Ion-ion-solvent interactions in solution. 8. Spectroscopic studies of the lithium perchlorate/N,N-dimethylformamide system

J . Phys. Chem. 1984, 88, 637-642 637

Ion-Ion-Solvent Interactions in Solution. 8. Spectroscopic Studies of the Lithium Perchlorate/N,N-Dimethylformamide System

David W. James* and Raymond E. Mayes

Chemistry Department, University of Queensland. Brisbane, Australia 4067 (Received: May 16, 1983)

Solvation interactions and ion association in solutions of LiCIO, in N,N-dimethylformamide (DMF) have been studied by using infrared spectra, Raman spectra, and NMR spectra for the nuclei 7Li, I3C, I4N, I7O, and 35Cl. The behavior of the OCN deformation vibration, the I3C NMR, and the 170 signal of the carbonyl support the proposition that solvation is through the carbonyl oxygen. A quantitative study of the OCN deformation vibrational band in the Raman spectra yielded a solvation number of 3.6 for the Li' ion with an indication that severe crowding and solvent-solvent interactions exist for the solvated cation. The solvation interactions are particularly strong and ion association is not significant below 2 M salt concentration. Above this concentration, evidence for both solvent-shared and contact ion pairs has been obtained. Consistent evidence from the perchlorate stretching vibrational band in the Raman spectra and 35Cl, 7Li, and I7O NMR supported a set of sequential equilibria (Li'), + C104- (Li+(DMF)ClO,-), =e (Li+.CIO,-), ( K l , K2) . Values of K1 of 0.06 dm3 mol-' at 2.5 M and 0.2 dm3 mol-' at 3.5 M were obtained while K2 appeared to have almost the same numerical values as Kl above 3.5 M. The behavior of the carbonyl stretching vibrational band was found to be complex and not in full accord with previous descriptions based on the infrared spectra.

Introduction

There have been a number of spectroscopic studies of lithium salts dissolved in N,N-dimethylformamide (DMF) which have been primarily concerned with solvation This is, in part, due to the importance of some amides as solvents in inorganic chemistry1° and in part because solutions of salts in monomeric amides have been used as models for the binding of salts to peptide groups in proteins and polypeptides.ls2 Because anion solvation is very weak in amides which cannot exhibit hydrogen the studies of solvation of D M F have been concerned with cat- ionsolvent binding. It has been generally proposed that solvation of the cation is through the carbonyl ~ x y g e n , ~ , ~ which would polarize the molecule and enhance the double-bond character of the N-C bond. The evidence which has been presented for binding at the oxygen has been the following: (i) A low-frequency IR band appears, which is regarded as characteristic of alkali-metal salts dissolved in oxygen-donor solvent^.^-^ However, a similar band also occurs in lithium salts dissolved in nitrogen-donor media.4 (ii) The torsional barrier to rotation in the D M F molecule (as measured from 'H NMR) increases for solutions \containing Li+.5-8 It has been inferred that this indicates an increase in C-N bond order. (iii) The energy of the C-0 stretching vibration (amide 1)2,335*9 decreases, and the energy of the band assigned as C-N stretch and N H bend2 and also the OCN deformation increases as the Li+ concentration is i n c r e a ~ e d . ~ , ~ ~ ~

These last studies highlight one of the difficulties in interpreting changes in energy of the DMF vibrational bands. There have been several studies of the vibrational spectrum of pure DMF including normal-coordinate analyses yielding potential energy distributions for the various vibrations of the molec~le . '~ - '~ It is evident from

(1) Bello, J.; Haas, D.; Bello, H. R. Biochemistry 1966, 5, 2539. (2) Balasubramanian, D.; Shaikh, R. Biopolymers 1973, 12, 1639. (3) Lassigne, C.; Baine, P. J. Phys. Chem. 1971, 75, 3188. (4) Rao, C. N. R.; Bhujle, V. V.; Goel, A.; Bhat, U. R.; Paul, A. J. Chem.

(5) Balasubramanian, D.; Goel, A,; Rao, C. N. R. Chem. Phys. Lett. 1972,

(6) Rode, B. M.; Fussenegger, R. J. Chem. SOC., Faraday Trans. 2 1975,

(7) Fussenegger, R.; Rode, B. M. Chem. Phys. Lett. 1976, 44, 95. (8) Rao, K. G.; Becker, E. D.; Rao, C. N. R. J . Chem. SOC., Chem.

(9) Perelygin, I. S. ; Osipov, V. S. Russ. J. Phys. Chem. (Engl. Trans1 )

(10) Covington, A. K., Dickinson, T., Eds. "Physical Chemistry of Organic

( 1 1 ) Rastogi, P. P. Z. Phys. Chem. 1970, 73, 163.

Soc., Chem. Commun. 1973, 161.

17, 482.

71, 1958.

Commun. 1977, 350.

1979, 53, 1036.

