A Tale of Two Ions: The Conformational Landscapes of Bis

Jan 12, 2008 - Weththasinghage Don Amith , Jeevapani J. Hettige , Edward W. Castner .... Shinobu Koda , Nobuyuki Serizawa , Shiro Seki , Kenta Fujii ,...
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J. Phys. Chem. B 2008, 112, 1465-1472

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A Tale of Two Ions: The Conformational Landscapes of Bis(trifluoromethanesulfonyl)amide and N,N-Dialkylpyrrolidinium Jose´ N. Canongia Lopes,*,†,‡ Karina Shimizu,†,‡ Agı´lio A. H. Pa´ dua,*,§ Yasuhiro Umebayashi,*,| Shuhei Fukuda,| Kenta Fujii,⊥ and Shin-ichi Ishiguro| Centro de Quı´mica Estrutural, Instituto Superior Te´ cnico, 1049 001 Lisboa, Portugal, Instituto de Tecnologia Quı´mica e Biolo´ gica, UNL, AV. Repu´ blica Ap. 127, 2780 901 Oeiras, Portugal, Laboratoire de Thermodynamique des Solutions et des Polyme` res, CNRSsUniVersite´ 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: August 31, 2007; In Final Form: NoVember 14, 2007

The conformational landscapes of two commonly used ionic liquid ions, the anion bis(trifluoromethanesulfonyl)amide (Ntf2) and the cations N-propyl- and N-butyl-N-methylpyrrolidinium, were investigated using data obtained from Raman spectroscopy, molecular dynamics, and ab initio techniques. In the case of Ntf2, the plotting of three-dimensional potential energy surfaces (PES) and the corresponding molecular dynamics (MD) simulations confirmed the existence of two stable isomers (each existing as a pair of enantiomers) and evidenced the nature of the anion as a flexible, albeit hindered, molecule capable of interconversion between conformers in the liquid state, a result confirmed by the Raman data. In the case of the N,N-dialkylpyrrolidinium cations, the PES show a much more limited conformational behavior of the pyrrolidinium ring (pseudorotation). Nevertheless, such pseudorotation produces two stable isomers with the propyl and butyl side chains in completely different positions (axial-envelope and equatorial-envelope conformations). This result was also confirmed by Raman spectra analyses and MD simulations in the liquid phase. The implications of the conformational behavior of the two types of ions are discussed in terms of the solvation properties of the corresponding ionic liquids.

Introduction In the past few years, the structural properties of ionic liquids have been probed by different experimental and theoretical techniques. For instance, molecular dynamics studies showed that ionic liquids are microsegregated, i.e., composed by a flexible polar network permeated by nonpolar domains,1,2 a fact that was later confirmed by direct experimental evidence.3 Another area where there is a fruitful collaboration between results emerging from simulation and experiments concerns the conformational analysis of the ions that compose the ionic liquids.4 One of the hallmarks of ionic liquids is that they are composed by at least one rather large cation or anion. In fact, it is the charge delocalization and/or asymmetry of at least one of the ions that generally determines the unusual properties of ionic liquids, viz., their low melting temperature as compared with common inorganic salts. The fact that those large ions are generally not rigid and can adopt different conformations is another key feature of ionic liquids. In a recent paper, a systematic force field developed for modeling ionic liquids based on the 1-alkyl-3-methylimidazo* Corresponding authors. E-mail: [email protected] (J.N.C.L.); [email protected] (A.A.H.P.); [email protected] (Y.U.). † Instituto Superior Te ´ cnico. ‡ UNL. § CNRSsUniversite ´ Blaise Pascal Clermont-Ferrand. | Kyushu University. ⊥ Saga University.

lium cation,5 [Cnmim]+, was tested against Raman spectroscopic data6 in order to check if the cations in the simulated ionic liquid would exhibit the same conformation distribution as that inferred by the spectroscopic results.4 A similar approach was performed in order to test the conformations adopted in the ionic liquid by the anion bis(trifluoromethanesulfonyl)amide (bistriflamide or [tf2N]-).7 In this case, the simulations were compared with neutron-diffraction data.8 In the present study, we will continue to study the structural behavior of bistriflamide-based ionic liquids by extending the simulations to different compounds at different temperatures, and by making the corresponding comparisons with recent spectroscopic data.7 On the other hand, ionic liquid systems based on N,N-dialkylpyrrolidinium bistriflamide, [CnCmpy][tf2N], will be used as the starting point for the analysis of the alkyl side chain and pyrrolidinium ring conformation distributions of the corresponding cations. The results will also be tested against Raman spectroscopy data.9 Experimental Section Simulation. The molecular force field used to represent the ionic liquids is based on the OPLS-AA model10 but with parameters specifically tailored for the ions in question.11 Following the spirit of OPLS-AA, intramolecular terms related to covalent bonds and angles are taken from the AMBER force field,12 and efforts are concentrated on carefully describing conformational and intermolecular terms. This strategy is justified because stretching and bending force constants, which are associated with fast intramolecular modes, have a minor

