10882
J. Phys. Chem. 1996, 100, 10882-10891
Theoretical Study of CF3SO3Li, (CF3SO2)2NLi, and (CF3SO2)2CHLi Ion Pairs R. Arnaud,* D. Benrabah, and J-Y. Sanchez LEDSS, UniVersite´ Joseph Fourier, Domaine UniVersitaire, BP 53, 38041 Saint Martin d’He` res, France, and LIES, Laboratoire d'Ionique et d’Electrochimie des Solides, URA 1213, CNRS ENSEEG-INP Grenoble BP 75, 38402 Saint Martin d’He` res Cedex, France ReceiVed: NoVember 6, 1995; In Final Form: April 16, 1996X
Ion pairs of lithium salts based on three organic anions, namely Tf-, TFSI-, and TFSM-, have been studied by ab initio quantum-chemical methods. These calculations allowed us to draw the conformational energy diagram for TFSM- and TFSI-. Geometries and binding energies have been evaluated at the restricted Hartree-Fock (RHF) level using standard basis set (3-21+G* and 6-31+G*). For the reference Tf--Li+, electron correlation effects have been determined at the MP2 level. Wave function analyses have been performed by the natural bond orbital (NBO) method. Bidentate structures involving two oxygen atoms (one of each SO2 group in the TFSI-,Li+ and TFSM-,Li+ systems) are preferred. The affinities of these anions for the lithium cation are strong and lie in the range 120-145 kcal‚mol-1, arising essentially from the electrostatic interaction. Charge transfer contributions are less important, only 14-17% of the electrostatic interaction.
Introduction High specific energy, lifetime, and above all safety are indisputably among the most important specifications of large batteries required for electrical cars. Owing (i) to the light weight of lithium, (ii) to its Li/Li+redox couple -3 V vs NHE, and (iii) to the small size of lithium cation which allows most of the intercalation materials to be used as cathode, lithium batteries are among the more promising secondary batteries. Nevertheless the high reactivity of lithium metal greatly limits the range of electrolytes usable in the cells. As liquid electrolytes significantly decrease the lithium battery lifetime and safety, all-solid state batteries appear henceforth as good candidates for high-capacity lithium batteries.1 Among these, one of the most promising for an industrial production seems to be the thin-film polymer batteries whose overall thickness ranges from 110 to 200 µm. In these devices the electrolyte is obtained by dissolution of a lithium salt in a polymer which acts (i) as a solid separator between the electrodes and (ii) as a polymeric solvent for the lithium salts. The redox stability requirement forbids the use of polar protic groups, such as acids, alcohols, thiols, and primary and secondary amines, which react with the lithium anode, and of most of the polar aprotics such as esters, nitrile, or carbonate. Owing to the absence of protic groups the host polymers have a rather low acceptor number (AN) and the anions are therefore poorly solvated. As for the exclusion of most of the polar aprotic groups, it results in a polymer with a low permittivity which does not favor the ionpair dissociation. The selection of salts to be dissolved in these polymeric solvents is therefore decisive in providing highly conductive polymer electrolytes. These salts are often chosen among salts of very strong acids or superacids such as LiClO4, LiI, LiBF4, LiAsF6, LiPF6, or CF3SO3Li. Nevertheless, some of them are unstable and sometimes dangerous. Thus LiI, although stable in contact with highly reducing lithium metal, has a limited stability toward oxidation (close to 2.8 V vs Li) which restricts the selection of cathode materials. As for anions of superacids such as AsF6-, BF4-, and PF6-, they are the result of an acid-base equilibrium1 with the corresponding Lewis acid, e.g., AsF6- f AsF5 + F-, influenced by the electron-donating X
Abstract published in AdVance ACS Abstracts, June 1, 1996.