Solvent Systems"; Plenum Press: New York, 1973; p 13.

these studies that there is extensive mixing of normal modes. The band which the above study assigned to amide I1 (C-N stretch and N H bend) is probably the band at 1407 cm-' for which the most reliable assignment appears to be CH3 deformation and C H deformation. A consideration of the motional composition of the bands being considered is obviously of great importance in studies of solvation and care must be taken to use the most reliable assignments.

Of the studies involving anhydrous solutions of LiC104 in DMF3,9,'6-'9 only two9J6 have been concerned with measuring the extent of ion association. Concentrations of up to 2 and 0.5 M were studied respectively and the conclusion in both cases was that ion association did not occur to a significant extent. For most salt solutions however ion association does occur to a significant extent before the solution reaches saturation and a study of association over the complete concentration range is expected to yield useful information on the association process.

We have recently studied solutions of LiCIO, in water,20 acetone,21 and diethyl ether.22 In these studies the infrared and Raman spectroscopic measurements were combined with examination of the multinuclear NMR. The combination of techniques enabled a more complete description of both the solvation interactions and the ion association equilibria. We report in this paper a similarly detailed examination of the ion-ion-solvent interactions in solutions of LiCIO, in D M F for solute concentrations up to 4.5 M.

Experimental Section

The D M F was purified by standing analytical reagent grade solvent over solid KOH for 24 h followed by fractional distillation at reduced pressure (-20 mmHg) under nitrogen. The purified

(12) Durgaprasad, G.; Sathyanarayana, D. N.; Patel, C. C. Bull. Chem.

(13) Chalapathi, V. V.; Ramiah, K. V. Proc.-Indian Acad. Sci., Sect. A

(14) Kaufmann, G.; Leroy, M. J. F. Bull. SOC. Chim. Fr. 1967, 2, 402. (15) Jones, R. L. J. Mol. Specrrosc. 1963, 11, 41 1. (16) Berman, H. A,; Stengle, T. R. J. Phys. Chem. 1975, 79, 1001. (17) Cahen, Y. M.; Handy, P. R.; Roach, E. T.; Popov, A. I. J. Phys.

(18) Maciel, G. E.; Hancock, J. K.; Lafferty, L. F.; Mueller, P. A.;

(19) Mishustin, A. I.; Kessler, Y. M. J. Solution Chem. 1975, 4 , 779. (20) Frost, R. L.; James, D. W.; Appleby, R.; Mayes, R. E. J. Phys. Chem.

(21) James, D. W.; Mayes, R. E. Aust. J. Chem. 1982, 35, 1775. (22) James, D. W.; Mayes, R. E. Aust. J. Chem. 1982, 35, 1785.

SOC. Jpn. 1971, 44, 316.

1968, 68, 109.

Chem. 1975, 79, 80.

Musker, W. K. Inorg. Chem. 1966, 554.

1982,86, 3840.

0022-3654/84/2088-0637$01.50/0 0 1984 American Chemical Society

638

D M F was stored over freshly activated molecular sieve. Infrared spectra were determined on a Perkin-Elmer 283 B

spectrometer. Qualitative spectra were taken by using an ATR technique while quantitative spectra were obtained b$ using a thin film solution cell with KRS-5 windows and a path length of 4.9 fim. Both methods ensured that the solution was isolated from the atmosphere. The spectra were transferred directly to a PDP 11-34 computer for signal processing. The Raman spectra were determined on a Cary 82 Raman spectrometer using a CRL Model CR4 argon ion laser operating at 514 nm with -500 mW at the sample. The sample cell was a standard 1 cm path glass cuvette. Constant-energy band-pass slits of -0.5 cm-l were used for strong bands and up to 2 cm-' for weak bands. Spectra were transferred directly to a computer for processing and for the bands which were component band analyzed several spectra were averaged to reduce noise. Magnetic resonance studies of the various nuclei were performed on a JEOL FXlOO spectrometer. Internal lock and reference signals for 7Li, 170, and 35Cl N M R were provided by a 4-mm capillary containing 2 M LiCl in D 2 0 centered within the 10-mm tube containing the sample. Measurements of I3C and I4N signals used the same locking procedure but here the chemical shifts, where measurable, were referenced to the pure solvent by the method of substitution. The necessity of making corrections for bulk susceptibility changes was examined. Preliminary results indicate that the chemical shift changes reported are too small but that the corrections are probably 10.08 ppm over the concentration range studied. These corrections will be discussed in detail in later communications. Viscosities of the solutions were measured by using an Oswald viscometer calibrated with various liquids of suitable viscosity. The densities of solutions were determined at 298 K with a standard density bottle method. LiC104 was prepared and dried by methods previously described.21

Results and Discussion

Solvation of the Lithium Ion. When the infrared and Raman spectra of solutions of LiClO, in DMF are examined, it is found that there is a solute-induced shift in several of the bands. Those which move to higher energy in both Raman and infrared spectra are 235 (unassigned), 355 (C'-N-C' rock), 660 (OCN deformation), 1093 (CH3 rock), 1407 (CH3 deformation + C H deformation), 2860 (CH stretch), and 2930 (CH3 stretch) cm-' while the band at 1685 cm-' (C=O stretch, C N stretch, C-H deformation) moves to lower energy in the infrared and to higher energy in the Raman spectrum. The bands at 235,355, and 1093 cm-' are obscured either by the vibrations of the C104- or by the absorption edge of the KRS-5 plates in the infrared.

