Influence of Alkyl Chain Length on Microscopic Configurations of the

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The Influence of the Alkyl Chain Length on the Microscopic Configurations of the Anion in the Crystalline Phases of PYR -TFSI 1A

Oriele Palumbo, Francesco Trequattrini, Giovanni Battista Appetecchi, and Annalisa Paolone J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 05 May 2017 Downloaded from http://pubs.acs.org on May 9, 2017

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The Journal of Physical Chemistry

The Influence of the Alkyl Chain Length on the Microscopic Configurations of the Anion in the Crystalline Phases of PYR1A-TFSI. O. Palumbo1,*, F. Trequattrini1, 2, G. B. Appetecchi3 and A. Paolone1 1

CNR-ISC, U.O.S. La Sapienza, Piazzale A. Moro 5, 00185 Roma, Italy

2

Physics Department, Sapienza University of Rome, Piazzale A. Moro 5, 00185 Roma, Italy

3 ENEA,

Materials and Physicochemical Processes Laboratory (SSPT-PROMAS-MATPRO), Via

Anguillarese 301, 00123 Rome, Italy

ABSTRACT The infrared spectra and their temperature dependence are measured for a series of pyrrolidinium based ILs sharing the bis(trifluoromethanesulfonyl)imide (TFSI) anion and having alkyl chains of different length. While in the liquid or glassy state both conformers of TFSI are retained for all compounds, in the solid state a strong predominance of trans-TFSI occurs in ionic liquids with alkyl chains shorter than five C-H groups; on the contrary, for alkyl chain longer than six C-H groups crystalline phases display only cis-TFSI, which is a rare configuration in solids. Moreover, a mixed system composed by a short chain liquid (PYR14-TFSI) with one having a longer chain (PYR18 -TFSI) in a mass ratio 1:1 is studied. The competition between the two conformers of TFSI hinders the crystallization and gives rise to a glass transition around 183 K.

* Corresponding author: Oriele Palumbo, CNR-ISC, Piazzale A. Moro 5, I-00185 Rome, Italy Email: [email protected] Fax: +39-06- 49694323 Tel.: +39-06-49914400

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INTRODUCTION Ionic liquids (ILs) have been largely investigated for their properties which make them good candidates for “environment friendly” applications. Moreover, the wide variety of possible anions/cations and/or functional/substitute groups allows the finely tuning of their physicochemical properties, such as viscosity, electrical conductivity, melting point and glass transition temperature, aiming to match particular operating conditions. These macroscopic properties are strictly related to the microscopic changes occurring in the materials and, indeed, the molecular configurations of anions and cations are the key point to understand the dependence of the macroscopic properties of the ILs on their ionic composition. In previous works1-6, we reported the temperature and pressure dependence

of

the

infrared

absorption

spectra

of

several

ILs

sharing

a

common

bis(trifluoromethanesulfonyl)imide (TFSI) anion, both in the liquid and in the solid phases in order to ascertain the changes occurring in the intramolecular structure at the phase transitions. Indeed the TFSI anion, is a flexible molecule that can adopt two energetically inequivalent conformations, the transoid (trans) and the cisoid (cis) forms, whose concentration affects the ILs physical and chemical properties in both the liquid and solid phases. The transoid conformer with a C2 symmetry is more stable than the cisoid one, which has a C1 symmetry, however due to the low energy separation 2.2 kJ/mol both conformers are present in the liquid state7-8. The infrared absorption spectra of ILs allow the detection of the different conformer of the ions and the evolution of their concentration as a function of pressure and temperature. In particular, the comparison1 between abinitio calculations of the infrared-active intramolecular vibrations and experiments confirmed that the spectral lines observed at 602 and 650 cm-1 can be assigned to the cis-conformer, whereas the line at 628 cm-1 can be assigned to the trans-TFSI1-8. Such infrared spectroscopy studies indicated2, 5 that different length of the alkyl chains in ammonium based ILs induces different relative concentration of the anion conformers. In particular, we showed that while the liquid phases of

N-trimethyl-N-propylammonium – TFSI (TMPA-TFSI)

and

N-trimethyl-N-

hexylammonium - TFSI (TMHA-TFSI) are all characterized by the presence of both TFSI ACS Paragon Plus Environment

