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The Journal of Physical Chemistry, Vol. 83, No. 26, 1979
Lawler, Nelson, and Trifunac
(9) R. H. Clarke, Chem. Phys. Lett., 6, 413 (1970). (10) V. Macho, unpublished results. (11) G. Dittrich, D. Stehlik, and K. H. Hausser, Z. Naturforsch. A , 32, 652 (1977). (12) C. P. Keijzers and D. Haarer, J. Chern. Phys., 67, 925 (1977); Chem. Phys. Lett., 49, 24 (1977). (13) D. M. Burns and J. Iball, Proc. R. SOC.London, Ser. A , 227, 200 (1955). (14) V. P. Chacko, C. A. McDowell, and B. C. Singh, Mol. Phys., 38, 321
(1979). (15) U. R. Bohme and G. W. Jesse, Chem. Phys. Lett., 3, 329 (1969). We have abbreviated the alternative name dibenzocyclohexadienyl by anthracenyl. (16) A. Otto,A. Hudson, R. A. Jackson, and N. P. C. Simmons, Chem. Phys. Lett., 33, 477 (1975). (17) See, e.g., A. Carrlngton and A. D. McLachlan, "Introduction to Magnetic Resonance", Harper and Row, New York, 1967, Chapters 6 and 7.
Carbon-I3 CIDNP from Reactions of Some Simple Organic and Inorganic Radicals during Pulse Radiolysist R. 0. Lawler,*$ D. J. Nelson, and A. D. Trifunac" Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439 (Received August 17, 1979) Publication costs assisted by Argonne National Laboratory
Examples of carbon-13 NMR spectra exhibiting CIDNP during pulse radiolysis of aqueous solutions of simple organic and inorganic compounds in D20are presented. The following aspects of carbon-13CIDNP are illustrated (A) detection of CIDNP in a reaction where all protons have been replaced by deuterons, (B) generation of a theoretically ideal radical pair with only one I = 1 / 2 nucleus, (C) use of isotopic enrichment to distinguish two different polarization pathways, and (D) comparison of proton and carbon-13 CIDNP from the same sample. The chemical systms studied include the radiolysis of CD30Din D20 (D. and CD20Dradicals) and the radiolysis of sodium formate in D20 (C02-.radical), sodium carbonate (C03-.), and sodium chloroacetate (.CH,COO-).
Introduction Some time ago1 we demonstrated the feasibility of detecting chemically induced dynamic nuclear polarization (CIDNP) in the protons of samples undergoing pulse radiolysis with 3-MeV electrons from a Van de Graaff accelerator. Development of this technique has now provided a direct link between the free radical intermediates detected optically2 or by EPR3,5during radiolysis and the reaction products formed from them. Subsequent reports have described lH CIDNP arising from radiolysis of aqueous solutions of alcohols,6 dimethyl ~ulfoxide,~ acetone,s and acetaldehyde.s Furthermore, in those cases where the mechanism of formation and reactivity of the free-radical intermediates is understood, radiolysis serves as a versatile, predictable method of producing high concentrations of reactive free radicals with initially uncorrected electron spins. These radicals may then be used in studies of the mechanism by which CIDNP is produced. Such studies to date have included the search for alternatives to the radical pair mechanismg and determination of the dependence of CIDNP intensities on applied magnetic field for simple radical pair^.^,^ It is of obvious interest to extend the investigation of CIDNP during radiolysis to nuclei other than protons. Toward this end, we have begun a program to explore the applications of 13C CIDNP to reactions of radiolytically produced radicals. Although there is a small, but growing, literature on 13CCIDNP from photochemicalloand thermall1 reactions, this constitutes the first report of observations of this effect during radiolysis. t Work performed under the auspices of the Office of Basic Energy Sciences of the U.S. Department of Energy. Department of Chemistry, Brown University, Providence, RI 02912.
