Potential Energy Landscape of Bis(fluorosulfonyl)amide - The Journal

J. Phys. Chem. B 2008, 112, 9449–9455

9449

Potential Energy Landscape of Bis(fluorosulfonyl)amide José N. Canongia Lopes,*,†,‡ Karina Shimizu,†,‡ Agílio A. H. Pa´dua,*,§ Yasuhiro Umebayashi,*,| Shuhei Fukuda,| Kenta Fujii,⊥ and Shin-ichi Ishiguro| Centro de Química Estrutural, Instituto Superior Te´cnico, 1049 001 Lisboa, Portugal, Instituto de Tecnologia Química e Biolo´gica, UNL, AV. Repu´blica Ap. 127, 2780 901 Oeiras, Portugal, Laboratoire de Thermodynamique des Solutions et des Polyme`res, CNRS-UniVersite´ Blaise Pascal Clermont-Ferrand, France, Department of Chemistry, Faculty of Science, Kyushu UniVersity, Hakozaki, Higashi-ku, Fukuoka, 812-8581, Japan, and Department of Chemistry and Applied Chemistry, Faculty of Science and Engineering, Saga UniVersity, Honjo-machi, Saga 840-8502, Japan ReceiVed: April 16, 2008; ReVised Manuscript ReceiVed: May 9, 2008

The conformational landscape of the bis(fluorosulfonyl)amide, [FSI]-, anion was analyzed using data obtained from Raman spectroscopy, molecular dynamics (MD), and ab initio studies. The plotting of three-dimensional potential energy surfaces and the corresponding MD simulation conformer-population histograms show the existence of two stable isomers, C2 (trans) and C1 (cis) conformers, and confirm the nature of the anion as a flexible molecule capable of interconversion between conformers in the liquid state. In ionic liquids, the two [FSI]- conformers coexist in equilibrium, a result confirmed by the Raman data. The implications of the conformational behavior of the ion [FSI]- are discussed in terms of the solvation properties of the corresponding ionic liquids. Introduction The most prominent property of ionic liquids, viz. their low melting temperature when compared with common inorganic salts, is usually caused by the size, charge delocalization, or asymmetry of the ions that compose them. The fact that those ions are generally nonrigid molecules and can adopt different conformations is another key feature that can partially explain that property. Traditionally, the different families of ionic liquids were defined in terms of their cations: the bulky, flexible, and organic ions. Different (small, rigid, inorganic) anions were usually employed to tune some of the properties of those families, for instance their hydrophilicity, viscosity, or stability.1 The bis(trifluoromethanesulfonyl)imide ion ([CF3SO2NSO2CF3]-, [TFSI]-, or bistriflamide) changed this state of affairs, and today the situation is somewhat more egalitarian, with many different anions gaining a classdefining status. Bistriflamide-based ionic liquids are particularly stable to thermal decomposition, have a relatively low viscosity, and exhibit many unique solvation properties, their relative hydrophobicity being one of them.2 In fact, the possibilities of this anion go beyond the scope of ionic liquids because when it is combined with small inorganic cations such as lithium, bistriflamide is a constituent of novel types of electrochemical devices (batteries, capacitors).3 The success of bistriflamide set off the search for anions with similar molecular structures: the triflate anion ([CF3SO3]- can be imagined as a rigid “half”-bistriflamide, whereas bis(perfluoroalkanesulfonyl)imides are “long-chain bistriflamides”. On the other hand, the bis(fluorosulfonyl)imide anion ([FSO2* Corresponding authors. E-mail: [email protected]; [email protected]; [email protected]. † Instituto Superior Te ´ cnico. ‡ Instituto de Tecnologia Química e Biolo ´ gica, UNL. § CNRS-Universite ´ Blaise Pascal Clermont-Ferrand. | Kyushu University. ⊥ Saga University.