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conformers, the solid phase shows different behavior for the two compounds: in solid TMPA-TFSI, the relative concentration of the conformers is strongly shifted toward a predominance of the transoid conformer; on the contrary, the solid TMHA−TFSI, with a longer chain, retains only the cis-TFSI, even if this rotamer is less thermodynamically stable. Actually this behavior is rather striking and has been reported in very few solid systems. Indeed, 1,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide (Tm = 22°C) was the first example of ionic liquid with the unusual cis-geometry9; also 1-Ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide adopts the cisoid conformation of the TFSI anion in the solid10. Ionic liquids can exhibit also glass or supercooled phases at low temperature. Previous investigations showed that in these physical states both conformers of TFSI are present3-6. In order to study the role of the cation alkyl chain on the relative concentration of the anion conformers and the possible link of the length of the alkyl chain with the conformer of TFSI retained in the solid phase, in the present work, we measure the infrared spectrum and its temperature dependence for a series of pyrrolidinium based ILs sharing the TFSI anion but with alkyl groups of different length. The PYR1A-TFSI (where A stands for the number of carbons in the alkyl side chains) liquids have been proposed as electrolyte components (in the place of volatile and hazardous organic solvents) for lithium batteries because of their ambient or sub-ambient melting point, high room temperature conductivity and suitable electrochemical stability11-16. These liquids have been largely studied17-20 demonstrating that their properties are strongly related to the nature of the alkyl side chain13, 20. Actually, the smaller and more symmetrical materials, the N,N-dimethyl pyrrolidinium (PYR11)+ and N-ethyl-N-ethylpyrrolidinium (PYR12 )+ - TFSI have relatively high melting point, above 90 °C, but, with the increasing of the alkyl chain length, the melting point decreases steeply on going from the material with the N-ethyl side chain to the one with the N-butyl side chain13, 17. However, at longer side chains the behavior of the melting point is not directly correlated with the length of the ACS Paragon Plus Environment


chain, but is more erratic13. Generally, thermal analysis measurements showed that materials having a linear side chain of three carbons or more have sub-ambient melting temperatures, as summarized in Table 1. Differential scanning calorimetry (DSC) measurements showed that most of these compounds usually exhibit a glass transition followed by devitrification before the endothermic melting process18. The glass transition temperature, the cold crystallization temperature and the melting point of the investigated PYR1A-TFSI samples are reported in Table 1. Once that the effects of the alkyl chain length on the relative concentration of the anion conformers are described, in the last part of this work, we further consider the effects of the mixing of two ionic liquids on the microscopic configurations occurring in the different phases. Indeed, it is known15, 21 that mixing of the liquids changes their properties. In a recent work21 we carried out a systematic investigation of mixtures of TMPA-TFSI and TMHA-TFSI, showing that when one of the two component is predominant the crystallization process is not hindered and the solid phase of the mixed system retain the properties of the main component. Conversely, for intermediate concentrations no crystalline phase can be obtained at low temperatures and the samples transforms into a glass in which both conformer of TFSI survive. This complex behavior at intermediate compositions was attributed to the competition between the two conformers of the TFSI anions21. In this framework, here we consider the case of a mixture of a short chain liquid (PYR14-TFSI) with one having a longer chain (PYR18-TFSI).

EXPERIMENTAL PYR1A with A = 3 and A > 4 were synthesized according to a procedure reported elsewhere22. Only PYR14-TFSI was purchased from Solvionic. As the purity was higher than 99.9 %, the sample was investigated as received without further purification.


