100
J . Phys. Chem. 1994,98, 100-1 10
Molecular Structures and Normal Vibrations of CF3S03-and Its Lithium Ion Pairs and Aggregates Weiwei Huang, Roger Frech,' and Ralph A. Wheeler' Department of Chemistry and Biochemistry, University of Oklahoma, Norman, Oklahoma 73019 Received: August 12, 1993; In Final Form: October 14, 1993'
The molecular structures, harmonic vibrational frequencies, infrared absorption intensities, internal force constants, and electronic charges of the trifluoromethanesulfonate (triflate) anion, its lithium ion pairs, and several aggregates have been studied using the a b initio self-consistent field Hartree-Fock method with 3-2 1+G*, 6-3 1+G augmented with polarization functions (d type orbitals) on the sulfur atom, and 6-31+G* basis sets. The calculated frequency shifts of the u,(SO3), u,(CF3), 6,(CF3), and u(CS) modes and the splitting widths of the u,,(SO3) modes in lithium triflate ion pairs and aggregates are compared with those observed in IR and Raman spectra of lithium triflate dissolved in some polar, aprotic solvents. A bidentate coordination of Li+ by triflate oxygens is the most stable structure for the isolated Li-Tf ion pairs, but a monodentate structure is apparently favored for Li-Tf ion pairs in polar, aprotic liquid solutions. Triflate ions in aggregates I and I1 of the lithium triflate are likely to have the bidentate bridging structure and the tridentate bridging structure, respectively. The calculated frequency shifts and splittings for the monodentate ion pair and bidentate bridging aggregate agree with the experimentally observed values for lithium triflate complexes in polar, aprotic liquids much better when each lithium cation is capped with an OH- group to model the effect of lithium coordination by solvent.
Introduction Lithium trifluoromethanesulfonate (triflate or Tf) is one of several important salts used in the preparation of liquid and polymer electrolytes with good electrical and chemical properties.l.2 Complexesof lithium triflate with various polyethers have been studied extensively by vibrational spectroscopy3" in order to infer the species present ("free" ions, ion pairs, and aggregates) and to find their relative concentrations in polymer electrolytes. It is believed that each type of ionic species present makes a different contributionto the total ionicconductivity. For example, the formation of neutral ionic pairs and higher aggregates decreases the number of charge carriers. The formation of charged, associated species may also affect the ionic conductivity through the change in ionic mobility of the associated charge carriers. A detailed knowledge of the interactions between the lithium cation and triflate anion at the molecular level will help us to understand the mechanism of ionic conduction in polymer electrolytes better and will also serve as a basis for calculating the dissociation constants among "free" ions, ion pairs, and aggregates by using the relative concentrations of these species obtained from vibrational spectroscopic studies. For a free triflate anion (C3J, the antisymmetric SO3stretching u,(S03) is doubly degenerate. In the presence of sufficiently large cation-anion interactions which do not preserve the axial symmetry of the triflate anion, the ua,(S03) band will split into two components as the degeneracy is lifted. The splitting of this mode has been observed in the IR spectra of LiTf in a series of solvents such as poly(ethy1ene oxide) (PEO),3 poly(propy1ene oxide) (PPO),' acetone,3 a~etonitrile,~ or tetrahydrofuran (THF).7a For lithium triflate ion pairs, the coordination of the lithium cation by triflate anion can be monodentate, bidentate, or tridentate. As the highest possible symmetry for both the monodentate and bidentate structures of the lithium triflate ion pairs is C,, it is impossible to distinguish between them solely from the splittingpattern of the u,(S03) modes in the IR spectrum. This is not the case for lithium perchlorate and lithium tetrafluoroborate ion pairs formed in low dielectric solutionssuch as PPO, PEO, and THF. The monodentate structure of LiC104 To whom correspondence should be addressed. Abstract published in Aduunce ACS Absrrucu. December 1, 1993.
0022-3654/94/2098-0100%04.50/0
or LiBF4can have a C3usymmetrywhereas the bidentatestructure belongs to the C, point group. The different splitting pattern of the degenerate modes observed in the C3" and C, structures can be used to differentiate the monodentate ion pairs from the bidentate ones. In lithium triflate aggregates, we would expect the same splitting pattern for the degenerate modes as in lithium triflate ion pairs as long as the aggregates do not have C3u symmetry. However, the magnitude of the splitting and frequencies of the two components may differ between the ion pairs and aggregates in such a manner as to allow differentiation between the two kinds of associated species. In the symmetric us(S03)stretching region, multiple bands have been observed in both the IR and Raman spectra of the systems mentioned above. As the u,(S03) mode is nondegenerate, the observed multiple bands in this spectral region must necessarily result from the triflate anions in different potential energy environments which, in turn, are interpreted as due to different ionic species. The bandsof the us(S03)stretchingmode appearing at about 1032, 1042, and 1052 cm-l in the LiTf-PPO system have been assigned to *free" ions, ion pairs, and aggregates, re~pectively.~ These assignments are based on the observations that the first band is cation independent whereas the last two bands are cation dependent. Multiple bands at about 752,757, and 762 cm-I in the 6,(CF3) spectral region were also reported recently for LiTf-glyme systems by two of US.^ Following the same reasoning for the assignments of the multiple bands in the us(S03)spectral region, we attributed these three bands also to the "free" ions, ion pairs, and aggregates of triflate, respectively. The distinct bands observed in the symmetric u,(SO3) and 6,(CF3) spectral regions do not provide any information about the symmetry of ion pairs and aggregates or the coordination of the lithium cations with the oxygen atoms of the triflate ions. Instead, the experimental observationsraisea fundamentalquestion-Why do the u s ( S 0 3 and ) 6,(CF3) bands of lithium triflate ion pairs and aggregates shift to higher rather than lower frequencies relative to thecorrespondingbands from the "free" triflateion? Toaddress this question and to infer the structure of the lithium triflate ion pairs and aggregates in polymer electrolytesand liquid solutions, we have performed ab initio molecular orbital calculations of the triflate anion, its lithium ion pairs, and aggregates. In this paper 0 1994 American Chemical Society
CF3S03- and Its Lithium Ion Pairs and Aggregates
The Journal of Physical Chemistry, Vol. 98, No. 1, 1994 101
I
I
I
I
1060
1040
1020
I
Wavenumber (cm-’) 1350
I
I
1300
1250
I
Wavenumber ( c m 1) Figure 1. IR spectra for the frequency interval from 1115-1350 cm-l: a, Bu4NTf-2MTHF (c = 1.3 mol/kg); b, LiTf-PPO trimer (c = 0.8 mol/kg); c, LiTf-2MTHF (c = 1.6 mol/kg). we report the calculated geometries, energies, normal vibrations, and internal force constants of the free triflate anion, lithium triflate ion pairs, and several lithium triflate aggregates. The calculated vibrational frequency shifts of the u,(S03) and 6,(CF3)modes and the splittingof the u,(S03) modes arecompared with those observed experimentally. Frequency shifts and mode splittings are correlated with changes of internal force constants originating from charge redistribution within the triflate ion upon complexation with lithium cation(s). On the basis of these comparisons, the coordination geometries of lithium cations by triflate oxygen atoms are proposed for the lithium triflate ion pairs, aggregate I and aggregate I1 (whose spectral evidence is reported here for the first time). Finally, experimental and computational details are provided in the Appendix.