NSO2F]- or [FSI]-) is a simplified (“shortened”) version of bistriflamide. [FSI]--based ionic liquids are even less viscous than bistriflamide-based ones; for instance, when these two anions are combined with the 1-ethyl-3-methylimidazolium cation ([C2mim]-) they yield ionic liquids with viscosities of 18 mPa s ([C2mim][FSI])4 and 33 mPa s ([C2mim][TFSI])5 and have important applications in the development of new electrolytes for devices such as the ones mentioned at the end of the last paragraph. One of the most conspicuous characteristics of the bistriflamide (and also [FSI]-) anions is their ability to adopt different conformations by rotation of the two groups attached to the central nitrogen atom. The “flexibility” of the bistriflamide anion, as opposed to the “rigidity” of most of the traditional anions that compose ionic liquids, can explain the unique solvation properties of bistriflamide-based ionic liquids. In the present work, we will extend a previous study6 on the structural behavior of bistriflamide-based ionic liquids to compounds containing the [FSI]- anion. In the first part of the work, we develop and validate a force field parametrization capable of modeling [FSI]--based ionic liquids within the framework of statistical mechanics (molecular dynamics, MD, or Monte Carlo) calculations. This parametrization is in the scope of the force field previously developed7,8 for ionic liquids, compatible with the OPLS-all atom (AA) model.9 In the second part, we calculate the conformational energy landscape of the isolated [FSI]- anion using ab initio calculations and we compare the results with condensed-phase data obtained by MD simulation and Raman spectroscopy experiments.10 To conclude, we also compare the torsion energy landscapes of [TFSI]- and [FSI]-, pointing out the similarities and differences between the two ions. No previous studies concerning the molecular structure or interactions of the [FSI]- anion were found in the literature.

10.1021/jp803309c CCC: $40.75  2008 American Chemical Society Published on Web 07/10/2008

9450 J. Phys. Chem. B, Vol. 112, No. 31, 2008 Experimental Section Simulation. MD simulations were used during the development and validation of the force-field parameters for the [FSI]anion and to obtain data concerning the distribution of conformers of that anion in imidazolium-based ionic liquids. All computer simulations were carried out using the molecular dynamics package DL_POLY.11 The development of a set of parameters capable of describing the torsions of articulated molecules or ions includes the ab initio calculation for a given dihedral angle of the compound of the corresponding torsion energy profile (see next point) and the calculation by molecular dynamics of that same torsion energy profile considering only nonbonded interactions in an isolated molecule or ion. The simulation procedure employed in this case consisted of a series of MD quench runs on isolated [FSI]anions during 10 000 timesteps of 0.5 fs under canonical ensemble conditions at 10, 1, 0.1, and 0 K. The dihedral angles being studied were constrained at the desired values by the addition of very steep harmonic terms. This method provides very accurate torsion energy profiles in a consistent manner, and thus the parameters obtained are transferable along a family of molecules or ions. This has been explained in detail in previous publications.7,8,12 As in previous cases,7,8,13 the validation of the new forcefield parameters was performed by comparing the molar densities of [FSI]--based ionic liquids obtained by MD simulation with experimental data.10 Both liquid and crystal densities were considered. In the case of simulations in the crystalline phase, the initial position, orientation, and conformation of each ion within the simulation box was the one defined by the lattice coordinates of the crystalline structure deposited in the Cambridge Structural Database (CSD).14 Since the overall size of the simulation box is defined by the dimensions of the unit cell of the crystal, several of these cells were stacked together to form a simulation box that would allow a cutoff distance of 160 pm, with the usual long-range corrections employed beyond this distance. The simulations were performed using a Nose´-Hoover thermostat coupled with an anisotropic Hoover barostat that allowed the simulation box to change volume and shape under constant (N, p, T) conditions. The temperature was fixed to match the one used during the crystallographic experiments, and the pressure was set to a null value. All runs were allowed to equilibrate for a period of 100 ps, followed by production times of 200 ps. These simulation times were found appropriate since the runs are started from a known initial configuration (the experimental equilibrium structure), and it was observed that the relaxation is complete before the end of the equilibration period. In the case of MD runs in the liquid phase, systems consisting of 250 ion pairs were simulated in periodic cubic boxes with cutoff distances of 160 pm and the appropriate long-range corrections were employed. Isotropic Nose´-Hoover thermostat and barostat were activated to maintain temperature and pressure, with time constants of 0.5 and 2 ps, respectively. The simulations were started from low-density initial configurations and allowed to equilibrate for 500 ps at 300 K. The final density was attained after about 50 ps. Further simulation runs of 100 ps were used to produce equilibrated systems at any desired temperature. The MD runs used to obtain the conformer distribution data followed those used to validate the force field (liquid-phase simulations). In this case, the systems were studied at 298 and 500 K; the last value corresponds to temperatures considered safe as regards the decomposition of many bistriflamide-based ionic liquids. The desired structural information was obtained

