March, 19GO
NOTES
more rapidly with decreasing ro/a than for the cylinder as demonstrated in Table I1 (now A in eq. 3 is subst#ituted with V / 2 a ) . Also from the formal point of view method I is less troublesome than method 11. We conclude that the approximation method suggested2 is very useful for a plane sheet to get a great accuracy. It has formal advantages to the steady-state approximation method for a cylinder and also a great,er accuracy in the range indicated here. But it lhas no formal advantage to the steady-state aplproximation method for a sphere and a greater accuracy in a rather narrow range.
Subsequent examination of the data failed to reveal any significant effectof over- or under-cycling or of unequal sweep rates, or of the degree of resolution upon the magnitude or direction of the effect. It should be emphasized that optimal resolution wtts not attained, nor waa bad resolution tolerated. “Ringing” waa never observed for any of the image lines, or for the Cla satellites of (CH,),Si, but was attained a t the ClS lines of the other compounds in about half of the measurements. The widths of the image signals a t half-maximum were 0.70 to 0.85 c./sec. in nearly all cases, while the CI3 satellites (when not ringing) were, if anything, narrower than the corresponding C12 images, except for (CH& Si, for which they averaged ca. 0.12 c./sec. wider. The effect of sweep direction is fairly large, the C13 isotope effect appearing to be larger by 0.0022p.p.m. when the sweep is toward decreasing field, or smaller by the sanie amount when the opposite sweep is used exclusively. Although such error was avoided, it would not have concealed the isotope effect.
PROTON NlJCLEAR SPIN RESONANCE SPECTROSCOPY. XI. A CARBON-13 ISOTOPE EFFECT BY GEORGE VANDYKETIERS Contrtbulion Yo. 166 from the Central Research D e p l . , A4~nncsota Mtning .E. Mfg. Co , Sl. Paul 19, Mrnnesotu Received October 16, 1969
Recently n nuclear spin resonance (n.s.r.) “isotope effect” of CI3upon attached fluorine atoms has been discovered.’ The rather unexpectedly large shifts were always found to be in the direction corresponding to greater shielding by CL3than by C12. Though no shift was found for protons, the much smaller effect anticipated by analogy with the deuterium shifts2would not have been detected. As both the sign and the relative magnitude of such a C13 effect might prove theoretically interpretable, a morc elaborate experimental procedure has been used in the present work. Experimental The compounds studied were examined neat in the customary 5 mm. 0.d. aample tubes, from whirh air had been swept by means of a brisk stream of bubbles of prepurified nitrogen; however, air-saturated CHClj and ( CH3),Si were found to give the same results. The C13 isotopic isomers were present at their natural abnndances. The n.8.r. spectrometer and measurement techniques have been d e ~ c r i b e d . ~For the present study separate reference compounds were not employed, the exceedingly strong sharp signal from the normal (CI2) compound being used instead; “image” lines (akio called “side-bands”) are readily produced from it by audio-oscillator modulation of the magnetic field. When Cl3 ia present, the proton signal is split by it into a doublet, the. coupling constant bring ca. 100 to 250 c./sec. The positions of these two weak CL3satellite lines are measured, sepaxately, rclative to the strong C1* central peak by use of the “image” lines.3 The small but reproducible difference found in earh case results from the isotopic shift, aa otherwise the high- and low-field C13 romponents would be equally spared from the C12central line. Errors random in nature were counteracted by multiple repetition of measurements; all data have been used and weighted e ually. Measurements were made over a threeweek perio8. A t each session six sweeps were run on each of the two C13 lineal for each of the compounds studied. In addition to the routine alternation of sweep direction,a which virtually eliminaki errors due to differential saturation or to “ringing” of the signals, in most rases rare waa taken in the magnet cyrling to obtain a “flat” field and hcnre very symmetrical peaks for both dirertions of sweep; ciweep rates also were controlled to be equal (within 10’70) in both directions. (1) P. C . Lauterbur, private communication: I am indeed grateful for this advance information, without which it is unlikely that the preaent work would have been begun. (2) G . V. D. Tiers, J . Am. Chcm. Soc., 79, 5585 (1957). J . Chcm. Phys.. 49, 963 (1958). (3) G. V. D. Tiers, ’ P H I 8 JOURNAL, 60, 1151 (1958).
