Research Article | Open Access
A Computational and Experimental Study of the Conformers of Pyrrolidinium Ionic Liquid Cations Containing an Ethoxy Group in the Alkyl Side Chain
We investigate the conformers of the N-methoxyethyl-N-methylpyrrolidinium () and N-ethoxyethyl-N-methylpyrrolidinium () ionic liquid cations by means of DFT calculations at the B3LYP/ level and we calculate their infrared vibration frequencies. The comparison with the absorbance spectra of two ionic liquids containing these ions indicates good performance of such a combination of theory and basis set. The lowest energy conformer of each pyrrolidinium cation displays equatorial-envelope geometry; however, in contrast with the prototypical PYR14, the main alkyl side chain is not in an all-trans configuration, but it tends to be bent. Moreover, calculations indicate that the LUMO orbital extends more along the alkyl side chain in and than in the parent ion 1-butyl-1-methylpyrrolidinium (PYR14).
Ionic liquids are salts with melting points lower than 100°C. The large variety of possible anions and cations allows tailoring their physiochemical properties, such as viscosity, electrical conductivity, melting point, or glass transition temperature. They are considered as promising safe electrolyte components for lithium batteries and, more broadly, for a wide variety of electrochemical devices, especially the family with pyrrolidinium cations and per(fluoroalkylsulfonyl)imide anions, due to the subambient melting temperature, high room temperature conductivity, and suitable electrochemical stability . One of the most studied ILs is 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR14TFSI). It displays a rich phase diagram as a function of temperature : on cooling it undergoes a glass transition around −85°C; on subsequent heating, it is devitrified and enters an undercooled region; at ~−63°C it crystallizes and around −30°C it undergoes a solid-solid phase transition; finally it melts at ~−7°C. PYR14TFSI has been largely investigated by means of vibrational spectroscopy, aided by DFT calculations [3–8]. Both the TFSI anion and PYR14 cation possess conformers, due to the flexibility of the chemical bonds. TFSI has two conformers: the transoid one with C2 symmetry is more stable than the cisoid one which has C1 symmetry [8–10]. However, the two conformers are energetically separated by only 2.2 kJ mol−1. The cisoid and transoid forms of TFSI give different Raman and infrared spectra, as confirmed by experimental and computational studies [8, 11–13]. In particular, the infrared spectral lines observed at ~600 and 650 cm−1 can be assigned to the cis conformer, whereas the line at ~620 cm−1 can be assigned to trans-TFSI [5–8].
Concerning the PYR14 cation, some detailed studies linking the geometry of the ion with the vibrational properties are available [3, 4, 14, 15]. In these studies, the Raman spectra and the computed Raman activities are reported [3, 4, 14, 15]. DFT calculations evidenced that the occurrence of conformers of the pyrrolidinium ion is due to the fact that the C4N ring is not planar and can therefore adopt either the envelope or the twist configuration [3, 4, 14–16]. In the first case, one of the atoms is located outside the plane defined by the other four atoms; on the contrary, in the twist configuration, two atoms are located above and below the plane constructed by the other three atoms. The envelope and twist conformers can easily convert one into the other by means of pseudorotations, as the difference in energy between the different conformers can be as low as ~2.6 kJ mol−1 [14, 15]. It has been reported [14, 16] that, among others, the equatorial-envelope and axial-envelope conformers with the butyl group at equatorial and axial positions against the four carbon atoms of the pyrrolidinium ring are relatively stable, and the equatorial-envelope conformer gives the global minimum. The same authors suggested that these two conformers are present in equilibrium in molten PYR14TFSI .
Previous systematic studies of the conformers of the ions composing ILs have been reported for TFSI and PYR14 [3, 4, 12–14], bis(fluorosulfonyl)imide (FSI) [15–17], N,N-diethyl-N-methyl-N-propylammonium , imidazolium [11, 19–22], and ammonium ions [23, 24]. It must be noted that the presence of different conformers can alter the physicochemical properties of ILs. For example, it has been reported that ionic liquids containing different conformers have different melting points . It is, therefore, important to investigate the possible occurrence of various conformers of new ions. In this context, a combination of DFT calculations and vibrational spectroscopies, such as Raman or infrared, are extremely useful.
Recently, looking for better performances of ionic liquids of the pyrrolidinium family, some modifications of the alkyl chain have been proposed with the aim of decreasing the temperature at which solid phases are formed and having greater flexibility of the chain. Indeed, when an oxygen atom is introduced in the alkyl chain of the pyrrolidinium ion, no crystallization was detected down to −150°C for TFSI and IM14 [26–30].