Results and Discussion Experimental Measurements of the u,(SO3), u,(SO3), and 6,(CF3) Modes. Figure 1 examines the experimentally observed splitting pattern of the antisymmetric u,,(S03) stretching mode in several solutions. The triflate anion in a tetrabutylammonium triflate solution (curve a) is described as a “free” ion since the bulky nature of the cation minimizes cation-triflate interactions. There is only a single peak observed at about 1270 cm-I. The peak at 1224 cm-I originates from the u,(CF3) mode of the‘free” triflate anion and is chosen as a reference frequency position for the splitting of the u,(SO3) modes. The reference peak is useful because the lower frequency band of the split u,(S03) for some divalent metal triflate salts dissolved in aprotic solutions is at the low-frequency side of the u,(CF3) band. The v,,(S03) band is split into two bands at 1257 and 1302 cm-I (Au = 45 cm-l) in the LiTf-PPO complexes (curve b). Curve fitting indicates that there is also a component at about 1273 cm-l which may be caused by the “free” triflate anions present in the system. In curve c of Figure 1, the IR spectrum of a LiTf-2-methyltetrahydrofuran (2MTHF) solution, there are two intense split components of 1270 and 1308 cm-1 (Av = 38 cm-I). As the frequencies of these two peaks are higher than those in curve b of Figure 1,the splittingis attributed to a lithium triflate aggregate. Two shoulders are seen at 1254 and 1318 cm-1, respectively. A weak feature also exists at 1289 cm-1 which is 19 cm-I higher than the usual u,(S03) “free” triflate frequency. This feature may be caused by another type of lithium triflate aggregate. The bands in the symmetric stretching u,(S03) spectral region are shown in Figure 2 for the systems BudNTf-2MTHF (curve a), LiTf-PPO (curve b), and LiTf-2MTHF (curve c). These three systems are chosen for comparison because the major component of the triflate species appears to be different in each solution. Curve a shows a single peak centered at 1033 cm-I
Figure 2. Raman spectra for the frequency interval from 1010-1080 cm-I: a, BwNTf-2MTHF (c = 1.3 mol/kg); b, LiTf-PPO trimer (c = 0.8 mol/kg); c, LiTf-2MTHF (c = 1.6 mol/kg).
.-E UJ C Q) c
-C I
I
I
770 765
I
I
I
760
755
750
745
Wavenumber (cm-’) Figure 3. Raman spectra for the frequency interval from 745-775 cm-1: a, Bu4NTf-2MTHF (c = 1.3 mol/@); b, LiTf-PPO trimer (c = 0.8 mol/kg); c, LiTf-2MTHF ( c = 1.6 mol/kg).
which was assigned to the “free” triflate peak because of the very weak ion pairing between the Bu4N+ cation and the triflate anion. Curve b has a main band at 1043 cm-1 plus a small band at 1034 cm-l and a shoulder at 1053 cm-l, respectively. The band at 1043 cm-1 is assigned as a lithium triflate ion pair and that at 1053 cm-I is attributed to an aggregate, designated as aggregate I. Curve c contains a main band at 1053 cm-1 with two shoulders at 1043 and 1062 cm-I. The shoulder at 1043 cm-I is due to ion pairs as in curve b, and the main component at 1053cm-I originates in aggregate I, also consistent with the assignment in curve b. A new band that is seen at 1062 cm-1 is attributed to a second kind of aggregate, designated as aggregate 11. This is the first time that two distinct aggregates have been reported for lithium triflate dissolved in solutions or polymers. Figure 3 shows the Raman spectra of the symmetric b,(CF3) deformation mode for Bu4NTf-2MTHF (curve a), LiTf-PPO (curve b), and LiTf-2MTHF (curve c). A pattern similar to the u,(S03) mode is observed in the 6,(CF3) mode. Consequently the components at 752, 758, 763, and 767 cm-I are assigned to the “free”triflate, ion pairs, aggregate I, and aggregate 11,respectively. The bands at 311 [u(CS)], 1152 [u,(CF3)], and 1224 cm-1 [u,(CF3)] for Bu4NTf in 2MTHF solution (spectra not shown) are all shifted to higher frequencies for LiTf in PPO solution (314,1162,and 1228cm-’,respectively) andin2MTHFsolution (316,1172,and 1233cm-’,respectively). Nodistinctcomponents have been observed for these modes. These experimental observations raise a number of important questions: (i) What are the structures of the various associated ionic species? (ii) Why do a number of triflate ion bands shift to higher frequencies upon interaction with the lithium cation? (iii) Why is the effect of ionic association seen in both the u,(SO3) and 6,(CF3) spectral regions? The ab initio molecular orbital calculations in the following attempt to address these questions. Ab Initio Molecular Orbital Calculations. The free triflate anion adopts a staggered conformation with C3, point group
102 The Journal of Physical Chemistry, Vol. 98, No. 1, 1994
Huang et al.
02p “43n o 3
n o 3
F1@xl
F3
Q Lil
Figure 4. Optimized geometries of the triflate anion, its lithium ion pairs, and aggregates.
symmetry. The lithium triflate ion pairs can have a variety of different structures, including the three highly probablestructures with monodentate, bidentate, and tridentate coordinationof the lithium cation. Two different triplets of CF3S03--Li+.-03SCF3 and Li+*CF$03--Li+ have also been proposed as the species for lithium triflateaggregates,4*9although the coordination of lithium cations to the oxygen atoms of the triflate in these two models is not certain. Considering the fact that only the u,(S03) modes are split in the IR spectra, we believe that lithium cations interact primarily with the SO3end rather than with the CF3 end of the triflate anion. Therefore, the structures of CF3S0rLi, CF3SOyLi2+, and CFpSOrLip+2 with Li+ coordinated by triflate oxygen atom@) have been suggested as the basic structures for theassociatedspecies in theLiTf-PPOand LiTf-2MTHFsystems and were used in the ab initio MO calculations. Molecular Geometries and Energies of CFSOJ- and Its Complexeswith Li+. The lowest energy geometries of free triflate anion, lithium triflate ion pairs, and aggregates calculated by using ab initio MO calculations are drawn in Figure 4. The optimized geometry parameters obtained with different basis sets are listed in Table l a and lb. A comparison of the results obtained with different basis sets for the free triflate anion shows that the C S bond increases by 0.009 A when the basis set changes from 3-21+G* to 6-31+G^ (the notationA in this paper means d type functions are added to the sulfur atom only) or 6-31+G*. The S-0bond length decreases by 0.03 or 0.016 8, and the C-F bond length decreases by 0.002 or 0.05 8,. respectively, when the basis set is from 3-21+G* to 6-31+W or to 6-31+G*. The -CF3 group is close to a regular tetrahedral geometry with a C S - F angle of 111.8O and an F-C-F angle of 107.0° (6-31+G*). The SO3 group is flatter than the CF3 group with a C S - 0 angle of 102.2O and an Os-0 angle of 115.4O.