The solvation of the lithium ion through the oxygen of DMF has been inferred from the shift to lower energy of the C=O stretching vibration in the infrared s p e c t r ~ m ~ ~ ~ ~ ~ ~ from estimates of the barrier to rotation about the C-N and from the presence of low-energy, infrared-active vibration^.^-^ Studies of the binding modes of the (SCN) m ~ i e t y ~ ~ - * ~ have established that the stretching vibration for the group CX is lowered in energy and increased in intensity26 when binding is through X. It is expected that the (OCN) moiety in DMF will behave in a similar fashion. Indeed, the lowering of the C=O stretching motion has already been used to indicate binding through the carbonyl oxygen. The C N stretching vibration makes contributions to several bands and is not suitable for analysis of energy shifts. The infrared band for the OCN deformation in DMF is a t 660 cm-' and, as the LiC104 concentration increases, a new broader band appears a t -670 cm-' as seen in Figure 1. The total intensity of the band complex increases by a factor of 4 in going from pure DMF to 4.5 M LiC104 solution. Because the nitrogen atom is already highly constrained due to the presence of the two methyl groups, it is unlikely that solvation through the nitrogen would cause a significant increase in this constraint and N bonding would thus

The Journal of Physical Chemistry, Vol. 88, No. 3, 1984 James and Mayes

~ ~~~~ ~~

(23) Turco, A.; Pecile, C. Nature (London) 1961, 191, 66. (24) Sabatini, A.; Bertini, I. Inorg. Chem. 1965, 4, 959. (25) Mitchell, P. C. H.; Williams, R. J. P. J. Chem. Soc. 1960, 1912. (26) Pecile, C. Inorg. Chem. 1966, 5, 210.

800 600 ' 400 Energykm-')

Figure 1. Infrared spectra of solutions of LiC104 in N,N-dimethylformamide: (A) pure DMF, (B) 1.5 M LiCIO, in DMF, (C) 4.5 M LiClO, in DMF.

TABLE I: NMR Data of Solvent Nuclei for LiCIO,/DMF Solutions at Various Concentrations of LiCIO,

i3pa 1 7 n b " concn

b M S(C') S(C2) S(C3) 1 4 N ~ , , 2 , b 6' V I 12

0.0 0.00 0.00 0.00 173 325.1 56 0.1 0.12 0.08 0.04 183 323.3 62 0.5 0.36 0.23 0.16 274 316.8 96 1.0 0.74 0.47 0.35 443 309.2 180 1.5 1.07 0.62 0.47 750 2.0 1.39 0.78 0.59 1300 2.5 1.66 0.94 0.70 -2300 3.0 1.93 1.13 0.86 3.5 2.16 1.37 0.98 4.0 2.40 1.48 1.13 4.5 2.61 1.72 1.25

a Chemical shift (ppm) measured with respect to pure solvent. (Carbons numbered as shown in Figure 2.) width (Hz) at half-height (fwhh). ' Chemical shift (ppm) measured with respect to 2 M LiCl in D,O (internal capillary).

produce little change in the band parameters. Solvation through the oxygen, on the other hand, would significantly decrease the freedom of motion in the OCN grouping. The Raman band assigned to N-CH3 does not change in position, intensity, or shape as the concentration of LiC104 increases. This behavior is not expected if cation binding is through the nitrogen atom. Thus, changes in parameters of several bands support binding through the oxygen atom.

Changes in the I3C, 14N, and "0 N M R spectra provide additional evidence for the mode of solvation and some results are collected in Table I. The changes in the 13C chemical shift are summarized in Figure 2 and it is evident that the amide carbon shows greater deshielding than the methyl carbons. It is expected that polarization by the Li+ cation will shift electron density toward the cation and the resulting deshielding will be most evident on the least electronegative carbon atoms. Solvation through the nitrogen atom would produce a similar deshielding effect in all three carbon atoms. The greater change in the amide carbon indicates that oxygen is the solvating atom. Minor support for the cation approaching through the oxygen atom is gained by the different deshielding of the two methyl carbons with that trans to the oxygen being most influenced. A similar trend observed for solutions of LiCl in DMFZ7 is consistent with little interaction between the anion and the solvent as previously described.

Both 170 and I4N are quadrupolar nuclei and so will show the influence of an electric field gradient in their half-width. Only qualitative I7O data could be obtained due to severe overlap with the I7O signal of C104-. The data suggest however that the signal

v l i 2 is the band-

(27) Rao, C. P.; Balaram, P.; Rao, C. N. R. J. Chem. SOC., Faraday Trans. I 1980, 76, 1008.

Ion-Ion-Solvent Interactions in Solution The Journal of Physical Chemistry, Vol, 88, No. 3, 1984 639

Conc (mol dm-3 1 Figure 2. Variation in "C chemical shift relative to pure DMF for solutions of LiC104 in DMF.

