NOTES
1573
anisotropies has not been experimentally determined, but theoretical calculations suggest a ratio of about 0.75. The present calculations were made for anisotropy ratios of 1.00, 0.75, 0.50, 0.25, and 0. Numerical integration was performed for free rotation of the methyl protons around the carbon-methyl bond. The geometry of the substituted cyclohexanes in this study is such that both chair conformations are identical. Therefore, it is reasonable to compare the chemical shift observed at room temperature with the averaged value of the calculated results for axial and equatorial substituents. Only with trans-1,3-dimethylcyclohexane, which shows separate peaks for axial and equatorial methyl groups at low temperature, can calculated and experimental results be compared for the individual conformatioiis. In Fig. 1 the observed
unique as the only methylcyclohexane with an observable chemical shift between axial and equatorial methyl groups. The reason for this is not obvious. Greater resolution a t low temperature than previously reported2 has allowed calculation of chair-chair inversion rates from peak coale~cence.~~ For the trans-l,3 isomer AH* = 4.8 i= 0.1 kcal./mole and log A = 6.7. The chemical shift of the methine protons as a function of temperature was obtained by proton double resonance techniques. Little change with temperature was noted except for cis-l14-dimethylcyclohexanewhere T decreased from 8.32 a t 2 5' to 8.14 a t - 110'. Acknowledgment. The authors thank Prof. N. Allinger and Prof. R. Y. Levina for kindly supplying samples of 1,3-di-t-butylcyclohexaneand 1,1,3,3,5,5hexamethylcyclohexane. Mr. M. Bednas purified, by gas chromatography, most of the compounds used in this investigation.
-
!I-
(7) J. Guy and J . Tillieu, J . Chcm. P h y s . , 24, 1117 (1956). (8) J. Tillieu, Ann, phys. (Paris), 2, 471 (1957). (9) R. F. Zttrchen, Helv. Chim. Acta, 44, 1755 (1961). (10) A. G. Rlonitr and N. Sheppard, Mol. Phys., 5, 361 (1962). (11) 13. S. Gutowsky and C. H. Holm, J . Chem. Phys., 2 5 , 1228 (1956). (12) R. Kaiser, Rev. Sci. Instr., 31, 963 (1960).
/
yo
LL
::
-
0,901
E
2 /
+
Y
',
+
AXcH/AXcc=O (r v)
1ooL.---J-
-0080
-0040
*
*
T R A N S - I ,3- EQUATORIAL (-98%
A
X
0.50
0.25
'
I.oo
0.75
! +0.040
I
1
+o I20
+0080
G Figure 1. Observed and calculated methyl chemical shifts.
Measurement of Formal Oxidation-Reduction Potentials of Cerium(1V)-Cerium(II1) System in Acetonitrile
chemical shifts of the methyl protons are plotted against the calculated geometrical factor of the anisotropy effect, G, as a function of AxCH/AxCC. G = Z: (gcc -Iagca), where g = (1 - 3 cos2 0)/3r3 and a = AxcH/ Axcc. From the slope of the line for each value of' a, A x C C may be obtained. It is 3.8 X ~rn.~/rnoXe for a = 0 and even larger for cy > 0. When a is greater than about 0.25 the calculated values of Axcc become negative. This possibility seems unlikely from the results of theoretical788 and experimental studies. Therefore, other effects a t least comparable in magnitude to the effect of bond anisotropy must be influencing the chemical shift of the methyl groups. In addition to spectra of the compounds listed in Fig. 1, the spectra of 1,1,3,3,5,5-hexamethylcyclohexane and 1,3-frans-di-t-butylcyclohexanein carbon disulfide solution were observed between - 120 and 25'. Throughout this temperature range only a single peak was observed for the methyl groups in each compound. trans-1,3-Dimethylcyclohexane appears 3~6,9910
by G. Prabhakar Rao and A. R. Vasudeva Murthy Department of Inorganic and Physical Chemistry, I n d i a n Institute of Science, Bangalore 12, I n d i a (Received November 4, 1963)
It has been observed by the present authors that a solution of ammonium hexanitratocerate in acetonitrile functions as a versatile oxidant. Analytical procedures have been standardized in the case of hydroquinone,1,2 potassium iodide, ascorbic acidj3oxalic acid, xanthate,5 and ferrous salts6 dissolved in acetonitrile or a mixture (1) G. P. Rao and A. R. V. Murthy. 2 . anal. Chem., 180, 169 (1961). (2) G. P. Rao and A. R. V. Murthy, ihid., 182, 358 (1961). (3) G. P. Rao and A. R. V. Murthy, ibid., 187, 96 (1962). (4) G. P. Rao and A. R. 5'. Murthy. ibid.. 195, 406 (1963). (5) G. P. Rao and ,4. R. V. Murthy, ibid.. 177, 86 (1960). (6) G. P. Rao and A. R . V. Rlurthy, unpublished observations i n these laboratories.