Although no experimental structure for the gas-phase triflate anion is availablefor comparison, there are a wide range of crystal structural data available for triflate anion bonded to various cations. Lundgren et a1.10 have determined the crystal structures of a series of triflate hydrates and found that average bond distances and bond angles for the CF3S03- ion are as follows: C S 1.827(5),S-0 1.445(9),andC-F 1.326(6)8,;CS-0 103.9( 7 1 , o s - O 114.6(16),S-C-F 110.2(6),and F-C-F 108.6(6)O. (The esd’s express the deviations of the individual values from the mean.) It can be seen that the calculated geometrical parameters at the HF/6-31+G1 level, shown in Table la, match the experimental data best and the other two calculations also give very satisfactoryresults. The C S bond distance,calculated at the HF/6-31+G* level, agrees with the experimental result, whereas the calculated S-0 and C-F distances are 0.023 and 0.005 8, shorter, respectively, than the experimental average distances. Similarly, the calculated C S - 0 and 0S-O angles (HF/6-31+G*) differ from the experimental values by only 1.3 and 0.8O, respectively, and the calculated S-C-F and F-C-F anglesvaryfrom theexperimentalaveragevalues by 1.6O. Despite this excellent agreement between calculated structural parameters and experimental values, one should bear in mind that we are comparing the calculated structure of an isolated triflate anion with the experimental crystalstructures. The interactions between the triflate ions and the countercations in crystals affect the geometrical parameters of the triflate ions and will be reflected in our comparisons. For example, Lundgrenlk discovered that in triflate hydrates the S-0 bond length increases as the number of hydrogen bonds to the oxygen atoms of the C F 3 S O j ion increases. Values of 1.429, 1.442, and 1.451 A or alternatively 1.435, 1.446, and 1.457 8, (with correction for thermal riding motion) were found for the average distances of the S-0 bonds with zero, one, and two hydrogen bonds, respectively. Therefore, one of the reasons that our calculated S-0 bond length is shorter than the average experimentalvalue in the triflate hydrate is due to the hydrogen bonding evident in the crystal structure. Nevertheless, the triflate ions in these hydrates are 0-bound to hydronium ions and/or water molecules through hydrogen bonding, so the geometries of the triflate ions are expected to be distorted less than in the structures with other counterions.lksd This is especially true for the -CF3 end, which is not bound to any hydronium ions or water molecules in these triflate hydrates. Compared to the structure of free triflate anion,the geometries of the triflate ions are severely distorted for the three types of lithium triflate ion pairs (4b-d) shown in Figure 4 because of the strong interaction between the lithium cation and the triflate anion. The shape of the SO3end changes more than that of the CF3 end, as expected. A comparison among the structural parameters of the free triflate ion and those of the lithium triflate ion pairs shows the following common features for the change of the bond lengths in the lithium triflate ion pairs. The Li-bound S-0 [ M ( b ) ] distances are lengthened whereas the unbound S-0 [S-O(ub)] distances are shortened when compared with thosein thefreetriflateion. ThetendencyofS-O(b) tobcgreater than S-O(ub) has been observed in numerous crystalscontaining triflate ions.lO.l*Both C-F distances and the C S distance are shortened in the lithium triflate ion pairs. For the monodentate and bidentate ion pairs, the C-F bond(s) at the trans position with respect to the bound S-0 bond(s) are shorter than the other C-F bond@). The order of the S-0 bond length for the Libound oxygen atom(s) is monodentate > bidentate > tridentate. This order is also true for the C-S bond length and the averaged C-F bond lengths. Bond angles at the SO3 end also change dramatically upon coordinating to the lithium cation. For the monodentate Li-Tf ion pair, the CS-O(b) angle decreases whereas the CS-O(ub) angles increase compared with C S - 0 angles in the free triflate ion. For both the bidentate and tridentate LhTf ion pairs, the
The Journal of Physical Chemistry, Vol. 98, No. 1, 1994 103
CF3S03-and Its Lithium Ion Pairs and Aggregates
TABLE 1: Optimized Ceometriers of the Free Tritlate Anion, Its Lithium Ion Pain, and APPrePPtes' Table Ia free anion ion pairs aggregates Figure 4a Figure 4a Figure 4b, monoden Figure 4c, biden. Figure 4d,triden. Figure 4e,biden. brdg Figure 4f, tri. brdg 3-21+G* 6-31+G* 3-21+G* 3-21+G* 3-21+G* 3-21+G* 3-21+G* 1.827 1.805 1.802 1.818 1.796 1.800 1.802 1.438 1.445 1.422 1.475 1.463 1.456 1.444 1.422 1.412 1.412 1.371 1.356 1.321 1.355 1.356 1.350 1.353 1.366 1.368 1.360 2.321 1.805 1.737 1.672 1.934 102.0 102.6 99.3 102.9 107.7 102.1 104.3 104.7 105.9 107.0 112.0 111.7 112.8 111.8 110.7 109.4 110.4 111.3 109.8 110.4 115.4 112.9 115.8 111.2 114.1 105.0 111.9 119.6 119.0 115.8 105.9 107.0 108.2 109.3 107.8 108.4 109.4 106.4 107.5 108.1 71.3 178.7 90.5 163.6 174.3 60.0 59.2 60.0 60.0 60.0 63.3 60.0 60.0 60.0 60.0 60.8 60.5 57.9 54.5 -87.8 180.0 103.7 180.0 0.0
R(C-9 R(S-Od R(S-02)
NC-FI) R(C-Fz) R(Li-01) L(Cs-01) w-2)
Figure 4a, freeb 1.827 1.435 1.369 102.2
4S-C-Fi) 4S-C-Fz)
112.5
@I-,)
115.7
L(0-d
Table Ib Figure 4b, monodenb 1.814 1.472 1.419 1.353 1.363 1.666 99.6 104.9 111.6 111.1 112.6 119.5 108.0 106.9 176.4 59.4 63.3
4Fi-C-S) 106.3 4FtC-F3) L(Li-0I-S) L(Oi-C*Fd 60.0 4S-Cs-01) 60.0 4OrS-C-Fi) L(FrC-S-Oi) L(Lil-0IS-C) 180.0 a Bond lengths in A and bond angles in degrees. Optimized with the 6-31+G' basis set. C S - 0 angles all open wider than they do in the free triflate ion. At the HF/3-21+G* level, the difference between the C S - 0 1 and C S - 0 2 angles is 5.4' for the monodentate structure and 3.0' for the bidentate structure; the difference between the 01S - 0 3 and 0 4 - 0 3 angles is 7.3' for the monodentate structure and 14.0' for the bidentate structure. The difference between the corresponding angles for the CF3 group, however, is less than 2.0' in both the monodentate and bidentate structures. For the tridentate structure, the conformations of both the SO3 group and CF3 group become closer to a regular tetrahedron than in the free triflate ion. The 6-31+G^ level calculations show essentially the same changes for the bond lengths and angles of the lithium triflate ion pairs as observed from the 3-21+G* level calculations. The calculated total energies for the monodentate, bidentate, and tridentate CF3SO3.Li structures are -961.077 378, -961.083 937, and -961.049 90 au at the HF/3-21+G* level and -965.605 872, -965.612 109 and -965.578 286 au at the HF/6-31+G^ level. Therefore, the energy order for the lithium triflate ion pairs is bidentate < monodentate < tridentate. The monodentate and tridentate LbTf structures are 17.22 and 89.37
Figure 4c, biden! 1.810 1.461 1.409 1.365 1.353 1.939 103.1 106.1 111.4 109.7 104.9 118.7 108.6 107.9 90.8 60.7 54.5 104.8
Figure 4d,triden.6 1.806 1.443 1.354 2.337 107.6 110.5 111.2 108.4 71.6 60.0 60.0 180.0
kT/mol higher in energy than the bidentate LieTf structureat the HF/3-21+G* level, or 16.37 and 88.80 kJ/mol higher in energy at the HF/6-31+G^ level. Francisco et a1.12and Spoliti et aI.l3 reported that the total energy order for lithium tetrafluoroborate ion pairs is bidentate < tridentate < monodentate, and a similar energy order was reported for the lithium perchlorate ion ~ a i r s . 1 ~ Apparently a common feature exists for the LieTf, Li-BFd, and LiC104 pairs, namely that the bidentate structure has the lowest total energy calculated for these isolated ion pairs. Structural changes (bond lengths and angles) similar to those in the ion pairs are found in the two types of aggregates (4e and 4f) illustrated in Figure 4. Briefly, the Li-bound S-0 distances in the triflate ion increase whereas the C-F,C-S,and unbound S-0 distances in the triflate ion decrease upon its complexation with lithium atoms. The C S - 0 angles increase whereas the S-C-F angles decrease in the two studied lithium aggregates compared to those in the free triflate ion. Normal Vibrations of C F m - and Its Complexes with LP. The HF/3-21+G* normal vibrational frquencies and IR absorption intensities of the structures in Figure 4 are listed in Table 2. The assignments of the normal vibrational modes for
104 The Journal of Physical Chemistry, Vol. 98, No. 1, 1994
TABLE 2
Huang et al.