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The mixed sample (PYR14 + PYR18)-TFSI was obtained by mixing the two starting ILs (PYR14-TFSI and PYR18-TFSI) with a 1:1 mole ratio15. Infrared spectroscopy measurements were performed by means of a Bruker IFS125 HR spectrometer at the AILES beamline of the SOLEIL Synchrotron23, 24. The spectra were recorded in the mid infrared at a resolution of 0.5 cm-1, by combining a KBr beamsplitter and a MCT detector. Thin layers of ILs were placed between the diamond windows of a vacuum tight cell. The transmission was calculated using the spectrum of the bare optical windows as a reference. Transmission measurements were converted to absorbance data. The samples were cooled down to 140 K by means of a Cryomec cryopump with a temperature rate of 5 K/min, and data were collected on heating at the same rate between this minimum temperature and 330 K. RESULTS AND DISCUSSION Figure 1 shows the temperature dependence in the range between 140 and 320 K of the infrared spectrum (550-700 cm-1) of samples PYR1A-TFSI measured on heating, after cooling at 5 K/min down to 140 K. In particular, spectra of samples PYR1A-TFSI with A in the range from 3 to 8 are reported on panels labeled with letters from “a” to “f” respectively. This spectral interval contains the markers of the anion conformers without almost any signature from the cations, as already reported both by computational and experimental works1-5, 7-8, 21. In particular, the bands around 602 and 655 cm-1 are ascribed to the cis-conformer, while the line around 618 cm-1 is due to the trans rotamer 1, 7-8. At low temperature, below the melting point of ~ 285 K (see Table 1), the spectrum of sample PYR13-TFSI (Fig. 1 panel a) does not display the lines at 602 and 655 cm-1, which are due to the cis-conformer, while the line at 618 cm-1 clearly appears, accompanied by a second narrow band around 630 cm-1, which is likely due to the cation (as displayed by DFT simulation reported in the SI). On heating, at 280 K the lines at 602, 618 and 655 cm-1 are all detectable in the measured



spectrum, while the narrow band at 630 cm-1 is no more detectable. This is the indication of the starting of the melting process, which is indeed completed at 290 K (Fig. 1 panel a), in agreement with values in Table 1. The experimental data indicate that in the solid phase only trans-TFSI is present, while above the melting point both TFSI conformers are present. Such behavior is consistent with the occurrence of only the trans conformer of TFSI in most solids containing the bis(trifluoromethanesulfonyl) imide anion1-6. The sample PYR14-TFSI (Fig. 1 panel b) shows a rather different behavior, as already reported in a previous work4, 6. At low temperature, both in the glass phase and below the cold crystallization temperature (~ 220 K), both TFSI conformers are present since the lines at 602, 618 and 655 cm-1 are all well detectable; on heating, the intensity of the line at 618 cm-1 increases while the other two bands completely disappear between 217 and 243 K, i.e., when the sample becomes a solid, as indicated by DSC measurements. At 270 K, the lines due to the cis-conformer are again present in the spectrum, in correspondence with the occurrence of the transition to the liquid state4, 6, 13, 18. Therefore, also PYR14-TFSI retains only trans-TFSI in the crystalline phases. The temperature dependence of sample PYR15-TFSI is similar to that observed for PYR13-TFSI. As displayed in Fig. 1 panel c, at low temperature, the spectrum of sample PYR15-TFSI presents only the line at 618 cm-1, due to the trans-conformer of TFSI. On heating the lines at 602, and 655 cm-1 are again detected in the spectrum measured at 300 K. This behavior suggests that at low temperature the sample is already in a crystalline state characterized by the presence of the low energy trans conformer. On heating , above 280 K, the sample melts and, as expected, in the liquid phase both conformers are present, in agreement with the melting point measured at 283 K by DSC measurements13. The spectra measured for samples PYR1A-TFSI with A ranging between 6 and 8 present a similar temperature dependence. The absorbance of samples PYR16-TFSI, PYR17-TFSI and PYR18-TFSI is showed in Figure 1, panels d, e and f, respectively. For all the three samples, at low temperature the ACS Paragon Plus Environment

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lines at 602, 618 and 655 cm-1 are all clearly detectable, indicating that both conformers of TFSI are present. This fact is consistent with the occurrence of a glass phase at low temperature13, 15, 20. On heating the intensity of the line at 618 cm-1, due to trans-TFSI, decreases and completely disappears around 240 K for PYR16-TFSI (260 K and 230 K respectively for PYR17-TFSI and PYR18-TFSI). On further heating, at 270 K for PYR16-TFSI (290 K and 260 K respectively for PYR17-TFSI and PYR18-TFSI) the line at 618 cm-1 increases again. The temperature range in which the band at 618 cm-1 disappears is the same in which DSC measurements evidenced the transition to a crystalline state13, 15, 20. Therefore, the present measurements clearly indicate that PYR16-TFSI, PYR17-TFSI and PYR18-TFSI retain only the cis conformer of TFSI in the crystalline state. These results clearly show that the configurations assumed by the TFSI anion in the different states of the PYR1A-TFSI materials are strongly influenced by the length of the cation alkyl chain. Indeed, after cooling at 5 K/min down to 140 K, samples PYR13-TFSI and PYR15-TFSI are solid and the relative concentration of the conformers is strongly shifted toward a predominance of the transoid conformer (see fig S3). In the liquid phase both conformers are present. All the other samples, after cooling at the same rate, are likely in a glassy state, characterized by the presence of both TFSI conformers2-3, 5. On heating, they undergo a cold crystallization and then melt. More indications about the liquid and glassy phase can be obtained by a quantitative analysis of the IR spectrum, which provides a detailed picture of the temperature evolution of the conformers in the samples1-6, 21. Indeed, the ratio of the intensities, , of the lines attributable to the different rotamers is proportional to their relative concentration.