S0022-3654(95)03259-X CCC: $12.00
ability of the solvent and the polarizing nature of Li+. Thus Abraham2 reported the rapid decomposition of diethyl ether/ LiAsF6 liquid electrolytes in lithium cells and proposed a multistep mechanism involving the ether function. On the other hand, LiBF4 undergoes hydrolysis in the presence of water traces, leading to acid products. Chemical toxicity (LiAsF6) and explosion hazards (LiClO4), moreover, make these salts unsuitable for use on a large scale. As for the lithium triflate, CF3SO3Li (LiTf), its solutions in polyethers are generally less conductive than most of the previous ones. Special attention has therefore been paid to new lithium salts meeting the previous requirements, i.e., thermal and electrochemical stability. These past years several organic bulky salts were therefore investigated in various host polymers allowing a neat enhancement of the polymer electrolyte conductivities. Thus, lithium trifluoromethanesulfonyl imide, (CF3SO2)2N- Li+ (LiTFSI), greatly enhances the conductivity of semicrystalline solvating polymers, such as poly(oxyethylene) (POE)3 and poly(oxyethyleneoxymethylene).4 LiTFSI, in which the nitrogen atom is substituted by two strong electron-withdrawing groups, decreases the crystallinity and slows down the crystallization rate. Moreover, in completely amorphous solvating polymers such as poly(oxypropylene) (POP)5 or cross-linked polyethers,6 LiTFSI provides more conductive electrolytes than LiClO4 and above all LiTf. Another salt, bearing three electron-withdrawing groups, the lithium tris(trifluoromethanesulfonyl) methanide, (CF3SO2)3C-Li+ (LiTriTFSM), has been investigated by Dominey in organic solvents7 and by Benrabah et al. in POE.8 Both LiTFSI and LiTriTFSM, as well as the disubstituted carbanion (CF3SO2)2CH-Li+, (LiTFSM), decrease the crystallinity content, slow down the crystallization rate and limit the Tg enhancement following the salt concentration increase. As a consequence, the ionic conductivities of their complexes, in semicrystalline POE, are higher than that of CF3SO3Li and LiClO4. As a matter of fact the number of investigations concerning the electronic structure of these anions, often conjugated bases of strong acids or superacids, and their lithium salts is somewhat limited. To the best of our knowledge, theoretical studies involve essentially the triflate anion and its lithium salt. Bencivenni et al.9 and Lindgren et al.10 reported an ab initio © 1996 American Chemical Society
Ion Pairs of Li Salts
J. Phys. Chem., Vol. 100, No. 26, 1996 10883
TABLE 1: Optimized Geometries of the Triflate Anion and Its Lithium Bidentate Ion Paira free anion (Figure 1a) parameterb C-S S-O1 S-O2(3) C-F1 C-F2(3) CSO1 CSO2(3) SCF1 SCF2(3) Li-O2(3) LiO2(3)S a
ion pair (Figure 1b)
HF/3-21+G*
HF/6-31+G*
MP2/6-31+G*
HF/3-21+G*
HF/6-31+G*
MP2/6-31+G*
1.828 1.450 1.450 1.371 1.371 102.3
1.832 1.443 1.443 1.324 1.324 102.6
1.860 1.482 1.482 1.359 1.359 102.0
112.7
111.8
112.0
1.809 1.424 1.476 1.366 1.355 106.4 103.2 109.5 111.5 1.933 90.7
1.823 1.417 1.469 1.318 1.309 106.1 104.0 109.7 110.3 1.895 89.3
1.848 1.455 1.510 1.354 1.342 105.6 103.1 109.8 110.5 1.927 88.1
Underlined values indicate that oxygen atom is bounded to Li atom. b Bond lengths are in Å, angles in deg.