Normal Vibrational Freauencies (em-’) and IR Intensities (km/mol) calculated with the Basis Set 3-21+6* 1 ion pairs aggregates free ion, Figure 4a Tf- Figure 4b monoden Figure 4c biden. Figure 4d triden. Figure 4e bid. brdg Figure 4f tri brdg ~
mode no. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 (I
1376 (495) 1317 (73) 1250 (141) 1114 (161) 758 (54) 675 (215) 599 (25) 529 (21) 367 (0.01)
328 (2) 234 (3) 80 (0.0)
1446 (397) 1353 (570) 1323 (241) 1297 (2) 1274 (171) 1152 (229) 789 (71) 730 (20) 666 (209) 603 (31) 558 (32) 529 (28) 514 (88) 370 (3) 368 (0.3) 342 (4) 240 ( 10) 223 (5) 101 (17) 78 (76) 46 (58)
1457 (372) 1327 (279) 1323 (156) 1283 (185) 1246 (204) 1088 (192) 786 (45) 687 (168) 645 (87) 592 (18) 566 (159) 532 (34) 519 (32) 370 (7) 361 (0.2) 348 (4) 303 (30) 252 (14) 215 (0.01) 112 (48) 81 (5)
1346 (530) 1326 (68) 1309 (21) 1124 (213) 808 (32) 697 (94) 605 (33) 532 (26) 423 (297) 360 (0.8) 335 (8) 200 (0.1)
i 197 (29) 70 (0.0)
1444 (425) 1364 (705) 1324 (43) 1313 (130) 1311 (115) 1152 (265) 805 (54) 697 (127) 686 (59) 641 (103) 547 (41) 543 (9) 512 (184) 487 (98) 371 (14) 367 (0.7) 349 (2) 255 (11) 228 (13) 127 (0.3) 107 (156) 98 (146) 85 (0.02) 50 (4)
1394 (722) 1340 (1) 1320 (17) 1155 (296) 807 (53) 686 (213) 643 (4) 531 (0.5) 465 (57) 450 (293) 369 (19) 352 (5) 256 (14) 162 (0.0) 130 (240) 126 (93) 95 (1) 43 (0.0)
The numerical values in parentheses are the IR intensities. The boldface frequencies are assigned in Tables 3, 4, or 5.
the free triflate ion have been discussed in detail elsewhere15and will not be repeated here. The following discussion will instead focus on frequency shifts and band splittings of some vibrational modes of the triflate ion upon coordination with the lithium cation(s). Only the calculated frequencies of stretching modes and the 6,(CF3) deformation modes will be compared to the experimental data because these modes are very intense in IR and/or Raman spectra and the frequencies are sensitive to ionic association. For ion pairs, only the tridentate structure has an imaginary vibrational frequency (i 197 cm-l with the 3-21+G* basis set and i 201 cm-1 with the 6-31+C basis set), which means that the tridentate structure is not located at a minimum position on the potential energy surface. Therefore the tridentate ion pair will not be discussed further. Themonodentateand bidentatelithium triflate ion pairs, on the other hand, are minimum energy structures. As the symmetry of the triflate ion is lowered from C3, to the C, symmetry of the monodentate and bidentate structures, splitting of the doubly degenerate modes is expected for each C, structure. Referring to Tables 3 and 4, where the potential energy distributions of some of the triflate vibrational modes are listed, one can see that the doubly degenerate ua8(S03) modes at 1376 cm-1 for the free triflate ion split into an A’ mode at 1446 cm-1 and an A” mode a t 1353 cm-1 (Av = 93 cm-1) for the monodentate Li-Tf ion pair, and into an A” mode a t 1457 cm-IandanA’mode 1246cm-l(Av=21 1 cm-1)forthebidentate Li-Tf ion pair. Similarly, the split u,,(CF3) modes appear at 1297 and 1274 cm-1 (Av = 23 cm-I) for the monodentate Li-Tf ion pair, and at 1327 and 1283 cm-1 (Av = 44 cm-1) for the bidentate LiSTf ion pair. The splitting of the ua,(S03) in the bidentate structure is so large that its lower frequency component is even lower than all the C-F stretching frequencies. Although the three A’ stretching modes (at 1297,1323, and 1353 cm-1) in the monodentate structure are mixed with contributions from both S-0 and C-F stretching, it can be seen that mode 2 is mainly an S-0 stretching motion, mode 4 consists mainly of C-F stretching, and mode 3 keeps most features of the u,(CF,) mode of the free triflate ion. Similar splitting widths have also been obtaiwdwith the6-31+G* basisset. FigureSshows thesplitting of the u,,(S03) modes calculated with basis sets 3-21+G* and
6-31+G^ as well as that observed experimentally in the IR spectrum of the LiCF3SOs-PPO system. The figure illustrates the key point from calculations of the splitting for the um(S03) modes: The splitting width of u,(S03) modes for the monodentate structure is much closer to the experimental value than that of the bidentate structure. Using the 3-21+G* basis set, the calculated symmetric SO3 stretching frequency shifts up 38 cm-I for the monodentate structure and down 26 cm-1 for the bidentate structure compared to the u , ( S 0 3 ) frequency of 1114 cm-I calculated for the free triflate ion. The direction of the calculated frequency shift of the u , ( S 0 3 ) mode for the monodentate LbTf ion pair is consistent with observations of the LiqTf ion pairs in the vibrational spectra of LiTf dissolved in polar, aprotic solvents, although the experimental shift is only about 10 cm-I. The calculated frequencies of the 6,(CF3) and u(CS) modes (see Table 5) in the three different Li-Tf ion pairs all shift higher when compared to the corresponding modes in the free triflate ion. The calculated frequency shift of the u,(CF3) is very small. The experimental data also show an upward shift for the 6,(CF3) and u(CS) frequencies and a small upward shift for the u,(CF3) frequency in the LiTF-PPO system. Very recently, Gejji et a1.I6 calculated the vibrational frequencies of the monodentate LiSTf ion pair at the HF/3-21G* level and the bidentate LieTf ion pair at HF/6-31G* and MP2/ 6-31G* levels. Because the low-frequency mode derived from u , , ( S 0 3 ) has a calculated frequency below that of the C-F stretching modes for both monodentate and bidentate structures, their relative calculated frequencies of these modes cannot be used to distinguish monodentate from bidentate coordination. Their calculated frequencies of the u , ( S 0 ~ )mode, on the other hand, shift to higher frequency for monodentate coordination (in agreement with experiment) but move to lower frequency for the bidentate structure. Our calculated vibrational frequencies of the bidentate Li-Tf with both 3-21+G* and 6-31+G^ basis sets match closely with their results at the MP2/6-3 lG* level. However, our calculated frequency shifts for the monodentate structure, described above, show some qualitative differences compared to theirs. In particular, the low-frequency modederived from ua,(S03) remains above the C-F stretching modes in our
The Journal of Physical Chemistry, Vol. 98, No. 1 , 1994 105
CF3SO3- and Its Lithium Ion Pairs and Aggregates