10.1021/jp076997a CCC: $40.75 © 2008 American Chemical Society Published on Web 01/12/2008

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Figure 1. (a) Depiction of the bistriflamide anion showing all of its atoms and the S-N-S plane of the molecule. The nomenclature of the backbone atoms (shown in parts b-i) is also given. (b) The C2 conformer (anti) of bistriflamide. The arrow shows the rotation of the S1-NS2-C2 dihedral angle to yield a (c) C1 conformer (gauche). (d) Newman-like projection of the anti conformer showing the possible rotations (light arrows) of both S-N-C-S dihedrals to yield the other conformers. The forbidden range of rotations is marked as a dark gray semicircle. (e and f) The gauche conformer obtained from part d by rotation of just one of the dihedrals. The two images shown represent the same conformation. (g-i) mirror images of parts d-f obtained by rotation of both dihedrals. The images in parts h and i represent the same gauche conformer.

effect on the configurational properties that lead to equilibrium thermodynamic quantities of a liquid phase. On the other hand, 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. Therefore, special attention has been paid to obtaining torsion energy profiles and electrostatic charge distributions, using ab initio calculations with a high level of theory (MP2 method) and large basis sets (ccpVTZ). For pyrrolidinium cations, which are a special case of quaternary ammonium cations, only electrostatic charge distributions were evaluated specifically for ionic liquids, since the dihedral angle torsions were already completely described in the OPLS-AA force field for amines.13 For the bistriflamide anions, a full set of torsion and electrostatic parameters were developed.5 The reader should be aware that electrostatic charges and Lennard-Jones terms in the force field also affect conformational energies, since atoms within the same molecule, if separated by three or more bonds, also interact through these terms. These nonbonded interactions within the same molecule play, in many cases, an important role in defining conformational energetics. The 1-alkyl-3-methylimidazolium bistriflamide, [Cnmim][tf2N] (n ) 2, 4, 6, and 8), systems were studied at 300 and 580 K; the last value corresponds to temperatures considered safe as regards the decomposition of many bistriflamide-based ionic liquids. N-propyl- and N-butyl-N-methylpyrrolidinium bistriflamide, [C3C1py][tf2N] and [C4C1py][tf2N], were simulated at 300, 400, and 500 K. All simulations were performed using molecular dynamics, implemented in the DL_POLY code.14 Starting from low-density initial configurations, systems composed of 250 ion pairs were equilibrated at constant NpT for 500 ps at 300 K (Nose´-Hoover thermostat and barostat with time constants of 0.5 and 2 ps, respectively). The final density is attained after about 50 ps. Further simulation runs of 100 ps were used to produce equilibrated systems at the studied temperatures. Electrostatic interactions were treated using the Ewald summation method considering six reciprocal-space vectors, and repulsivedispersive interactions were explicitly cut off at 16 Å (longrange corrections were applied assuming the system had a

uniform density beyond this cutoff radius). Then, 1000 configurations were stored from production runs of 300 ps. Successive 300 ps runs showed no drift in the corresponding equilibrium properties at this stage. Density Functional Theory (DFT) Calculations. The geometry optimization and normal coordinate analyses for the isolated single bistriflamide or N,N-pyrrolidinium ions were performed using density functional theory according to Becke’s three-parameter hybrid method15 with LYP correlation (B3LYP).16 DFT calculations were carried out using the Gaussian 03 program package.17 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 at 1000 mW throughout the 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 thermostated within (0.3 K at a given temperature. Water content checked by the Karl Fischer method was less than 90 ppm for all samples examined. No appreciable decomposition was detected after the measurements. Details of Raman data analyses such as the band assignment and deconvolution were as described elsewhere.6 Results Bistriflamide. a. DFT. Conformational analysis of the bistriflamide anion poses a peculiar and very interesting problem. Due to the symmetry of the molecule, one tends to focus on population distributions around the C1-S1-S2-C2 dihedral angle (Figure 1a), and that is the type of information one can easily retrieve from Raman spectroscopy or molecular dynamics (MD) simulation data: In fact, two stable conformers of group symmetry C2 (anti) and C1 (gauche) (Figure 1b and c) were postulated by Hartree-Fock (HF) calculations as early as 1998 by Johansson et al.18 and confirmed recently by MP2level calculations,5 MD simulation,8 and DFT calculations and Raman spectroscopy.7 However, the atoms that form such a dihedral angle are not directly connected to each other in succession: there is the sp3 nitrogen atom between the two sulfur atoms, conferring the molecule an extra pivotal point. The pseudo-dihedral C1-S1-

