J. Phys. Chem. 1983, 87, 4694-4699
4694
Multinuclear Nuclear Magnetic Resonance Study of LICI0,-Solvent-Nitroxide Systems
Radical
Waclaw KoYodzleJskl Department of Chemistry, University of Warsaw, 02-093, Warsaw, Poland
and Gary E. Maclel' Department of Chemistry, Colorado State University, For? Collins, Colorado 80523 (Received: December 17, 1982)
A study is reported of the effects of 2,2,6,6-tetramethylpiperidinyl-l-oxy (Tempo) radical on systems consisting of LiC104in three different solvents, tetrahydrofuran (THF), CH3CN,and CH3N02. Chemical shift and line width effects on NMR parameters were studied for various nuclides-'Li, 'H, 13C,170,and 35Cl. On the basis of effects that added Tempo radical has on these LiClO,-solvent systems one can conclude that the ability of solvent and Tempo to compete in satisfying the solvation of LiC10, increases in the order CH3NOz< CH3CN < Tempo < THF. On the basis of temperature-dependencedata, the enthalpies of formation of the complex involving the lithium cation and TemDo are found to be 4.6 f 0.2 k J mol-' in THF and -3.3 f 0.8 k J mol-' in CH3CN.
Introduction 7Li dynamic polarization studies' have shown that nitroxide radicals can effectively compete with alcohols in the solvation of lithium cations. A strong scalar coupling between 7Li and the unpaired electron of the radical has been found, suggesting that it should be possible to observe 7Li NMR contact shifts. Recent NMR studies of 23Na contact shifts induced by a nitroxide radical2 have helped to explain interactions in the system NaC104-THF-nitroxide radical. Popov et al. have found a correlation between 23Na,39K, and 133Cschemical shifts and the Gutmann donor numbers of solvent^.^-^ However, the relationship fails completely in the case of 7Li.6 This combination of circumstances has encouranged us to study lithium cation solvation in nonaqueous solvents by 7Li NMR, using as a spin probe the 2,2,6,6-tetramethylpiperidinyl-1-oxy radical, Tempo.
n 0. Tempo
Each solution contained 10 vol% of C6D6for an internal deuterium lock signal, as had been used earlier.2 The NMR contact shift was determined as the shift change from the diamagnetic solution (in the absence of nitroxide radical) to the given solution doped with the nitroxide radical. A negative contact shift means a downfield (higher frequency) shift and it corresponds to positive spin density. Uncertainties (error estimates) in the contact shifts are indicated in the figures. 7Lilongitudinal relaxation times were measured by the inversion recovery technique, with alternating phases of 90° pulses; a three-parameter fit8to the data was applied. The errors were estimated to be about 6%. Transverse relaxation times were measured from half-widths of the NMR signals, and the uncertainties are estimated to be less than about 10%. Paramagnetic relaxation rates, Tl,;' (vide infra), were obtained from the observed relaxation rates by substraction of respective diamagnetic relaxation rates. Tempo was prepared from 2,2,6,6-tetramethylpiperidine (Aldrich) by oxidationg and purified by sublimation. Tetrahydrofuran (THF), acetonitrile, and nitromethane were reagent-grade materials, carefully dried and distilled. LiC104was reagent-grade material, heated before use for several hours at 150 OC. The measurements were carried out shortly after the samples had been prepared. Many measurements were repeated with newly prepared samples from freshly dried solvents to make sure that the results are reproducible and that moisture was avoided.
Experimental Section 7Li, 1 7 0 , and 36Cl NMR spectra were recorded on a Nicolet NT-150 spectrometer a t 58.32, 20.34, and 14.72 MHz, respectively. 'H and 13C NMR spectra were recorded on a Jeol FX-100 spectrometer at 99.55 and 25.00 MHz, respectively, or on a Jeol FX-9OQ spectrometer at 89.55 and 22.50 MHz, respectively. The temperature was stabilized and measured with an accuracy of f l K, using the Nicolet or Jeol variable-temperature units. Additionally, the temperature was checked with a thermometer before and after each series of measurements. The measurements were carried out in Wilmad 10- and 5-mm glass tubes.