TABLE 3: Approximate Descri tioes and Potential Energy Distributions (PED)of the Stretching Modes of the Trinate Ion for Free Triflate, Its Lithium Ion Pa!m, and Aggregates, Calculated by using a 3-21+C+ Basis ion pairs ‘solvated” species free LiTf LiTf Li2TfC LiTf(0H)LiTf(0H)Li2Tf(OH)2 ion Tfmonoden biden. bid. brdg monoden biden. biden. brdg Modes Originating in u,(SO3) 1378 (E) 1446 (A”) 1457 (A’) 1444 (A’) 1405 (A”) 1413 (A’) 1418 (A’) freq (sym) description b~(sod U(SO2) U(S0) U(S0) U(SO2) 4SO) U(S0) mode no. 1 1 1 1 2 2 3 St ( C S ) St (S-01) St (S-02)
St (S-Q) freq (sym) description mode no. St ( C S ) St ( s - 0 1 ) St (s-02) St (S-03) St (C-FI) St (C-F2) St (C-F3)
0.23 0.09 0.63 1374 (E)
0.50 0.47
0.10 0.74 0.10
0.50 0.45
0.14 0.68 0.13
0.13 0.66 0.16
U(s01)
1246 (A”)
1364 (A”)
1365 (A’)
1336 (A”)
1377 (A”)
UdSO3)
U(SO2)
U(SO2) u(CF2)
U(W
u(S02)
U(SO2)
2
2 0.05 0.21 0.14 0.15 0.34
5
2
3
3
4
0.44
0.19
0.43
0.45
0.44
0.20 0.29
0.59 0.13 0.18 0.05
0.41 0.03
0.44 0.04
0.06
0.03
0.40 0.53
1353 (A’)
0.09 0.78 0.08
4CF)
0.24
Modes Originatingin u,(SO3) freq (sym) description mode no. St
(s-01) (s-02)
St St 6 - 0 3 ) St (C-FI) St (C-F2) St (C-F3)
1 152 (A’) u(SO3)
1088 (A‘)
1152 (A’)
1149 (A‘)
U(s00)
u(SO3)
u(sod
1110 (A’)
h(so3)
U(sos)
u(sod
6 0.31 0.31 0.30
6 0.27 0.23 0.24
6 0.36 0.16 0.37
6 0.27 0.17 0.26 0.03
7 0.27 0.28 0.29
7 0.30 0.26 0.3 1
8 0.27 0.26 0.25
0.04 0.04
0.04
11 14 (Ai)
calculations. Including electron correlation by using the MP2 method would probably increase our calculated bond distances slightly and thus increase the differences between calculated and experimental structures. The splitting pattern of ua,(S03) and ua,(CF3) modes for aggregate 4e is complicated by the mixing of the S-0 and C-F stretching in some of the modes (see Tables 3 and 4). The higher frequency component of the split u,(SO3) modes is at 1444 cm-1. Its lower frequency partner is mixed into modes 2-5. The splitting pattern of the u,,(CF3) is also not clearly discernible except for some indication of its split, higher frequency component in mode 2. The frequencies of modes 4 and 5 are very close, and neither of these two modes has a clearly discernible A’ symmetry, which is required when the E modes in the C3” structure are split into an A’ and an A” mode in the C, structure. The u,(S03), &(CFp), and u(CS) shift up 38,47, and 21 cm-I in the bidentate bridging triplet, respectively. The calculated shift of the u,(CF3) stretching frequency is again very small ( 5 cm-1). For aggregate 4f, which has the same symmetry as the free triflate ion, the frequency shifts relative to the free triflate ion W S ) , and u ( W for the uaS(SO3),u,(CFd, u,(CF3), GQ), modes are at 18, 3, 90, 41, 49, and 25 cm-1, respectively. The two degenerate S-0and C-F stretching modes are slightly mixed with each other, and the frequency order for the u,,(CF3) and u,(CF3) is reversed in the aggregate 4f compared to free triflate because of the large interaction between the lithium cations and the triflate ion. Overall, calculated vibrational frequency shifts and splittings for structures 4e and 4f agree qualitatively with experimentallyobserved frequency shifts and splittings attributed to aggregate I and aggregate 11. Internal Force Constants. To trace the origin of the observed frequency shifts and splittings, internal force constants for the triflate ion are obtained from the force constant matrices calculated in Cartesian coordinates. All the stretching and bending coordinates plus a minimal number of torsional coor-
1131 (A’)
0.07 0.03
dinates, as recommend by Boatz and Gordon,17were used in the force constant transformation. Table 6 includes the main internal force constants of the triflate ion calculated at the HF/3-21+G* level. The C-S and C-F force constants all increase by 10- 17% when the triflate ion is coordinated with lithium cation(s) for each geometry studied. The changes of these force constants are consistent with the shortening and strengthening of the C S and C-F bonds and the increase of frequencies of these modes consisting mainly of the C S and C-F stretching motions in the lithium triflate ion pairs and aggregates. The force constants of the Li-coordinated S-0 bond(s) decrease by 5-218 whereas those of the uncoordinated S-0 bond@) increase by about 10%. Therefore, the frequency of the symmetric SO3stretching mode can be higher (monodentatepair, bidentate bridging and tridentate bridging aggregates) or lower (bidentate ion pair) than that of the free triflate, depending roughly on whether the averaged S-0 bond force constant (or more accurately the averaged S-0 bond force constant with weighting factors from the PED contribution) is higher or lower than that in the free triflate ion. For the split u,,(S03) modes, the higher frequency mode consists primarily of the stretching motion of the uncoordinated S-0 bond and the lower frequency component originates from the Li-coordinated S-0 stretching motion. The very large splitting of the u,(S03) modes is therefore caused primarily by the very large difference among force constants of the uncoordinated S-0 bond@) and those of the Li-coordinated S-O bonds. Atomic Chargeswithin the Tritlate Ion. In order to understand the molecular-level origins of the changes of bond lengths and force constants which are reflected in the observed frequency changes in the IR and Raman spectra of the lithium triflate complexes, it is useful to look at the redistribution of electronic charge in the triflate ion perturbed by lithium cation(s). The calculated Mullikenl* charges and LtiwdinI9 charges are listed in Table 7. There are large differences between the calculated Mulliken and Lawdin charges on the carbon, sulfur,
106 The Journal of Physical Chemistry, Vol. 98, No. 1, 1994
Huang et al.
TABLE 4: Approximate Descriptions and Potential Energy Distributions (PED) of the C-F Stretching Modes of the Triflate Ion for Free Triflate, Its Litbium Ion Pairs, and Aggregates, Calculated by Using a 3-21+6* Basis* ion pairs "solvated" species LiTf free LiTf Li2TfC LiTf (OH)LiTf(0H)LiZTf(0H)zmonoden biden. ion Tfbiden. brdg monoden biden. biden. brdg Al[u,(CFa)] and Its Shift 1323 (A') 1323 (A') 1324 (A') 1326 (A') 1317 (Ai) 1323 (A') 1331 (A') freq (sym) description 4CF3) 4CF) WFz) 4CFd 4CFd 4CFd 4CS) mode no. St ( C S ) St ( s - 0 1 ) St ( s - 0 2 ) St (W3) St (C-FI) St (C-F2) St (C-F3) freq description mode no. St ( C S ) St ( s - 0 1 ) St ( s - 0 2 ) St ( s - 0 3 ) St (C-FI) St (GF2) St (C-F3) freq description
4SO) 3 0.19 0.24
3 0.24
0.15 0.15 0.15
0.03 0.13 0.16
1252 (E)
1297 (A')
h(CFd
4CF) ~(SOJ)
4
4
3 0.22 0.09
0.07 0.54
0.06 0.22 0.05 0.16
4 0.24
4 0.25
5 0.24 0.05
0.13 0.13 0.15
0.05 0.13 0.15 0.13
0.04 0.14 0.12 0.12
1275 (A") 4CFd
4CF)
5
5
6
0.36
0.06 0.53 0.24
0.51 0.18 0.10
E[uM(CF3)]Modes and Their Splitting 1327 (A") 1313 (*) 1272 (*) 4CF) dCFd 4CFd
.