*
0022-3654/79/2083-3444$01 .OO/O
The four examples given here illustrate the following applications of 13C CIDNP: (A) detection of CIDNP in a reaction where all protons in the reactants have been replaced by deuterons, (B) generation of theoretically ideal reaction conditions such that only one type of radical is present and only one I = 1/2 nucleus (carbon-13)is present per pair, (C)use of labeling to distinguish two polarization mechanisms, and (D) comparison of the lH and I3C CIDNP arising from the same set of radiolytically produced radicals. Experimental Section All chemical reagents were of the highest purity commercially available and were used without further purification. NMR Measurements. Radiolysis was carried out as previously describedl~~ in a variable magnetic field (0-8000 G) and the sample was rapidly transferred to the NMR spectrometer in a recirculating flow system. The total volume of the recirculating sample employed was 100-500 mL. This size sample could usually be irradiated for at least 1 h without producing noticeable changes in CIDNP spectra. Adequate signal/noise could usually be obtained with a few minutes of spectrum accumulation, e.g., 100-2000 spectra taken ca. 0.5 s apart. NMR spectra were obtained with a Bruker WP-80 pulsed fourier transform NMR spectrometer equipped for detection of either lH or 13Cby use of 5- and 10-mm tubes, respectively. A flow rate of 1mL/s was employed for all spectra described here. At this flow rate the sample was transferred from the radiolysis zone to the spectrometer probe in ca. 1 s. The residence time in the NMR probe was ca. 0.08 s for lH and 0.3 s for I3C. This led to line width contributions of 4 Hz for 'H and 1 Hz for 13C. Since both the transfer and residence times were shorter than the spin-lattice relax0 1979 American Chemical Society
The Journal of Physical Chemistry, Vol. 83, No. 26, 1979 3445
CIDNP of Sample undergoing Pulse Radiolysis
ation times for either ‘H or 13Cin these samples (in excess of 5 s for all lines), the observed CIDNP is essentially free of the distorting effects of relaxation. This was confirmed by observing that (a) an increase or decrease of the flow rate by a factor of 2 on either side of 1mL/s produced the same absolute integrated CIDNP intensity, provided that the Van de Graaff pulse rate was increased or decreased by the same amount to keep the dose per unit volume constant; and (b) the intensity of the background ‘H spectrum arising from thermal polarization in the solvent or starting material (Figure 4) varied inversely with flow rate, as expected if the residence time in the probe, i.e., the time available for recovery of thermal polarization in the high magnetic field, were shorter than T1.12 The signal from thermal polarization of natural abundance 13Cwas below the noise level in all CIDNP spectra. When reference 13C spectra were desired, the flow was stopped and conventional prolonged signal averaging under nonsaturating conditions was employed. Since the sample is continuously replenished in the probe under flow conditions, a radio-frequency pulse repetition time approximately equal to the residence time could be employed. A lower pulse repetition rate simply wasted the sample since portions of it flowed undetected through the probe. Furthermore, since heteronuclear CIDNP multiplet effects are not affected by the angle of the rf p u l ~ e , ’ ~it- was ~ ~ possible to use 90’ pulses for maximum sensitivity in both proton-coupled and -decoupled 13C spectra. For lH spectra, however, this was possible only when the lines of interest were known to be singlets.’* In ‘Hspectra with the potential for multiplet effects, a pulse angle of 30” or less was employed to avoid distortion~.~~ Irradiation Conditions. The sample was typically irradiated with pulses from the Van de Graaff accelerator lasting 200 ns and spaced 5 ms apart. Estimates of the radiation dose and/or radical concentration per pulse16 were based on (a) actinometry by using the known G value for disappearance of chl~roacetate,’~ (b) radical decay rates estimated from relative intensities of combination and radical-radical scavenging products’* from -CH2CO2-,or (c) beam current and sample geometry: All three methods M primary radicals per produced estimates of ca. 1 X 200-1-1spulse for the spectra described here. At a constant flow rate the CIDNP intensity varied linearly with Van de Graaff pulse rate, even at pulse rates high enough to irradiate the same volume several (typically 5-10) times before it flowed from the irradiation zone. This is good evidence that events from individual electron beam pulses were well separated in time and that the observed spectrum is adequately described as the sum of independent radiolysis events. [This is not always the case, however, when the concentrations of substrates, or impurities such as 02,are comparable to the concentration of radicals produced per pulse. In these cases “local depletion” of reactants apparently occurs and the observed CIDNP intensity may depend strongly on Van de Graaff pulse width, pulse repetition rate, and flow rate.lg]
Results and Discussion The radicals responsible for the CIDNP spectra described here are all formed by further reactions of either the hydrated electron, eaq,or hydroxyl radical, HO., which are the primary species generated in electron radiolysis of water. Since we are concerned here with deuterium oxide solutions, the primary radiolysis event may be adequately described (after ca. 1 ns) by the reaction20 D20
--
D+ + eaq + DO.