study of CF3SO3- and CCl3SO3- including vibrational frequencies. Huang, Frech, and Wheeler11 using ab initio HF method up to 6-31+G* level examined lithium salt of the triflate anion and performed a detailed analysis of the effect of Li+ complexation upon harmonic vibrational frequencies and electronic distribution. Burk et al.12 studied the prototropic tautomerism and acidity of tris(fluorosulfonyl)methane at both semiempirical (PM3) and ab initio (HF/STO-3G* and 3-21+G*) levels. In preliminary work,13 we discussed by means of ab initio techniques the molecular structures and electronic charges of the three free anions: Tf-, TFSI-, and TFSM-. We showed that these anions are strongly delocalized species and this is probably the main reason for the superacid behavior of the conjugated acids. In this report we will address the following issues regarding the TFSI- and TFSM- anions and their lithium salts: (a) the conformational characteristics of the two anions, (b) the molecular structures and the electronic distributions of the stable TFSI-Li+ and TFSM-Li+ ions pairs, and (c) the nature of the bonding corresponding to each local energy minimum by estimating electrostatic and charge transfer contributions to the binding energy. For completeness, the calculated data relative to these two salts are discussed with special reference to the Tf--Li+ complex. For this less computationally demanding system, basis set and correlation effects (MP2) on the structure and binding energy have been examined. Methods Ab initio Hartree-Fock (HF) self-consistent field molecular orbital calculations were carried out by using the GAUSSIAN 92 system of programs.14 All geometries were fully optimized using the internally stored 3-21+G* basis set.15 It is widely recognized that a reliable description of negatively charged species requires inclusion of diffuse functions in the basis set.16 Thus, HF/3-21+G* geometries were further used for single point calculation with the 6-31+G** basis set.15 For the smallest systems (triflate anion and its lithium salt) geometry optimization was also performed at the HF/6-31+G* and at the second-order Moller-Plesset MP2(fc)/6-31+G* levels. Wave function analysis was performed by the natural population analysis (NPA)/ natural bond orbital (NBO) method of Weinhold and coworkers17 with the program NBO version 3.1 which was built into link 607 of the GAUSSIAN 92 program. Quantitative analyses of delocalization interactions between NBOs were obtained using the NBO Fock matrix deletion procedure.18 The deletion procedure is not self-consistent, but as long as the particular interactions in the Fock matrix that have been zeroed are not strongly coupled with other interactions, the error in the energy is not significant.19 The deletion procedure permits one to study both intramolecular delocalizing interactions by coupling any two orbitals on the separate fragments in lithium
Figure 1. Atomic numbering and equilibrium geometries of triflate anion (a) and its bidentate lithium salt (b).
salts. The energy difference between calculations performed with and without deletion of all off-diagonal blocks of the NBO Fock matrix for the anion-Li+ salt represents the charge transfer contribution (ECT) to the binding energy. To interpret computational information about anion-cation interactions, it is important to be able to estimate the electrostatic contribution to binding energy. This estimation can be done in two different ways: (i) from the computation of the electrostatic potential created by the anion at the point corresponding to the Li+ position in the salt; (ii) according to the technique suggested by Horn and Ahlrichs20 and applied recently for alkaline cationpolar molecules systems21 by calculating the total energy of the salt without valence shell basis for the cation. In the following, these two quantities will be labeled respectively as Ees1 and Ees2. Results A. Triflate Ion and Its Lithium Salt. First, we shall present and discuss the results obtained for the “model” system. The other two systems studied here were discussed with reference to this first system. As mentioned in the introduction, the CF3SO3-‚‚‚Li+ ion pair has been studied by Frech et al.;11 this recent work shows that the stability order (Figure 1b) is bidentate > tridentate > monodentate at various level of calculations; our results agree with this ordering. Thus, only the optimized geometries of the bidentate ion pair are given in Table 1. In a previous report10a basis set effects on geometrical parameters of the triflate anion (C3V) have been analyzed; hence, we limit the discussion to the effects of the electron correlation and to the distortions induced by the presence of Li+ on the geometry of the triflate anion. With respect to HF/6-31+G* results, MP2 correlation does not affect the values of bond angles significantly but causes a lengthening of the S-C, S-O, and C-F bonds by about 0.03 Å. The bidentate lithium salt belongs to the C2V point group; the equilibrium Li-O2(3) distance is longer than that obtained for BF4-‚‚‚Li+ which lies in the range 1.750-1.813 Å according
10884 J. Phys. Chem., Vol. 100, No. 26, 1996
Arnaud et al.