=

=



where Ix indicates the integrated IR intensity of the band centered at wave number x2-3, 5, 21 after subtraction of a background. In the present case it was calculated as:

=

=

+

It can be observed that in the liquid state the relative concentration of the two conformers of TFSI follows the Boltzmann law, which leads to the following Van't Hoff relation:

ln = −

1 ∆ ∆! + + "

where ∆H and ∆S are the enthalpy and entropy difference between the two TFSI conformers, but on entering the glass state, the distribution of the conformer population is completely frozen and the quantity ln(r) does not depend on the temperature2-3, 21. In the supporting info (see Fig. S2), the plot of ln(r) vs 1/T for all the samples is reported together with an example of the fit of the absorbance spectrum obtained by adding three Lorentzian, one for each of the three lines corresponding to the TFSI conformer. The slope of the linear regression of ln(r) vs 1/T provides the value of ∆H. The ∆H values obtained for the liquids (ranging between 3.1 and 5.3 kJ/mol) are reported in the supporting info (Table S1) and are all very close within the errors, except for a slightly lower value obtained for PYR14 – TFSI. These values are compatible with the ones reported for other ILs containing the TFSI anion, which range between 3 and 7 kJ/mol2-3, 21. Moreover they are all positive, confirming an increase of the cis concentration with the temperature in the liquid phase, even if a strong dependence of their on the alkyl chain length is not observable. The liquid or supercooled phases of all the samples are characterized by the presence of both TFSI conformers, while the solid phases show different behavior for the different compounds: indeed in solid PYR1A-TFSI with A ranging between 3 and 5 the TFSI anions are mainly in their trans configuration; on the contrary, increasing the chain length, the solid PYR1A-TFSI with A ranging between 6 and 8 retains only the cis-TFSI, even if this rotamer is less ACS Paragon Plus Environment

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thermodynamically stable. Such a behavior might be related with the ability of cations, exhibiting long aliphatic side chains (A > 4), to form nanoscale alkyl domains embedded into polar regions of the ionic liquid bulk. Therefore, these nanoaggregates, previously evidenced by SWAXS measurements performed on piperidinium TFSI ILs25, could lead to internal structural reorganization, stabilizing the less thermodynamically unstable, but kinetically favored cis-TFSI conformer. This issue is worthy, however, to be more deeply investigated. Solid phases in which the cisoid conformer of TFSI is retained are extremely rare, and only a few examples are available. The solid phases of the alkali metal salts of TFSI were reported to show the cisoid conformer9, even though in those crystals a large variety of solvent inclusion was observed. Two imidazolium based ionic liquids display the occurrence of cis-TFSI in the solid: 1,3-dimethylimidazolium9 and 1-Ethyl-3-methylimidazolium10. Xue et al. attributed the occurrence of cis-TFSI in the solid phases of the alkali metal salts to a strong cation-anion interactions26. Holbrey et al.9 suggested that the presence of the cis-conformer in solid 1,3-dimethylimidazoliumTFSI is due to the formation of an extended hydrogen-bonding network generated by the interaction between the O atom of the anion and C-H group of the cation. Vitucci et al.2 proposed that cis-TFSI is preferred in solid TMHA-TFSI because of different force balance experienced by the TFSI, compared to that one experienced with the ion with shorter alkyl chain TMPA-TFSI. The present study of the ionic liquids PYR1A-TFSI confirms the similar behavior previously reported for ammonium based ILs sharing a common TFSI anion2. In particular, we suggest that in the solid phase the longer alkyl chains stabilize the less stable conformer of TFSI by changing the coulombic interactions between anions and cations, thus modifying the local potential profile experienced by the anion and therefore its conformer distribution. The effects of inter and intra-molecular interactions on the ions conformer distribution has been suggested in several works27-29. Indeed, vibrational spectroscopy experiments29 on protic TFSI based ionic liquids, having imidazolium [Cnim] ions with increasing length of the alkyl chain as