Conformational Landscapes of Ionic Liquid Ions S2-C2 can therefore be decomposed into the two proper dihedrals C1-S1-N-S2 and S1-N-S2-C2. These two dihedrals are symmetrical, but the corresponding torsion energy profiles are far from simple due to the extra degree of freedom introduced by the possibility of rotation of the dihedral angles relative to each other. Several authors5,7,18 analyzed such torsion energy profiles using ab initio calculations with different levels of theory and basis sets and reached similar conclusions. The most conspicuous feature of the torsion profile is the existence of a quite high energy barrier (around 35 kJ/mol) as the C-S-N-S dihedral angles approach 0°. On the other hand, the strong correlation between the two dihedrals makes the mapping of the region between 90 and 270° highly dependent on the value of the second dihedral angle. When the second dihedral is allowed to relax to the most stable conformation, a rather flat region between 90 and 230° is present, with minima at those angles and a rather low energy barrier between them of around 8 kJ/ mol (Figure 1d). The C1 and C2 conformers based on the composite dihedral angle C-S-S-C can therefore be rationalized in terms of this scenario: when the C-S-N-S proper dihedral angles are 90 and 270°, one obtains the “trans” C2 conformer; when one is at 90° and the other at 130° (or 270 and 230°), one obtains the “gauche” C1 conformer. It must be noted that due to the presence of the nitrogen atom each conformer will have a mirror image elevating to four (two trans, two gauche) the number of most stable conformers of the moleculesparts d, e (or f), g, and h (or i) of Figure 1. On the other hand, if instead of a simple energy profile one plots the potential energy surface (PES) of the two C-S-N-S dihedralssJohannson et al.18 performed a similar analysis in 1998sone is able to see clearly the four conformers of the molecule (Figure 2a), with each anti conformer (a round basin) surrounded by the corresponding gauche conformer (two elongated pools). One of the most interesting features of the 3D suface plot is that the energy barriers (saddle points) between the different minima (basins) in the diagram do not have the same height. Two groups (one the mirror image of the other) of three basins are separated by a higher energy barrier than the one that exists between the pools of the anti and gauche conformers. The relevance of this fact will become apparent in the following discussion of the MD results. b. MD. Distributions of the CSSC dihedral angle obtained by MD simulation of different bistriflamide-based ionic liquids are presented in Figure 3. The plots show two peaks, one at 180° (270° minus 90°) corresponding to the anti conformers and the other at 40° (130° minus 90° or 270° minus 230°) corresponding to the gauche conformers. It must be stressed that, due to the geometry of the molecule, the value of the CSSC dihedral is only approximately equal to the difference between the two CSNS dihedrals (the deviation can be as high as 18°). The MD results clearly show the existence of two main conformers of the bistriflamide anion in the liquid phase of a pure ionic liquid, with dihedral angles close to those predicted by DFT. A direct comparison between torsion energy profiles in the gas phase calculated ab initio or via MD simulations can be found in ref 5. The simulations also show that, although the rotation of the two carbon atoms around the corresponding S-N axis is hindered, it is still possible, yielding non-negligible distributions of off-peak conformers. This is in agreement with the threedimensional potential surface presented above where the different conformers can interconvert into each otherssampling

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Figure 2. 3D representation of the potential energy surface of bistriflamide, as a function of the two SNSC dihedral angles. In part a, the basins in black represent the four most stable conformersstwo anti (two round basins) and two gauche (four elongated basins)swith energies up to 5 kJ/mol. In part b, the limit is raised to 10 kJ/mol, showing the interconversion route between conformers.

Figure 3. Dihedral distribution functions of the CSSC dihedral angle of bistriflamide obtained by MD simulation at 300 (gray) and 580 K (black) of 1-alkyl-3-methylimidazolium bistriflamide, [Cnmim][tf2N], ionic liquids: (a) [C2mim][tf2N]; (b) [C4mim][tf2N]; (c) [C6mim][tf2N]; (d) [C8mim][tf2N].

all possible CSSC dihedral anglesswithout having to overcome energy barriers (saddle points) higher than 10 kJ/mol (going around the lake and circling the central “island” of Figure 2b). It must be stressed that in order to sample all possible CSSC dihedral angles the anion never needs to adopt conformations