TABLE 2: Energetics of Triflate Anion and Its Bidentate Lithium Salt ET (triflate)c ET (salt)c,d Ebindinge Edefe Ees1e Ees2e ECTe
HF/3-21+G*a
HF/6-21+G*b
HF/6-21+G*a
MP2/6-21+G*a
-953.56302 -960.96109 132.2 8.5 -135.5 -144.4 -19.3
-958.29834 -965.76400 139.9 6.7 -139.1 -139.9 -23.2
-958.30456 -965.76400 140.5 7.1 -142.3 -142.7 -25.7
-959.65974 -967.11766 139.6 7.0
a Geometry optimized at the same level of calculation. b Geometry optimized at the HF/3-21+G* level. c In hartrees. d Total energies (hartrees) of Li+ are the following: -7.187 33 (3-21+G*) and -7.235 54 (6-31+G*). e In kcal/mol.
to the level of calculation.22 Compared to the geometry of free triflate anion, the structural changes in the salt are the following: 1. The Li-unbound S-O1 distance is shortened by ca 0.025 Å whereas the Li-bound S-O2(3) distances are lengthened by the same extent; at the same time, the CSO angles increase. One can notice than these variations are practically insensitive to the change of basis and to inclusion of electron correlation. 2. As expected, the CF3 moiety is less distorted than that of SO3; the C-F2(3) bond length decreases by 0.015 Å. This shortening of the C-F is accompanied by a flattening of the CF3 group, in contrast to the SO3 moiety. 3. Finally, the C-S distance is shorter than in the free triflate ion by 0.012 Å at the highest level of calculation. It is interesting to notice that the structural changes are correctly reproduced at the lowest level of theory; this result is encouraging insofar as for the largest systems studied, geometry optimization with 6-31+G* basis is prohibitive in computation time. Table 2 provides total and binding energy data at various computational levels for the triflate ion and its bidentate lithium salt. Comparison of the binding energies shows that HF/321+G*//3-21+G* calculations underestimate the stability of the ion-pair structure; the other three values are very close; this finding implies that HF/6-31+G*//3+21+G* calculation are adequate for the purpose of evaluating the binding energies of the lithium salts. In addition to the total and binding energy data, Table 2 also includes the anion deformation energy Edef in the salts and the electrostatic (Ees1 and Ees2) and charge transfer (Ect) contributions to the binding energy. Ect is associated with the covalent delocalization of electrons of the Tf--Li+ association whereas Ees arises from the ionic interaction of the Tf-,Li+ charge distribution. Thus, the degree of covalent bond character in the salt can be judged approximately from the percentage of charge transfer stabilization. The deformation term Edef, which is calculated as the difference between the energies of Tf- at its optimized geometries in the free anion and in the complex one, is roughly independent of the level of calculation, in accordance with the geometrical distortions discussed above. Ees and Ect values are basis set dependent, the stabilizing contributions increasing with the enlargement of the basis set. When the 6-31+G* basis is used, Ees1 and Ees2 are very close; in addition, the percentage of covalent binding (Ect/(Ees + Ect)) is almost independent of the level at which the optimization of geometry has been made (14% at the HF/6-31+G*//3-21+G* level, 15% at the HF/631+G*//6-31+G* level). The absolute value of the sum (Ees + Ect) is larger than the binding energy; in molecular associations, there is a repulsive contribution associated with Pauli principle that prevents the interpenetration of the electronic clouds of the two fragments. Finally, in order to analyze the redistribution of the electronic charge in the anion perturbed by the lithium cation, we have calculated the atomic charges in the triflate anion and its lithium salt. Table 3 shows the Mulliken and natural charge populations (NPA). As expected, because of the nonbinding between fluorine atoms and Li+, Mulliken and NPA charges indicate
TABLE 3: Mulliken and NPA (in Parentheses) Atomic Charges of the Triflate Anion and Its Bidentate Lithium Pair Calculated at the HF/6-31+G* Level atoma Li C S O1 O2(3) F1 F2(3) a
triflate anion (Tf-)
ion pair
0.941(0.935) 1.850(2.645) -0.839(-1.099) -0.839(-1.099) -0.425(-0.428) -0.425(-0.428)
0.652(0.960) 1.006(0.990) 1.760(2.603) -0.706(-1.024) -0.797(-1.166) -0.383(-0.409) -0.367(-0.393)
Atomic numbering as in Figure 1.