Volume 68, Number 6 J u n e , 1964
NOTEB
1574
of acetonitrile and glacial acetic acid. Oxidationreduction indicators such as ferroin, diphenylamine, methyl red, and janus green function as useful indicators to register the end point in the titrations. Potentiometric titrations can easily be carried out in all these cases making use of bright platinum as the indicating electrode while glass or antimony electrodes could serve as reference electrodes. The interesting feature of this nonaqueous cerimetry is that there is no need for the presence of a strong mineral acid in such systems, and the cerate solutions in acetonitrile can be stored without deterioration for a considerably long period. It was, therefore, of interest to measure the oxidation-reduction potential of ceric-cerous system in nonaqueous medium employing the usual potentiometric method. In the present investigation, oxidation-reduction potentials have been measured employing silver-silver nitrate (0.01 M in acetonitrile) (Pleskov's reference electrode) and aqueous-saturated calomel electrodes as reference half-cells. The other half-cell consisted of ceric-cerous mixtures in acetonitrile. The formal oxidation-reduction potentials of these systems have been measured and are reported in this communication.
+
Ce(IV)(Cz) Ce(III)(C1) (in acetonitrile)
Pt (2)
1.23
I
Experimental Reagents and Apparatus. A standard solution of aninionium hexanitratocerate (0.025 M ) was prepared in acetonitrile and its exact strength was determined against a standard solution of hydroquinone taken in acetonitrile as described earlier.1*2 Solutions of cerous salt of known concentrations were obtained by reducing the ceric reagent with oxalic acid in acetonitrile, to prepare suitable ceric-cerous mixtures. Cerous perchlorate was prepared' and solutions of appropriate molarities were prepared in acetonitrile to study the formation of cerous complex. Silver-Silver Nitrate Cell in Acetonitrile. An allglass cell fitted with appropriate ground-glass joints similar in design to that described by BrittorP was found to be convenient to serve the present purpose. The capacity of the cell was limited to about 20 nil.; the cell was filled with 0.01 176 silver nitrate solution in acetonitrile. A pure silver wire on which a thin coating of fresh crystals of silver was electroplated was introduccd into the ceK9 Potentials were measured with a Leeds and Northrup potentiometer at 25 f 0.2" in an air thermostat. A Hilger Uvispeck H700 spectrophotometer was used for spectral measurenicnts of cerous coniplexes. Procedure. Mixtures containing both ceric and cerous salt in various proportions were put, in the tiThe Journal of Physical Chemistry
log
$
Figure I. Variation of oxidation-reduction potential with log Ce(IV)/Ce(III): cell 1, 0-0-0; cell 2, 0-0-0; cell 4, X-X-X. The scale of the left-hand ordinate refers to cell 1 and that a t the right-hand ordinate refers t o the cells 2 and 4. (7) L. J. Heidt and J. Berestecki, J . Am. Chem. Soc., 77,2049 (1955). ( 8 ) €1. T. S. Britton, "Hydrogen Ions," Chapmann and Hall, London, 1955, p. 35. (9) A. Findlay, "Experimental Physical Chemistry," Longmans. Green, and Co.. 1955, p. 254.