To the best of our knowledge, the conformers of pyrrolidinium cations containing an ethoxy group in the main aliphatic side chain and their vibrational spectra have never been studied. The aim of the present work is to fill this gap, by means of a combination of DFT calculations and experimental investigations of the infrared spectra of two ILs as a function of temperature: TFSI and FSI. For comparison, also the conformers of PYR14 will be investigated again by means of DFT calculations.
2. Materials and Methods
Infrared spectroscopy measurements on FSI were performed by means of a Bruker Vertex 70 V spectrometer at the SISSI beamline of ELETTRA Synchrotron, while measurements on TFSI were performed by means of a Bruker Vertex 125 HR spectrometer at the AILES beamline of Soleil Synchrotron [31, 32]. In both cases, we used an MCT detector and a KBr beamsplitter; the spectral resolution was fixed at 1 cm−1. At ELETTRA, a thin layer of ionic liquid was placed between two CsI windows and mounted on a Heflow Helitran LT3 cryostat. At Soleil, a thin ionic liquid film was placed in a vacuum sealed cell (for liquids) with diamond windows and mounted on a CryoMac cryopump. In both cases, the samples were cooled down to ~130 K with a temperature rate of ~5 K min−1, which allows reaching a glass state at low temperatures [26–28, 33].
Preliminary calculations of the conformers of PYR14, , and cations were performed using the Spartan software [34, 35] at the molecular mechanics level. They show 33, 27, and 40 possible conformers, respectively. All possible geometries were optimized by means of DFT calculations, using the B3LYP functional and the basis set. This particular choice of basis set and theory was extensively used in previous literature in order to calculate the infrared and Raman spectra of ions composing ILs [2–4, 8, 36]. We calculated the infrared vibration frequencies and intensities for all the geometries having energy within 3.5 kJ mol−1 from that of the lowest energy conformer of each ion. The visualization of the ions was performed by means of the wxMacMolPlt software . For each cation, the IR spectrum was simulated by summing Gaussian curves centered at each calculated IR vibration frequency with a fixed 10 cm−1 peak width. As reported in Section 3, the good coincidence of the calculated vibration lines with the experimental absorption of the two ILs is itself a proof of the validity of the model based on the combination of theory and basis set presently used, which allows an easy study of the conformers of the cations. However, one must remember that, using the approach based on the calculations on single ions, one cannot obtain information about the interaction between anions and cations.
3. Results and Discussion
3.1. DFT Calculations of the Conformers of the Cations
In Figure 1, we report the structure of the lowest energy conformers of PYR14. As already reported in [14, 16], the equatorial-envelope conformer has the lowest energy. The axial-envelope and the twist configurations possess slightly higher energies: 1.8 and 3.0 kJ mol, respectively. In all cases, the side chain is in all-trans configuration; geometries with different configurations of the alkyl chain have higher energies.
Figures 2 and 3 display the geometries of the lowest energy conformers of the and cations, respectively, calculated at the B3LYP/ level, together with their relative energy differences. For both ions, the lowest energy conformer has equatorial-envelope geometry. However, differently from the case of PYR14, the side chain is not in an all-trans configuration: for it presents a cis-trans structure (Conf. 1 in Figure 2) while, for , it displays a cis-trans-trans geometry (Conf. 1 and Conf. 2 in Figure 3). For both cations, the conformers with the next higher energy are those with the twist structure of the pyrrolidinium ring (Conf. 2 in Figure 2 and Conf. 3 in Figure 3) and, subsequently, with the envelope pyrrolidinium geometry and the side chain at the axial position with respect to the ring (Conf. 3 and Conf. 4 in Figure 2 and Conf. 4 in Figure 3). For higher energies, the pyrrolidinium is in the envelope configuration, while the chain starts to be folded (see Figures 2 and 3). Among the conformers of and one can also find those with the side chain in the all-trans configuration; however, those conformers have higher energy than that of the lowest conformer by at least 20 kJ mol−1, so that at ambient temperature they have a negligible population.
The calculated vibration frequency and IR intensity values of the lowest energy conformers of and are reported in Tables 1 and 2, for the frequency range of 800–1100 cm−1, while the calculated absorbance is reported in Figures 4 and 5. The choice of this particular spectral range was motivated by the absence of absorption lines due to TFSI (from ~790 to ~1060 cm−1)  or due to FSI (from ~830 to ~1100 cm−1) .
As conformers 3 and 4 of and conformers 1 and 2 of differ only on the orientation of the side chain, their IR spectra are identical and, therefore, in the following, we will report only the spectrum of one rotamer for each couple.