that the oxygen atoms are more negative than the fluorine atoms in triflate anion. Upon complexation, the charge population (in electron unit) of the lithium atom is in the range 0.652 (Mulliken)-0.960 (NPA), a value strongly dependent on the method of analysis.23 This value is consistent with Ect mentioned above; Reed et al.24 discussed the relationship between energy and population demonstrating that the energy stabilization ∆E associated with a change in population of 0.001 e corresponds to ∆E ) 0.001 hartree or ≈0.6 kcal/mol. This estimate gives a Ect ) -25 kcal/mol. The Mulliken population analysis probably overestimates the charge transfer triflate f Li+. Concerning the relative changes of the charges, both methods indicate (i) an electronic depletion on the unbound O1 center and to a less extent on the carbon and fluorine atoms and (ii) an increase of the sulfur electronic density. However, a discrepancy between the two population analyses arises for the bounded O2(3) atoms which are less (Mulliken) or more (NPA) negatively charged in the salt. In the following, we will compare only the relative changes of the charges within the same kind of population analysis. B. Conformational Study of TFSI- and TFSM-. The structures of the absolute energy minima of the two anions are presented in Figure 2 together with the numbering scheme used for the atoms. The equilibrium geometry of the TFSI anion is reached for a C4S2S3C5 angle of 94.6° while the corresponding value in the TFSM anion is 166.6°. In our preliminary study, we have tentatively related these values to the number and the direction of the lone pairs borne by the N(TFSI-) and C2 (TFSM-) anionic centers.13 In order to specify the role played by the lone pair delocalization on the stabilization of the equilibrium structures, we have undertaken a conformational study of these anions. Figures 3 and 4 display the relative energies ∆ET of TFSI- and TFSM- as a function of the dihedral angle CSSC, denoted Θ. For TFSI-, the curve obtained is very flat, the two transition structures corresponding to Θ ≈ 0° and Θ ≈ 180° lying only 0.9 and 1.2 kcal/mol, respectively, above the absolute minimum. This result suggests that TFSI- structure is very flexible. In the case of TFSM- (Figure 4), on the contrary, a local minimum is found for Θ ≈ 8° which lies 1.1 kcal/mol above the absolute minimum with a high-energy barrier (≈12 kcal/mol) separating it. We shall now examine the extent to which conformational preferences can be interpreted in terms of nN(C2) f σ*
Ion Pairs of Li Salts
Figure 2. Atomic numbering and equilibrium geometries of TFSI(a) and TFSM- (b).
J. Phys. Chem., Vol. 100, No. 26, 1996 10885
Figure 4. Relative energies as function of dihedral angle Θ for TFSM at the HF/3-21+G* level.
TABLE 4: NBO Energetic Analysis for Various Values of θ (CSSC) Angle (Energies in kcal/mol) TFSI- θ values vicinal interactionsa n1N f σ*SC(F) (1) n2N f σ*SC(F) (1′) (1) + (1′) n1N f σ*SO (2) n2N f σ*SO (2′) (2) + (2′) n2O f σ*NS (3) n3O f σ*NS (3′) (3) + (3′)
0.0 -1.5 -21.2 -22.9 -47.5 -18.2 -68.9 -51.3 -51.7 -103.8
94.6 -10.7 -12.9 -23.8 -32.8 -31.4 -66.9 -51.4 -51.4 -104.0
180.0 0.0 -25.2 -25.2 -44.7 -13.0 -59.4 -49.6 -49.6 -100.3
TFSM- θ values vicinal interactionsa nC2 f σ*SC(F) (1) nC2 f σ*SC(F) (1′) n2O f σ*C2S (3) n3O f σ*C2S (3′) (3) + (3′)
0.0 -27.0 -14.1 -34.3 -34.3 -69.4
90.0 -17.6 -28.4 -33.0 -33.0 -66.7
166.6 -26.8 -13.3 -33.8 -33.8 -68.5
a 1 and n2 refer, respectively, to the σ (≈sp5.0) and π (p) orthogonal nN N lone pairs of the nitrogen atom; n2O and n3O refer to the p lone pairs of the oxygen atom and nC2 refers to the lone pair of the C2 anionic center.
