NOTES
3663
constant for a t least 20 min. The values of E" for the cell reaction were obtained at several temperatures and are listed in Table I. Each value for the E" is the average of at least five and in Some cases six measure ments of the e.m.f. at known oxide ion concentrations.
E o (v.)
= 0.7759
- 2.557 X
lO-*T 530OK.
< T < 639%
With these additional data the oxygen electrode is Sufficiently defined to be a Useful electrode for electrochemical studies in alkali nitrate solvents. (6)R. N. Kust, Inorg. Chem., 3 , 1036 (1964).
Table I: Comparison of the Eo Values for the Oxygen Electrode Obtained by Chemical and Coulometric Methods EO,
v.
Ea, V.
Temp., OK.
(chemical)
(ooulometric)
536 543.5 545 553 565 578 585 589 595 602 610 616 621 630 639
0.6390 0.6370 0.6367 0.6346 0.6314 0.6280 0.6260 0.6254 0.6236 0.6221 0.6200 0.6182 0.6171 0.6151 0.6128
0.6388 0.6369 0.6365 0.6345 0.6314 0.6281 0.6263 0.6253 0.6238 0.6220 0.6200 0.6185 0.6173 0.6150 0.6127
Discussion The chemical addition of oxide ion to an equimolar sodium-potassium nitrate melt is complicated by the insolubility of most metallic oxides. Also, the introduction of cations different from the solvent cations would lead to possible complex formation, the formation constants of which would be unknown. Hence, one is limited to either NaO or KzO as a source of oxide ion. It is very diflicult to prepare either of these oxides so that they are free from peroxide and superoxide contaminants. However, it has been shown that sodium carbonate has an unusually large dissociation constant in this solvent in the temperature range of i n t e r e ~ t . ~ The dissociation constant for reaction 2 is on the order of at 300". Thus the addition of Na&O, to the solvent and the subsequent removal of the COZ produced is equivalent to the addition of NazO directly. Also, no contamination by peroxides or superoxides is likely to occur. A comparison of the E" values obtained in this manner to the values obtained by the coulometric generation of oxide ion is given in Table I. I n every case the difference between the two values is 0.3 mv. or less. The E o can be calculated for any temperature between 530 and 639°K. from the equation
Aluminum-27 Nuclear Magnetic Resonance
of Trialkylaluminum Compounds. 11. Variable-Temperature Studies
by Charles P. Poole, Jr., Harold E. Swift, and John F. Iteel, Jr. Gulf Reeearch & Developntent Company, Pittsburgh, Pennsylvania (Received May 10, 1966)
I n a previous publication' several aluminum alkyl compounds were studied by aluminum-27 n.m.r. both in the pure state and dissolved in various solvents. In low-viscosity solvents the line width was found to be proportional to the viscosity times the cube of the molecular radius, and the dominant relaxation mechanism in these solvents was attributed to quadrupolar relaxation through molecular rotation. In high-viscosity solvents the line width became much less dependent on the viscosity. The present study employed variable-temperature techniques to obtain the temperature dependence of the line width of pure aluminum alkyls and mixtures of triethylaluminum in solution.
Experimental Section The n.m.r. measurements were made on a Varian V-4200-A wide-line n.m.r. spectrometer equipped with a V-4257 variable-temperature accessory. The experimental arrangement and spectrometer settings were identical with those employed in the room-temperature studies of these same chemical systems.' The temperature was monitored by a thermocouple located below the sample, and a correction was made for the temperature difference between the thermocouple position and the actual sample location. The sample tubes used in the variable-temperature studies had inside diameters of 8 mm., whereas the tubes used for the room-temperature studies had inside (1) C. P.Poole, Jr., H.E.Swift, and J. I?. Itael, Jr., J . Chem. Phys., 42,2676 (1966).
Volume 69, Number 10 Odober 1966
3664
NOTES
diameters of 12 mm. The decrease in the diameter of the tube resulted in a decrease in sensitivity. The sources of the aluminum alkyls were also previously reported.'
