Quaternary Ammonium Room-Temperature Ionic Liquid Including an

Jul 18, 2008 - ... Seiji Tsuzuki , Yasuo Kameda , Shinji Kohara , and Masayoshi Watanabe ... Hajime Matsumoto , Wataru Shinoda and Masuhiro Mikami...
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9914

J. Phys. Chem. B 2008, 112, 9914–9920

Quaternary Ammonium Room-Temperature Ionic Liquid Including an Oxygen Atom in Side Chain/Lithium Salt Binary Electrolytes: Ab Initio Molecular Orbital Calculations of Interactions between Ions Seiji Tsuzuki,*,† Kikuko Hayamizu,† Shiro Seki,‡ Yasutaka Ohno,‡ Yo Kobayashi,‡ and Hajime Miyashiro‡ National Institute of AdVanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8568, Japan, and Materials Science Research Laboratory, Central Research Institute of Electric Power Industry (CRIEPI) 2-11-1, Iwado-kita, Komae, Tokyo 201-8511, Japan ReceiVed: May 2, 2008; ReVised Manuscript ReceiVed: June 12, 2008

Interactions of the lithium bis(trifluoromethylsulfonyl)amide (LiTFSA) complex with N,N-diethyl-N-methylN-(2-methoxyethyl) ammonium (DEME), 1-ethyl-3-methylimidazolium (EMIM) cations, neutral diethylether (DEE), and the DEMETFSA complex were studied by ab initio molecular orbital calculations. An interaction energy potential calculated for the DEME cation with the LiTFSA complex has a minimum when the Li atom has contact with the oxygen atom of DEME cation, while potentials for the EMIM cation with the LiTFSA complex are always repulsive. The MP2/6-311G**//HF/6-311G** level interaction energy calculated for the DEME cation with the LiTFSA complex was -18.4 kcal/mol. The interaction energy for the neutral DEE with the LiTFSA complex was larger (-21.1 kcal/mol). The interaction energy for the DEMETFSA complex with LiTFSA complex is greater (-23.2 kcal/mol). The electrostatic and induction interactions are the major source of the attraction in the two systems. The substantial attraction between the DEME cation and the LiTFSA complex suggests that the interaction between the Li cation and the oxygen atom of DEME cation plays important roles in determining the mobility of the Li cation in DEME-based room temperature ionic liquids. Introduction Room temperature ionic liquids have attracted considerable attention due to a variety of expected applications in many fields of chemistry.1–4 Room temperature ionic liquids have the potential to become important industrial solvents for synthesis, catalysis, extraction, and purification because of the negligible vapor pressure and unusual catalytic properties. They also have potential for applications to electrolytes for lithium batteries because of their ionic conductivity and low flamability.5–9 Cyclic and acyclic quaternary ammonium based room temperature ionic liquids have been studied as candidates of electrolytes for lithium batteries because of the higher electrochemical stability compared with imidazolium- and pyridiniumbased room temperature ionic liquids.10,11 Some quaternary ammoniums including an oxygen atom in a side chain were used for cations for room temperature ionic liquids.12–15 N,NDiethyl-N-methyl-N-(2-methoxyethyl) ammonium (DEME) is one of the most widely used quaternary ammonium cations including an oxygen atom for ionic luquids.16–19 The oxygen atom decreases melting temperature and improves ionic conductivity.16 Recently, DEME-based room temperature ionic liquids and lithium salt binary electrolytes have been used for lithium secondary batteries. Polymer electrolytes consisting of poly(ethylene oxide) with ionic liquids were also used for lithium batteries.20,21 The large mobility of the Li cation in the polymer electrolyte and DEME-based ionic liquids suggests that the oxygen atoms play an important role in the high mobility of the Li cation. * Corresponding author. E-mail: [email protected]. † AIST. ‡ CRIEPI.