Figure 3. Relative energies as function of dihedral angle Θ for TFSI at the HF/3-21+G* level.
hyperconjugation. Of particular interest are the interactions between N or C2 lone pairs and vicinal antibonding orbitals. The magnitude of the nO f σ*NS interactions which are altered by rotation about the S-S axis are also considered. The magnitudes of these interactions are shown in Table 4 for the
values of Θ corresponding approximately to the stationary points of the potential surface. Their variations as functions of Θ are also given in Figures 3 and 4. We shall first discuss the results obtained for TFSI-. One can see that the contribution of the nN1 f σ*SC(F3) interaction to the total vicinal delocalization is small. For the stable conformation, the overlaps 〈n1N|σ*SC〉 and 〈n2N|σ*SC〉 are almost the same; the weaker n1N f σ*SC(F3) interaction can be attributed to the lower energy of the n1N orbital because of its s character. Among the delocalization interactions, the n02(3) f σ*NS are the most stabilizing. Examination of Figure 3 shows that the variations of nN f σ*SC(F3) and nO f σ*SN hyperconjugations as functions of Θ are very weak and give opposite differential
10886 J. Phys. Chem., Vol. 100, No. 26, 1996
Arnaud et al.
TABLE 5: Main Geometrical Parameters of TFSI- and TFSI- Moiety in TFSI--Li+ Salta,b
TABLE 6: Main Geometrical Parameters of TFSM- and TFSM- Moiety in TFSM--Li+ a,c
parameter
TFSI-
a1
a2
a3
a4
a5
N1-S2 N1-S3 S2-C4 S3-C5 S2-O6 S3-O8 S2-O7 S3-O9 C4-F11 C4-F12 S2N1S3 C4S2N1 C5S3N1 O6S2O7 C4S2N1S3 C5S3N1S2 C4S2S3C5
1.520 1.520 1.817 1.817 1.439 1.439 1.441 1.441 1.362 1.361 156.2 99.7 99.7 118.1 134.8 134.8 94.6
1.491 1.573 1.804 1.812 1.472 1.424 1.471 1.437 1.349 1.350 142.3 101.7 99.0 105.5 176.5 108.4 79.3
1.578 1.578 1.826 1.826 1.432 1.432 1.425 1.425 1.349 1.404 136.4 94.9 94.9 121.9 130.8 130.8 104.8
1.524 1.500 1.834 1.811 1.427 1.477 1.432 1.425 1.415 1.352 172.3 96.2 102.7 120.6 148.7 135.3 76.9
1.541 1.541 1.810 1.810 1.422 1.422 1.470 1.470 1.362 1.349 136.6 101.7 101.7 117.9 107.8 107.8 155.3
1.540 1.540 1.810 1.810 1.471 1.423 1.423 1.471 1.351 1.358 137.2 102.0 102.0 117.5 239.3 119.6 -1.1
a
Atomic numbering and nomenclature as in Figure 2. b Underlined values indicate that the terminii center is bounded to Li+; see also Figure 4a.