Results and Discussion
(1)J. A. Potenza and J. N. Linowski, J. Chem. Phys., 54,4095(1971). (2)W.Koyodziejski, P. Laszlo, and A. Stockis, Mol. Phys., 45,939 (1982). (3)R. H. Erlich and A. I. Popov, J.Am. Chem. SOC.,93,5620(1971). (4)J. S.Shih and A. I. Popov, Inorg. N u l . Chem. Lett., 11,105(1977). ( 5 ) W.J. DeWitte, L. Liu, E. Mei, J. L. Dye, and A. I. Popov, J. Solution Chem., 6 , 337 (1977). (6)Y. M. Cahen, P. R. Hardy, E. T. Roach, A. I. Popov, J. Phys. Chem., 79,80 (1975).
(7)J. D.Cutnell, H. E. Bleich, and J. A. Glasel, J. Magn. Reson., 21, 43 (1976). (8) G. Levy and I. Peat, J. Magn. Reson., 18,500 (1975). (9)E.G. Roszantzev and M. B. Neiman, Tetrahedron, 20,131(1964). (10)G. E. Maciel, J. K. Hancock, L. F. Lafferty, P. A. Mueller, and W. K. Musker, Inorg. Chem., 5,554 (1966). (11)J. W.Akitt and A. J. Downs in "The Alkali Metals Symposium", The Chemical Society, London, 1967,p 199.
0022-3654/83/2087-4694$0 1.5010
Major Interactions in the LiC104-Solvent-Tempo Systems. An addition of Tempo to the solutions of LiC104 in THF or in CH3CN induces large downfield 7Li NMR shifts (Figures 1 and 2) which are beyond the usual shift range found for this salt in diamagnetic solvents.6J0J1 Hence, they are contact shifts due to positive spin density
0 1983 American Chemical Society
The Journal of Physical Chemistry, Vol. 87, No. 23, 1983 4895
LiCI0,-Solvent-Nitroxide Radical Systems
0
6, wm
-2t \
-5
-I 0
-4
-I 5
I \
349 K 329 K
3 1 3K
-20
298 K
I
\ 256 K
L
.05
.I5
.IO
.20 ,[R]* mole
/
dm
Figure 1. Plot of the 'Li NMR contact shift vs. Tempo concentration in the LCIO,-CH,CN-Tempo system for 0.1 M LiCIO, at various term peratures. The contact shift error depends on line wldth and does not exceed 8%.
I
2
3
4
5
[R],
Flgure 2. Plot of the 'Li NMR contact shifts vs. Tempo concentration in the LiCI0,-THF-Tempo system for 0.1 M LiCIO, at various tempertures. The contact shift error depends on line width end does not exceed 1%.
on 'Li nuclei. The contact shifts prove the existence of Li+ complexation by the radical and suggest a significant direct spin density transfer from the radical to the 2s orbital of the lithium cation. The Tempo-Li' and solvent-Li' interactions can be studied and interpreted most directly in systems in which an anion does not interfere appreciably. Lithium perchlorate has been chosen for this study, because the C104anion is known to be solvated only weakly, and, compared to other anions, it has only a slight tendency to come into the solvation sphere of alkali cations.12 The measurements of this study have been performed on 0.1 M LiClO, solutions in THF, CH3CN,and CH3N02. These solvents represent a wide range of donor numbers (DN) and dielectric constants (e), and correspondingly differ in their ability to solvate the lithium cation. Accordingly, they influence differently the equlibrium between contact ion pairs and solvent-separated ion pairs. The predominant ionic species in a 0.1 M solution of LiC104in THF (DN = 20.0,13 E = 7.3914)have been shown (12)(a) P. Dryjanski and Z. Kecki, J. Mol. Stmct., 12,219(1972);(b) M. K. Wong, W. J. McKiney, and A. I. Popov, J. Phys. Chem., 75, 56 (1971). (13)V. Gutmann, 'Coordination Chemistry in Nonaqueous Solvents", Springer-Verlag, Vienna, 1968,p 19.