Detailed information on the intermolecular interactions between the Li and DEME cations is essential for understanding causes of the high mobility of the Li cation in DEME-based ionic liquids since the motion of the Li cation is controlled by the interactions with neighboring ions. Although physicochemical and electrochemical properties for DEME-based ionic liquids were studied extensively by several experimental measurements, the interactions between the Li and DEME cations are still unclear. There remain many unsettled issues: (1) It is not clear whether the Li cation prefers to have a close contact with the oxygen atom of the DEME cation in ionic liquids. Although the Li cation prefers to have close contacts with the oxygen atoms of neutral poly(ethylene oxide), the positive charge of DEME may prevent the contact with the Li cation. (2) It is unclear whether the DEME cation interacts with an isolated Li cation or the Li complex with an anion. (3) The difference between the interactions of the Li cation with the DEME cation and that of the Li cation with a typical cation of ionic liquids (for example EMIM) is unclear. (4) It is expected that the interactions between the Li and DEME cations is weaker than that between the Li cation and neutral ether molecules (for example DEE) because of the positive charge of DEME. The comparison of interactions in the Li-DEME and Li-DEE systems is necessary since a quantitative evaluation of the effects of the positive charge of DEME on the interactions was not reported. In this article, we studied the intermolecular interactions between the Li and DEME cations by ab initio calculations. It is not an easy task to reveal the details of the interactions only by experimental measurements. Recently, ab initio molecular orbital calculation is becoming a powerful tool for studying intermolecular interactions. Ab initio calculations provide very accurate

10.1021/jp803866u CCC: $40.75  2008 American Chemical Society Published on Web 07/18/2008

Ab Initio Molecular Orbital Calculations

J. Phys. Chem. B, Vol. 112, No. 32, 2008 9915

interaction energies for small clusters, if a reasonably large basis set is used, and electron correlation is properly corrected. We studied the size of interaction energy, its orientation dependence and the origin of attraction. The interactions of Li cation with ethylmerhylimidazolium (EMIM) cation and neutral diethylether (DEE) were also studied for comparison. Computational Methods The Gaussian 03 program22 was used for the ab initio molecular orbital calculations. The basis sets implemented in the program were used for the calculations. Electron correlation was accounted for by the second-order Mφller-Plesset perturbation (MP2) method.23,24 Interaction energies (Eint) for the LiTFSA complex with DEME, EMIM cations, and neutral DEE were calculated by the supermolecule method at the MP2/6311G** level, if not otherwise noted. The basis set superposition error (BSSE)25 was corrected using the counterpoise method.26 Geometries of isolated DEME, EMIM cations, neutral DEE, and LiTFSA complex were optimized at the HF/6-311G** level. The optimized geometries were kept frozen in the calculations of intermolecular interaction energy potentials. The potentials were calculated by changing the intermolecular distance. The formation energies (Eform) of the DEME-LiTFSA and DEE-LiTFSA complexes from isolated species (DEME cation, neutral DEE, and LiTFSA complex) were calculated using the fully optimized geometries of the complexes at the HF/6311G** level. Eform was obtained as the sum of the interaction energy of the complex (Eint) and the deformation energy (Edef). The Edef is the sum of the increase of the energies of DEME cation, neutral DEE, and the LiTFSA complex by the deformation of geometries associated with the complex formation. The Edef was calculated at the MP2/6-311G** level. Electrostatic energy was calculated as the interactions between distributed multipoles27,28 of the DEME cation, nutral DEE, and LiTFSA complex using ORIENT, version 3.2.29 Distributed multipoles up to hexadecapole on all atoms were obtained from MP2/6311G** wave functions of the isolated species using the GDMA program.30 Induction energy was calculated as the interactions of polarizable sites with the electric field produced by the distributed multipoles of the DEME cation, neutral DEE, and LiTFSA complex.31 The atomic polarizabilities of carbon (R ) 10 au), nitrogen (R ) 8 au), oxygen (R ) 6 au), fluorine (R ) 3 au), and sulfur (R ) 20 au) were used for the calculations. The distributed multipoles were used only to estimate the electrostatic and induction energies.32 Atomic charge distributions were calculated by electrostatic potential fitting using the Merz-Singh-Kollman scheme33,34 from the MP2/6-311G** wave functions of isolated species. Results and Discussion Basis Set and Electron Correlation Effects. HF and MP2 interaction energies for the DEME cation with the LiTFSA complex (DEME-LiTFSA, 1) and neutral DEE with the LiTFSA complex (DEE-LiTFSA, 2) (Figure 1) were calculated using several basis sets for evaluating basis set and electron correlation effects as summarized in Table 1. The basis set effects are very small, if basis sets including polarization functions are used. Negligible basis set dependence is also observed in calculations of the interaction energy for the 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIM-BF4) complex.35 The calculations show that basis set effects are negligible if the 6-311G** or larger basis sets are used. The calculations of the EMIM-BF4 complex also show that electron correlation effects beyond MP2 are very small. We studied the basis set dependence of the interaction