(1)
TABLE I: Conditions Employed for Production of Radicals
example
A
B C
radical precursor
D,O+ CD,OD HC0,Na HC0,Na Na,CO,
D
ClCH,CO,Na Na,CO,
priconcn, mary secondary M radical radical 0.1 1.0 0.05 0.05 0.01‘ 0.5 0.1‘ O.ld 0.3e l.Od
l.Oe
eag DO. DO.b DO.! DO. DO,b DO.b eaq eaq DO. DO.
D. .CD,OD CO,-. CO,.. CO,-. CO,-. CO,-. .CH,CO; .CH,CO,CO,-. CO,-.
r,,‘
ns 0.4 2 8 8 40 6 30 8 3 3 3
Half-life for reaction of primary species with radical precursor. Calculated from concentration of precursor and second-order rate constant for reaction with primary Solution species. Rate constants taken from ref 2. saturated with N,O (ca. 0.02 M) to convert eaq to DO. within 6 ns. Solution also 0.015 M in 90% enriched Na,13C0,. Carbon-13 spectrum. e Proton spectrum. a
[We shall ignore small amounts (15-20%) of D. apparently formed directly from D20 molecule^.^^] Each of these species then reacts with a radical precursor within a time T~ to give the radicals which eventually lead to products exhibiting CIDNP. Table I summarizes these radicals, their precursors, and estimates of T~ under the conditions prevailing when the spectra were run. In the case of DO. the secondary reactions are either hydrogen abstraction (A, B) or electron transfer (C, D). The hydrated electron in all cases undergoes addition to a substrate molecule either concurrently with, or followed immediately by, dissociation to give De (A), DO. (B, C), or .CH2C02-(D). In examples B and C the DO- formed by dissociative addition to nitrous oxide reacts in the same fashion as that formed initially, although the second “pulse” of DO. lags behind the first by several nanoseconds. The chemical picture presented above and in Table I thus says that within a maximum of ca. 50 ns (substantially less than the width of a typical single electron beam pulse applied to the sample, see Experimental Section), the sample contains a mixture of radicals R1. and R2. (or possibly, as in example B, only one type of radical). CIDNP then results from the radical-radical reactions which this mixture undergoes. It seems reasonable to assume that the situation at the stage where radical pairs begin to react is formally identical with that described by FischerZ2and by C10s.s~~ for mixtures of radicals generated by steady-state photolysis and thermolysis, respectively. We shall not repeat their quantitative description of the expected product distributions or CIDNP intensities. An adequate determination of all of the kinetic parameters needed to describe CIDNP from a mixture of two radicals would require variations of precursor and radical concentrations which are beyond the scope of the present report. We shall instead describe how the CIDNP shown in each of the examples is consistent with the predictions of the radical pair theory applied to the anticipated radical intermediates. Each of the radicals shown in Table I has also been characterized by EPR. The pertinent g factors and hyperfine splittings are presented in Table 11. A. D., *CD20D. In Figure 1 is shown the 13C NMR spectrum of CD30D obtained from the radical pair reaction D. + sCD’20D D-CD’ZOD (2) 2 in a field of 8000 G, where the enhanced nuclei are italicized. This reaction, and indeed any radical reactions at all in this system, would, of course, go undetected if lH
-
The Journal of Physical Chemistry, Vol. 83,No. 26, 1979
3446
Lawler, Nelson, and Trifunac
TABLE 11: g Factors and Hyperfine Splittinas of Radicals radical D. .CD,OD
co, -. co, -.
a
g factor
ref
hfs, G
ref
2.00223 2.00322
32 33
aD = t 7 7 ac: = t 4 7 a,D = - 2 . I a a 0 J J = ( + ) b 0.02a a c = +148
32 32 33 34 36 37 38 38 38
2.00045 2.0113 2.00324
,CH,CO,
Estimated from a H .
I
160
,
1
140
CALCULATED
120
-11 a,c = + 3 2 apc = ( - ) b 1 4 aH = - 2 1 aC =
0
I
1
80
1
60
,
40
1
20
1
1
Oppm
Figure 1. Carbon-13 CIDNP observed from CD30D during radiolysis The pH was adjusted to 1 by addition of D,SO,. This is the only signal observed over the chemical shift range from 0 to 200 ppm. The sample was irradiated in a magnetic field of 8000 G.