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Figure 4. Dihedral distribution function of the CSSC dihedral angle of bistriflamide obtained by MD simulation at 300 (gray) and 500 K (black) of N,N-butylmethylpyrrolidinium bistriflamide, [C1C4pyrr][tf2N], ionic liquid. At the lower temperature, the histogram is still slightly asymmetric, indicating that the interconvertion between the enantiomeric forms of [tf2N]- was still incomplete after more than 500 ps of simulation (cf. discussion in the text).

with CSNS dihedral angles close to 0°: the rotation of the C groups around the S-N axis is always performed in the direction of the nitrogen atom (Figure 1d), thus avoiding the high-energy regions surrounding the “lake”. A more careful analysis of the MD data can even disclose the presence of two groups of mirror conformers. In fact, if the simulations are started with all bistriflamide anions in a given conformation (only one of the two possible mirror images of that particular conformation), then the conformer distribution histograms like those presented in Figure 3 are no longer symmetrical in relation to 180° (if one defines a positive and negative direction of the dihedral angle in order to differentiate between mirror-image conformers, one can draw the diagrams from 0 to 360°). Many of the simulations at the lower temperature (300 K) still presented such asymmetry after simulation times of 300 ps, thus showing the difficulty of converting from a (gauche) conformer to its mirror image by passing the corresponding saddle points between two adjoining elongated basins in Figure 2a; cf. also Figure 4. On the other hand, the equilibrium between anti and gauche conformers on the “same side of the mirror” was attained much faster (the smaller energy barriers between the round and elongated basins). X-ray diffraction data on bistriflamide-based salts also show evidence of the difficult conversion between mirrorlike conformers and the apparent ease of the anti-gauche conversion. The MD data show (corroborating the Raman data, see below) that in the liquid there are always considerable amounts of the gauche and anti conformers at equilibrium. However, all of the crystalline structures deposited at the Cambridge Structural Database19 (CSD) containing the bistriflamide anion show the existence of bistriflamide only at a given conformation (in the vast majority of cases, the crystals contain only the anti conformer, while some other crystals contain only the (sometimes distorted) gauche conformer). This means that during the crystallization process the molecules are able to adopt the conformation that best fits the crystal structure; sometimes the price for a better fit is the adoption of the energetically less favorable gauche conformation. The conversion between anti and gauche is therefore unhindered even at the rather low crystallization temperatures, and this fact is reflected in the discrepancy between the conformer fractions in the liquid (always between 0 and 1, generally close to 0.5; see below) and in the solid (only 0 or 1). The same does not happen if one looks at the mirror-imageconformer ratio. In the liquid, the fraction of mirror-image

Canongia Lopes et al. conformers (both anti and gauche) is 0.5; conversion is hard but not impossible, and given enough time, the liquid should contain exactly the same amounts of each pair of mirror-image conformers. On the other hand, almost all bistriflamide-based crystals at the CSD exhibit that same ratio (0.5). This means that during the crystallization process the bistriflamide anion is not able to adopt one of the two mirror-like conformations that better suits the structure of the crystal. Both mirror images of a given conformer already present in the liquid are incorporated in the crystal in equal amounts. A noteworthy exception is the N,N-butylmethylpyrrolidinium bistriflamide crystal that was obtained using an in situ cryo-crystallization method,20 that avoids the formation of ionic liquid glasses on cooling and allows the growing of single crystals. In this case, single crystal diffraction analysis showed only one of the two possible anti conformations of bistriflamide. Probably the zone-melting technique used to melt and regrow the crystals allows the interconversion between the mirror-like conformers of bistriflamide. Another important fact emerges from the MD simulation results: the conformer distribution is not greatly affected by the nature of the cation. In the case of the MD results presented in Figure 3, the alkyl side chain of the methylimidazolium cation was progressively lengthened from 1-ethyl- to 1-octyl-3methylimidazolium. The C8mim cations occupy more than double the volume occupied by C2mim.21 Nevertheless, such profound structural change seems to not disturb bistriflamide. This fact suggests that the extra length of the alkyl side chain is accommodated for in regions far from the bistriflamide anion. Since the bistriflamide anion has to remain close to the charged parts of the cation, this is another piece of evidence supporting the idea of ionic liquids as microsegregated media, with nonpolar domains permeating a polar network.2 When 1-alkyl-3-methylimidazolium is substituted by a N,Ndialkylpyrrolidinium cation, the MD results continue to show the presence of the two main conformers of bistriflamide (Figure 4). Here, the change is in the charged part of the cation, with profound consequences for the interaction patterns between cations and anions (although we have five-membered rings in both cases, only imidazolium is aromatic with charge delocalization between two heteroatoms). This suggests that the polar network is flexible enough not only to accommodate different sizes of nonpolar domains but also to allow for its constituent ions to adopt their preferred conformations (the conformational complexity of the pyrrolidinium cation is discussed below). c. Raman Spectroscopy. A Raman spectroscopy study based on the [C2mim][tf2N] ionic liquid7 confirms the DFT and MD results: the spectroscopic data are consistent with the existence in the ionic liquid of both the anti and gauche conformers of bistriflamide. In this study, the authors were able to assign the different wagging vibrations of the SO2 group characteristic of the C1 and C2 conformers, with calculated wavenumbers of 387.22 (C2 conformer, 4.64 Raman activity), 396.04 (C1, 2.62), and 401.76 (C2, 2.03) cm-1, to a composite Raman band in the 380-420 cm-1 region. Using spectra taken at different temperatures and spectral deconvolution in the above-mentioned region, they were also able to estimate the Raman intensity ratio between thee two conformers and the corresponding energy difference between them, ∆Hm ) 3.5 kJ/mol in the 300-400 K range, a value close to that calculated by ab initio methods. On the other hand, if one takes into account the intrinsic Raman activity of each peak, one can also estimate the mole fraction of each conformer in the liquid. Taking the intensity ratios, I(C1)/I(C2), reported in Figure 5 of ref 7 and the