Figure 1. Optimized geometries for the (a) DEME-LiTFSA (1) and (b) DEE-LiTFSA (2) complexes at the HF/6-311G** level.

TABLE 1: HF and MP2 Level Interaction Energies Calculated for DEME-LiTFSA and DEE-LiTFSA Complexesa basis set DEME-LiTFSA (1) 6-31G* 6-311G* 6-311G** 6-311++G** cc-pVDZ cc-pVTZ DEE-LiTFSA (2) 6-31G* 6-311G* 6-311G** 6-311++G** cc-pVDZ cc-pVTZ

bfb

EHFc

EMP2c

438 544 604 728 472 1068

-15.70 -15.51 -15.67 -15.20 -16.52 -14.93

-19.13 -18.40 -18.39 -18.36 -19.22 -19.66

343 424 454 548 352 778

-21.16 -21.02 -21.15 -20.99 -21.55 -20.45

-22.05 -21.44 -21.05 -21.21 -21.38 -21.45

a Energy in kcal/mol. Geometries of complexes are shown in Figure 1. b Number of basis functions used for calculation. c BSSE corrected HF and MP2 level interaction energies.

energies for the LiTFSA complex with the DMEM cation and neutral DEE since the interactions in these systems may have different basis set dependence. We studied interaction energies of the LiTFSA complex with DEME, EMIM cations, and neutral DEE at the MP2/6-311G** level in this work because of the negligible basis set dependence. HF and MP2 level interaction energy potentials calculated for the DEME-LiTFSA system are shown in Figure 1S, Supporting Information. The equilibrium distance calculated at the HF level is nearly identical to that at the MP2 level, though the HF level calculations slightly underestimate the attraction. The nearly identical equilibrium distances show that the optimized geometries obtained by HF level calculations are sufficiently accurate. Interactions between the Li Cation and the TFSA Anion. Optimized geometries and calculated formation energies (Eform) for five structures of the LiTFSA complex (3) are shown in Figure 2. The five structures were obtained by geometry optimizations from 23 initial geometries. Optimized structures show that the Li cation favors close contact with oxygen and nitrogen atoms of the TFSA anion. In the five stable structures, the Li cation does not have contact with a fluorine atom. Similar stable structures were obtained from HF/6-31G* level geometry optimizations.36 More recently, MP2/aug-cc-pVDZ level potentials for two orientations of the Li-TFSA complex were reported.37 The Li cation prefers to have close contact with

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Tsuzuki et al.

Figure 2. HF/6-311G** level optimized geometries and MP2/6-311G** level formation energies (Eform ) Eint + Edef) of five orientations of the Li-TFSA complex from isolated ions (kcal/mol). The Eint is the MP2/6-311G** level interaction energy between ions. The Edef is the increase of the energy of TFSA by the deformation of geometry in complex formation. The Eint is shown in parentheses.