1290 (*)
U(SO2) 2 0.06
0.12 0.09 0.10 0.36 0.08 0.12
0.35 0.46
3 0.17
4 0.09 0.13 0.26 0.39
0.09 0.64
0.43
1248 (E)
1274 (A")
1283 (A')
1311 (*)
1269 (*)
1267 (A')
1287 (*)
b(CF3)
4CFd
4CF)
4CS)
4CF)
dCFd
WFd
U(SO2) 5 4 5 6 6 mode no. 5 St ( C S ) 0.04 0.03 0.18 St (S-01) St 6 - 0 2 ) 0.03 0.15 St ( s - 0 3 ) 0.17 0.48 St (C-Fl) 0.20 0.07 0.23 0.46 0.55 0.08 St (C-F2) 0.28 0.39 0.68 0.30 0.11 0.55 St (C-F3) #The asterisks indicate that the vibrational symmetries of the modes are not clearly discernable. See text for details,
1400
I
1200
Y b
e.
a
c
..... .....
l
b
a
3-
.....
.I...
1100
Figure 5. Comparison of the calculated splitting of the uU(so3)modes in lithium ion pairs with that observed in the IR spectrum of UTFPPO trimer elcctrolytes. Thedotted lines mark the frequenciesof the u'(CF3) mode used as a referencc position for the splitting of the U,(so3) modes. The calculated frequencies with basis sets 3-21+G* and 6-31+G^ are scaled by 0.926 and 0.935, respectively. For the calculations, a is free triflate anion, b is monodentateCF3S03-Li. and c is bidentateCF3SOrLi. For the experimental data, a is Bu4NTf-ZMTHF and b is LiTf-PPO trimer.
and oxygen atoms. The LBwdin charges indicate that the oxygen atoms are more negative than the fluorine atoms in triflate ion. The Mulliken charges show the opposite results. This difference is caused by the different partitioning method used for electronic charge shared between atoms in the Mulliken and LBwdin charge calculations. However, if the Lawdin charge of the CF3 group
7
0.38 0.45
is compared to that of the SO3group, the latter is still much more negatively charged (-0.92)than the former (-0.08). From Table 7 one can see that electrons flow from the fluorine atoms, the unbound oxygen atom(s), and the sulfur atom to the lithium cations when a lithium cation is coordinated to the oxygen atom(s). Although it is well known that the calculated Mulliken or LGwdin charges are affected greatly by the basis sets used, we are comparing the relative changes of the charges within the same basis sets and the comparison is valid. Geometries, Energies, and Normal Vibratiolls of (CFsSo3)Li*(OH)-and (CF&303)*Lir(OH)~-. The calculated frequency shifts and splittings for the Li.Tf ion pairs and aggregates are all too large when compared to what is shown in Figures 1-3. Furthermore, the calculated splitting of the u,(SOs) modes and the frequency shift of the us(S03)mode of the bidentate Li-Tf ion pair are contradictory to the experimental observations, although ab initio calculations indicate it is the most stable structure among the lithium ion pairs in the isolated state. The bidentate LhTf ion pair structure seems very unlikely to exist in liquid solutions because of the high strain coming from the LiOs-0 ring. In aprotic, low-dielectric solvents such as PPO, THF, and ZMTHF, the interactionsbetween the solvents and the triflate ion are very weak and even the weak interactions will have a negligible effect on the frequency shifts of the triflate vibrations in these solvents. In fact, experimentally observed u(S03) frequency shifts change very little for the "free" triflate anion in aprotic solvents with different dielectric The small solvent dielectric constant will also have only a small effect on the tightness of bonding between the lithium cation and
The Journal of Physical Chemistry, Vol. 98, No. 1, 1994 107
CF3S03-and Its Lithium Ion Pairs and Aggregates
TABLE 5 Approximate Descrlpths of the CF3 Deformation and the CS Stretching Modes of the Triflate Ion for Free Triflate, Its Ion Pairs, and Aggregates, Calculated by Using tbe 121+C*Basis ion pairs “solvated”species LiTf LiTf aggregate free LiTf(0H)LiTf(0H)LizTf(0H)zLi2Tf+biden. brdg monoden biden. ion Tfmonoden biden. biden. brdp ~~
758 (Ai)
789 (A’)
786 (A’)
805 (A’)
769 (A‘)
770 (A’)
783 (A’)
WF3)
WF3) v(CS)
WF3) 4CS)
w33)
NCF3) U(CS)
WF3) 4CS)
WF3) 4CS)
7 0.16 0.13 0.12 0.13 0.12
7 0.19 0.15 0.15 0.15
9 0.17 0.15 0.15 0.15
9 0.17 0.15 0.15 0.15
11 0.18 0.15 0.15 0.15
0.05 0.05 0.05 0.03 0.03 0.03 0.02 0.02 0.02 0.03 0.03 0.03
0.03 0.07 0.07
0.04 0.05 0.05 0.02 0.02 0.02 0.02 0.05 0.02
0.04 0.05 0.05 0.02 0.02 0.02 0.02 0.02 0.02 0.03 0.03 0.03
0.04 0.04 0.04 0.02 0.02 0.02 0.02 0.02 0.01 0.03 0.03 0.03
0.05 0.04 0.05 0.02 0.02 0.02 0.02 0.03 0.02 0.02 0.02 0.02
328 (AI)
342 (A’)
348 (A’)
349 (A’)
337 (A‘)
344 (A‘)
4CS)
4CS)
as)
339 (A’)
4CS)
4CS)
4CS)
4CS)
15
16
16
17
19
19
23
4CS) 7 0.15 0.15 0.15 0.15
0.02 0.02 0.05 0.05
4CS) 7 0.20 0.14 0.14 0.14 0.02 0.02 0.04 0.07 0.04 0.01 0.01 0.01 0.02 0.05 0.02 0.01 0.01 0.01
0.01 0.01 0.01
v,(CS)
TABLE 6 Internal Force Constants of the Triflate Anion, Its Lithium Ion Pairs, and Aggregates Calculated with the Basis Set f21+C** internal force free Tf-, LiTf (M), LiTf (B), LiZTf+l, (H0)Li Tf-l (M), (H0)Li Tf-1 (B), (H0)2Li2Tf-1, constants Figure 4a Figure 4b Figure 4c Figure 4e Figure 6a Figure 6b Figure 6c 3.36 3.78 3.85 3.94 3.61 3.61 3.78 AC-9
AMI) As-02) AM31
AC-Fi) AC-Fz) AC-F3) ACS-01) ACs-02) ACs-03) AS-C-FI) AS-C-Fz) AS-C-F3) AOls-02) jtois-03)
10.77 10.74 10.77 6.51 6.51 6.51
9.50 11.46 11.41 7.13 6.74 6.76
8.47 12.05 8.45 7.10 6.77 7.11
10.23 11.99 10.27 7.30 7.12 7.28
10.41 10.97 10.93 6.72 6.69 6.71
9.51 11.13 9.45 6.71 6.84 6.73
10.71 11.32 10.77 6.96 6.94 6.94
0.56 0.56 0.56 0.38 0.37 0.38 0.66 0.67 0.67 0.73 0.75 0.75
0.55 0.52 0.52 0.38 0.40 0.40 0.66 0.65 0.58 0.69 0.70 0.73
0.38 0.44 0.38 0.38 0.42 0.38 0.58 0.3 1 0.58 0.7 1 0.67 0.71
0.5 1 0.48 0.5 1 0.40 0.41 0.40 0.65 0.68 0.65 0.69 0.66 0.69
0.57 0.54 0.54 0.39 0.39 0.39 0.68 0.68 0.63 0.72 0.73 0.73
0.57 0.49 0.52 0.38 0.38 0.38 0.63 0.28 0.63 0.73 0.71 0.73
0.55 0.53 0.54 0.40 0.40 0.40 0.65 0.71 0.65 0.72 0.7 1 0.72