I I

/ I

1 : : : : : l 1 2

Conc(mol d m-3) Figure 3. Variation in I4N half-width (fwhh) for solutions of LiC104 in DMF.

broadens rapidly and moves to higher shielding. This indicates that the field gradient already existing in the carbonyl oxygen is increased by the presence of the cation and that there is an increase in electron density about the oxygen atom. The change in half-width of the 14N signal is shown in Figure 3; however, measurements of changes in the chemical shift were not possible because of the breadth of the signal. The change in half-width indicates that there is an increase in field gradient a t the nitrogen atom as the salt concentration increases. While this is consistent with cation binding at the nitrogen, there are a t least two other sources of increased field gradient. The first of these is associated with the increased a character in the C-N bond which has been inferred from estimates of the barrier to rotation about the C-N b ~ n d . ~ - ~ The other is in the steric crowding of D M F molecules which occurs in the solvated cation, which will be discussed below. This crowding may well require that the three carbon atoms about the nitrogen atom no longer have a planar structure. It is believed that these changes in electron density and structure are sufficient to account for width changes in the 14N signal.

Soluation Number of Li+. The variations of the 6(OCN) vibration in the Raman spectra which resemble those seen in the infrared are shown in Figure 4 and the gradual replacement of the D M F band at 659 cm-' by a broad feature at higher energy is evident. The new bands appearing below 650 cm-' are due to C10,. The spectrum of a 2 M solution of the C104- salt indicated that the higher energy band may have two components and this was confirmed when a component band analysis was performed with the results listed in Table 11. The narrow half-width of the band at 659 cm-' is consistent with the slow dephasing characteristic of a nonassociated liquid.28 The two new bands which grow with increasing salt concentration have a much greater half-width, which indicates a more rapid dephasing process consistent with an unsymmetrical environment due to polarization

(28) It has been suggested that DMF is dimerized in the liquid form (Kopenci, M. M.; Laub, R. J.; Petkovic, Dj. M. J . Phys. Chem. 1981, 85, 1595) but the broadening of the 6(OCN) expected for dimerization is not

Energy (cm-' 1

Figure 4. Variation in the OCN deformation band in the Raman spectra of DMF/LiC104 solutions: (A) pure DMF, (B) 2 M LiCIO, in DMF, (C) 4 M LiCIO, in DMF.

by Li+. When an internal standard was used, as described later, it was shown that the total intensity of this band complex per mole of D M F did not change with added salt. Assuming that the 659-cm-' band is due to free DMF and the bands at 666 and 673 cm-' are associated with DMF solvating Li+, a value for the solvation number, RB, can be obtained. The determined value of 3.6 f 0.1 is in excellent agreement with previously published values of 3.7 and 3.8 obtained by using proton N M R of Li- C104/DMF mixtures in dioxanes3 A somewhat smaller value of 3.24 for solutions of LiCl in DMF was obtained from conductance m e a s ~ r e m e n t . ~ ~ This lower value may reflect the greater tendency of LiCl toward ionic association.

The appearance of two components for the bound solvent deformation band is unexpected and has not been observed for other solvents studied. There are at least two possible explanations for these bands. If the solvation is very strong, coupling could occur between the vibrations of the bound molecules to give several components. Alternatively, crowding of the solvent molecules could result in some solvent molecules bonding more strongly than others to the lithium ion. We favor the latter explanation because intramolecular coupling in a solvation complex in solution has not previously been reported. The lower energy of the band at 666 cm-I and its Lorentzian character support the view that this corresponds to DMF molecules having weaker bonding to the Li+. The second band both is a t higher energy and has more Gaussian character. The ratio of intensities of the 666/673-cm-l bands is -1:1.6.

The solvation number for Li+ appears to remain constant at 3.6 up to the salt concentration where the ratio of solvent to salt falls below 3.6:l. This indicates that the solvation complex is uncommonly stable. In part this will be due to the bulk of the solvent which makes the exchange of solvent molecules by a rotational mechanism a relatively unlikely event. The free energy of formation of solvated Li+ in D M F has been calculated30 to be -40 kJ mol-', indicating that Li+ in D M F is an exceptionally strongly solvated cation. At concentrations of LiC10, above 3 M we find that essentially all of the DMF appears as bound solvent.

Zon Association. Various N M R studies principally of the 35Cl resonance have indicated that there is little or no ion association up to 2 M salt. From conductivity measurements3' however it has been suggested that, for LiCl in DMF, a formation constant for ion pairs is 35 while calorimetric m e a s ~ r e m e n t s ~ ~ give an even

(29) Paul, R. C.; Singla, J. P.; Narula, S. P. J . Phys. Chem. 1969, 73,741. (30) Butler, J. N.; Synnott, J. C . J . Am. Chem. SOC. 1970, 92, 2602. (31) Prue, J. E.; Sherrington, P. J. Trans. Faraday SOC. 1961, 57, 1795. (32) Held, R. P.; Criss, C. M. J . Phys. Chem. 1967, 71, 2487. observed.