Lopes et al. from 1000 configurations that were stored from production runs of 300 ps. Successive 300-ps runs showed no drift in the corresponding equilibrium properties at this stage. Quantum Mechanical Calculations. Ab initio calculations were used during the development of the force field and also during the calculation of the potential energy profiles and surfaces corresponding to the conformational landscape of the [FSI]- ion. Geometry optimizations for an isolated [FSI]- anion were carried out at the RHF/6-31G(d) level and were followed by single-point energy calculations at the MP2/cc-pVTZ(-f) level.7 The calculations were carried out using the Gaussian03 program package.15 Raman Spectroscopy. Raman spectra were recorded employing an FT-Raman spectrometer (Perkin-Elmer GX-R) equipped with a Nd:YAG laser operating at 1064 nm, whose power was kept 1000 mW throughout measurements. The optical resolution was 2.0 cm-1, and spectral data were accumulated 1024 times to obtain sufficient signal-to-noise ratio. The sample liquids in a quartz cell were thermostatted within ( 0.3 K at a given temperature. Water content was checked by Karl Fischer coulometry and was less than 90 ppm for all samples examined. No appreciable decomposition was detected after the measurements. Raman spectra and their analyses such as band assignment and deconvolution were described elsewhere.6,10 Results and Discussion Force-Field Development and Validation. DeWelopment. The molecular force field used to represent the ionic liquids studied in this work is based on the OPLS-AA potential function,9 with parameters specifically tailored for the [FSI]anion. The function has the general form given in eq 1, with the traditional decomposition of the potential energy into covalent bonds, valence angles, torsion dihedral angles, and atom-atom pairwise repulsive, dispersive, and electrostatic contributions. The repulsive and dispersive terms are described by the Lennard-Jones 12-6 potential. bonds

uRβ )

∑ ij

angles kij θij (rij - r0,ij)2 + (θijk - θ0,ijk)2 + 2 2 ijk

∑

dihedrals

4

Vm,ijkl [1 + (-1)m cos(mφijkl)] + 2 m)1

∑ ∑ ijkl

nonbonded

∑ ij

{ [( ) ( ) ] 4εij

σij rij

12

-

σij rij

6

+

1 qiqj 4πε0 rij

}

(1)

The parameters describing bonds, angles, and Lennard-Jones interactions were taken directly from the AMBER/OPLS data set16 or from our previously published work on the parametrization of bistriflamide.7 On the other hand, we used extensive ab initio and MD calculations to carefully describe the conformational and electrostatic terms. This strategy is justified because stretching and bending force constants, which are associated with fast intramolecular modes, have a minor effect on the configurational properties that lead to equilibrium thermodynamic properties of a liquid phase. In contrast, accurate conformational energies and electrostatic charge distributions are relevant to render subtle energetic or conformational features, such as those responsible for many of the particular properties of ionic liquids. In Table 1, we give our set of parameters for the [FSI]- anion. For comparison purposes, X-ray diffraction data (RX) are also

Potential Energy Landscape of Bis(fluorosulfonyl)amide

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TABLE 1: Force Field Parameters for the [FSI]- Aniona Atoms

Q (e)

ε (kJ mol-1)

σ (Å)