Figure 4. Seven geometries of the DEME-LiTFSA system used for the calculations of interaction energy potentials. The N · · · Li distances in the orientations 1a-1e and O · · · Li distances in orientations 1f and 1g are shown in Figures 5 and 6. Figure 3. Optimized geometries for DEME, N4221 cations, neutral DEE, and the LiTFSA complex at the MP2/6-311G** level and atomic charges obtained by electrostatic potential fitting using the Merz-SinghKollman scheme from the MP2/6-311G** level wave functions. Atomic charges with hydrogen atoms are summed into heavy atoms.

oxygen and nitrogen atoms because of the large negative charges on these atoms. Recently reported ab initio calculations show that the negative charges on the nitrogen atom (-0.71 e) and oxygen atoms (-0.53 to -0.54 e) in TFSA are significantly larger (more negative) than those on fluorine atoms (-0.14 to -0.20 e).35 Eform calculated for the most stable orientation of LiTFSA complex (3a) is -137.2 kcal/mol. The interaction between the Li cation and the TFSA anion is significantly larger than the interaction in ion pairs of typical room-temperature ionic liquids. For example, the Eform calculated for the EMIM-TFSA complex at the same level is -78.8 kcal/mol. The Eform values for the ion pairs of typical room temperature ionic liquids do not largely

depend on the choice of anion and cation (in most cases less than 10 kcal/mol).35 The very strong interactions between the Li cation and TFSA anion suggests that the Li cation binds strongly with the TFSA anion in DEMETFSA/LiTFSA mixed electrolyte and that the DEME cation does not interact with an isolated Li cation but interacts with the LiTFSA complex in the binary electrolyte. Therefore, we studied the interactions of the LiTFSA complex with DEME, EMIM cations, and neutral DEE in this work. The Li cation has close contact with two oxygen atoms, one from each of the different SO2 groups of the TFSA anion, in the most stable 3a structure, which has C2 symmetry. The Cs structure (3b) is slightly less stable (Eform ) -135.7 kcal/mol). Although the nitrogen atom has a larger negative charge than the oxygen atoms, the structures 3c and 3d, in which the Li cation has close contact with the nitrogen atom and an oxygen atom of the SO2 group, are 8.7 and 9.6 kcal/mol less stable than 3a, respectively. The structure 3e, in which the Li cation has close contact with two oxygen atoms of the same SO2 group, is 18.8 kcal/mol above 3a.

Ab Initio Molecular Orbital Calculations

Figure 5. MP2/6-311G** level interaction energy potentials for the DEME-LiTFSA system. Geometries of the DEME-LiTFSA system are shown in Figure 4.

Figure 6. MP2/6-311G** level interaction energy potentials for the DEME-Li, DEME-LiTFSA (1f) and DEE-LiTFSA (2a) systems. Geometries of the three systems are shown in Figures 4 and 9.

Charge Distributions of DEME and N4221 Cations. Atomic charge distributions of the DEME cation are compared with those of the N4221 (N-butyl-N,N-diethyl-N-methylammonium) cation as shown in Figure 3. Although the total charge of the DEME cation is +1.0 e, the oxygen atom of DEME cation has a large negative charge (-0.33 e). The methyl and methylene groups of the DEME cation connected to the oxygen atom have substantial positive charges due to the electron-withdrawing nature of the oxygen atom. Similar charge distributions were calculated for neutral DEE. The oxygen atom of DEE has a large negative charge (-0.48 e), and the methylene groups of DEE connected to the oxygen atom have substantial positive charges. The negative charge on the oxygen atom of the DEME cation is smaller than that of neutral DEE. Methyl and methylene groups of the DEME cation connected to the nitrogen atoms have substantial positive charges as in the case of N4221 cation.

