NOTES
1151
are 46.5 and 187.8 cps, respectively. Both these calculated Avid values are for nonequivalent fluorine nuclei and no observed or calculated values have been reported for protons. Indeed, it has been suggested that for protons Avid will be close to zero.ld We wish to report the direct observation of a hightemperature limit of Avtotal for the diastereotopic benzylic protons in benzylmethylphenylmethoxysilane (1). Ph
I
CH30-Si-CHzPh
I CHs
I A norganosilicon compound was chosen for this study since because of the comparatively large covalent radius of silieon it was hoped that the limit of equal conformer population would be approached at a relatively lower temperature than for a carbon compound. The pmr spectra of I were recorded on a Varian Associates HAlOO spectrometer.3 At temperatures below 100" a clear AB quartet was obtained for the benzylic protons (J = 14.0 f 0.1 cps). At temperatures above 100" the central lines of the quartet began to overlap and this together with a decrease in resolution at higher temperatures made it difficult to determine accurately the positions of the central doublet. However, since the position of the outer lines was not masked by overlap and the decrease in resolution, the chemical shift values were calculated from the outer line positions and J = 14.0 cps.
Table I: Variation of Temp, OC
-70 -50 -30 - 10 10 30
40 50 60
70 80 80 90 100 120 140 160 170 180 190
200
AVtotal
with Temperature
Solvent Toluene Toluene Toluene Toluene Toluene Toluene Neat Toluene Neat Toluene Neat Toluene Toluene Neat Neat Neat Neat Neat Neat Neat Neat
AYtotali CPB
17.7 15.4 13.7 12.3 11.2 10.3 10.0 9.6 9.2 9.1 8.7 8.5 8.2
8.0 8.0 7.7 7.3 7.1 7.1 6.9 7.1
The results obtained are shown in Table I. As the temperature increases from -70" the rate of change of Avtotal decreases until finally the five values obtained between 160 and 200' are constant within experimental error. This value of 7.1 f 0.2 cps should represent the chemical shift difference when conformer populations are approximately equal and thus may be assigned to the intrinsic diastereotopic nonequivalence of the benzylic protons of I. Since the maximum value observed for Avtotal was under 20 cps, at least in I the contribution of Avid to Avtotal is a major one over the whole range of temperatures studied.
Acknowledgment. We thank the Petroleum Research Fund and the National Science Foundation for support of this research, and Mr. D. Gifford for providing the HAlOO spectra. (3) The variable temperature probe was calibrated using an ironconstantan thermocouple, and line positions were determined using an electronic counter in conjunction with a variable oscillator.
Ion-Solvent Interactions. Relation to Solvent Dielectric Constant1 by C . N. Hammonds and M. C. Day Department of Chemistry, Louisiana State University, Baton Rouge, Louisiana '70803 (Received November I d , 1 9 6 8 )
The importance of solvent dielectric constant on ionic conductance has long been recognized and was well demonstrated by the classic studies of Fuoss and Kraus on the conductivity of N (iamyl)4N03 in dioxane-water rnixtures,O but at the same time, specific ion-solvent interactions undoubtedly can play an important role. In some instances they may be more significant than the solvent dielectric constant. Gilkerson has clearly shown the importance of such interactions on the ionpair equilibrium ~ o n s t a n t ,and ~ examples of ion-size effects in which the ion must be treated as a solvated entity in order to correlate data are quite c0mmon.~-6 A particularly good example of this latter case can be seen from the study of Griffiths and Scarrow on the ease of formation of the NiBr40- complex i0n.7 They propose that the Li+ ion in acetone must be a larger entity than the (octyl3propyl)N+ ion. (1) Reprint requests should be sent to M. C. Day. (2) R. M. Fuoss and C. A. Kraus, J . Amer. Chem. Soc., 5 5 , 21 (1933). (3)W.R. Gilkerson, J . Chem. Phys., 2 5 , 1199 (1956). (4) F. Accascina, A. D'Aprano, and R. Triolo, J . Phys. Chem., 71, 3469 (1967). ( 5 ) D. Nicholls, C. Sutphen, and M. Szwarc, ibid., 72, 1021 (1968). (6) J. B. Ezell, and W . R. Gilkerson, ibtd., 72, 144 (1968). (7) T. R. Grifflths and R. K. Scarrow, "International Conference