S O F N

1.02 -0.53 -0.13 -0.66

1.046 0.879 0.222 0.711

3.55 2.96 2.95 3.25

Bonds N-S S-O F-S

R0 (Å) 1.57 1.437 1.575

kr (kJ mol-1 Å-2) 3137.0 5331.0 1879.0

r0 (Å) XR18 1.579 1.426 1.563

Angles O-S-O O-S-N S-N-S F-S-N F-S-O

θ0 (deg) 118.5 113.6 125.6 103.0 104.1

kθ (kJ mol-1 rad-2) 969.0 789.0 671.0 902.0 1077.0

θ0 (deg) XR18,19 115.0 112.6 125.0 103.9 104.2

Dihedrals O-S-N-S F-S-N-S

V1 (kJ mol-1) 0 11.445

V2 (kJ mol-1) 0 -15.186

V3 (kJ mol-1) -0.015 -3.212

a The parameters in normal script were taken directly from the OPLS-AA force field or adapted from the parameterization of bistriflamide, whereas those in bold were the result of ab initio and MD calculations performed in the present study.

included in the table. The details of the parametrization procedure for each class of interaction are discussed in the following paragraphs. The equilibrium distance and angle values taken from previous parametrizations were confirmed by geometry optimization using ab initio calculation at the RHF/6-31G(d) level and compared with X-ray diffraction data of [FSI]--based crystals. The corresponding stretching and bending force constants were estimated through the calculation of the frequencies of the different internal molecular modes. In some cases, ill-conditioned matrices render these calculations ineffective when converting from normal into the required internal molecular coordinates. Whenever this occurred, the correlation proposed by Halgren17 was used to assign the corresponding angle-bending force constants. Dihedral angle parameters were calculated using the following procedure: (i) calculation for a given dihedral angle of the corresponding torsion energy profile using ab initio (Figure 1a), (ii) calculation by molecular dynamics of the same torsion energy profile considering only nonbonded interactions in an isolated ion, and (iii) the fitting of the dihedral angle paramenters (V1 to V3 in eq 1) to the difference between the profiles obtained in (i) and (ii). The three steps of this process are illustrated in Figure 1b. The parameters describing the electrostatic forces acting on each atom of the ion (atomic point charges) were calculated ab initio at the MP2 theoretical level using extended basis sets (ccpVTZ(-f)). This yields an accurate description of the electron density surrounding the ion for a given conformation. The point charges placed at the center of mass of each atom of the ion are then calculated from the electron density using the CHelpG electrostatic surface potential methodology. In the case of [FSI]-, it was possible to keep all the atomic point charges that modeled the bistriflamide anion7 without incurring errors larger than a few hundredths of acu. Only the fluorine atom was changed from -0.16 acu in bistriflamide to -0.13 in [FSI]- to preserve the overall charge of -1 of the anion. This kind of transferability between analogous ions is one of the hallmarks of the present force field.

Figure 1. Torsion potential energy profiles (TPEP) as a function of the FSNS angle of the [FSI]- ion. (a) Total TPEP calculated ab inito in this work (MP2 calculation, 0 and full line), and from ref 10(B3LYP calculation, 4 and dotted line). (b) Total TPEP (0 and full line as in (a)), nonbonded TPEP obtained by MD (O and dotted line): total TPEP obtained by MD with the fitted dihedral parameters (dashed line).

The Lennard-Jones parameters were taken directly from the OPLS-AA database9 and are the same as those previously proposed for the different types of atom in the bistriflamide ion.7 Validation. The force field was validated using experimental density data available for two [FSI]--based salts: an ionic liquid of the 1-alkyl-3-methylimidazoloium family, [C2mim][FSI],10 and a triphenylphosphonium crystal, [PH(C6H5)3][FSI].19 The limited number of validation systems reflects the scarcity of experimental data for ionic liquids based on the [FSI]- ion. In the case of [PH(C6H5)3][FSI], the objective of the simulations was to test the performance of the force field in predicting both the crystal density and its structural properties such as the dimensions and director angles of the crystalline unit cell. The simulation boxes and initial configurations were set by taking into account the corresponding crystalline structure deposited in the Cambridge Structural Database19 (ref code ZUWCAV). The temperature was fixed to match that of the crystallographic experiments (143.15 K), and the pressure was set to a null value. The simulation results are given in Table 2. The density is predicted with an accuracy better than 1.6%. This level of agreement is very good considering that the simulation results are purely predictive; all the force-field parameters were either taken as such from our previous force field7,8,13 or calculated ab initio and none was adjusted to match the kind of experimental data against which the comparison was made. For [C2mim][FSI], the objective was to reproduce the density of the liquid. The equilibration period in the liquid-phase simulation runs is very important. Different approaches were used to ensure that the ergodicity of the simulation was properly