J. Phys. Chem. B, Vol. 112, No. 32, 2008 9917 The optimized structure (Figure 1) suggests that the attractive electrostatic interaction between TFSA anion and positive charge near the nitrogen atom of DEME also contributes to the large Eint for the fully optimized DEME-LiTFSA complex. The large negative charge on the oxygen atom of the DEME cation suggests that the Li atom in the LiTFSA complex favors close contact with the oxygen atom of the DEME cation in the electrolyte. However, ab initio calculations show that the imidazolium ring and ethyl and methyl groups in EMIM do not have negative charges.35 These results suggest that the negative charge on the oxygen atom of the DEME cation plays important roles in determining the characteristic properties of DEME-based ionic liquid electrolytes. Interactions between the DEME Cation and the LiTFSA Complex. Interaction energy (Eint) potentials calculated for the DEME cation with the LiTFSA complex (Figure 4) show that the LiTFSA complex and DEME cation form a stable complex when the Li atom in the LiTFSA complex has close contact with the oxygen atom of DEME (1f). MP2/6-311G** level interaction energy potentials calculated for seven different orientations of the DEME-LiTFSA system (1a-1 g) are shown in Figure 5. Seven detailed structures of the DEME-LiTFSA system are shown in Figure 2S in Supporting Information. The seven potentials are repulsive when the DEME cation and the LiTFSA complex are well separated. The potential for the preferential orientation 1f has a minimum when the Li · · · O distance is 2.0 Å. The interaction energy calculated at the minimum is -5.9 kcal/mol. The potentials for the other six orientations are always repulsive. The DEME-LiTFSA interactions favors the 1f orientation because of the attractive electrostatic interaction between the Li cation and the negatively charged oxygen atom in the DEME cation. Charge distributions calculated for the LiTFSA complex show that the Li atom has a substantial positive charge (0.92 e) as shown in Figure 3. Interaction energy potentials calculated for seven orientations of the DEME-Li systems show that an isolated Li cation does not form the stable complex with the DEME cation. The interaction energy potentials for the DEME-Li system were calculated using the same geometries of the DEME-LiTFSA system while removing the TFSA. The potential calculated for the DEME-Li system (1f) was compared with that for DEMELiTFSA (1f) in Figure 6. The potential is always repulsive, though the potential has a local minimum when the Li · · · O distance is 2.0 Å. The local minimum is 37.5 kcal/mol less stable than the isolated Li cation and DEME cation. The fully optimized geometry of the DEME-LiTFSA complex at the HF/6-311G** level is shown in Figure 1. Eint calculated for the complex is -18.4 kcal/mol. The electrostatic and induction interactions are the major source of the attraction, as

Figure 7. Eight geometries of the EMIM-LiTFSA system used for the calculation of interaction energy potentials. The X · · · Li distances are shown in Figure 8. X is the midpoint between the nitrogen atoms of EMIM.

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Tsuzuki et al. TABLE 2: Interaction Energies for DEME-LiTFSA and DEE-LiTFSA Complexesa complex

Eintb

Edefc

Eformd

Eese

Eind f

Eother g

DEME-LiTFSA (1) DEE-LiTFSA(2)

-18.4 -21.1

2.3 0.8

-16.1 -20.3

-15.0 -21.0

-9.3 -9.1

5.9 9.1

a Energy in kcal/mol. Geometries of complexes are shown in Figure 1. b BSSE corrected interaction energies for complexes calculated at the MP2/6-311G** level. c Sum of increases of energies of monomers (DEME, DEE, and LiTFSA) by the deformation of geometries associated with complex formation. See the text. d Formation energy of complex from isolated monomers. Sum of Eint and Edef. e Electrostatic energy. See the text. f Induction energy. See the text. g Eother ) Eint - Ees - Eind. Eother is mainly exchange-repulsion and dispersion energies.

Figure 8. MP2/6-311G** level interaction energy potentials for the EMIM-LiTFSA system. Geometries of the EMIM-LiTFSA system are shown in Figure 7.

Figure 9. Geometry for the DEE-LiTFSA system (2a) used for the calculation of interaction energy potentials. The O · · · Li distance is shown in Figure 6. The C2 axis of the LiTFSA complex is on the bisector of the C-O-C angle of the DEE.