cations, claimed that the TFSI conformer distribution is governed by the balance between the intramolecular Coulombic forces and the intermolecular forces, like van der Waals or hydrogen bond. For these protic liquids an increase of the alkyl chain length favored the trans- conformation, but a different behavior is reported for imidazoliom based liquids27 having an aprotic character similarly to those ones studied in the present work. Moreover an effect of the alkyl chain length also on the conformational equilibrium of the cations has been reported in imidazolim based Ils at high pressures28. Likewise, in this case the changes in the conformer distribution were attributed to the competition between the coulombic interaction and the alkyl-chain packing effect, highlighting its role in the occurrence of pressure induced glass transitions28. Once the nature of the conformers of TFSI in the different phases of the various ionic liquids PYR1A-TFSI is established, it is interesting to investigate their temperature dependence in a mixed system, since it is well known that mixing of different ILs hinders the crystallization process. In particular we focused on a binary mixture composed by two liquids having different composition of the solid phase. We selected a mixture composed in a mass ration 0.50 - 0.50 by PYR1A-TFSI, whose solid phase is characterized by a predominance of the trans-TFSI, and PYR18-TFSI, which instead in the solid phase retains the cis-conformer and we measured the infrared spectrum as a function of the temperature for the mixture, as reported in Fig. 2. The three lines at 602, 618 and 655 cm-1 are all present in the measured spectrum in the whole investigated temperature range, indicating that the two conformers are present both in the liquid and in the glassy phase of the sample. Indeed, it has been reported that blending pyrrolidinium ionic liquids with differing cation aliphatic side chain length, remarkably hinders or prevents the IL mixtures from crystallizing15. In particular mixed systems of PYR14-TFSI and PYR18-TFSI do not present any crystalline phase and remain amorphous, thus showing an extended liquid range down to Tg, which is measured around - 80 °C (190 K), with no dependence on the (PYR18)+ content. As previously described for the pure ionic


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liquids, a quantitative analysis of the IR spectrum provides a detailed picture of the temperature evolution of the conformers in the mixed samples1-6, 21. In Figure 3 the plot of ln(r) vs 1/T for the mixture (PYR14 + PYR18)-TFSI is reported together with an example of the fit of the absorbance spectrum obtained by adding three Lorentzian, one for each of the three lines corresponding to the TFSI conformer. The slope of the linear regression of ln(r) vs 1/T provides the value of ∆H. For the sample (PYR14+PYR18)-TFSI one obtains an enthalpy value of ∆H = 5.2±0.1 kJ/mol, which is compatible with the ones reported for other ILs containing the TFSI anion, which range between 3 and 7 kJ/mol3,

8, 21, 30

. Moreover, Figure 3 provides a clearly indication of the glass transition

temperature, detected around 190 K, in perfect agreement with the value obtained by DSC measurements15. This result confirms that in mixed systems composed by combining two ILs, which display different TFSI conformers in the solid phases, the crystallization process is hindered, and provides a quantitative analysis of the conformers distribution. Indeed, in the liquid with the cation having the shorter alkyl chain the trans-conformer is the more stable, while the cation with longer alkyl chains stabilizes the less stable conformer of TFSI by means of stronger interactions between anions and cations. As previously suggested21, in the mixed systems, the presence of cations with different alkyl chains induces the occurrence of competition between the two TFSI conformers at low temperatures, which can be the origin of the lack of crystalline phases.

CONCLUSIONS In the present study we investigated the infrared spectrum and its temperature dependence for a series of PYR1A-TFSI (with A ranging between 3 and 8) to understand the role of the cation alkyl chain on the TFSI conformers distribution during the occurrence of the different physics phase. ACS Paragon Plus Environment


Results clearly show that cation with longer alkyl chains stabilize in the solid phase the less stable cis-conformer of TFSI, likely by means of different balance of forces between anions and cations. In a mixed systems, the competitive effect on the stability of the TFSI conformers induced by the presence of cation with different alkyl chains hinders the crystallization process. ACKNOWLEDGEMENTS We wish to thank P. Roy and J.-B. Brubach for assistance at the AILES beamline of Synchrotron Soleil during beamtime # 20150313.