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TABLE 2: Validation of the Force Field against Density and Structural Data of FSI-Based Ionic Liquids in the Crystalline ([PH(C6H5)3][FSI]) and Liquid ([C2mim][FSI]) Phases density data (crystal and liquid) [PH(C6H5)3][FSI] coderef

CSD space group ions/unit cell stacked cells cutoff/Å T/K ion pairs/box Vexp/box Å3 Vsim/box Å3 Fexp/mol dm-3 Fsim/mol dm-3 δF/%

ZUWCAV19 P21/n 8 4×3×3 16.0 143.15 144 69995 68953 3.416 3.471 1.6

structural data (crystal)

[C2mim][FSI]

16.0 298.15 250 79577 79772 5.217 5.204 -0.2

attained: Initially, the ions are placed at random in the simulation box, at very low density, and the equilibration starts by a short relaxation of a few picoseconds at 1 K and constant (N, V, E) to allow internal modes to relax. Then, an equilibration period is imposed at the final temperature of the simulation, followed by the activation of the thermostat and barostat. Alternatively, other simulations of the same system were allowed to evolve from initial configurations based on the expanded structure of an analogous ionic crystal. Another method to ensure the ergodicity of the simulation is to perform several temperature annealing cycles or to scramble/unscramble the two components (ions) of the system by switching off/on the electrostatic charges. The equilibration was considered successful only after stable and consistent results over periods of at least 100 ps. Table 2 also lists the liquid-phase results, where an almost perfect match between experimental and simulated densities was achieved (0.2% relative difference). Although such a match can be considered partially fortuitous (the performance of the present force field for other classes of ionic liquids usually shows accuracies better than 2-3%), it also validates the extension of the parametrization to [FSI]--based ionic liquids.7,8,13 Ab Initio and Conformational Potential Energy Surfaces. The conformational analysis of the [FSI]- anion poses a more complex problem than anticipated for a symmetrical, nine-atom ion. Because of that symmetry, one tends to focus on the possible conformations of the ion obtained by rotation of the FSSF dihedral angle, and that type of information can easily be matched to Raman spectroscopy or MD simulation data. Two stable conformers of group symmetry C2 (trans) and C1 (cis) were recently postulated by DFT calculations and Raman spectroscopy (cf. Figure 2).10 Nevertheless, if one wants to calculate the paths of interconversion between the different conformations of the ion, it has to be recognized that the FSSF dihedral angle mentioned in the previous paragraph is not a proper dihedral since the four atoms are not directly connected to each other. In fact, the nitrogen atom that bridges the two sulfur atoms confers an extra pivotal point to the ion, which means that the pseudodihedral FSSF can be decomposed in two proper FSNS dihedrals. These two dihedrals are symmetrical but the corresponding torsion energy profiles (cf. Figure 1a) are far from simple because of the extra degree of freedom introduced by the possibility of rotation of each dihedral relative to the other. When the second dihedral is at its most stable angle (72°), the most conspicuous feature of the torsion profile is the existence of two energy barriers (around 14-18 kJ/mol) as the first FSNS dihedral angle approaches 0 or 160°. The barriers separate two minima at 72 and 288°. Another important characteristic of the plot is the lack of

[PH(C6H5)3][FSI] aexp/Å asim/Å bexp/Å bsim/Å cexp/Å csim/Å Rexp/(deg) Rsim/(deg) βexp/(deg) βsim/(deg) γexp/(deg) γsim/(deg)