shown in Table 2. Electrostatic (Ees) and induction (Eind) energies are -15.0 and -9.3 kcal/mol, respectively. The induction energy by polarization of the DEME cation is -6.7 kcal/mol, while that of the LiTFSA complex is -2.6 kcal/mol. The strong electric field produced by the Li cation is responsible for the large polarization of DEME. The Eint for the fully optimized geometry is remarkably larger than that for 1f (-5.9 kcal/mol). Interactions between the EMIM Cation and the LiTFSA Complex. The interactions in the EMIM-LiTFSI system (Figure 7) was calculated as a model of the interactions of the LiTFSA

complex with a typical cation of ionic liquids. Interaction energy potentials (Eint) calculated for the eight orientations of the EMIM-LiTFSA system (4a-4h) are shown in Figure 8. Eight detailed structures of the EMIM-LiTFSA system are shown in Figure 3S, in Supporting Information. In contrast to the DEMELiTFSA system, the interactions in the EMIM-LiTFSA systems are always repulsive, when the Li atom in the LiTFSA complex faces the EMIM cation (4a-4f). Ab initio calculations show that all the CH, CH2, and CH3 groups and nitrogen atoms of the EMIM cation have positive charges. The electrostatic interaction between the Li cation and the positively charged atoms and groups in the EMIM cation is responsible for the repulsive potentials. Clearly, the DEME-LiTFSA system has significantly different interactions from that in the EMIM-LiTFSA system because of the negatively charged oxygen atom of the DEME cation. The comparison suggests that the characteristic attraction found in the DEMELiTFSA system plays important roles in the high mobility of the Li cation in DEME-based ionic liquids. Relatively high lithium ion transport numbers were reported for the DEMETFSA-LiTFSA binary electrolyte.38 Although the potentials for the six orientations of the EMIM-LiTFSA system (4a-4f) are always repulsive, the potentials for the two orientations (4g and 4h), in which TFSA faces to the EMIM cation, are attractive probably because of the attractive electrostatic interaction between the TFSA anion and the EMIM cation. The calculations show that the TFSA in the LiTFSA complex prefers to face the EMIM when the LiTFSA complex approaches the EMIM cation. Interactions between Neutral DEE and the LiTFSA Complex. The attraction between the LiTFSA complex and the neutral DEE is greater than that between the LiTFSA complex and the DEME cation. Interaction energy potential (Eint)

Figure 10. Stable geometries calculated for the DEMETFSA-LiTFSA complex at the HF/6-311G** level and MP2/6-311G** level formation energies (Eform ) Eint + Edef) for the complex from the isolated DEMETFSA complex and the LiTFSA complex (kcal/mol). The Eint is the MP2/ 6-311G** level interaction energy between the DEMETFSA complex and the LiTFSA complex. The Edef is the increase of energies of the two complexes by the deformation of geometries associated with the DEMETFSA-LiTFSA complex formation. The Eint is shown in parentheses.