3.2. Comparison with Infrared Spectroscopy Measurements
In order to validate the previously reported computational approach, we compared the calculated absorption of the two pyrrolidinium cations with the absorbance of TFSI and FSI. To avoid contributions from the TFSI or FSI anions, we limited our comparison to the spectral range between 800 and 1060 cm−1 for TFSI and from 860 to 1100 cm−1 for FSI. Indeed, in this spectral range, no contributions from the anions were previously reported [7, 38].
The absorbance spectra measured at two temperatures (e.g., around room temperature and at 130–140 K) for the two selected ionic liquids are reported in Figures 4 and 5. Both TFSI and FSI display several absorption bands in the considered spectral range. They are broader around room temperature, while they become sharper at low temperature. In many cases, at low temperatures, it is evident that the spectral lines are structured and composed of more than one contribution. Tables 3 and 4 report the frequencies of the absorption peaks found in the two compounds at both temperatures.
In Figure 4, one can observe that the peaks found in the experimental spectrum of TFSI nicely agree with those calculated for the lowest energy conformer, except for the peak located around 880 cm−1, which is completely absent in Conf. 1 of . However, sharp features around 870 cm−1 are present in the calculated spectra of Conf. 2, Conf. 3, and Conf. 4 (the last one is not shown as it is identical to Conf. 3). Therefore, we suggest the presence of at least these four conformers of the pyrrolidinium cation. Indeed, the low energy difference among them supports the occurrence of dynamic equilibrium of those rotamers. In this framework, one can easily explain the asymmetry of the experimental absorption band located around 1000 cm−1, which is evident at room temperature, and its splitting at low temperatures.
Also in the case of FSI, there is a general good agreement between the experimental spectrum and the calculated absorbance of the lowest energy conformer. However, the presence of the experimental vibration line centered around 880 cm−1 suggests the occurrence of Conf. 3 and Conf. 4, which display absorption bands around this value. Such features are maintained also at 130 K.
The comparison of the experimental absorbance with the calculated vibrations lines evidences for both ILs that they exhibit more than one conformer of the pyrrolidinium cation at both measured temperatures. One can note that at low temperatures both ILs enter into a glass state. Therefore, the present measures indicate that both in the liquid and in the glassy state more than one conformer of pyrrolidinium is present.
For both ions, the presence of conformers of higher energy, starting from Conf. 5, cannot be excluded; indeed according to the Boltzmann distribution function their occurrence is possible. However, their low concentration and the superposition of most of their absorption lines with those of the lower energy conformers prevent easy detection.
It must be noted that the good coincidence of the calculated vibration lines with the experimental absorptions of the two ILs is a proof of the validity of the DFT calculations based on the B3LYP functional and the basis set for the studied ions.
3.3. Calculations of the HOMO and LUMO Orbital
Once the structures of the lowest energy conformers have been confirmed by infrared spectroscopy measurements, the investigation is extended to the electronic properties. In Figure 6, the comparison among the calculated LUMO orbital of PYR14, , and calculated at the B3LYP/ level is reported. One can note that in all ions the LUMO extends on the whole pyrrolidinium ring and on the methyl group directly attached to it. For PYR14, the orbital extends slightly also on three of the CH groups of the side alkyl chain. In the case of and , the LUMO extends also on the fourth group of the side chain and displays bonding orbital between the first group and the second group of the chain, in contrast to PYR14.
Also, the HOMO orbital of the three ions, reported in Figure 7, displays some differences. In all three ions, it extends on the second, third, and fourth groups of the side chain. However, while in PYR14 the orbital on the second group is smaller than that of the third and fourth atoms of the chain, in and this difference is much smaller.
To summarize, an investigation of the conformers of two pyrrolidinium cations containing an ethoxy group in the main alkyl side chain has been performed by means of DFT calculations and validated by means of infrared absorbance measurements as a function of temperature. The lowest energy conformer of both pyrrolidinium cations has equatorial-envelope geometry. However, the side chain does not have all-trans geometry, as in the case of the prototype PYR14, but has a cis-trans configuration for and a cis-trans-trans structure for . For both ions, the conformers with the next higher energies are those with the twist structure of the pyrrolidinium ring and subsequently with the envelope geometry of pyrrolidinium and the side chain at the axial position with respect to the ring. Moreover, calculations indicate that the LUMO orbital extends more along the alkyl side chain in and than in the parent ion 1-butyl-1-methylpyrrolidinium (PYR14).
The authors declare that they have no competing interests.
The authors wish to thank P. Roy and J.-B. Brubach for assistance at the AILES beamline of Synchrotron Soleil during beamtime no. 20150313 and A. Perucchi and P. Di Pietro for assistance at the SISSI beamline of ELETTRA Synchrotron. This study was partially financed by ELETTRA Synchrotron for beamtime no. 20145151.
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