9.94 10.35 15.98 15.04 12.34 12.19 90 90.05 97.02 98.51 90 89.97

symmetry of the profile, indicating the coupling between the two dihedrals. This conformational analysis along the torsion energy profile of the F-S-N-S dihedral contrasts with that of bistriflamide (C-S-N-S dihedral) where the two almost identical 13-14 kJ/mol peaks10 are replaced by a very high energy barrier of 35 kJ/mol at 0° and a smaller barrier of 8 kJ/mol at around 150°.6 This means that, compared to that of bistriflamide, the interconversion between conformers in [FSI]- is less favorable but can proceed through a larger variety of routes (see below). The combination of two torsion profiles allows one to identify the two conformers (and their mirror images) of the FSI anion: when both FSNS dihedrals are 72° (or both 288°) one obtains the more stable trans isomer, whereas when the angles are 72 and 288° (or 288 and 72°) one obtains the cis isomer (Figure 2). Finally, if instead of a simple energy profile one plots the potential energy surface (PES) of the two F-S-N-S dihedrals, one is able to clearly see the conformers of the molecule (Figure 3a): the four basins represent the two conformers (cis and trans) and their respective mirror images. One of the most interesting features of the 3D surface plot is that the energy barriers (saddle points) between the different minima (basins) in the diagram show a gridlike structure. This means that the two mirror images of a given conformer cannot interconvert directly into each other, without passing through the other type of conformation. This PES of [FSI]- is also quite different from that of bistriflamide (Figure 3b), with the “anti” and “gauche” conformers of the latter being replaced by the “trans” and “cis” conformers of the

Figure 2. Two main conformations of the [FSI]- anion and their relation with the two FSNS dihedral angles defining the conformational energy landscape of the ion. (a) C2 “trans” conformer. (b) C1 “cis” conformer. The red bars indicate the position of the oxygen atoms that lie close to the SNS plane of the ion.

Potential Energy Landscape of Bis(fluorosulfonyl)amide

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Figure 3. (a) PES of [FSI]- as a function of the two FSNS dihedral angles of the ion. (b) PES of bistriflamide (CSNS dihedrals). (c) Potential energy profile (section of the PES) of [FSI]- with one of the dihedrals set to 72°. (d) Potential energy profile of bistriflamide with one of the dihedral angles set to 90°. The areas shaded in dark blue represent the location of the two most stable conformers in each case and those in light blue the interconversion paths between them (see text).

former, and “the round lake surrounded by high mountains” being replaced by the above-mentioned gridlike reservoir system. The energetics of interconversion between the two molecules is also quite different (Figure 3c,d, to be compared with Figures 1a and 3b of ref 20). In the case of bistriflamide, the gauche and anti conformers can interconvert by dihedral angle rotation in only one direction, the path around the central island, by overcoming an energy barrier of just 8 kJ mol-1 (or a bit higher when converting between mirrorlike gauche conformers6). In the case of the [FSI]- ion, interconversion is much more difficult (14-18 kJ mol-1) but can proceed using two distinct paths: around the central island or by exiting the surrounding mountain range at the adequate cols (saddle points). Molecular Dynamics and Conformer Distribution Surfaces. Conformer distributions profiles (CDP, population histograms as a function of the F-S-S-F dihedral angle of [FSI]-) were obtained at different temperatures by MD simulation of the liquid [C2mim][FSI] salt and are presented in Figure 4. The plot at 298 K shows two peaks: one around 120° (both F-S-N-S dihedrals at 72° or both at 288°) corresponding to the trans conformers, the other at around 0° (one F-S-N-S dihedral at 72°, the other at 288°) corresponding to the cis conformers. It must be noted that, because of the presence of the “bridge” nitrogen atom of the molecule, the value of the F-S-S-F dihedral is only approximately equal to the sum of the two F-S-N-S dihedrals (the deviation can be as high as 20°). In other words, the MD results clearly show the possibility of the coexistence of two main conformers of the [FSI]- anion in the simulated liquid phase of [C2mim][FSI], with dihedral angles close to those predicted ab initio. The simulation at 298 K also shows that the rotation of the two fluorine atoms around the corresponding S-N axis is possible, yielding non-negligible distributions of off-peak conformers. This is in agreement with the three-dimensional potential surface for the isolated ion presented in the previous section where the different conformers can interconvert into each other, sampling all possible F-S-S-F dihedral angles, without

Figure 4. Conformer distribution histograms of [FSI]- as a function of the FSSF dihedral angle at 298 K (9) and 500 K (O) calculated by MD simulation of [C2mim][FSI].