stabilization. The nN f σ*SO contribution is more Θ dependent and seems responsible for the destabilization of the anti conformer. However, when considering the variations of the total delocalization energy ∆Edel vs Θ, it appears that most of this effect disappears and ∆Edel is almost constant. Because of a large SNS bond angle (vide infra), small steric effects are expected, which counterbalance the small differences in delocalization energy. This explains the flatness of the potential energy surface. When the anionic center bears only one lone pair (C2), the situation is quite different (see Figure 4); a decrease, for a given value of Θ, of the stabilizing interaction nC f σ* is not compensated by an increase of the delocalization energy resulting from the interaction of the other orthogonal lone pair with the same antibonding σ* orbital (compare for example n1(2)N f σ*SO and nC2 f σ*SO interactions for Θ ) 0°); consequently, one notices large variations of the nC f σ*SC(F3) and nC f σ*SO interactions vs Θ (see Figure 4). Another point of discussion concerns the magnitudes of the nO f σ*C2S and nO f σ*NS delocalization energies. The larger nO f σ*NS interaction can be attributed to a larger overlap between oxygen lone pairs and σ*NS (N is more electronegative than C2 and thus σ*NS is more concentrated on the S atom). In addition, considering that nC2 f σ*CO relative stabilizing interaction is also reduced in TFSM-, the nC2 f σ*SC(F3) contribution to the total delocalization energy is most important in this anion. Nevertheless, the inspection of Figure 4 does not permit one to attribute the stabilization of the ≈syn and ≈anti conformations of TFSM- to this interaction. Indeed, the nC2 f σ*SO vicinal interactions annihilate this stabilizing effect. Finally, there is clear evidence that Edel is the factor determining the relative stabilities of the TFSM- conformers but the changes in total delocalization energy cannot be ascribed to C2 lone pairvicinal stabilizing interactions. Thus, one can conclude that lone pair-vicinal interactions cannot play a major role in this conformational problem. C. Li+- TFSI- and Li+- TFSM- Associations and Their Binding Energies. Geometries. Five locally stable minima were located for Li+_TFSI- and are depicted in Figure 5a; details of the optimized geometrical parameters are given in Table 5. For comparison, the geometrical parameters of the equilibrium structure of the free anion are also included in Table 5. The bidentate structure a1 is similar to the one obtained for the lithium-triflate ion pair; the Li+_TFSI- association is somewhat looser with a larger Li-O distance (1.966 Å vs 1.933 Å). This resemblance can be extended to the geometrical changes of the
parameter
TFSMb
b1
b2
b3
b4
b5
C2-H
1.070 (1.068) 1.651 (1.653) (1.653) 1.831 (1.831) (1.831) 1.447 (1.447) 1.441 (1.441) (1.441) 1.367 (1.361) 128.2 (131.0) 105.6 (106.3) (106.3) 120.1 (120.1) 270.3 (272.8) (272.8) 180.0 (172.6) 166.6 (4.5)
1.069
1.070
1.069
1.070
1.069
1.619
1.649
1.652
1.620
1.653
1.677 1.821
1.655 1.815
1.652 1.822
1.680 1.821
1.653 1.822
1.825 1.475 1.435 1.470
1.869 1.485 1.436 1.433
1.822 1.431 1.431 1.470
1.821 1.471 1.438 1.471
1.822 1.432 1.470 1.470
1.436 1.361
1.430 1.423
1.470 1.354
1.438 1.362
1.432 1.360
126.0
127.6
126.4
127.1
127.8
110.8
105.9
105.9
108.6
108.2
102.8 107.4
102.2 116.4
105.9 119.4
104.8 107.4
107.2 119.4
273.8
307.3
275.9
290.7
271.9
270.2 181.5
258.5 168.2
275.9 180.0
104.4 171.9
80.6 174.2
167.7
194.2
175.8
29.7
-7.2
C2-S3 C2-S4 S3-C5 S4-C6 S3-O7 S4-O9 S3-O8 S4-O10 C6-F15 S3C2S4 C5S3C2 C6S4C2 O7S3O8 C5S3C2H C6S4C2H S3C2S4H C6S4S3C5
a Atomic numbering and nomenclature as in Figure 2. b Values in parentheses correspond to the least stable conformer. c Underlined values indicate that the terminii center is bounded to Li+; see also Figure 4b.
anions. Another stable complex is a2 (C2 symmetry) in which Li is bounded with the N atom and with two fluorine atoms each belonging to a CF3 group. The common feature for the change in bond lengths in the anion moiety (i.e., lengthening of the A-B distance when B is bounded to Li) seems to be enhanced in this tridentate structure. a3 is also a bidentate structure reached for rather shorter interfragment distances (Li-O ) 1.805 Å and Li-F ) 1.903 Å); one can notice for this structure a large SNS bond angle (172.3° vs 156.2° in free TFSI-). Finally, bidentate structures a4 and a5 illustrate the flexibility of the anionic fragment mentioned above: in these two structures which belong respectively to the C2 and Cs point groups, the TFSI- moiety undergoes drastic changes; particularly noticeable are the variations of the values of dihedral angles