1.6
, I
Results and Discussion The peak-to-peak full line widths of the first derivative n.m.r. spectra obtained from pure triethylaluminum (TEA), tri-n-propylaluminum (TNPA), and triisobutylaluminum (TIBA) varied with temperature in the manner shown on Figure 1. Triethylaluminum was dissolved in the four hydrocarbon solvents: isopentane, hexane, cyclohexane, and hexadecane (cetane) ; and the line widths of the n.m.r. spectra from these solutions varied with temperature in accordance with Figure 2. Each of these two figures is drawn to the same scale and has the logarithm of the line width AH as the ordinate. The data for each system fit straight lines which are almost parallel. One should note that the slopes of the lines in Figure 1 are greater than those in Figure 2. The lowest temperature triisobutylaluininum point in Figure 1 and the lowest temperature hexadecane point in Figure 2 are considerably above their corresponding lines. These two points were obtained from broad weak resonances where the experimental error is large. The resonances of tri-n-butylaluminum and tri-n-hexylaluminum were too broad to furnish meaningful line widths. All of the spectra recorded in this study contained a single Lorentzian-shaped resonance. Figures 1 and 2 show that the data fit the relationship2
AH =
mQeAEm/RT
(1)
2.5
30
71 =
w eA E v w / R T
I "
I.-
08
I
I
2.5
A
3.0
3.5
The J o u Tof~Physical ~ ~ Chemistry
4.O
4.5
O K
Figure 2. The temperature dependence of the peak-to-peak full line width ( A H ) of triethylaluminum ( 2 ml.) in variom solvents (2 ml.): A, hexadecane; 0, cyclohexane; 0, hexane; and X, isopentane.
ture dependence of the line widths of pure TEA is the same as the exponential temperature dependence of the viscosity of TEA resulting in the same values of AEnmr and AEvi,. This agreement for AE,,, and AEvis does not hold for pure TIBA and the solutions of TEA. O'Reilly and Schacher found AE,i, to be consistently greater than the a,,, values obtained from C135n.m.r. line widths. If eq. 1is divided by eq. 2, then one obtains
(2)
and 710 are listed in Table I. VisThe values of cosity data were not available for tri-n-propylaluminum. It is interesting to note that the exponential tempera-
4.5
O K
Figure 1. The temperature dependence of the peak-to-peak full line width ( A H ) of pure triethylaluminum ( x ), tri-n-propylaluminum (0), and trikobutylaluminum (A) a t 7.2 Me.
IOOO/T,
where R is the gas constant, AE,,, is the activation energy for nuclear relaxation, and AH0 is a constant. From the data in Figures 1 and 2 the activation energy (LEnmr)and pre-exponential constant (AH,) were calculated, and the results are listed in Table I. The three pure aluminum alkyls have the same activation energy (3.4 kcal./mole) while each of the triethylaluminum solutions have an activation energy of about two-t hirds of this value. The logarithms of the viscosities, 7, of the aluminum alkyls and solvents were plotted against the reciprocal of the absolute temperature, and the resulting straight lines were used to calculate MVisand the pre-exponential viscosity constants 71, using the equation2
4.0
3.5 IOOO/T.
fils =
A H , / ~ , ~ AE A~ E~v i s~/ R T
(3)
(2) D. E. O'Reilly and G. E . Schacher, J . Chem. Phys., 39, 1768
(1963).
NOTES
3665
From Table I one can see that A E n m r = A E v i a for pure TEL4which means that AH/q equals AHo/qoand, therefore, is independent of temperature. For the other pure aluminum alkyls and TEA solutions AE,,, # AE,i, making the ratio A H / q temperature dependent. Triethylaluminum, in isopentane and hexane, has AE,,, > AE,i, and so for it the ratio AH/v decreases with increasing temperature, whereas, pure TIBA and TEA in cyclohexane have AE,i, > AE,,, and a ratio AH/? which increases with increasing temperature.