having to overcome energy barriers (saddle points) higher than 15 kJ/mol (going around the gridlike system of basins; Figure 3b). It should be stressed that to sample all possible F-S-S-F dihedral angles the anion never needs to adopt conformations with both F-S-N-S dihedral angles close to 0°; the rotation of the two fluorine atoms around the corresponding S-N axis can always be performed in such a way that they never move in the direction of the nitrogen atom at the same time. However, a more careful analysis of the dihedral distribution at 298 K and the inclusion of the results at 500 K shows that the population of the off-peak conformers is higher than expected, especially in the region between 120 and 240°. How can these MD F-S-S-F distributions be reconciled with the existence of just two main conformers (two pairs of mirrorlike images), predicted by the ab initio calculations and confirmed by Raman spectroscopy? This can be answered by replacing (again) the improper F-S-S-F dihedral angle by the two proper F-S-N-S dihedral angles and to generate conformer distribution surfaces (CDS) instead of CDP.

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Figure 5. (a) CDS of [FSI]- as a function of the two FSNS dihedral angles at 298 K, calculated by MD simulation of [C2mim][FSI]. (b) Similar CDS at 500 K. (c) CDS of bitriflamide as a function of the two CSNS dihedral angles at 298 K, calculated by MD simulation of [C2mim][TFSI].

In Figure 5, we show the conformer distribution surface (population histogram plotted as a function of the two F-S-N-S dihedral angles) for the [C2mim][FSI] ionic liquid in the liquid phase at 298 and 500 K. For comparison purposes, we also included the CDS for the [C2mim][TFSI] ionic liquid at 298 K. Unlike the CDPs, the CDSs clearly show the existence of the two main conformers of [FSI]- (cis and trans) as two pairs of two mirrorlike images. The off-peak population is now correctly represented, and the four “islands” of Figure 5a can even be regarded as the “reverse” of the four basins of Figure

Lopes et al. 3a, again emphasizing the consistency between ab initio and MD results. The CDS at 500 K shows the “erosion” of the four islands (i.e., the possibility at higher temperatures of conformer interconversion and the higher probability of off-peak conformers). Raman Spectroscopy and Thermodynamic Relations between Conformers. A Raman spectroscopy study based on the [C2mim][FSI] ionic liquid confirms the ab initio and MD results. The spectroscopic data available are consistent with the existence in the liquid phase of both the trans and cis conformers of the [FSI]- anion. The Raman spectrum of [C2mim][FSI] shows three strong bands at 293, 328, and 360 cm-1 characteristic of the [FSI]ion (Figure 3 of ref 10). These bands are in fact the result of band overlap from both the trans and cis conformers, and their asymmetrical shape indicates that the characteristic vibrations of each conformer are slightly shifted in relation to each other. By analyzing the position and intensity of the corresponding theoretical bands obtained ab initio, it was possible to deconvolute the experimental Raman spectra into different contributions and assign the peaks at 293, 328, and 360 cm-1 to the trans conformer and the smaller peaks (shoulders in the nondeconvoluted spectra) at 305, 320, and 353 to the cis conformer. By studying the effect of temperature on the shape and intensity of the bands, it was also possible to estimate the difference in molar enthalpy, entropy, and Gibbs energy trans trans between the conformers: ∆cis Hm ) -4.5 kJ mol-1, ∆cis Sm trans -1 -1 -1 ) -19 J K mol , and ∆cis Gm ) +1 kJ mol at 298 K. In addition, Raman spectra of the N-methyl-N-propylpyrrolidinium bis(fluorosulfonyl)imide ionic liquid, [P13][FSI], were also measured at various temperatures between 298 and 367 K, and their temperature dependence was analyzed. The peak positions of the Raman bands ascribable to the [FSI]anion were similar to those for the [C2mim][FSI] ionic liquid, although band overlapping in the case of [P13][FSI] was slightly more complicated than that for [C2mim][FSI]. By careful curve fitting analysis, the ∆H° and ∆S° values were evaluated to be 6.8(6) kJ mol-1 and 33(3) J K-1 mol-1, respectively, and thus the ∆G° value of -3(2) kJ mol-1 at 298 K was obtained, which indicates that there is no appreciable dependence of the conformational isomerization of the [FSI]- anion on the cation species composing ionic liquids. These analyses confirm the ab initio and molecular dynamics results: although the trans isomer is more stable than its cis counterpart by about 4.6 kJ/mol (cf. the ab initio results of Figure 3), the two populations of cis and trans isomers are almost identical at 298 K (cf. the MD results in Figure 5a). The relations between the enthalpy and entropy of the two conformers can even be accessed in a semiquantitative basis by looking at the shape of the basins depicted in the PES of Figure 3a: the trans basins are deeper (lower energy) but the cis basins occupy larger areas (higher entropy). The same applies to the peaks in Figure 5a: the trans conformer exhibits higher but narrower peaks relative to the peaks of the cis conformer, lower but with a wider (crescent-shaped) base. In a previous publication,6 we published the PES surface of bistriflamide calculated ab initio (Figure 2 of ref 6) with the corresponding CDP obtained from MD simulations. At the time, we based our conformational analysis on that CDP, which means that while the general conclusions are correct, a more refined analysis, based on the two individual CSNS of bistriflamide, was not performed. Figure 5c shows the CDS