373
TABLE I THE EXCESSN.S.R. SHIELDING,A7, PRODUCED BY 0 1 , RELATIVE TO C1*, IN SEVERAL COMPOUNDS Compound
No. of meas.
A T , p.p.m.0 (Cia-Cl*)
J(C”H), c/sb
Shielding value, I C
(CH3)rSi 18 f0.0042 118.20 1O.OOO CHjI 18 .0012 151.17 7.843 CHZClz 12 ,0042 178.24 4.720 CHCla 18 ,0059 209.17 2.755 a Standard deviat,ion of the averaged value was ~k0.0012 p.p.m. in each case. Standard deviation of the averaged value was f0.09 r./sec. in each case. Measured in dilute solution in cc14, as described in ref. 3.
+ + +
Results and Discussion The results presented in Table I demonstrate a small but statistically significant excess shielding by CI3 in three molecules, namely, CHC13, CHBC12 and (CH3)$i. I n the case of CH31the shift is not large enough to be considered as established. The center of the doublet corresponding to protons attached directly to C13 is found a t shielding values higher by ca. 0.004 p.p.m. than the line due to the normal (CI2)compound. This effect is of the same sign but only about 1/44 as large as the corresponding effect upon fluorine.’ The same ratio, 1/40, has been observed for the deuterium isotope effect upon proton and fluorine shieldings.2 I t appears unrelated to the ratios found for the coupling constants in the same compounds, ca. 1/2 for J(C1a-H)/J(C13-F) and ca. 1/4 for J (D-H) /J (D-F) . The data in Table I seem to indicate a significant variability in the magnitude of the effect. The variation observed is not simply related either to the coupling constants, to the relative shieldings, or even to the number of substituents; however, the experimental uncertainty is such as not to warrant more detailed studies at this time. The conclusions reached above are of course entirely dependent upon the successful elimination of directed error in the measurements. Potential errors and the precautions taken have already been discussed in the Experimental section. There may well be further, unrecognized, sources of error; for example, treatment of the C13HC13 spectrum as an “AX” case rather than as an “AB” case‘ in fact must result in an apparent excess shielding by C13, even if there were no isotope effect. By (4) J. A . Pople. W. G. Schneider and H. J . Bernstein. “High-Resolution Nuclear Magnetic Resonance,” McCraw-Hill, Book C o . , Inc., N e w York, N . Y., 1969. pp. 118-123.
374
Vol. 64
NOTES
algebraic manipulation of the relations appropriate to the two cases, the error is shown to be only O.oooO1 p.p.ni. for CHC13, and should be equally negligible for the other molecules studied. The most serious effect of bad resolution should be an increase in random errors, with the result that the CL3effect would be rendered highly uncertain. If resolution varied substantially with sweep direction, :L directed error might be produced; however, no evidence for such could be found by careful analysis of the data. The apparently poor resolution of the C13satellites for the (CH3)& (as judged by the excess line width, ca. 0.12 c./sec., observed for them) is actually due to the exceedingly weak spin-spin coupling between protons on C13 and those on C12. This splitting becomes observable as a result of the "effective chemical shift," produced by the magnetic moment of individual C13 atoms upon their attached pro* tons. The coupling constant, J(C12H3SiC13H3), must be approx. 0.12 c./sec., the multiplet being assumed to be 10-fold, with binomial intensity distribution; J = (0.902- 0.782)i/2/3.6. Acknowledgment.l-I am indebted to Donald Hotchkiss for the excellent careful operation of the 1i.s.r. equipment so nccessary in this work. T H E S't'ANDARD ELECTRODE POTENTIAL OF THE QUINHYDRONE ELECTRODE FROM 225 to 55" BY JoriN