Conformational Landscapes of Ionic Liquid Ions

Figure 5. Pseudorotation of the N,N-alkylmethylpyrrolidinium ring. The figure shows the 10 envelope (outer circle) and 10 twist conformers (inner circle). Each conformer can be assigned a pseudorotational phase angle.21 The bold lines join contiguous coplanar atoms of the ring. The larger and smaller dots represent the alkyl and methyl groups connected to the nitrogen atom, respectively.

corresponding Raman activity ratio (given in the previous paragraph), one finds that the mole fraction of C1, xC1, varies from 0.48 (at 300 K) to 0.56 (at 400 K). Please note that if the two conformers had the same energy or at the high-temperature limit the proportion of C1:C2 would be close to 2:1 (xC1 ) 0.66(6)). With regard to the pyrrolidinium-based ionic liquids, similar analyses were applied to Raman spectra obtained by varying the temperature from 298 to 360 K. The isomerization enthalpies from the C2 to the C1 conformer of the anion were successfully evaluated to be 4.2(2) and 3.5(2) kJ mol-1 for the N-propyland N-butyl-N-methylpyrolidinium ionic liquids, respectively. The values are similar to the one calculated for the 1-ethyl-3methylimidazolium ionic liquid, which suggests that the population distributions and/or the isomerization enthalpies are practically the same in these ionic liquids and independent from the nature of the cation, a fact also corroborated by other authors using different bistriflamide-based ionic liquids.22 It must be emphasized at this point that the MD simulations obtained with different ionic liquids (see Figures 3 and 4) show exactly the same conformational behavior of the anion; i.e., the conformer populations and their temperature dependence are practically independent of the cation nature, as supported by the Raman experimental evidence. Nevertheless, the MD results show a more complex picture as regards the distribution of conformers. Figure 3 (specially those at higher temperatures) show the existence of a whole range of possible conformers, not only the C1 and C2 conformers. Nevertheless, if one assigns the areas below each probability density function to the corresponding C1 or C2 conformers (for instance, by assigning the probability function between 0 and 140° to C1 and that between 140 and 180° to C2), then the ratio C1:C2 is around 3:2; i.e., xC1 is around 0.6, consistent with the results estimated in the previous paragraph from the Raman data. In conclusion, both Raman and MD results show that the amounts of anti and gauche conformers of bistriflamide in the liquid phase of ionic liquids are not far removed from an