AOfi-03) AFI-C-F~) AFi-C-S) AFrC-Fd The units are mdyn/A. The bending force constants are normalized by (RiRj), where Ri and Rj are the lengths of the bonds forming the angle. the triflate ion. Therefore, the differencebetween the calculated and the observed frequency shifts and splittings may arise from the difference between the solvation of the lithium cationsomitted from thecalculation but present in the real experimental situation. The Of the triflate ion be affected by the amount of electron charge withdrawnby the cation as well as the particular coordinationof the cation by the oxygen atoms of the triflate. In crystalline oxide solids, lithium cations are often tetrahedrally coordinatedby oxygen atoms. In our recently determined crystal structure of LjCF3S03(CH3CN),7athe lithium cation is COOTdinat4 by three oxygen atoms, each coming from a separate triflate ion and by one nitrogen atom from the CH3CN molecule. As a crude description of the donation of electrons from oxygen atoms of the solvent molecules to the lithium cations, we have
performed exploratorycalculationswith one hydroxide ion added to each lithium cation. Figure 6 shows the monodentate and bidentate ion pairs as well as the bridging triflate aggregate used in the calculations. The calculated geometrical parameters are listed in Table 8. The trend of bond length and angle changes is similar to that of the corresponding structures in Figure 4* As expected, the shapes of the triflate structure in Figure 6 are distorted much less than they are in Figure 4. The total energies calculated at the HF/3-21+G* level for structures 6a and 6b are -1036.238 292 and-1036.229 567 au, respectively. Interestingly, the monodentate (CF$3O&Li-(OH)- is now 22.9 W/mol lower in energy than the bidentate (CF3SO,).Li-(OH)-, indicatingthat
108 The Journal of Physical Chemistry, Vol. 98, No. 1, 1994
Huang et al. TABLE 8: Optimized Geometries of the Free Triflate Anion, Its Lithium Ion Pairs (CFfiOs)Li(OH)-, and Aggregate (CF3SO3)Li,( H0)z- (3-21+G*)* ion pairs +%reg, Figure 6a Figure 6b Figure 6c Figure 4a Monoden Biden. Bid. Brdg ~
1.818 1.438
(6a)
&3
1.371
102.0 01
112.8
‘’Eib-0,
F3
115.8 (6 b )
105.9
3H2 60.0 60.0
1.810 1.452 1.430 1.366 1.367 1.813 1.657 0.956 100.9 103.2 112.0 112.4 117.4 114.6 106.5 106.6 174.2 179.7 179.3 60.0 61.3 180.0 0.0 0.0
1.809 1.446 1.426 1.366 1.366 2.184 1.658 0.956 102.9 103.1 112.1 112.1 109.7 117.7 106.7 106.7 147.1 179.2 147.1 60.0 57.0 108.6 175.9 93.7
1.804 1.443 1.423 1.362 1.363 1.832 1.647 0.956 102.0 104.4 111.6 111.8 113.5 116.0 107.3 107.1 175.0 179.4 179.7 60.0 58.8 -1 16.0 53.6 177.3
Bond lengths in A and bond angles in degrees. F2 F3
(6 c)
Figure 6. Optimized geometries of CFsS03Li(OH)- and CF3SO3Liz(OH)z-.
TABLE 7: Mulliken and Uwdin (in Parentheses) Charges for Atoms of the Triflate Anion, Its Lithium Ion Pairs, and Anerenates Calculated with tbe Basis Set 3-21+G* Figure 4a, Figure 4b, Figure 4c, Figure 4d, Figure 4e, atom CF3SO3-l LiTf (M) LiTf (B) LiTf (T) LiZTfel C 0.408 0.422 0.477 0.501 0.449 S
01
02 FI Fz Lil
(0.769) 2.193 (-0.260) -0.928 (-0.217) -0.928 (-0.217) -0.272 (-0.283) -0.272 (-0.283)
(0.786) 2.180 (-0.170) -0,990 (-0.359) -0.860 (-0,137) -0.232 (-0.243) -0.237 (-0.260) 0.816 (0.786)
(0.789) 2.000 (-0.224) -0.844 (-0.238) -0.837 (-0.095) -0.226 (-0.237) -0.239 (-0.260) 0.731 (0.741)
(0.798) 1.778 (-0.320) -0.818 (-0.190) -0.818 (-0.190) -0.217 (-0.236) -0.217 (-0.236) 0.825 (0.803)
(0.795) 2.215 (-0.090) -0.988 (-0.286) -0.781 (-0.084) -0.205 (-0.226) -0.197 (-0.236) 0.850 (0.819)
the monodentate triflate ion pair is morestable than the bidentate pair when the OH- ligand is attached to the lithium cation. The calculated frequencies and IR intensities for vibrational modes of structures 6a-c are listed in Table 9. The imaginary frequency at 57 cm-1 for the bidentate structure indicates that structure 6b is actually located at a saddle point on the energy surface. The potential energy distribution of selected normal vibrational modes of the triflate ion in structures 6a-c are tabulated in Tables 111-V. The S-0 and C-F stretching modes are not mixedwitheachotherasstronglyasintheion pairsandaggregates shown in Figure 4. The splitting widths of the u,(S03) for structures 6a-c are 40, 77, and 41 cm-’, respectively. The CF3 group in structures 6a and 6c is virtually undisturbed by the lithium cation(s) which interacts with the SO3 group. For structure 6c, the frequencies of modes 5 and 6 are once again close to each other, as seen previously in structure 4f, and neither
mode shows a clearly discernible A’ symmetry. The symmetric u,(S03) mode for the monodentate CF3SO,(Li)(OH)- structure and the bidentate bridging C F ~ S O ~ ( L ~ Z ) ( ~structure H ) Z - shifts up 17 and 35 cm-I, respectively, which is supported by our experimental data (Figure 2). A downward shift (4 cm-1) of this mode for the bidentate structure 6b is again contradictory to the experimental data (Figure 2). Therefore, the results from both the energy and frequency calculations of the model structure CF3S03Li(OH)- indicate the monodentate structure is favored over the bidentate structure for Li-Tf ion pairs in polar, aprotic liquid solutions. Figures 7 and 8 provides a concise summary of the calculated splitting of the u,,(S03) modes and the frequency shifts for the u,(S03) and 6,(CF3) modes and compares these values with data from IR and Raman spectra. Clearly, the addition of one OHgroup to each lithium cation brings the calculated results much closer to experimental observations. A full comparison between the calculated vibrational frequencies and those observed in IR and Raman spectra of solutions indicates the following for the structure of LiTf in low-dielectric,aprotic solvents: (i) LbTf ion pairs possess the monodentate structure, which has its characteristic experimental frequencies at 758 cm-I [6,(CFp)], 1042 [uS(SO3)], and a splitting at 1257 and 1302 cm-1 [A’u(SO) and A ” u ( S O ~ ) ] ;(ii) ~ ~ aggregate I is likely to have the bidentate bridging structure 4e, which has its experimental characteristic frequencies at 758 cm-l [6,(CF,)], 1052 cm-i [us(SO,)],and a splitting at 1270 and 1308 cm-1 [A”u(S02) and A’u(SO)];(iii) aggregate I1 may very well have a tridentate bridging structure similar to 4f, which has its experimentalcharacteristic frequencies at 765 cm-1 [6,(CF3)], 1062 cm-i [u,(SO3)],and 1085 cm-’ [uu-
(sod].