NMR alone were applied to the sample. The A/E phase of the symmetrical multiplet is as predicted by the simple rules for CIDNPZ4for this radical pair. Furthermore, it is easily shown by use of the quantitative radical pair theoryz5that the relative intensities within the multiplet predicted for the above pair should be 1,2,2,0,-2,-2,-1, in agreement with those observed. The absence of detectable CIDNP from deuterated ethylene glycol formed in reaction +
DOCD2CDzOD
(3)
3 may be attributed to the much smaller 2H hyperfine splitting in this radical compared to that in the deuterium atom. B. 2C02-.. The low natural abundance of 13C and the absence of nuclear spins in 12Cand l60make possible the formation for the first time of the theoretically ideal radical pair, 2COp, which is symmetrical except for the incorportion of one I = 1 / 2 nucleus, in this case 13C, in one fragment. COz. radicals, formed from sodium formate (Table I), are known to decay by dimerization to form oxalate ion.26 .-OzC
-
+ 13C02-.
-02C-13COz-
300
200
400
Flgure 2. Carbon-13 CIDNP from oxalate ion (ac = 175 ppm) formed during radiolysis of 0.05 M sodium formate saturated with N,O. The duplicate scan at 200 G demonstrates the reproducibility of the experiment. Each peak was obtained from the sum of 1600 FID's collected 0.5 s apart by use of a 90' rf pulse. The total accumulation time required for each peak was thus ca. 13 min. The dotted line is the theoretically predicted field dependence.
Of 1.0 M CD30D in D,O.
2CDzOD
IO0
GAUSS
Sign uncertain.
1
100
-
35 35 38
------- --_________
(4)
Since this pair is symmetrical and possesses only a single nuclear spin, it is predicted, and observed, that no CIDNP is produced in high magnetic fields. When the radiolysis is carried out in a magnetic field approximating the 13C hyperfine splitting in 13COz-.,however, a fairly strong absorption line is observed for oxalate ion but no other lines are observed in the spectrum. The presence of only a single CIDNP line actually presents somewhat of a problem in FT NMR because improper phase adjustment following transformation may turn an absorption line into emission and vice versa. The correct sign of the enhancement may be determined only by using phase parameters obtained by carefully adjusting the phase of the
spectrum of an unpolarized sample which has absorption lines both in the region of the CIDNP line and far removed from it. It is instructive to compare the observed magnetic field dependence of the oxalate line, shown in Figure 2, with that predicted theoretically. We see that the observed field dependence possesses three characteristics: (1) the line is in absorption at all four fields shown in the plot, (2) the maximum intensity occurs at a field near the ca. 140-G 13C hyperfine splitting in 13C02-.,and (3) as the field increases beyond the maximum, the intensity falls off inversely with the field. All three of these characteristics, incorporated in the dotted curve in Figure 2, are in fact predicted by any of several different versions of the radical pair theory for a pair with one I = 112 nucleus in the limit where the electron exchange interaction is negligible.27-30It should be emphasized that, although these theoretical descriptions differ greatly in the models used to describe the radical pair lifetime and the exchange interaction, they all predict the observed field dependence, within an adjustable scale factor, for this simplest of all radical pairs. While it is possible, in principle, to determine the absolute CIDNP enhancement per radical pair encounter and therefore fix the scale factor, to be useful it would be necessary to know the concentration and kinetic behavior of the radicals with sufficient accuracy to distinguish among the predictions of the various theoretical models. It seems unlikely, however, that the predictive power of any of these models is sufficiently good at present to warrant such an investigation. Our conclusion that the exchange interaction is unimportant for low field CIDNP of a recombination product from independently generated radicals supports similar conclusions for thermally28,31and photochemically28130generated radical pairs possessing several nuclear spins. C. Cop,COB--. As noted above, a symmetrical, one-spin radical pair cannot produce CIDNP in a high magnetic field. This restriction no longer exists, however, for an unsymmetrical radical pair with one spin, provided that the two radicals have substantially different electron g factors, A particularly simple example of such a pair is that arising from COz-. and COB-., formed as indicated in Table I. In this case, however, two unsymmetrical pairs are formed. [13c02-. co3-.] I
[co2-.13c03-.] I1
Figure 3 shows the CIDNP spectrum observed in high field when equal amounts of the two radicals are formed by radiolysis. The emission and absorption lines are due to COa2-and oxalate ions, respectively. Consideration of the relative magnitudes of the hyperfine splittings in the radical pairs in I and I1 (Table 11) leads to the conclusion that I should produce much stronger enhancement than
The Journal of Physical Chemisfry, Vol. 83,No. 26, 7979 3447
CIDNP of Sample undergoing Pulse Radiolysis -02CEO;
-
13
OECOUPLER ON
DECOUPLER OFF
Figure 4. Proton and carbon-13 CIDNP from solutions of sodium chloroacetate and sodium carbonate (see Table I). Numbers refer to protons at carbon atoms identified in eq 9 and 10. The dotted line in the 'H spectrum indicates the peaks from HOD and chloroacetate which are observed with the electron beam off. Figure 3. CIDNP from oxalate and carbonate ions formed during radiolysis of solutions of sodium carbonate and sodium formate in N,O-saturated D20 (see Table I). The total accumulation time was 6 min for each spectrum. T b magnetic field was 8000 G.