Harned and Wrightll determined the normal electrode potentid of the quinhydrone electrode from 0 to 40' by combining their data with activity coefficients determined by Harned and Ehlers12 by a different method. In the present investigation the standard electrode potentials of the quinhydrone electrode were measured from 25 to 55 a t 5" intervals, using a AgAgCl reference electrode. The establishment of standard electrode potentials a t each interval allows the direct calculation of activity coefficients a t these temperatures. In the calculations, it has been assumcd that the cell reaction is Quinone
(1)
The electromotive force for this reaction is given by the Nernst equation in the forrr.. E = EO,,II
+ RT - In a m F
(2)
Since the quinhydrone used is an equimolar compound of quinone and hydroquinone, and since these substances are non-electrolytes of low solubility in contact with the solid, their activities should be constant. At higher temperatures, the solubility of quinhydrone becomes appreciable (7 g. per 100 g. Hz0),2but as long as the ratio of the activities of quinone and hydroquinone is nearly constant, these activities may be ncglected. Therefore, the Nernst equation may now be written
+
2RT E = Eoc0i~ -F- In
c. HAYES*AND M. H. LIETZKE
Contribalion f r o m the Chemistry Divisinn, Oak Ridoe National Lnborntoru. Oak Ridge, Tennessee
+ 2HC1 + 2Ag r'Hydroquinone + 2'4gC1
~I~CIYHCI
(3)
If one assumes that
RPceived October 17, 1969
(4)
Since the discovery3 and development4 of the quinhydrone ckctrode, it has found frequent use as a substitute for the hydrogen electrode for pH where A and B are parameters and I = ionic measurements. The electrode is convenient to use strength, S = 1.17202 (23375.2/DT)'/2,and D = dielectric constant of water (at temperature T ) , and it gives results which are easily reprodu~ible.~-~ The earlier research established the fact that meas- determined by the Akerlof equation13then equation urements with the electrode in certain solutions 3 may be rearranged to give contained a "salt error,5 which was due* to a change in the ratio of the activity of hydroquinone and quinone caused by t,he presence of other dis(5) solved substances in solution. While other invesIf the quantities on the right are equated to EO", tigations werc concerned with the "salt-error," they also established the normal potential of the then quinhydrone e l e ~ t r o d eas , ~well as the normal potentials of t.hc hydroquinhydrone and the quinoquinhydrone electrodes. The Eoat any one temperature may be determined ( 1 ) 1'111s paper is hasrd upon work performed for the Atomic Energy by extrapolating the Eo" values to zero ionic Coinrnission at tlie Oak IWae National Laboratory operated by Union strength. In this work, however, the parameters Car'Gic. Coi jmration. A and B and the values of Eo in equation 5 were (2) Reseal-sh I'a,rtiriiiant for the Sumnier, 1969, from IIainline Univi nlity. S t . I'aul. Alinnrsota. determined by a non-linear least squares method (3) 1;. 1laht.r and R. Rims, Z . phvsik. Chrm.. 47, 257 (1904). on a high speed computer (the ORACLE). (4) S ( ; r a n g e - &nd .i. AI. Nelson, J . A m . Chem. Sor., 49, 1401 t5
12.
( IR'L I J .
(A) 15. 1riilcn:in. A n n . C h ? m . ,[9] 16. 109 (1921). ( 6 ) .1. I,. It. Morgan, 0. h l . I,ainmert and hI. A . Campbell. J . A m . Cham. Soc.. 63. 454 (1931). (7) E. Biilnran and .4. L. Jensen. Bull. soc. chrm.. 41, 151 (1027). ( 8 ) S. P. I,. Sijren,scn, AI. Sorensen and K. Iinderstrom-Lang, A n n . Cham., 16, 283 (1!321). (9) F. Ilovorka and W. C. nearing, J . A m . Chcm. Soc.. 67, 440 (1935). ( 1 0 ) 11. 1. Stonchill. T m n s Faradav Snc.. 39, F7 (1943).
Materials and Apparatus Quinhydrone.-The quinhytironr n s r d in this projwt waa Enstman KO.217, rcrrystnllincd from wttcr hcntcd to (11) I[. S. Ilarncd anti D. D. Wright, J . A m . Chem. Soc., 66, ,4849
(1933). (12) H. 8. Harncd and R . W. Ehlcrs. J . A m . Chem. Soc., 66, 2179 (1933~. (13) G. C. Akcrlof and TI. 1. Oshry, J . A m . Chcm. Soc.. 79, 2844 (1950).