on Non-Aqueous Solvent Chemistry," Hamilton, Ont., Canada, June 1967. Volume 75, Number 4 April 1969
NOTES
1152 Table I: Equivalent Conductance of NaAlBu4-THF Solutions as a Function of Salt Concentration
A, mho om2
0.05216
0.09525
0.1145
19.62
21 .os
21.03
In spite of an awareness of the importance of tthese two quantities on ionic systems, the separation of the two effects has proven difficult. This is primarily because the usual procedures for altering the solvent dielectric constant also result in a change in the extent of ion-solvent interaction. Consequently, there is considerable interest in the independent correlation of the two effects with changes in conductance and, more importantly, with reaction rates which are observed to occur in going froni one solvent to another. It would now seem that the solubility of sodium tetrabutylaluminate (NaAlBu4) in saturated hydrocarbon solvents offers a somewhat unique approach to the problem. Nmr studies of the complexation of the Na+ ion by tetrahydrofuran (THF) using cyclohexane as a solvent have shown that a very stable 1:l complex exists with three additional molecules of THF in equilibrium with the 1 : l complex.8 In the same manner, it has been shown that there is no significant interaction between the NaAlBu4 and the hydrocarbon solvent. The solvent can thus be considered to be essentially a dispersing medium for the salt.* A knowledge of the nature of the T H F complex with the Na+ ion permits us to interpret the conductance curves of the system NaA1Bu4-cyclohexane-THF in a manner that indicates it is possible to distinguish between a specific ion-solvent interaction and a solvent dielectric constant effect. We wish to report here our studies on this system.
Experimental Section The preparation of NaAIBur and the solvents has been reported earlier.8v9 The conductance measurements were made in conventional cells. Cell constants were determined with aqueous KC1 solutions. For the different cells, these ranged between 0.01042 and 1.2238 cm-l. The cells were filled in a nitrogen drybox and transferred to a constant temperature bath controlled a t 25.00 f 0.05O and containing a mineral oil, marcol. Conductance measurements were made with a Leeds and Xorthrup Co. Model 4666 Jones modified conductivity bridge and an Industrial Instruments Inc. RC16 conductivity bridge. The RC16 bridge was used for resistance measurements in the range of 1 X lo5 to 2.5 X lo6 ohms. Solution and solvent densities were obtained with pycnometers calibrated with distilled water. Viscosities were measured with Cannon-Fenske viscosimeters calibrated with appropriate pure organic solvents. The Journal of Physical Chemistry
21.20
-
~-
Concn of NaAlBuc, M 0.1674 0.2094
20.26
0.2881
0.3348
0.4687
19.46
17.91
15.57
All sample preparations and solution transfers were carried out in a nitrogen drybox. A circulating system passed the drybox atmosphere through a copper oven to remove oxygen and through 3A molecular sieves and a Dry Ice trap to remove moisture and organic vapors.
Results and Discussion In Figure 1, the equivalent conductances of several different concentrations of NaA1Bu4 are shown as a function of the ratio of THF:salt using cyclohexane as a solvent. In all cases a peak occurs at a ratio of approximately 1 : l followed by a drop and short plateau. Analogous studies were made a t salt con-
0
1
2
3
4
5
6
Ratio T H F : NaAlBu, Pigure 1. Equivalent conductance of NaAlBur in cyclohexane-THF mixtures as a function of the mole ratio THF:salt: 0 ,0.1525 M ; . , 0.09524 M ; A,0.05213 M NaAlBur.
centrations of 0.209, 0.265, 0.288, and 0.468 M . Although different shapes of the peak and plateau regions occur, as well as considerably greater equivalent conductances, indicating possibly a different conductance mechanism at higher concentrations, the inflection point and rise in conductance are in general accord with those shown here. At 25O, the dielectric constant of T H F is 7.39 and that of cyclohexane is 2.05. Thus the addition of T H F would be expected to increase the solvent dielectric (8)E. Schaschel and M. 0.Day, J. Arner. Chem. Soc., 90, 503 (1968). (9) M. C. Day, H. M . Barnes, and A. J. Cox, J. Phys. Chem., 68, 2595 (1964).