J. Phys. Chem. B, Vol. 112, No. 5, 2008 1469 “equimolar composition”. The MD results also show that, although the anti and cis conformers are the predominant species, other conformers exist in non-negligible amounts, evidencing the rather easy interconversion between the gauche and anti conformations of the molecule. This fact does not imply that the conversion between the mirror images of each conformer is easy; cf. the previous section. Finally, the evolution with the temperature of the dihedral distributions obtained by MD, where the C1 conformer peak seems to decrease more rapidly than that of the C2 conformer, apparently contradicts the Raman results that show an increase of the C1 conformer fraction with increasing temperature. However, these facts must be interpreted taking into account the presence of the other “intermediate” conformers and also the potential energy landscape depicted in Figure 2a and b. As the temperature increases, the interconversion between species is enhanced and in this case it is not just the most stable species (C2) that is “depleted” but also the C1 conformers that can sample new (and wider) regions of the energy landscape. When one compares the “cold” and “hot” dihedral distributions depicted in Figure 3, one can see that the decrease of the C1 peak is compensated by an increase of probability of conformations between 70 and 140° (the “baseline” shifts up). These conformations are not anti conformations, and thus, their signals in the Raman will continue to be felt just as “C1” (gauche) contributions. This fact reconciles the differences observed in the temperature dependence of the Raman and MD results. It must also be stressed that the uncertainty associated with the conformer distributions obtained by MD simulation make the estimation of the corresponding changes with temperature difficult. Pyrrolidinium. a. DFT. Conformational analysis of the pyrrolidinium cation is dominated by the possibility of ring pseudorotation. Evidence that cyclopentane and other related five-membered ring systems are rather puckered than planar and can pseudorotate is now more than half a century old,23 and according to the theory of ring puckering developed by Cremer and Pople,24 the conformational space of those systems can be described as a linear combination of two types of conformations: envelope (E) and twist (T), Figure 5. The interconvertion between the E and T conformers (there are 10 of each) allows five-membered rings to undergo the conformational process usually referred to as preudorotation. In order to compare DFT and MD results, we will employ a method similar to the one used for the bistriflamide anion, i.e., plot the potential energy surface as a function of two dihedral angles of the ion. In fact, one can define two dihedral angles along the pyrrolidinium ring in a completely analogous way to the dihedrals used for the bistriflamide anion (C3-C1-N-C2 and C1-N-C2-C4 in pyrrolidinium instead of C1-S1-NS2 and S1-N-S2-C2 in bistriflamide). As in the case of bistriflamide, we will also represent the conformers using a plane centered on the nitrogen atom (S1-N-S2 in the case of bistriflamide, C1-N-C2 for pyrrolidinium), Figure 6. The relative internal energies of the 10 E conformers of N-propyl- and N-butyl-N-methylpyrrolidinium were calculated by DFT and are presented in Table 1. The reported values correspond to the propyl and butyl side chains in fully “transoid” conformations. The table also lists values of the C3-C1-NC2 and C1-N-C2-C4 dihedral angles as the ring undergoes pseudorotation. When the information contained in Table 1 is plotted in 3D, the PES shown in Figure 7 is obtained. The PES also assumes that dihedral combinations “outside” the elliptical route depicted

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Figure 6. Two envelope and one twist conformations of the N,N-alkylmethylpyrrolidinium ring represented using the C1-N-C2 plane and emphasizing the two dihedral angles that can be rotated in order to obtain any of the 20 possible conformers. R1 and R3/4 represent the methyl and propyl/butyl groups, respectively. (a) Envelope conformation with the propyl/butyl in the equatorial position relative to the ring (phase angle 0°). (b) Twist conformation (phase angle 90°). (c) Envelope conformation with propyl/butyl in the axial position (phase angle 180°).

Figure 7. PES of the N,N-butylmethylpyrrolidinium ring as a function of the C3-C1-N-C2 and C1-N-C2-C4 dihedral angles.

TABLE 1: Relative Internal Energy of the Envelope Conformers of the N-Propyl- and N-Butyl-N-methylpyrrolidinium Cations (R3 and R4, Respectively) as a Function of the Pseudorotation Phase Angle, O, and the C3-C1-N-C2 and C1-N-C2-C4 Dihedral Angles, τ φ, deg

τC3-C1-N-C2, deg

τC1-N-C2-C4, deg

∆UR3, kJ/mol

∆UR4, kJ/mol

0 36 72 108 144 180 216 252 288 324

318 318 334 0 26 42 42 26 0 334

42 26 0 334 318 318 334 0 26 42

0 1.8 2.4 4.0 4.8 2.7 4.8 3.9 2.4 1.8

0 2.4 3.3 5.1 5.6 3.1 5.5 5.1 3.5 2.3

in Figure 7 have very high energies, since they would correspond to the elongation or breaking of the C3-C4 bond, and that a conformation with both dihedrals equal to 0° (corresponding to a planar ring) is around 20 kJ/mol less stable than the equatorial-envelope conformation. Unlike the DFT results of bistriflamide (with a complex system of basins and saddle points), the message is quite clear in this case: the only route available to the molecule is the canyonlike crevasse corresponding to conformations undergoing pseudorotation. At the bottom of the canyon, there are just two minima, both corresponding to E conformations with the nitrogen atom at the “tip” of the