Potential Energy Landscape of Bis(fluorosulfonyl)amide for the bistriflamide anion based on those two dihedral angles. The two main conformers of bistriflamide, anti and gauche, and their mirror-image conformations are now visible as the six different peaks (the two higher ones corresponding to the two mirror images of the anti conformer, the four lower ones to the two mirror images of the gauche conformer. The most conspicuous feature of the CDS is the disposition of the peaks around a circular ridge (almost like an atoll in the Pacific Ocean), a situation quite different from the cell-like distribution of the [FSI]- anion. Again, the CDS of Figure 5c has its counterpart in the PES (circular lake with a central island) of Figure 3b. Conclusions We showed in this study that the conformational landscape of an isolated [FSI]- ion calculated ab initio is very similar to the corresponding landscape, inferred from Raman spectroscopy data or obtained using MD simulation, when the ion is a component of an ionic liquid. In other words, the different conformations adopted by the [FSI]- ion are ruled by its internal structure even when the ion is included in a highly interactive media such as an ionic liquid (i.e., albeit being part of a polar network, the ions enjoy enough freedom to interconvert between different conformations). This freedom can be related to the weak coordinating nature of the [FSI]- anion with imidazolium cations, indicating the absence of tightly bound ion pairs, and rather an ionic liquid structure in which several counterions interact rather equivalently with each ion. The adoption of different conformations does not happen in crystalline phases of ionic liquids, where only one of the possible conformations is usually present due to constraints imposed by the crystalline lattice. When compared with the previously studied [TFSI]- ion, [FSI]- exhibits only slightly different conformers (cis and trans in [FSI]- instead of gauche and anti in [TFSI]-). However, [FSI]- displays a distinct (more hindered but more varied) interconversion path between conformers: both ions are flexible but in different ways. The relationship between the flexibility of [FSI]- and [TFSI]and the nanostructured nature of ionic liquids (polar network permeated by nonpolar domains) can help explain many of the unique physical and solvation properties of ionic liquids based on those anions. Acknowledgment. K.S. acknowledges financial support from the FCT, Portugal (postdoctoral Grant SFRH/BPD/38339/2007). This work has been financially supported by Grants-in-Aids for Scientific Research No.18850017, 19003963 and 19350033, and the Global COE Program “Science for Furture Molecular Systems” from the Ministry of Education, Culture, Sports, Science and Technology of Japan. References and Notes (1) Ionic Liquids in Synthesis; Wasserscheid, P., Welton, T., Eds.; Wiley-VCH Verlag: Weinheim, Germany, 2003. (2) Bonhoˆte, P.; Dias, A. P.; Papageorgiou, N.; Kalyanasundram, K.; Gra¨tzel, M. Hydrophobic, highly conductive ambient-temperature molten salts. Inorg. Chem. 1996, 35, 1168–1178. (3) Sakaebe, H.; Matsumoto, H. N-Methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl)imide (PP13-TFSI)-novel electrolyte base for Li battery. Electrochem. Commun. 2003, 5, 594–598.

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