Flgure 3. Plot of the 'Li NMR contact shift vs. Tempo concentration in the LiCI0,-THF-Tempo system at 301 K for 0.01 (e)and 0.1 (A) M LiCIO,.
to be ion pairs.14J5 The conductance datal4 do not definitely distinguish whether ion pairs are solvent separated or in contact. One's estimate of the minimum approach distance, a, between the center of the Li' and C104-ions in the ion pairs depends on the theory used for the association constant. Values of a ranging from 2.0 to 3.74 A are obtained, compared to the estimated sum, Er = 3.3 A, of the crystallographic radius of Li+, the C1-0 distance, and an oxygen van der Waals radius. Jagodzinski and Petrucci14concluded that only the order of magnitude is significant in such estimates of a values. The 7Li chemical shift for LiC104 in THF is nearly concentration independent, and the 'Li signal is considerably upfield compared to those of other Li+ salts.6 In our view, this behavior shows that the Li+C104-ion pairs are separated by THF. The same conclusion is drawn from our 35ClNMR studies. The bulky Tempo molecule, entering into the solvation shell of the lithium cation, should separate the Li+C104-contact ion pair, if it existed. The result would be some line-narrowingeffect of the W1 NMR signal of the C104- anion due to a reduced quadrupolar interaction,6 and a significant line-broadening effect because the nitroxide group would approach the C104- anion quite close and exert a dipolar paramagnetic influence. It is unlikely that both of these contributions would cancel, so the half-width of the 35ClNMR signal should change upon variation of the Tempo concentration if contact ion pairs were involved. However, it has been found that the half-width, Avl = 41 f 3 Hz, of the 35ClNMR signal of c104- in the Lih04-THF-Tempo system is independent of the radical concentration within the limits of expected error. This concentration independence has been checked in a 0.1 M solution of LiC104 up to a 0.5 M concentration of the radical. Furthermore, the 7Li contact shift in this system is found to be the same for 0.01 and 0.1 M LiC104 solutions, as seen in Figure 3. This is evidence that the radical does not influence any equilibrium between ion pairs in the LiC104/THF system; i.e., contact ion pairs do not exist in the LiC104/THF system. Therefore, we assume that the Li+ cation in THF is fully solvated by solvent molecules. This is not so for LiClO., in CH3CN (DN = 14.1,13 E = 36.716). The 7Li chemical shift for LiC10, in CH,CN is dependent on salt concentration,6 indicating that contact ion pairs are present in the solutions. The 7Li chemical shift for LiCIOI in CH3N02(DN (14)P. Jagodzinski and S. Petrucci, J . Phys. Chem. 78,917 (1974). (15)H.Faber and S. Petrucci, J.Phys. Chem., 79, 1221 (1975). (16)J. E. Gordon, 'The Organic Chemistry of Electrolyte Solutions", Wiley, New York, 1975.
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The Journal of Physical Chemistty, Vol. 87, No. 23, 1983
Kot'odziejski and Maciel
6.
6, PPm
.
ppm
I
-1.5
mole/ dm3
.I
Flgure 4. Plot of the 'H NMR contact shift at 301 K vs. Tempo concentration for CH&N (a), CH3N02(O),and THF (+, (uCH,, upper line; 0, 0-CH,, lower line).
.2
.3
9
.'5
[R],
Flgure 6. Plot of the I7O NMR contact shift of THF vs. Tempo concentration for THF solutions at 301 K.
4000 ""I!'
HZ
Flgure 7. Plot of the dependence of the 7Li NMR half-width on Tempo system for 0.1 M LICIO, concentration in the LiCI0,-CH,NO,-Tempo at 298 K. .2
.I
.3
.4
.5
[Ri
mole/dm
Flgure 5. Plot of the 13C NMR contact shift at 301 K vs. Tempo concentration for CH3CN (0, CH,; A,CN), CH3N02(O),and THF (+, (u-CH~;0, PGH,).
= 2.7,13 E = 35.916) has the strongest dependence on salt concentration; and contact ion pairing in this case is evident.6 It is possible to estimate the strength of the solventTempo interactions on the basis of contact shifts of the nuclei of solvent molecules. This can be seen in Figures 4-6. These shifts are smaller in magnitude than the 7Li contact shifts in corresponding solutions (Figures 1 and 2). This is especially evident for protons. Unfortunately, we cannot make the comparison for CH3N02,because in this case the 7LiNMR signal is very broad in the presence of the radical and its contact shift cannot be measured with suitable accuracy. One should also take note of the fact that the same contact shifts in 7Li, 13C,and 170resonances would correspond to considerably smaller spin densities on outer s orbitals of carbon and oxygen atoms than in the case of lithium; the reason is that the contact shift (6,) is proportional to the spin density ( p ) in the outer s orbital & by the relation17
6,
-
I4B(O)I2P
where 1q5,(0)12 equals 0.318 (Le., 7r-l) for hydrogen, 0.145 for (17)B. R. McGarvey and R. J. Kurland, in 'NMR of Paramagnetic Molecules", G. N. LaMar, W. Dew. Horrocks,Jr., and R. H. Holm,Eds., Academic Press, New York, 1973,p 558.