Ab Initio Molecular Orbital Calculations calculated for the DEE-LiTFSA system 2a (Figure 9) is compared with that for the DEME-LiTFSA system 1f as shown in Figure 6. The potential for the DEE-LiTFSA has a minimum when the Li · · · O distance is 2.0 Å. The Eint at the potential minimum (-19.2 kcal/mol) is about 3 times larger than that for 1f. The fully optimized geometry for the DEE-LiTFSA complex is shown in Figure 1b. Eint calculated for the complex is -21.1 kcal/mol, which is 2.7 kcal/mol greater than that for the DEMELiTFSA complex. Table 2 shows that the attraction in the DEELiTFSA complex is enhanced by the stronger electrostatic interaction. The Ees for the DEE-LiTFSA complex (-22.0 kcal/ mol) is greater than that for the DEME-LiTFSA complex (-15.0 kcal/mol) because of the large negative charge on the oxygen atom of DEE (-0.48 e) compared with that of DEME (-0.33 e) as shown in Figure 3.The repulsion between the positive charge of the DEME cation and the Li atom of LiTFSA complex is also a cause of the weaker attractive electrostatic interaction in the DEME-LiTFSA complex compared with the DEELiTFSA complex. Interactions between the DEMETFSA Complex and the LiTFSA Complex. In the bulk ionic liquid environment, the LiTFSA complex interacts with the DEME cation, which is surrounded by TFSA anions since the DEME cation strongly interacts with TFSA anions in the ionic liquid. The interactions between the DEMETFSA complex and LiTFSA complex were calculated for evaluating the effects of the TFSA anion. Before starting the geometry optimizations of the DEMETFSA-LiTFSA complex, geometries of the DEME-Cl complex (5) were optimized. Three stable geometries calculated for the DEMECl complex (5a-5c) are shown in Figure 4S in Supporting Information. Initial geometries for the geometry optimizations of the DEMETFSA-LiTFSA complex were prepared from the optimized geometries for the DEME-Cl complex by replacing the Cl anion with the TFSA anion and adding the LiTFSA complex. Three stable geometries calculated for the DEMETFSALiTFSA complex (6a-6c) are shown in Figure 10.39 Eint for the most stable orientation 6b (-23.2 kcal/mol) is substantially larger than that for the DEME-LiTFSA complex (-18.4 kcal/ mol) probably because of the attraction between the Li cation and the additional TFSA anion. The structure 6c is slightly less stable than 6b. The Li cation has close contact with the oxygen atom of the DEME cation and with two TFSA anions. The small energy difference between 6b and 6c suggests that the TFSA anion interacting with the DEME cation does not strongly prevent the Li cation approaching the oxygen atom of the DEME cation. The Eint for the DEMETFSA-LiTFSA complex (6b) is slightly greater than that for the DEE-LiTFSA complex (-21.1 kcal/mol). The large Eint for the DEMETFSA-LiTFSA complex suggests that the Li atom of the LiTFSA complex prefers to have close contact with the oxygen atom of the DEME cation in ionic liquid. The repulsive electrostatic interaction between the Li cation and DEME cation was largely screened by the TFSA anions. Conclusions Interactions of the LiTFSA complex with DEME, EMIM cations, neutral DEE, and the DEMETFSA complex were studied by ab initio molecular orbital calculations. The attraction between the Li cation and the TFSA anion is very strong, which suggests that the DEME does not interact with an isolated Li cation in the binary electrolytes, but can interact with the LiTFSA complex. Calculations show that the DEME-Li