envelope (phase angles 0 and 180°). The absolute minumum has the propyl or butyl group positioned in an equatorial position relative to the ring; the other minimum (around 3.5 kJ/mol above the former) presents those groups in an axial position. b. MD. It would be interesting to see if the force field model used to characterize ionic liquids in general and the pyrrolidinium cation in particular can adequately describe the conformers corresponding to the ring pseudorotation. As explained above, intramolecular parameters were taken from the OPLSAA database and not adjusted directly to pyrrolidinium ions. Therefore, a good agreement with experiment would demonstrate how the force field model is robust and its parameters transferable within a family of compounds. In Figure 8 are shown some distribution populations around chosen dihedral angles. The C2-C3-C4-C5 dihedral angle corresponds to the four carbon atoms of the pyrrolidinium ring, and it has a distribution centered around 0°. These four atoms are therefore planar. The dihedral N1-C2-C3-C4, on the other hand, is not planar, indicating that the pyrrolidinium ring adopts an envelope conformation with the N atom pointing away from the plane of the four C atoms. These dihedral angle distributions show that the most stable conformers are either the axial envelope or equatorial envelope (see Figure 7a and also Figure 2 of Fujimori et al.). Another conformational aspect concerns the orientation of the alkyl side chain with respect to the pyrrolidinium ring. Once again, agreement with experiment would demonstrate the transferability of force field parameters. The rightmost plot in Figure 8 shows a distribution of dihedral angles related to the position of the alkyl side chain with respect to the pyrrolidinium ring. The most likely occurrence around 160° corresponds to the alkyl chain in the equatorial position with respect to the ring, whereas the smaller peak around 80° is attributed to the conformers having the alkyl chain in the axial position. This result shows that the most stable conformer is the equatorial envelope and that a conformational equilibrium is present. The temperature dependence is as expected, with broadening and flattening of the distribution peaks. Raman spectra showed variations with temperature compatible with the presence of conformational equilibrium in the pyrrolidinium cation. Integration of the areas under the two peaks in Figure 8c and f yields the following values for the relative populations of equatorial to axial conformers of the alkyl chain relative to the pyrrolidinium ring: for [C4C1py]+, 4.3, 3.3, and 2.6; for [C3C1py]+, 21, 5.9, and 3.0; each set of three values corresponding to 300, 400, and 500 K, respectively. The set of observations derived from computer simulation are in total agreement with the DFT calculations and also with the experimental results of Raman spectroscopy of Fujimori et al.,9 discussed in the next point.

Conformational Landscapes of Ionic Liquid Ions

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Figure 8. Population distributions around chosen dihedral angles in [CnC1py]+ obtained by MD simulation of [CnC1py][Ntf2]. Atoms C1-C4 belong to the pyrrolidinium ring together with N, and atom C6 is the first carbon atom of the longer alkyl side chain (propyl or butyl), attached to N.

c. Raman Spectroscopy. In order to show the presence of different conformations of the pyrrolidinium ring in the ionic liquid [C4C1py][tf2N], Fujimori et al.9 assigned four Raman bands in the 860-950 cm-1 wavenumber range to the equatorial-envelope and axial-envelope conformers of [C4C1py]+ (the [tf2N] anion shows no Raman bands over that range). DFT calculations showed that the 892, 905, and 930 cm-1 bands are assignable to both conformers, whereas the band at 883 cm-1 is characteristic of the axial-envelope conformer. Finally, by obtaining spectra at different temperatures and noticing that the relative magnitude of the bands at 892, 905, and 930 cm-1 decreased with increasing temperature relative to the band at 883 cm-1, they could infer the presence of both conformers at equilibrium. This analysis is in full agreement with the MD resultssthe temperature-dependent spectrum of Figure 5 of ref 9 has its perfect analog in the temperature-dependent histograms of Figure 8f. Both clearly show the relative depletion of the most stable conformer as temperature is raised. Band deconvolution based on the theoretical Raman bands of each isomer (at the above-mentioned frequency range 860950 cm-1 and shown in Figure 5 of ref 9) was used to estimate the isomerization enthalpy from the equatorial envelope (phase angle 0°, Figure 6a) to the axial envelope (180°, Figure 6c). A value of 4.2(3) kJ mol-1 was obtained for the [C4C1py]+ cation from Raman spectra obtained in the 298-360 K temperature range. It must be stressed at this point that, although the equatorial- and axial-envelope conformers of the [CnC1py]+ cations represent local and absolute minima in the PES (cf. Figure 7), other conformations exist at relatively low energies, namely, those with phase angles between -72 and +72° (cf. Table 1, and also the large pool at the front of the PES of Figure 7). These conformers have Raman bands similar to the axialenvelope conformer, which means that a substantial part of the observed spectra (and corresponding shifts) can also be attributed to them. This is particularly valid for the [C3C1py]+ cation, where the conformer at 72° has a lower energy than the axialenvelope conformer (180°). In this case, an isomerization enthalpy from the equatorial envelope to the isomer with a 72° phase angle was evaluated from the Raman data to be 2.9(6)