NOTES
1153
constant, However, because of the stability of the 1:1 complex, it can be assumed that there is essentially no free T H F up to the 1 : l ratio. Consequently, the increase in equivalent conductance up to this point cannot be attributed to an increase in solvent dielectric constant. Rather it is the result of the formation of a stable 1:1 complex between the Na+ ion and the THF. The decrease in equivalent conductance after the 1:1 ratio may be attributed to a decrease in mobility of the Na+ ion arising from additional complexation. A rapid rise in equivalent conductance begins at or just prior to a 4: 1 ratio of T H F :salt and in all cases it rises to very nearly the same maximum in pure THF, as can be seen from the concentration dependent studies shown in Table I. In terms of the proposed equilibrium [Na.THF]+
+ 3THF
The increase in equivalent conductance beginning at approximately the 4: 1 ratio may then be attributed to an increase in solvent dielectric constant. However, in this region the separation of the two effects is not straightforward since the ion aggregates include the solvated rather than the free S a + ion, and a separation of the effects will require an analogous study as a function of cation size using sufliciently large cations that ion-solvent interaction can be neglected. Acknowledgment. Support of this work by National Science Foundation Grant G P 6421 and a National Science Foundation Science Faculty Fellowship for C. N. Hammonds is gratefully acknowledged.
[h’a*4THF]+
in which a relatively stable 4: 1 complex is formed, the rise in equivalent conductance after the 4: 1 ratio would be expected because of the increase in solvent dielectric constant corresponding to the increase in free T H F in the bulk solvent. In Figure 2, the equivalent conductance of the 0.1525 Jf salt solution is extrapolated to that of pure T H F as solvent. An increase is seen relative to the maximum at the 1 : 1 ratio of a factor of approximately 300. Qualitatively, ion-solvent interaction may here be interpreted in terms of an ion-size effect. The stability of the ion aggregates will decrease with an increase in the effective cation size. This will result in an increase in the number of charge carriers, but complexation by a second T H F molecule leads to a decrease in cation mobility. Thus the conductance behavior prior to the 4 : l ratio of THF:salt can be considered primarily in terms of ion size rather than solvent dielectric constant.
/‘ T /
Ah8316
AE.5.3
n
Ratio THF: NaAIBuS Figure 2. Comparison of the equivalent conductance of THF:NaAlBud a t low mole ratios to the equivalent conductance of NaA1Bu4 in pure THF.
Nuclear Magnetic Resonance Study of Concentrated Lithium Chloride Solutions by Robert G. Bryant Department of Chemistry, Stanford University, Stanford, California 94305 (Received J u n e $1, 1 9 6 8 )
Nuclear magnetic resonance has made significant contributions to the study of aqueous solutions containing electrolytes. While much of the work has employed proton resonance to observe changes in the solvent, an increasing amount of information is coming from the nuclear resonance of the solute Most work thus far has focused on the more dilute solutions and the data analysis has drawn heavily on models appropriate for the limit of infinite dilution. Although the theoretical treatments of more concentrated solutions are less precise, nuclear magnetic resonance measurements in concentrated solutions reflect the environment of the nucleus observed and thus provide useful information about the structure of the solution. Since aqueous lithium chloride solutions may be made exceedingly concentrated, measurements of the ?Li+ and Wl- resonances were made as a function of lithium chloride concentration to look for unusual behavior of the solute resonances a t high concentrations. ?Li and aKC1both possess a nuclear spin quantum number of $ and nuclear quadrupole moments of 0.1 and -0.079, respectively, so that the nuclear magnetic relaxation for both is usually dominated by the interaction of the nuclear quadrupole moment with random (1) J. F. Hinton and E. S. Amis, Chem. R e v . , 67, 367 (1967). (2) R. A. Craig and R. E. Richards, Trans. Faraday Soc., 59, 1972 (1963).
(3) B. F. Fabricand and 9. 9. Goldberg, Mol. Phys., 13, 323 (1967). ( 4 ) P. A. Speight and R. L. Armstrong, Can. J . Phys., 45, 2493 (1967). ( 5 ) D. E. Woesner. B. 8. Snowden, Jr., and A. G. Ostroff, J. Chem. Phys., 49, 371 (1968). Volume 75,Number 4 April 1989