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TABLES

alkyl mp [K] Sample

side

mp [K]

Tg [K]

Tcc [K]

18820

2232

(this work)

chain

PYR13-TFSI

n-propyl

2851

280

PYR14-TFSI

n-butyl

2704

270

n-pentyl

283 13

300

PYR15-TFSI

2 cold crystallization 192 20 at 215 and 243 K 20 Broad

PYR16-TFSI n-hexil

27113

270

191 20

recrystallization from 233 to 271 K 20

PYR17-TFSI

cold crystallization n-heptyl

28515

290

19320 from 256 to 289 K

PYR18-TFSI

n-octyl

25915

260 18315

(PYR14 + PYR18) -TFSI

Table 1. Physical Properties (melting point (mp), Glass transition temperature (Tg) and Recrystallization temperature (Tcc)) of synthesized PYR1A-TFSI materials. Superscript indicates the reference for the reported values.



FIGURES

170 K 220 K 270 K 280 K 290 K 300 K

trans

absorbance (a. u.)

Pyr13 cis cis

170 K 210 K 217 K 243 K 270 K 297 K 320 K

trans

PYR14-TFSI

absorbance (a.u.)

PYR13-TFSI

cis

cis

550

560

570

580

590

550 560 570 580 590 600 610 620 630 640 650 660 670 680

600

610

620

630

640

650

660

670

680

-1

wavenumber (cm )

-1

wavenumber (cm )

b)

a)

absorbance (a. u.)

140 K 240 K 280 K 300 K 330 K

cis cis

PYR16-TFSI

140 K 230 K 240 K 260 K 270 K 280 K 330 K

trans

cis absorbance (a. u.)

trans

PYR15-TFSI

cis

550 560 570 580 590 600 610 620 630 640 650 660 670 680

550 560 570 580 590 600 610 620 630 640 650 660 670 680

wavenumber (cm-1)

wavenumber (cm-1)

c)

d)

cis

140 K 220 K 230 K 260 K 280 K 290 K 300 K 330 K

cis

550 560 570 580 590 600 610 620 630 640 650 660 670 680

PYR18-TFSI

absorbance (a. u.)

trans

PYR17-TFSI

absorbance (a. u.)

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

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trans

160 K 220 K 230 K 250 K 260 K 290 K 330 K

cis

cis

550 560 570 580 590 600 610 620 630 640 650 660 670 680

-1

wavenumber (cm-1)

wavenumber (cm )

e)

f)


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Figure 1. Temperature dependence of the absorbance of samples PYR1A-TFSI measured on heating at 5 K/min.

(PYR14 +PYR18)- TFSI

160 K 220 K 230 K 250 K 260 K 290 K 330 K

trans

cis cis

550 560 570 580 590 600 610 620 630 640 650 660 670 680

wavenumber (cm-1)

Figure 2. Temperature dependence of the absorbance of sample (PYR14 + PYR18)-TFSI measured on heating at 5 K/min.



-0.6

0.2

T = 310 K

trans absorbance

-0.8 -1.0

0.1

cis cis

-1.2 0.0

ln(r)

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

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600

-1.4

610

620

630

640

650

660

670

680

-1

wavenumber (cm )

-1.6 -1.8 -2.0 -2.2 3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5

7.0

-1

1000/T (K )

Figure 3. Temperature dependence of the logarithm of the ratio of the intensities of the bands (sample (PYR14 + PYR18) TFSI) due to the transoid and cisoid TFSI conformers and best fit lines. In the inset an example of the fit of the absorbance spectrum and deconvolution into the contributions of the two conformers is reported.


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SUPPORTING INFORMATION Figure. S1 Upper panel: experimental absorbance of PYR13-TFSI as a function of temperature; lower panel: calculated infrared intensities of PYR13, cis- and trans-TFSI. Table S1. Unscaled calculated vibration frequencies (in cm-1) and infrared intensities (km/mol) of the PYR13 ion at the B3LYP/6-31G** level. Figure S2. Temperature dependence of the quantity ln(r) of samples PYR1A-TFSI Table S2. Enthalpy values obtained from the best fit lines for all the samples. Figure S3. Fraction of cis-conformer fcis calculated at T = 250 K and T = 330 K, and plotted as a function of n, which is the length of the alkyl chain.



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Influence of Alkyl Chain Length on Microscopic Configurations of the

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