640

TABLE 11: Results of Analysis of the 670-cm-’ Band System of DMF at Various Concentrations of LiCIO, in DMF

The Journal of Physical Chemistry, Vol. 88, No. 3, 1984 James and Mayes

bound DMF bound A bound B

free DMF concn, 56 area % area

M posn hwhh free posn hwhh % A a posn hwhh % B b bound % B / % A R T ~ R B 0.0 659.1 2.2 100 1.0 659.1 2.3 71 666.6 4.2 12 672.7 4.5 17 29 1.4 12.51 3.6 1.5 659.1 2.4 55 666.1 4.3 17 672.3 4.6 28 45 1.6 8.19 3.7 2.0 659.1 2.5 42 666.2 4.2 21 672.3 4.7 37 58 1.8 6.02 3.5 2.5 659.2 2.6 21 666.3 4.5 30 672.5 4.9 49 79 1.6 4.71 3.7 3.0 659.2 2.65 9 666.5 4.9 37 672.9 5.0 54 91 1.5 3.83 3.5 3.5 659.1 2.7 2 667.1 5.5 37 673.6 5.4 61 98 1.6 3.20 3.1 4.0 667.5 6.0 35 674.3 5.7 65 100 1.9 2.72 2.7

2.34 2.3 4.5 668.8 6.0 37 675.0 5.7 63 100 1.7

% area bound A. %area bound B. Determined from the solution density which is described by the expression p = 0.944 + 0 . 0 8 0 ~ - 0 . 0 0 3 ~ ~ when c is in mol dm-3. Analy5es are based on band components at 659 ,667 , and 673 cm‘’ , with average shape ratios [ X , / ( X , + X,)] of 0.93, 0.96, and 0.71, respectively. Band positions (cm-’) and half-widths (cm-l) at half-height (hwhh) are as indicated. R B =moles of DMF bound per mole of LiCIO,. RT =total moles of DMF per mole of LiClO,.

TABLE 111: Viscosities and NMR Data of Solute Nuclei for LiCIO,/DMF Solutions at Various Concentrations of LiCIO,

?Li 35c1

M ity, cp ul,,a g b u l i 2 a 1’0 u1,2a,d concn, viscos-

0.0 0.797 0.1 <0.7 -0.02 4.1 1009.4 0.5 1.221 <0.7 -0.02 4.5 1009.2 1.0 1.945 <0.7 -0.01 5.0 1009.1 1.5 3.365 <0.7 -0.01 6.8 1009.0 c ( 8 5 ) 2.0 <0.7 -0.02 14.4 1008.8 330 (72) 2.5 14.88 1.0 -0.05 46.8 1008.4 330 (71) 3.0 2.0 -0.12 137 280e 3.5 72.4 3.7 -0.22 440 120 4.0 7.3 -0.32 1120 220 4.5 240 10.2 -0.45 1920 3 20

a u l i z is the bandwidth (Hz) at half-height (fwhh). Chcmical shift (ppm) measured with respect to 2 M LiCl in D,O (internal capillary). Overlap o f signal with I7O of DMF prevents iiicasure- ment at lower concentrations Bandwidth arises from chlorine to oxygen coupling (coupling constant (Hz) shown in parentheses). e Intermediate bandwidth as chlorine/oxygen coupling collapses.

higher value of 2 X 10’. These measurements may reflect the loss of mobility of the Li+ caused by the very strong solvation.

We have recently shown that analysis of the band profiles for the symmetrical stretching vibrations of small polyatomic anions in conjunction with multinuclear NMR can give useful information on ion association. The approach of the strongly polarizing lithium ion induces an asymmetric perturbation on the C104- which is manifested by a loss of degeneracy in the antisymmetric stretching vibration, broadening of the 35Cl NMR signal, and the appearance of additional components for the nondegenerate symmetrical stretching vibration. The 7Li N M R signal is also shifted by the ion association.

Examination of the infrared spectra shows that the antisymmetric stretching vibration undergoes a loss of degeneracy at salt concentrations above 2.5 M. If this perturbation is from the cation, it should be reflected in the 7Li N M R and should also be ob- servable in the 35Cl N M R bandwidth.

The behavior of the width of the 35Cl resonance with concentration is shown in Table I11 and Figure 5A. Relaxation in C104- is dominated by quadrupole effects. The quadrupolar coupling constant is influenced by field gradients produced both by the environment and by the covalent bonds, which means that small distortions from Td symmetry will greatly increase the field gradient. In addition to the field gradient effects the bandwidth reflects the time correlation factor 7,. It is frequently assumed that 7, a 7, the viscosity, and so line widths divided by viscosity should enable the estimation of quadrupolar relaxation. The appropriate viscosity is of course the inicroscopic viscosity, which is not experimentally accessible. The macroscopic viscosity a t higher concentrations may well be strongly dependent on the same ion association processes which are producing the field gradient.

A t

- -.--_ -.. -~ *..*d 1 2 3 4 Conc [mol drn-3)

Figure 5. Variations in and 7Li NMR signal for DMF/LiClO, solutions: (A) bandwidth (fwhh) of 35Cl signal, (B) chemical shift of 7Li signal relative to 2 M LiCl in D20.

