structure of the [C3H7NH3][NO3] cluster with n = 4 ionic pairs. It is clear in this structure that the [NO3]− anions interact with the −NH3 Figure 4. (A) Anti and (B) gauche conformers of the [C3H7NH3]+ cation and displacement vectors of the characteristic normal modes at 837 and 803 cm−1, respectively, calculated by the DFT method.

of displacement vectors calculated for these two normal modes. The DFT calculation provides a clear assignment of the two bands observed in the 800−900 cm−1 range of the experimental spectra whose relative intensity is strongly dependent on the thermodynamic state of [C3H7NH3][NO3] (see the left panel of Figure 2). The high-frequency band, observed at 868 cm−1 in the experimental spectra, is assigned to the anti conformer, whereas the low-frequency band, observed at 828 cm−1 in the experimental spectra, is assigned to the gauche conformer. Figure 5 shows the vibrational frequencies of the δ(NO3) and νs(NO3) normal modes obtained by DFT calculations with different numbers of ionic pairs [C3H7NH3][NO3]. In line with previous studies,7,13 we also found the Raman spectrum in the whole spectral range calculated for the n = 4 cluster to be in reasonable agreement with the experimental data. The splitting of the δ(NO3) normal mode is clear in the calculated vibrational frequencies shown in the top panel of Figure 5. For instance, for the n = 4 cluster, there is a group of four bands within the 707−712 cm−1 range and another group of four bands within the 720−733 cm−1 range. Considering the n = 4 cluster, the vibrational frequencies calculated for the νs(NO3) normal mode cover the 1040−1070 cm−1 range, so that the

Figure 6. Optimized structure of a cluster with n = 4 pairs of ions obtained by DFT calculations. Short H···O distances are given in angstroms. The vibrational frequencies of the νs(NO3) normal mode for the anions labeled A−D are given in Figure 5. 10908



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comparison to those in the normal liquid phase, is not necessarily related to a corresponding inversion of population of gauche/anti conformers because the Raman cross sections for these normal modes might be different at such high pressure. This warning is particularly important in light of the significant inversion of relative intensities seen in the left panel of Figure 7 for these cation bands and the δ(NO3) band between the liquid and the high-pressure crystalline phases. Nevertheless, the high-pressure results indicate again that structural rearrangements with more or less ordered local structures might occur in the polar and nonpolar domains of [C3H7NH3][NO3] independently from each other. Figure 8 shows other measurements in which the pressure was increased in a stepwise manner to 1.0 GPa. We obtained

group of the cations and the alkyl chains point toward the outside of the cluster. Short O···H distances are given in Figure 6, and we found O···H−N angles smaller than 180°, in agreement with similar results obtained by Bodo et al.7,13 The [NO3]− anion indicated as D in Figure 6, which has the lowest νs(NO3) vibrational frequency in Figure 5, has the shortest distances to the nearest-neighboring cation, whereas [NO3]− anion A, which has the highest νs(NO3) vibrational frequency, has the longest distance to cations in the cluster of Figure 6. This finding agrees with conclusions of Bodo et al.7 obtained from DFT calculations of clusters of [CH3NH3][NO3] that a relatively strong hydrogen bond implies weakening of the anion N−O bond. Therefore, these theoretical results strongly suggest that a complex Raman band shape of the νs(NO3) normal mode will result as long as a distribution of anion− cation distances appropriate to the very distorted network of hydrogen bonds exists in the bulk of alkylammonium nitrates. III.B. Pressure Effects. There are several studies in the literature showing crystallization of nonprotic ionic liquids under high pressure in diamond anvil cells (DACs).21,22,24,25 In the case of [C3H7NH3][NO3], the fact that the anions experience a complex distribution of environments with different strengths of anion−cation interactions in the liquid phase has a significant impact on the structural arrest under high pressure. We observed that microscopic heterogeneity develops in the sample depending on the rate of increase of pressure. We first show in Figure 7 the Raman spectrum

Figure 8. Inset: Photograph of the sample chamber in the DAC with [C3H7NH3][NO3] at 1.0 GPa and the resulting micro-Raman imaging using the spectral region indicated by the blue square of the Raman spectra in the right panel. The Raman spectra in red and green correspond to different regions of the mapping. For comparison purposes, the Raman spectrum of the liquid phase at room temperature and atmospheric pressure is shown by the dashed line. The intensities of the Raman spectra in each panel have been arbitrarily normalized.

different Raman spectra by focusing the laser beam in different parts of the sample. The component observed at 1062 cm−1 in the band contour of the νs(NO3) normal mode (red line in the right panel of Figure 8) is close to the single sharp band observed in the high-pressure spectrum of Figure 7. However, there remains an intense component at 1051 cm−1, and the double-peaked band contour is different in different parts of the sample (red and green lines). The clear difference of the band shape of the νs(NO3) mode in the spectral range indicated by the blue rectangle in Figure 8 allowed for micro-Raman mapping of the sample in the DAC. The picture shown in the inset of Figure 8 is the Raman imaging of the gasket in the DAC with different colors corresponding to the regions of each Raman spectrum. It should be noted that both of the highpressure spectra in Figure 8 indicate cations in the anti conformer despite the different contours of the anion bands. Therefore, these results again show that different local structures in the polar domains allow for the same anti conformation in the nonpolar domains. Low-frequency Raman spectroscopy provides evidence that the two phases observed by the Raman mapping of [C3H7NH3][NO3] under pressure correspond to crystalline phases, rather than glassy phases. In the normal liquid phase, the low-frequency range of the Raman spectrum is dominated by the quasielastic scattering due to fast (picosecond)