Table I: Values of M,,, aHo,
Aluminum irlkyl solute
Triethylaluminuma Tri-n-propylaluminum Triisobutylaluminuma Triethylaluminum Triethylaluminum Triethylaluminum Triethylaluminum
At high viscosities (large line widths), both the roomtemperature1 and variable-temperature results (Figure 1) indicate a breakdown in the simple Debye theory. The monomeric character of TIBA may be the reason why its values of AE,is and yo deviate from the others shown in Table I. We have shown that both A E n m r and AEvi, are nearly the same for TEA in isopentane and n-hexane. Therefore, these two systems also have nearly the same value for the ratio AHo/va3 as determined from eq.
and 90 for Several Aluminum Alkyls and Hydrocarbons Concn.
...
... ... b b b b
Hydrocarbon solvent
AE-,
None None None Isopentane n-Hexane Cyclohexane Hexadecane
kcal./mole
3 . 3 i0 . 2 3.4 3.4 2.0 2.0 2.4 2.4
AHQ,
AEvisr
gauss X IO*
0.80 3.5 2.3 4.6 4.7 3.3 4.4
kcal./mole
3.3
rlo,
cp. X 108
1.5
...
...
7.0 1.6“ 1.6“ 3.0“ 2.4“
0.002 1.3c 2.0c 0.13” 2.65”
+
a Viscosity data furnished by Texas Alkyls Inc. b 2 ml. of alkyl 2 ml. of hydrocarbon solvent. Values apply to the hydrocarbons and not to the alkyl-hydrocarbon mixtures. Viscosity data for the hydrocarbons were obtained from the “American Institute of Physics Handbook” and viscosity data for hexadecane were obtained from “Selected Values of Physical and Thermodynamic Properties of Hydrocarbons and Related Compounds,” American Petroleum Institute, Carnegie Press, Pittsburgh, Pa., 1953.
It was previously shown that at room temperature the ratio AH/qu3 remained constant for several trialkylaluminum compounds (TEA, TNPA, TNBA, and tri-n-hexylaluminum) dissolved in isopentane and normal hexane, where a is the effective radius of the trialkylaluminum compounds. It was also found that the ratio AH/qas w&s less for TEA dissolved in cyclohexane and hexadecane than for TEA dissolved in isopentane and normal hexane. These room-temperature studies indicated that at low viscosities the Debye relation for the correlation time T~
(4) is a good approximation in these systems. The general correspondencebetween the n.m.r. and viscosity results shown on Table I for the n-trialkylaluminum compounds supports the assumed approximate proportionality between 7 and rc. The C4 and lower normal aluminum alkyls are predominantly dimeric3 and participate in an alkyl exchange proce~s.l*~ The effect Of the temperature dependence Of this exchange process on the activation energies and pre-exponential constants may he Similar for all these compounds.
1 and 2. The ratio AH/71a3 calculated from eq. 3 (including the a value) at T = 300°K. is over three times as great for TEA in cyclohexane as it is for TEA in isopentane or normal hexane. Thus if the activation energy and pre-exponential term are taken into account, the AH/va3 ratio for TEA in cyclohexane is too large, while if these terms are ignored as was previously done, the ratio is too small. In other words the change in activation energy from one system to another appears to be partially compensated by changes in AHo and qo so that the over-all effect on the ratio AH/ qaS is minimized. Values of AH/va3 for the various systems previously reported obtained at various temperatures would give more information about the linebroadening mechanism. A more accurate explanation of the results presented in this paper would take into account the viscosities of the mixtures instead of merely the solvent vi~cosities.~
(3) G. E. Coates, “Organ&Metallin Compounda,” John Wiley and Sons, Inn., New York, N. Y.,1956,p. 132. (4) N. Muller and D. E. Pritchard, J . Am. C h . Soc., 82, 248 (1960). (5) R.W. Mitchell and M. Eianer, J . Chem. Phye., 33,86 (1960).
Volume 69, Number 10 October 1966