kJ mol-1. Both isomerization enthalpies for [C4C1py]+ and [C3C1py]+ are in agreement with those predicted by DFT calculations (cf. Table 1) especially if we consider that most probably we have a distribution of conformers, with the axialenvelope conformer being more common than the one at 72° in the case of [C4C1py]+ and the opposite situation being valid for [C3C1py]+. This subtle difference between the two types of cations can also be observed in the conformer distribution histograms obtained by MD: the comparison of parts c and f of Figure 8 shows that at low temperatures there is a much smaller relative amount of the axial conformer (peaks around 80°) in the case of [C3C1py]+. The 72° phase angle conformer peak is superimposed to the large peak at 180° (belonging to the equatorialenvelope conformer) and cannot be observed separated from the latter. On the other hand, parts a and d of Figure 8 show that the dihedral distributions around 30° increase with increasing temperature, indicating that the isomers in the -72 to +72° phase angle range (corresponding to angles of 36° in Figure 8a and d) exist in equilibrium and, as expected, are more common at higher temperatures. Conclusions Raman spectroscopy and molecular dynamics simulation results have shown that the anions and cations that compose some commonly used ionic liquids exhibit in the liquid phase a conformational landscape very similar to the one calculated ab initio (DFT) for the isolated ions. This means that, despite the structured nature of the ionic liquids (composed of a polar network permeated by nonpolar domains), the ions enjoy enough freedom to adopt the conformations dictated by their internal structure and even interconvert between different conformations. This is not valid for the crystals of ionic liquids, where only one of the possible conformations is usually present due to constraints imposed by the crystalline lattice. The interplay between the structured nature of ionic liquids and the conformational freedom (flexibility) of its ions can help

1472 J. Phys. Chem. B, Vol. 112, No. 5, 2008 explain in the future many of the physical and solvation properties of these fluids. For instance, the “flexibility” of the bistriflamide anion (as opposed to the “rigidity” of most of the anions that compose ionic liquids) can explain the “special” properties of bistriflamide-based ionic liquids. Acknowledgment. The authors affiliated in Japan would like to acknowledge the financial support from Grant-in-Aids for Scientific Research Nos. 17350037, 18850017, and 19750062, and also by a Grant-in-Aid for the Global COE Program, “Science for Future Molecular Systems” from the Ministry of Education, Culture, Sports, Science and Technology. J.N.C.L. and K.S. acknowledge the support of grant POCI/QUI/57716/ 2004 (FCT/Portugal). A.A.H.P. and J.N.C.L. were supported by the PICS 3090 cooperation program between the CNRS France and GRICES Portugal. A.A.H.P. benefited from access to the IDRIS and CINES computing centers in France. References and Notes (1) Wang, Y.; Voth, G. A. J. Am. Chem. Soc. 2005, 127, 12192. (2) Pa´dua, A. A. H.; Canongia Lopes, J. N. J. Phys. Chem. B 2006, 110, 3330. (3) Triolo, A.; Russina, O.; Bleif, H-.J.; Di Cola, E. J. Phys. Chem. B 2007, 111, 4641. (4) Canongia Lopes, J. N.; Pa´dua, A. A. H. J. Phys. Chem. B 2006, 110, 7485. (5) Canongia Lopes, J. N.; Pa´dua, A. A. H. J. Phys. Chem. B 2004, 108, 16893. (6) Umebayashi, Y.; Fujimori, T.; Sukizaki, T.; Asada, M.; Fujii, K.; Kanzaki, R.; Ishiguro, S. J. Phys. Chem. A 2005, 109, 8976. (7) Fujii, K.; Fujimori, T.; Takamuku, T.; Kanzaki, R.; Umebayashi, Y.; Ishiguro, S. I. J. Phys. Chem. B 2006, 110, 8179. (8) Deetlefs, M.; Hardacre, C.; Nieuwenhuyzen, M.; Padua, A. A. H.; Sheppard, O.; Soper, A. K. J. Phys. Chem. B 2006, 110, 12055. (9) Fujimori, T.; Fujii, K.; Kanzaki, R.; Chiba, K.; Yamamoto, H.; Umebayashi, Y.; Ishiguro, S. I. J. Mol. Liq. 2007, 131, 216. (10) Jorgensen, W. L.; Maxwell, D. S.; Tirado-Rives, J. J. Am. Chem. Soc. 1996, 118, 11225.

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