Figure 7. Inset: Photograph of the sample chamber in the DAC after crystallization of [C3H7NH3][NO3] at 2.0 GPa. The solid line shows the corresponding Raman spectrum, and for comparison purpose, the Raman spectrum of the liquid phase at room temperature and atmospheric pressure is shown by the dashed line. The intensities of the Raman spectra in each panel were arbitrarily normalized.

obtained after crystallization of [C3H7NH3][NO3] when the pressure was increased quickly to 2.0 GPa. Crystallization was easily identified by visual inspection of the sample in the microscope, and the inset of Figure 7 shows a photograph of the sample chamber in the DAC. In this case, the Raman spectrum was the same in different parts of the sample without any indication of microscopic heterogeneity. The homogeneity of local environments experienced by the anions is indicated by the sharp Raman band of the νs(NO3) mode at 1070 cm−1. It is also worth noting that there is no split of the Raman band corresponding to the δ(NO3) normal mode at 727 cm−1. On the other hand, the occurrence of cations in both the gauche and the anti conformers is clear from the Raman bands at 858 and 883 cm−1, respectively. The inversion of the relative intensities of these two bands in the high-pressure crystal, in 10909



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relaxational processes (green line in the inset of Figure 9).35−37 The quasielastic scattering intensity decreases as the liquid is

Raman spectra given by the red and green lines correspond to the regions in the Raman mapping of the sample chamber. In this case, part of the sample has cations in anti conformation (red line), whereas part of the sample has a distribution of cations in anti and gauche conformations (green line), as in the normal liquid phase. The Raman imaging in Figure 10 provides a nice illustration of the microscopic heterogeneity appearing in [C3H7NH3][NO3] under high pressure. It strongly suggests that nuclei are randomly generated in the sample with different local structures, but their growth becomes frustrated. The actual band shape of the resulting Raman spectrum depends on the pressure rate and the final pressure in a complicated way because of the random nature of nucleus formation. The microscopic heterogeneity shown in Figure 10 for the [C3H7NH3][NO3] sample at 1.5 GPa was obtained after a quick increase of pressure, but we also observed heterogeneity in another run following a stepwise increase of pressure to 1.5 GPa. Previous spectroscopic and DSC investigations of ionic liquids have shown that slow dynamics at crystallization is related to the cooperative nature of molecular rearrangements and conformational changes at low temperatures.26,32,38,39 One expects that the very high viscosity of ionic liquids under pressure should imply analogous slow collective dynamics as previously observed in low-temperature crystallization. We found that, when a [C3H7NH3][NO3] sample was left inside the DAC under high pressure once the pattern of microscopic heterogeneity had been generated, the pattern remained at least for several weeks because the dynamical arrest at such high pressures precludes full crystallization of the sample.

Figure 9. Low-frequency ranges of the Raman spectra of the two regions of the mapping (red and green) for the same sample of [C3H7NH3][NO3] at 1.0 GPa shown in Figure 8. The inset shows corresponding data for the liquid phase (green) and the two lowtemperature crystalline phases of [C3H7NH3][NO3] at 180 K (black) and 220 K (red).

supercooled, and a broad intermolecular vibrational contribution at ∼20 cm−1, the so-called boson peak, is observed in Raman spectra of glassy phases of ionic liquids that do not crystallize.35−37 The low-frequency Raman spectrum of an amorphous solid phase does not exhibit sharp peaks as observed in the Raman spectrum of an ordered crystalline phase. As the ionic liquid [C3H7NH3][NO3] is easily crystallized, Raman spectra of this system at 180 and 220 K (black and red lines in the inset of Figure 9) indicate that they indeed correspond to two different crystalline phases. The red and green lines in the main Figure 9 are the low-frequency Raman spectra of the same [C3H7NH3][NO3] sample shown in Figure 8 that exhibits microscopic heterogeneity under pressure. The spectral features observed in the low-frequency Raman spectra of both of these high-pressure phases of [C3H7NH3][NO3] are clearly different from typical broad bands observed in Raman spectra of high-pressure glassy phases of ionic liquids.35 Therefore, the low-frequency range of the Raman spectra strongly suggests that the microscopic heterogeneity observed in [C3H7NH3][NO3] under pressure corresponds to crystalline phases. Figure 10 shows another example of microscopic heterogeneity in a sample of [C3H7NH3][NO3] at 1.5 GPa. The

IV. CONCLUSIONS Crystal polymorphism of the protic ionic liquid [C3H7NH3][NO3] was followed by signatures in Raman spectra as a function of temperature, revealing structural rearrangements taking place in both the nonpolar domains, in which there is dominance of van der Waals interactions between the atoms of the alkyl chains of the cations, and the polar domains, in which there is dominance of Coulombic and hydrogen-bonding interactions between the anions and the −NH3 group of the cations. The distorted network of hydrogen-bonded anion− cation interactions implies a distribution of local environments around the anions. Such different local structures can be arrested in isles of microscopic heterogeneity under high pressure. Raman spectra obtained for [C3H7NH3][NO3] under high pressures show that such structural rearrangements in the polar and nonpolar domains can be independent of each other. As the main contribution to the potential energy comes from the Coulombic interactions and hydrogen bonds, the changes in the local structures of the polar domains should be a more important driving force for polymorphism in [C3H7NH3][NO3] than different conformations of the alkyl side chains in the nonpolar domains.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Laboratório Multiusuário de Espectroscopia Ó ptica Avançada at UNICAMP for the availability of

Figure 10. Same as Figure 8 for a sample of [C3H7NH3][NO3] at room temperature and 1.5 GPa. 10910



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the Jobin-Yvon T64000 Raman spectrometer with a coupled microscope. The authors are indebted to FAPESP and CNPq for financial support.

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