J. Phys. Chem. B, Vol. 112, No. 32, 2008 9919 complex is less stable than isolated DEME and Li cations. However, calculations show that the DEME cation and the LiTFSA form a stable complex. Interestingly, the interactions between the DEME cation and the LiTFSA complex are significantly different from those between the EMIM cation and the LiTFSA complex. Interaction energy potentials for the EMIM-LiTFSA system are always repulsive, while that for the DEME-LiTFSA system is attractive when the Li atom of LiTFSA has close contact with the oxygen atom of DEME. The attraction between the DEME cation and the LiTFSA complex is weaker than that between the neutral DEE and the LiTFSA complex. The interaction of the LiTFSA complex with the DEMETFSA complex is greater than that with the DEME cation. The repulsive electrostatic interaction between the Li cation and DEME cation is largely screened by TFSA anions. The comparison suggests that the characteristic attraction between the DEME cation and the LiTFSA complex plays important roles in the high mobility of the Li cation in DEMEbased ionic liquids. Acknowledgment. We thank Tsukuba Advanced Computing Center for the provision of computational facilities. Supporting Information Available: HF and MP2 level interaction energy potentials for the DEME-LiTFSA system; geometries of the DEME-LiTFSA and EMIM-LiTFSA systems used for the calculation of interaction energy potentials; and stable geometries and formation energies for the DEME-Cl complex. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Welton, T. Chem. ReV. 1999, 99, 2071. (2) Holbrey, J. D.; Seddon, K. R. Clean Products Processes 1999, 1, 223. (3) Wasserscheid, P.; Keim, W. Angew. Chem., Int. Ed. 2000, 39, 3772. (4) Xu, K. Chem. ReV. 2004, 104, 4303. (5) Chum, H. L.; Koch, V. R.; Miller, L. L.; Oesteryoung, R. A. J. Am. Chem. Soc. 1975, 97, 3264. (6) Wikes, J. S.; Levisky, J. A.; Wilson, R. A.; Hussey, C. L. Inorg. Chem. 1982, 21, 1263. (7) Ryan, D. M.; Reichel, T. L.; Welton, T. J. Electrochem. Soc. 2002, 149, A371. (8) Lagrost, C.; Carrie, D.; Vaultier, M.; Hapiot, P. J. Phys. Chem. A 2003, 107, 745. (9) Tokuda, H.; Tsuzuki, S.; Susan, M. A. B. H.; Hayamizu, K.; Watanabe, M. J. Phys. Chem. B 2006, 110, 19593. (10) Matsumoto, H.; Kageyama, H.; Miyazaki, Y. Chem. Commun. 2002, 1726. (11) Zhou, Z.-B.; Matsumoto, H.; Tatsumi, K. Chem. Eur. J. 2006, 12, 2196. (12) Matumoto, H.; Yanagida, M.; Tanimoto, K.; Nomura, M.; Kitagawa, Y.; Miyazaki, Y. Chem. Lett. 2000, 922. (13) Zhou, Z.-B.; Matsumoto, H.; Tatsumi, K. Chem. Lett. 2004, 33, 1636. (14) Zhou, Z.-B.; Matsumoto, H.; Tatsumi, K. Chem. Eur. J. 2004, 10, 6581. (15) Lang, C. M.; Kohl, P. A. J. Electrochem. Soc. 2007, 154, F106. (16) Sato, T.; Maruo, T.; Marukane, S.; Takagi, K. J. Power Sources 2004, 138, 253. (17) Sato, T.; Masuda, G.; Takagi, K. Electrochem. Acta 2004, 49, 3603. (18) Seki, S.; Kobayashi, Y.; Miyashiro, H.; Ohno, Y.; Mita, Y.; Usami, A.; Terada, N.; Watanabe, M. Electrochem. Solid-State Lett. 2005, 8, A577. (19) Seki, S.; Kobayashi, Y.; Miyashiro, H.; Ohno, Y.; Usami, A.; Mita, Y.; Watanabe, M.; Terada, N. Chem. Commun. 2006, 544. (20) Shobukawa, H.; Tokuda, H.; Tabata, S.; Watanabe, W. Electrochem. Acta 2004, 50, 1. (21) Shin, J.-H.; Henderson, W. A.; Tizzani, C.; Passerini, S.; Jeong, S.-S.; Kim, K.-W. J. Electrochem. Soc. 2006, 153, A1649. (22) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.;

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Tsuzuki et al. (30) Stone, A. J. J. Chem. Theory Comput. 2005, 1, 1128. (31) Stone, A. J. Mol. Phys. 1985, 56, 1065. (32) van Duijnen, P. T.; Swart, M. J. Phys. Chem. A 1998, 102, 2399. (33) Singh, U. C.; Kollman, P. A. J. Comput. Chem. 1984, 5, 129. (34) Besler, B. H.; Mertz, K. M.; Kollman, P. A. J. Comput. Chem. 1990, 11, 431. (35) Tsuzuki, S.; Tokuda, H.; Hayamizu, K.; Watanabe, M. J. Phys. Chem. B 2005, 109, 16474. (36) Gejji, S. P.; Suresh, C. H.; Babu, K.; Gadre, S. R. J. Phys. Chem. A 1999, 103, 7474. (37) Borodin, O.; Smith, G. D. J. Phys. Chem. B 2006, 110, 6293. (38) Seki, S.; Ohno, Y.; Miyashiro, H.; Kobayashi, Y.; Usami, A.; Mita, Y.; Terada, N.; Hayamizu, K.; Tsuzuki, S.; Watanabe, M. J. Electrochem. Soc. 2008, 155, A421. (39) The positions of the TFSA anion interacting with the DEME cation in 6a and 6b are close to the positions of the Cl anion in DEME-Cl complex 5a and 5b. The C3-N2-NTFSA angle in 6a and the C1-N2-NTFSA angle in 6b are 176° and 162°, respectively. The C3-N2-Cl angle in 5a and the C1-N2-Cl angle in 5b are 177° and 161°, respectively. See Figure 10 and Figure 4S in Supporting Information.

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