lithium, 2.767 for carbon, and 7.638 for oxygen. Hence, due to the small magnitude of proton contact shifts and because of the reason given on the basis of the above equation, the 'H, 13C,and 170contact shifts observed in solvent molecules reveal only relatively small spin density transfer from Tempo radicals. Consequently, the present data point out that the interactions between Tempo and the solvents used are considerably weaker than Tempo-Li+ and solvent-Li+ interactions. It is worth commenting on the reasons for the character of the lH, 13C,and 170contact shifts in the three solvents represented in Figures 4-6. Upfield proton contact shifts and downfield 13C contact shifts in CH3N02and CH3CN agree well with a spin polarization mechanism advanced by Morishima et al.ls The reason for downfield proton shifts in THF are not clear. The observed proton shifts are rather small and can be obscured considerably by experimental aberrations, e.g., small changes of C6D6internal lock position caused by the presence of the radical. The 13Ccontacts shifts for aliphatic carbon atoms in ethers and hydrocarbons have been observed and explained by collisional interactions.1i20 This explanation seems to be appropriate in the case of THF. The 1 7 0 contact shift in THF may be due in part to spin density transferred from the adjacent carbon atoms, similar to the effect found for carbon atoms that cannot be directly en(18)(a) I. Morishima, K. Endo, and T. Yonezawa, Chem. Phys. Lett., 9,203 (1971);(b) i. Morishima, K, Endo, and T. Yonezawa, J . Chem. Phys., 68,3146 (1973).
(19)W. Koyodziejski, Chem. Phys. Lett., 78,586 (1981). (20) W. Koyodziejski, Ber. Bunsenges. Phys. Chem., 85, 70 (1981). (21)D. Draney and C. Kingsbury, J.Am. Chem. SOC.,103,1041(1981).
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LiCi0,-Solvent-Nitroxide AV,,, 3000--
Radical Systems
The Journal of Physical Chemistry, Vol. 87, No. 23, 1983 4697
Hz
2000
2500.: I500
-5
vol.%H,O 2
4
.6
8
IO
Figure 10. Plot of the 'LI NMR contact shift vs. volume percent of water added to the LICI0,-THF-Tempo system for 0.1 M LiCIO, and 0.54 M Tempo at 298 K.
measure. In CH3CN, 7Li NMR line widths are 1 order of magnitude smaller and for THF 2 orders of magnitude .05 .IO .I5 .20 [Rl0 smaller than in CH3N02,for corresponding radical concentrations. These large variations in line width are too Figure 8. Plot of the dependence of the 'Li NMR haif-width on Tempo concentration in the LiCi0,-CH,CN-Tempo system for 0.1 M LiCiO, big to be explained by viscosity differences among the at various temperatures. solvents. Paramagnetic relaxation is undoubtedly most intense if the Tempo radical is close to the Li+ cation, i.e., in a Li+-Tempo complex. Formation of such a complex probably depends on the extent of contact ion pairing and on the solvation properties of the solvent, which competes with the Tempo radical. Furthermore, an electrostatic attraction of the polar Tempo molecule by a solvated Li+ cation is likely to be stronger than by a solvated contact ion pair. If the extent of contact ion pairing were the dominant influence on Li+-Tempo encounters, then the probability of some degree of Li+-Tempo contact (and consequently paramagnetic broadening) should decrease in the order THF > CH3CN > CH3N02,which corresponds to the increasing tendency from THF to CH3CN to CH3N02for the formation of contact ion pairs in these solvents; this order is opposite to what is found for the line widths. On the other hand, 7Li contact shifts (Figures 1 and 2) are higher in CH3CN than in THF, despite the Y mole dm3 increasing tendency from THF to CH,CN for the formation of contact ion pairs. This confirms an easier approach .I .2 .3 9 .5 [q0 of the nitroxide radical toward the Li+ cation in CH3CN Figure 9. Plot of the dependence of the 'Li NMR half-width on Tempo that in THF. The above considerations suggest that the concentration in the LiCI0,-THF-Tempo system for 0.1 M LiCIO, at 7Li line widths and contact shifts reflect different accesvarious temperatures. sibility to the Li+ cation of the Tempo radical, controlled by the solvation properties of the solvent. A higher gaged in the collisional interaction with the r a d i ~ a 1 . l ~ ~ mainly ~~ solvating ability of the solvent should cause more difficult Comparison of 13C contact shifts suggests that THF inaccess of the radical