A I

Figure 6. Component band analysis for the symmetrical stretching band of C10, in the Raman spectra of DMF/LiC104 solutions: (A) 1.5 M LiCIO, in DMF, (B) 3.0 M LiCIO, in DMF, (C) 4.5 M LiCIO, in DMF.

We are in strong agreement with the s u g g e ~ t i o n ~ ~ , ~ ~ that viscosity corrections to experimental line widths are not appropriate for these salt solutions. Viscosities of most of the solutions have however been measured and are included in Table 111.

A previous studyI6 concluded that ion pairs were not formed

(33) Arnold, M. J.; Packer, K. J. Mol. Phys. 1968, 24, 241. (34) Burke, T. E.; Chan, S. I . J . Magn. Reson. 1970, 3, 5 5 .

Ion-Ion-Solvent Interactions in Solution

TABLE IV: Results of Component Analysis of the C10; A , Band at Various Concentrations of LiC10, in DMF"

The Journal of Physical Chemistry, Vol, 88, No. 3, 1984 641

band 1 band 2 band 3

concn, M posn hwhh "/o area posn hwhh St area posn hwhh 54 area K I b K ,

1.0 932.2 1.15 100 1.5 932.1 1.21 100 2.0 932.2 1.30 95 938 3.0 5 2.5 932.2 1.41 89 938.1 3.3 11

0.03 0.06

3.0 932.3 1.58 81 93 8.1 3.3 19 0.1 3.5 932.5 1.75 67 938.3 3.3 28 944.0 5.3 5 0 .2 0.2 4.0 932.9 1.95 56 938.5 3.5 34 944.5 5.4 10 0.3 0.3 4.5 933.1 2.10 39 938.8 3.9 43 945.2 5.6 18 0.6 0.4

a Analyses are based on band components at 932, 938, and 944 c n - ' with averagc shape ratios of 0.91, 0.75, and 0.99> respectively. Band positions (cm-I) and half-widths (cm-I) at half-height (hwhh) are as indicated. Association quotients as defined in t h e tes t .

TABLE V: Results of Analysis of the CIO; A , Band and the DMF 866-cm-' Band at Various Concentrations of LiC10, in DMF"

0 60;

a

% * m 0 .

30-

Conc (mol dm-3)

Figure 7. Variation in the relative intensities of the band components of the symmetrical stretching band of Clod- with concentration: (A) free perchlorate, (B) solvent-shared ion pair, ( C ) contact ion pair.

up to a concentration of 0.5 M because of the sharpness of the 35Cl N M R signal (<5 Hz). It is clear from Figure 5A that the signal remains relatively sharp up to concentrations = 2 M but that rapid broadening occurs a t higher concentrations. This broadening is attributable to the strong field gradient produced by the close approach of the lithium ion. The chemical shift for 35Cl has been measured for the lower concentrations and a small increase in shielding is noted as the concentration increases.

The chemical shift for 7Li is reported17 not to change up to a concentration of 0.5 M. Our measurements shown in Figure 5B and Table I11 agree with this and show that the chemical shift is almost constant up to a concentration of 2 M, indicating that the environment of the Li' does not change over this range. At higher concentrations however there is a small but significant increase in the shielding, corresponding closely to the behavior of the 35Cl resonance. The half-width of the 7Li resonance shows a slight broadening as the concentration increases. The parallel changes in the resonances of 35Cl and 7Li give a strong indication that mutual interactions are responsible for the changes.

The Raman spectra of the symmetric v(C1-0) vibration shown in Figure 6 clearly show the growth of higher energy bands above 2.0 M concentration of salt. Component band analysis of this band yielded the results listed in Table IV. In addition to the dilute-solution band at 932 cm-' there are two higher energy bands, the first at 938.5 cm-' which appears for salt concentrations above 1.5 M and the second at 945 cm-l which occurs for solutions above 3 M. The band at 866.3 cm-' has been assigned as mainly N-C stretching vibration of the N-Me groupings in DMF.12 The three components analyzed for the perchlorate band are similar to those found for solutions of various perchlorate salts in ~ a t e r , ~ ~ ~ ~ ~ acetone:',35 and diethyl ether22 and they are assigned to the species free C104- (932 cm-I), solvent-shared ion pairs [Li+(DMF)- C104-],Iv (938.5 cm-I), and contact ion pairs [Li+C104-]solv (945 cm-'). The variation of the relative areas of these bands with concentration is shown in Figure 7 . Values of equilibrium quotients were calculated for the two equilibria

K (Li+)solv + C104- & [Li(DMF)CI04-],,~,

(35) James, D. W.; Cutler, P. G., unpublished results.

c10; D M T (866-ciii-' band)

concn, :/c %, M pom SR h ~ i h h areaC arcabjc Rd

0.0 1 .o 1.5 2.0 2.5 3.0 3.5 4.0 4.5

866.3 866.4 866.5 866.2 866.5 866.2 866.6 866.4 866.5

0.88 0.81 0.80 0.74 0.72 0.68 0.68 0.65 0.64

2.6 100 3.2 6 9 31 5.6 3.4 59 41 5.7 3.1 51 49 5.8 3.9 45 55 5.8 4.1 40 60 5.7 4.3 35 65 5.9 4.4 32 6 8 5.8 4.5 28 72 6 .0

" Band positions (cm-I), shape ratios ( S R ) , and half-widths (cn- ' ) at half-height (hwhh) arc as indicated. of component bands 1, 2, and 3 of the C10,- A , band. cd as a percentage of the combined areas of the DMF (866-cmP band) and thc C10; A , band. cni" band) ;RT is defined in Table 11.