XOTES
1575
+
Nernst equation: E = Eo RT/nF log Ce(IV)/Ce(111). The formal oxidation-reduction potential of the system Ce(lV)-Ce(II1) was observed to be 0.755 3 0.002 v. us. Ag/AgN03 half-cell given by the point on the graph corresponding,to the ratio of oxidant to reductant equal to unity, i.e., log Ce(IV)/Ce(III) = 0. Oxidation-Reduction Potential agaznst Aqueous Xaturated Calomel Electrode. When a similar graph W R B drawn with measured potentials ( E ) us. aqueous saturated calomel electrode against log Ce(IT')/Ce(III) using cell 2, a straight line was obtained. The formal oxidation-reduction potential in this case was found to be 1.056 f 0.002 v. This value corresponds to 1.300 f 0.002 v. on the standard hydrogen scale (1.056 -t 0.244). For purposes of cornparison of the potential measurements made in nonaqueous medium with the measurements involving aqueous medium, it became necessary to measure the potential of the system silver-silver nitrate in acetonitrile with reference to a saturated calomel electrode. Hg, HgzCMs) saturated KCl
1
+
Ce (IV)(G) C e ( W (Cd in acetonitrile varying amounts of glacial acetic acid
+
Pt
(4)
KC1 bridge agar-agar AgN03(0.01 44 in aceton itrile)
1
Ag
(3)
The potential of this system was found to be 0.300 v. at 25". This value was found to be quite stable and reproducible, and compared favorab iy with the value reported by Adams, et al.1° It is possible to compute the formal oxidationreduction potential of the following system from the experimental values obtained using cells 1 and 3.
I
saturated KC1 H g l HgzC12(s)
KCl agar-agar bridge
+
Ce(IV) (C,) Ce(II1) ('21) in acetonitrile
Pt
This can easily be seen to be the algebraic sum of the potentials obtained in the two systems viz., 0.755 0.300 = 1.055 v. The same potential has been observed with cell 2. Thus, the computed formal potential is in very good agreement with the experimentally observed value, although the liquid-junction potential in these systems may not be the same. This value corresponds to 1.299 v. on the standard hydrogen scale. Oxidation-Reduction Potential in Acetonitrile and Acetzc A c i d M i x e d Solvents. While studying several oxidation-reduction reactions with cerate in acetonitrile with various types of reducing agents in the same solvent, it was found advantageous to add glacial
+
(10) R. C. Larson, R. T. Iwamoto, and R. N. Adams, Anal. Chim. Acta, 25, 371 (1961).
Volume 68, Number 6
June, 1964
NOTES
1576
observed that the peak around 264 nip is sensitive to acetic acid concentration. With the increase in the concentration of acetic acid, the intensity of this peak increases considerably, suggesting possible complex formation. It may be mentioned that these results are in general agreement with those reported by Benson and Sutcliffe,'l who have also observed complex formation between cerous ion and acetate in acetic acid media. It is, therefore, reasonable to expect a reduction in the concentration of cerous species and consequent increase in the oxidation-reduction potential of ceric-cerous system in the presence of acetic acid.
Acknowledgment. G. P. R . is indebted to the Unjversity Grants Commission of India for the award of a research fellowship, (11) D. Benson and L. H . Sutcliffe, T r a n s . Faraday Soc., 5 6 , 246 (1960).