toward the Li+ cation and a decrease teracts less strongly with the radical than do CH3CN and in paramagnetic line broadening or contact shift. Thus, CH3N02. qualitatively the 7Li NMR line widths and contact shifts 7LiNMR Half- Widths and Contact Shifts. The most allow one to conclude that the solvating ability toward the striking features of the systems studied here are the huge lithium cation increases in the series CH3NOZ< CH3CN differences in 7Li half-widths for solutions of LiC104and < THF. This order agrees qualitatively with solvent donor Tempo in different solvents, as seen in Figures 7-9. These numbers (2.7, 14.1, and 20.0, respectively) for this series differences cannot be explained by changes in the quadof solvents. rupolar contribution to the 7Li line width. A magnitude It has also been found that a small addition of water into of the quadrupolar contribution can be estimated from the LiC104-THF-Tempo system causes a sharp decrease data on a 0.1 M LiC104 solution in CH3N02without the in the 7Li contact shift, as shown in Figure 10. This fact radical. In this case the quadrupolar relaxation is a domconfirms the suggestion of strong preferential solvation of inant factor because of considerable contact ion pairing, Li+ by water advanced earlier.1° Any direct study of 7Li but the 7Li resonance is quite narrow (Avl,,, = 2.9 f 0.2 contact shifts in solutions of LiC104 and Tempo in water Hz). Any interference of the nitroxide radical with the is not practicable because of the very low solubility of the existence of the contact ion pair would only decrease this radical in water. value, because an electrostatic perturbation by the radical Another interesting phenomenon is an increase of the should be less intense than that exerted by the C104-anion. 7Li contact shift in THF and a decrease in CH3CN when Therefore, the observed 7Li line width changes with varthe temperature is increased. The contact shift of a comiation of the Tempo concentration have to be discussed plex is known to be proportional to the inverse absolute in terms of paramagnetic dipolar and paramagnetic scalar temperature and to the mole fraction of the complex.22 In relaxation. For CH3N02as solvent, the paramagnetic broadening (22) E. DeBoer and C. MacLean, J. Chem. Phys., 44, 1334 (1966). is largest, and therefore contact shifts are difficult to
4698
The Journal of Physical Chemistry, Vol. 87, No. 23, 1983
KoYodziejski and Maciel AT,-!
100
:
:
-I\\
75
3 2 3.4 3 6 3 8 40T':IO3
50
Figure 11. Graphic representatlon of the data corresponding to eq 2 for the LiCI0,-THF-Tempo system.
THF the mole fraction of lithium in the form of the Li+-radical complex apparently increases with increasing temperature, similar to the effect found in the NaC104THF-Tempo system.2 The increased mole fraction of the complex gives an averaged contact shift of larger magnitude with higher temperature, in spite of the decrease that would be observed for a pure paramagnetic species. According to the above considerations, CH3CN is a weaker donor than THF, and, if it is also a weaker donor than Tempo, and Tempo in turn weaker than THF, then the observed temperature effects could be explained as follows. A replacement of CH3CN by Tempo in the solvation shell of the lithium cation should be an exothermic process, so the mole fraction of Li+ in the complex would decrease as the temperature is increased, leading to lower contact shifts. The reverse temperature trend would be observed in the THF case. This type of interpretation is straightforward only when a Tempo radical replaces the same number of THF or CH3CN molecules. THF and CH3CN molecules do not differ much in size, and spectroscopic method^^^?^^ give the same solvation number of 4 for Li+ in both these solvents. Therefore, the explanation is resonable. Enthalpy of Formation of the Li+-Tempo Complex. Formation of a 1:l complex can be expressed by the equlibrium K'
Lis4++ R LiS,_,R+ + nS (1) where S represents a solvent molecule. A clear similarity between the behavior of 23Nacontact shifts in the NaClod-THF-Tempo system2 and the 7Li contact shifts in the LiC10,-THF-Tempo system suggests that the Li+ data can be analyzed on the basis of eq 2, used previously in In (-K6,T) = - ( A H f / R ) T ' + C (2) the 23Nastudies.2 This equation is valid for 1:l complex formation, if the condition K[Rl0