From the calculated downward shift of the u,(S03) mode, the upward shift of the 6,(CF3) mode, and a very large u,(SO3) splitting for the bidentate lithium triflate pair, it seems that the bidentate structure is a good candidate for some of the divalent or trivalent cation triflate complexes6.20 because similar frequency shifts and band splittings have been observed for SnTfii1 and PbTf222 dissolved in some polar and aprotic solvents. However, the possibilityof a monodentatestructure for thesedivalent triflate
The Journal of Physical Chemistry, Vol. 98, No. 1, 1994 109
CF3S03- and Its Lithium Ion Pairs and Aggregates
TABLE 9 Normal Vibrational Frequencies (cm-l) and IR Intensities (km/mol) of Structures 6 a (Monodentate Ion Pair), 6b (Bidentate Ion Pair), and 6c (Bidentate Bridging Triplet) Calculated with Basis Set 3-21+CS mode no.
Figure 6a, Monoden
Figure 6b, Biden.
Figure 6c, Bid. Brdg
mode no.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
4015 (0.1) 1405 (440) 1365 (630) 1326 (63) 1272 (103) 1269 (132) 1131 (210) 907 (208) 769 (60) 675 (233) 604 (39) 600 (29) 530 (32) 529 (27) 400 (291) 393 (307) 374 (8) 368 (4)
4012 (.01) 1413 (436) 1336 (484) 1323 (75) 1275 (147) 1267 (89) 1110 (172) 859 (235) 770 (64) 675 (260) 618 (51) 592 (18) 538 (24) 522 (28) 382 (279) 373 (3) 363 (.02) 340 (295)
3993 (0.5) 3988 (1) 1418 (483) 1377 (695) 1331 (49) 1290 (85) 1287 (121) 1149 (239) 922 (90) 919 (372) 783 (62) 677 (264) 608 (49) 606 (40) 532 (43) 530 (34) 417 (454) 410 (148)
19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36
a
Figure 6a, Monoden
Figure 6b, Biden.
Figure 6c, Bid. Brdg
337 (2) 252 (2) 237 (1 1) 225 (4) 193 (12) 181 (123) 89 (1) 39 (4) 34 (1)
339 (4) 249 (6) 222 (3) 208 (6) 175 (3) 152 (5) 74 (0.4) 44 ( 5 ) i 57 (0.5)
407 (619) 406 (11) 377 (0.01) 370 (8) 344 (5) 256 (4) 238 (0.8) 234 (0.8) 222 (1 5) 185 (6) 179 (15) 177 (14) 167 (0.1) 86 (1) 38 (0.5) 29 (0.1) 28 (3) 10 (2)
The numerical values in parentheses are the IR intensities. The boldface frequencies are assigned in Tables 3, 4, or 5.
c
b
I
830
Exptl
3-21+G' ~~
a
d
h
d
c
0
0
b
a
e
3
810 v
..... ..... ..... ....,
23.....
..... .,...
c $cp 3E
0
790 770 *
e
+
ion pairs cannot be ruled out because the bonding between Sn2+, for example, and the oxygen atoms of the triflate is more covalent. A structure intermediate between the monodentate and bidentate structure without rigid C, symmetry, in which the divalent cation interacts with one or two oxygen atoms and possibly with the sulfur atom, is also possible. Further investigation of the relation between the frequency changes and coordination of Sn2+to oxygen atoms is currently underway.
Conclusions Experimentally obtained vibrational spectra for lithium triflate in polar, aprotic solvents show evidence of at least four species: "freen triflate ions, LbTf ion pairs, aggregate I, and aggregate I1 (reported here for the first time). Ion pairs and aggregates are identified by splitting of the doubly degenerate u,,(SO3) modes (1 270 cm-I for "freen triflate) into two distinct bands at 1257 and 1302 cm-1 ( A v = 45 cm-I) for LieTf ion pairs in PPO and a t 1270 and 1308 cm-I (Au = 38 cm-I) for aggregate I in 2MTHF. The u,(S03) band shifts from 1033 cm-1 for "free" triflate to 1043 cm-1 for ion pairs, 1053 cm-I for aggregate I, and 1062 cm-1 for the newly identified aggregate 11. Similarly, the frequency assigned to the G6(CF3)-appearing a t 752 cm-1 for "freen triflate-shows components at 758,763, and 767 cm-I attributed to ion pairs, aggregate I, and aggregate 11, respectively. Although the monodentate structure of the LiSTf ion pair is calculated to have higher energy than the bidentate structure, calculated vibrational frequencies for a monodentatecoordination
t
e 0
750
Figure 7. Comparison of the calculated splitting of the u,(SOJ) modes in the free triflate (a), monodentate CF,SOsLi(OH)- (b), bidentate bridgingstructureCF3SOLiz(OH)? (c), and bidentateCF3SO3Li(OH)(d) with that observed in the IR spectra of BudNTf-2MTHF (a), LiTfPPO trimer (b), and LiTf-2MTHF (c). The dotted lines are the frequenciesof the u,(CFa) mode. The calculated frequencies are scaled by 0.926.
0
+
1100 1 -
d
c
b
:
q
0
e
lo00
~
a
8
O
t
~
0
+e
- .
~
Figure8. Comparison of the 3-21+GS frequencyshift of the u,(SO3) (A) and&(CFs)(B) modeswith thoseobservedinexperiments.Thecalculated frequencis of ur(S03)and b,(CF3) are scaled by0.93 and 0.99, mpectively.
Labels a+ indicate free, monodentateTf-Liion pair, bidentate bridging aggregate I (TfLi2+'), tridentate bridging aggregate I1 (TfLi3t2), and bidentate TfeLi ion pair, respectively: ( 0 )-experimental, (0) -computational,(+) -computational with each Licapped with anOH-group. Experimental and computational points overlap for free triflate (a). of Li+ by triflate oxygen are most consistent with experimental results. The calculated splitting of u,,(S03) in monodentate Li-Tf (Au = 93 cm-1) is much closer to experiment than the splitting for bidentate LieTf ( A v = 21 1 cm-I). In fact, the splitting for the bidentate structure is sufficiently large to place the lowfrequency component below the u"(CF3) band, in conflict with experiment. Moreover, u,(SO3) shifts to lower frequency for bidentate LLTf but shifts to higher frequency for the monodentate structure, in accord with experiment. Finally, calculations incorporating an OH- ligand coordinated to Li+ to model the effect of solvent place the monodentate structure at lower energy
-
110 The Journal of Physical Chemistry, Vol. 98, No. 1, 1994
than the bidentate structure and bring calculated vibrational frequencies for the monodentate structure into closer agreement with experiment. Similar comparisons of calculated and experimental vibrational frequency shifts and splittings imply that aggregate I may involve the monodentate coordination of two lithium cations to triflate as in Figure 4e, whereas aggregate I1 may possess the structure of Figure 4f, with monodentate coordination of three lithium cations. Calculated frequencies of the u,(SO3), u,(CF3), 6,(CF3), and u(CS) increase in the order "free" triflate < monodentate ion pair < bidentate bridging aggregate < tridentate bridging aggregate, in parallel with frequencies for the experimentally inferredspecies: "free" triflate < ion pairs < aggregateI