Conibincd areas Express-

R = RT(% Cl0;)/% D M F (866-

and are presented in Table IV. When these quotients are com- pared with those determined in water and acetone, it is found that they follow the trend

Kwater < KDMF < L e t o n e

which follows the trend of increased association as the dielectric constant of the solvent decreases. The values determined for the equilibrium quotients are however very much smaller than those estimated from conductance or calorimetric measurements.

The data presented in Figure 7 and Table IV rely on the molar intensities of the different C10, vibrators being equal. Justifi- cation of this assumption can be demonstrated as follows: Listed in Table V are parameters for the 866.3-cm-I (v,(NCH,)) band of D M F appearing in Figure 6. The constancy in position, and the relatively small changes in both SR and hwhh, of this band allowed it to be used as an internal intensity standard. Variation in the values of the expression (I?,)(% C104- (total))/(% DMF) is a measure of the trend in molar intensity of the C10, envelope. The fact that this ratio remains essentially constant at (5.8 * 0.2):l is reasonable evidence for constancy in the molar intensity of components within the C104- envelope.

Other Spectral Features. The infrared spectra in Figure 1 clearly show the band at -450 cm-' due to the Li-0 vibrations in the solvent cage which grows as the salt concentration increases. Also the replacement of the C'-N-C' rocking vibration at 350 cm-' by a more intense broader band at -370 cm-' is clearly demonstrated. The breadth and movement to higher energy of this band for molecules solvating the lithium ion is most probably due mainly to the constraint imposed by the crowding of DMF molecules in the solvent complex.

The I7O signal of the C104- ion shows variation with concentration which is listed in Table 111. At low concentrations there is coupling to both 35Cl and 37Cl nuclei which results in two broad

642 The Journal of Physical Chemistry, Vol. 88, No. 3, 1984 James and Mayes

overlapping quartets having coupling constants of -85 Hz. As the concentration increases and the relaxation rate of C1 increases due to the quadrupolar coupling, the ‘70-chlorine coupling decreases and the overall line width decreases. This decrease con- tinues up to a concentration of -3.5 M above which the line, now a singlet, broadens rapidly. This broadening of the signal of the quadrupolar I7O is undoubtedly due to the electric gradient produced by the close approach of the Li+ ion in the formation of the contact ion pair.

The stretching vibration of the carbonyl group has been ex- tensively used in studies of solvation in DMF and other solvents. The vibration is very intense in the infrared and it is difficult to obtain quantitative information on its variation. It is reasonably clear however that the band moves to lower energy as the concentration of salt increases. The behavior of the band in the

Raman spectrum is however quite different. As the concentration of salt increases, the band reduces rapidly in intensity, initially moves to higher energy, and then moves to lower energy. A description of the origin of the changes requires a more detailed knowledge of both infrared Raman spectra than is at present available. We are currently undertaking such a study which will be reported at a later time.

Acknowledgment. The ARGS is thanked for grants enabling the purchase and maintenance of the Raman spectrometer. R.E.M. acknowledges financial support from a Commonwealth P.G. Award.

Registry No. LiC104, 7791-03-9; DMF, 68-12-2; 7Li, 13982-05-3; 14N, 7727-37-9; I7O, 13968-48-4; 35Cl, 13981-72-1.

Other Presentations

In addition to the papers printed in this issue of The Journal of Physical Chemistry, the following lectures and contributed papers, listed in alphabetical order by first author, were presented at the conference. For papers with more than one author, the presenter is denoted by an asterisk. Abstracts for most of these papers are available as supplemental material for this issue.

Kenneth G. Brown, ”The Effect of Hydration upon the Structure of Dipalmitoyl Phosphatidyl Ethanolamine by FTIR.

S. Califano. “Anharmonic Effect in Molecular Crystals”. V. V. P. Chacko* and R. G. Bryant, “Quadrupole Relaxation

in Symmetrical Complexes”. S. Ganapathy,* V. P. Chacko, and R. G. Bryant, “Carbon-13

CP-MAS N M R Spectra of Paramagnetic Solids”. Gregory Hall,* Kopin Liu, Michael J. McAuliffe, Clayton F.

Giese, and W. Ronald Gentry, “Pulsed Molecular Beam Study of State-to-State Vibrational Excitation in I, + He Collisions- Energy Dependence of v = 0 - 1, 2, 3 Cross Sections”.