Radiolysis of Methane-CI4
by William P. Hauser Radiation Research Laboratories, Mellon Institute, Pittsburgh, Pennsylvania (Received November 6 , 1063)
Within recent years the radiation chemistry of methane has received considerable attention. 2-12 Although some insight into the mechanism has been obtained, the role of the products as intermediates in the radiolysis of methane still remains a question. I n this respect the yield of ethylene in pure methane radiolysis and in the radiolysis of methane with added scavengers is of interest. Values for the yields of ethylene produced when pure methane is irradiated generally range from 0.004 to 0.3 molecule/100 e.v.4--7,11although recently yields have been reported which linearly increase from 0.3 to 3 molecules/100 e.v. as the pressure is reduced from 800 to -3 mm.l0 I n addition, a significant dose rate dependence of ethylene production has been observed at 5 nun. pressure.1° When methane is irradiated in the presence of added nitric oxide,6s11 oxygen,1° or unsaturated hydrocarbons,lo the ethylene yield is enhanced (G = 0.6-1.5 at initial pressures near 1 atm.). This suggests that when pure methane is irradiated, the ethylene produced participates in secondary reactions. The present study was undertaken to determine the initial yield of ethylene and the role of products in methane radiolysis. The radiolysis of pure methane T h e Journal of Physical Chemistry
labeled with carbon-14 was first examined a t low conversion. Methane-Cl4 was then irradiated in the presence of small amounts of added unlabeled ethylene in order to determine the initial yield of eth~1ene-C'~. (The unlabeled ethylene was added to protect from further reaction any e t h ~ 1 e n e - Cproduced ~~ during the radiolysis.) Unlabeled methane was also irradiated in the presence of added ethylene-CI4 to determine the fate of ethylene in methane radiolysis.
Experimental Research grade methane and ethylene were obtained from Phillips Petroleum Co. Methane was further purified by repeated sublimation from - 182 to - 196" and irradiation in the liquid phase with 2.8-Mev. electrons. After subsequent sublimation during which sizeable end fractions were discarded the sample was degassed on activated charcoal held a t - 196 ". Analysis by mass spectrometry and gas chromatography, using a molecular sieve column, indicated that impurities were less than 0.001% except for nitrogen which was less than 0.03%. Ethylene was passed through a trap a t -78" and was thoroughly degassed a t - 196". E t h ~ 1 e n e - C ' ~( E O . 1 mc./mmole) previously purified a t this laboratoryT3was used without further purification. Methane-C14 was obtained from Volk Radiochemical Co. (specific activity, 1 mc./ mmole) and from New England Nuclear Corporation (specific activity, 2 mc./mmole). The 1 mc./mmole sample was purified by irradiation with Coao y-rays to remove traces of oxygen. Similarly the 2 mc./ mmole sample was purified by irradiation with 2.8Mev. electrons; however, even after extensive irradiation carbon monoxide produced from oxygen impurity could not be eliminated in this sample. Samples were prepared for each experiment by expanding methaneCI4 from the storage bulb held a t - 196" into a known ~~~
(1) Supported, in part, by the U. S.Atomic Energy Commission. (2) L. H. Gevantman and R. R. Williams, Jr., J . P h y s . Chem., 5 6 , 569 (1952). ( 3 ) G. G. Meisels, W. H. Hamill, and R. R. Williams, Jr., ibid., 61, 1456 (1957). (4) F. W. Lampe, J . Am. Chem. Soe., 79, 1055 (1957). (5) K. Yang and P. J. Manno, ibid., 81, 3507 (1959). (6) G. J. Mains and A. S. Newton, J . P h y s . Chem., 6 5 , 212 (1961). (7) J. Maurin, J . C h i m . Phys!, 59, 15 (1962). (8) R. R. Williams, J r . , J . P h y s . Chem., 6 6 , 372 (1962). (9) P. J. Ausloos and S. G. Lias, J . Chem. P h y s . , 38, 2207 (1963). (10) R. W. Hummel, Discussions Faraday Soc., to be published. (11) L. W. Sieck and R. H . Johnsen, J . P h y s . Chem., 6 7 , 2281 (1963). (12) P. J. Ausloos, S. G. Lias, and R. Gorden, Jr., J . Chem. P h y s . , 39, 3341 (1963). (13) R. A. Holroyd and G. W. Klein, J . Am. Chem. Soc., 84, 4000 (1962).