G. Herzberg, ”Rydberg Spectra of Triatomic Hydrogen and the Ammonium Radical”.

T. A. Holme* and M. S. Gordon, “Transition State Structures for Isomerization Pathways Leading to Silaacetylene”.

Cynthia J. Jameson, “The Dependence of Nuclear Magnetic Shielding on Rotation, Vibration and Isotopic Substitution”.

M. Jarvis and R. G. Bryant,* “Solvent-Macromolecule Dy- namical Coupling in Water”.

Russell D. Johnson HI,* W. Ronald Gentry, and Glayton Giese, “Infrared Photodissociation of Weakly Bound Molecular Clusters”. H Leo Laux,* D. Harradine, M. Dubs, and J. I. Steinfeld, “Diode Infrared Laser Double Resonance of Substituted Methanes: Spectroscopy and Relaxation”.

Carmay Lim* and Donald G. Truhlar, “An Invariant Vector Characterizing the Nonequilibrium Vibrational-State Distribution of Homonuclear Diatomic Dissociation-Recombination Reactions in Excess Inert Gas”.

Elmer D. Lipp, Daryl A. Young, and Laurence A. Nafie,* “Fourier Transform Vibrational Circular Dichroism”.

William N. Lipscomb, “Electron Correlation Effects in Boron Hydride Structures, Intermediates and Reactions”.

Isao Noda, A. E. Dowrey, and Curtis Marcott,* “Dynamic Infrared Linear Dichroism of Polymer Films under Oscillatory Deformation”.

R. Pariser, “Polymer Dreams”. Robert G. Parr* and Ralph G. Pearson, “Absolute Hardness:

Companion Parameter to Absolute Electronegativity”. George C. Pimentel, “Multiphoton Excitation of Bimolecular

Reactions”. Sachchida N. Rai* and Donald G. Truhlar, “Variational

Transition State Theory Calculations for 0 + OH - H + O2 Based on a Vibrational-Rotational and Potential Interpolation Scheme and on an Ab Initio Potential Surface”.

Richard L. Redington* and Delphia F. Hamill, “Infrared Spectra of Matrix Isolated Hydrogen Fluoride and Vibrational Predissociation of the Dimer”.

S. J . Riley, E. K. Parks,* L. G. Pobo, and S. Wexler, “The A - X Transition on CR2: Predissociation, Isotope Effects and the 1-1 Sequence Band”.

Hubert Romanowski* and Joel M. Bowman, “Vibrational Energies of Polyatomic Molecules: Self-Consistent Field and Configuration Interaction Calculations for H20” .

Joel W. Russell, “An Infrared Study of the Electrosorption of C O on Pt”.

Kerin Scanlon, Robert A. Eades, John Overend, David A. Dixon,* and Andrew Komornicki, “Ab Initio Calculations of Force Field and Intensities for CH4, NH4+, BH4-, and CHI”.

David W. Schwenke* and Donald G. Truhlar, “Quantum Scattering Calculations of the Vibrational Excitation of I2 by He”.

Rex T. Skodje,* David W. Schwenke, Donald G. Truhlar, and Bruce C. Garrett, “Vibrational Predissociation of Collinear Re- active Resonances”.

A. Lee Smith* and Dennis R. Anderson, “Vibrational Spectra of Me2SiCI2, Me3SiC1, Me3SiOSiMe3, (Me2Si0)3, (Me2Si0)4, (Me2SiO),, and their Deuterated Analogues”. w A. J. G. Strandjord* and P. F. Barbara, “Excited State Dy- namics of 3-Hydroxyflavone”.

Todd C. Thompson and Donald G. Truhlar,* “Stabilization Calculations of Vibrational Wave Functions and Probability Density Maps for Reactive Resonances in H + FH, H + H2, and

H Todd C. Thompson,* Donald G. Truhlar, and C. Alden Mead, “Vibrational Energies Computed with the Electronic Adiabetic Approximation in the Presence of an E @ e Jahn-Teller Effect”. H David Werst,* Paul Barbara, and W. Ronald Gentry, “Proton Transfer of 3-Hydroxyflavone in a Free Jet”.

Patrick G. Zimmerman* and William E. Farneth, “Laser Induced Heterogeneous Catalysis”.

Richard L. Zimmerman and Thomas M. Dum,* “The Vapor Phase Infrared and Raman Spectra of p-Difluorobenzene-h4 and

The Symposium organizers, Paul Barbara, John Overend, and Donald G. Truhlar, and the Symposium secretary, Mary Hof- bauer, wish to join their colleagues in congratulating Bryce Crawford on this happy occasion. The Symposium was supported in part by E. I. du Pont de Nemours and Company, Inc., and by the Graduate School, the Department of Chemistry and the In- stitute of Technology of the University of Minnesota.

F + H2”.

-d4”.

Supplementary Material Available: Abstracts for the presentations listed above that have the symbol are available (32 pages). Ordering information is given on any current masthead page.

Documents

Ion-ion-solvent interactions in solution. 8. Spectroscopic studies of the lithium perchlorate/N,N-dimethylformamide system