11. One can, in fact, explain both enhanced products by reactions 5 and 6
-
-02C-13C02A
cop
2DO-
[13C02-.COS-.]
-
+ 13C02
-
+
[13C02] CO2-
E
(5)
13C02- + D 2 0
(6)
where nuclei exhibiting CIDNP are italicized and A or E refer to the sign of enhancement, This interpretation is confirmed by the observation that addition to the system of carbonate which is enriched fourfold in 13Cproduces no change in the relative enhancements of carbonate and oxalate. If carbonate CIDNP enhancement arose from reaction 7, however, a fourfold increase in COS2-CIDNP
-
[co2-.13c03-.] coz+ 13co 2E3
(7)
relative to oxalate would have been expected. It should be noted that the simultaneous change of sign of ac and the g factor difference for the pairs in reactions 5-7 is expected to produce CIDNP of the same sign (E) for CO$from either pair. The simple rules can therefore not distinguish these two pathways. In principle, a contribution of (7) to the CIDNP might be detected by addition of even larger amounts of 13C02-. It is interesting to speculate that the observed CIDNP in CO2- might also have occurred via the formation of carbonic anhydride rather than the two-step sequence in ( 5 ) and (6).
--
[13C02-,COB-.] 2DO-
+ [-Oz13C-O-C02-]
[-Oz13C-O-CO;] D2O
(84
+ 13C032-+ CO$-
(8b) While such an intermediate cannot be ruled out by our results, its lifetime under the conditions used here would have to be less than the 1-2 s which elapse between the time radiolysis occurs and polarized carbonate ion is detected in the NMR spectrometer. D. COS-., .CH2C02-. As a final example we compare the 'H and 13CCIDNP obtained from a sample in which equal amounts of C o p and .CH2C0; are expected to be present. These spectra are shown in Figure 4.
The enhancements in the lH spectrum and the 13C spectrum with lH decoupling are quite adequately explained by a simple reaction scheme analogous to (5), (6)) and (8): combination
-
[ C o p CHzC02-] scavenging
5
-02C-O-CH2-C023 2
(9a)
-
6 -02CCH2-CH2-CO; 4 1 (9b) Atoms giving CIDNP are italicized and the numbers refer to the corresponding peaks in Figure 4. As in example C above, the addition of 13C-enriched carbonate to the mixture produced no obvious change in CIDNP. All 13C lines thus arise from .CH2C02-. In the absence of IH decoupling it becomes possible in this case also to detect in the 13C spectrum the presence of one of the two symmetrical radical pairs which should be formed in this system. [-02CCHz. 13CH2C02-] -02CCH2-13CH2C0< (10)
+
[C03-. .CH2C02-] -02CCH2.
-
4
This pair cannot, however, contribute to the lH singlet, line 6, from succinate and is therefore invisible to lH NMR. The other pair, [2C03-+],would, of course, not be detectable in high field in any case. The analysis of the CIDNP in Figure 4 may be carried one step beyond the qualitative statements of consistency presented above. The following additional points should be made regarding the magnitudes of the CIDNP lines: (1) The net effect from succinate (lines 4 and 6) is substantially lower than the oppositely signed net effect for the corresponding lines from the carbonate ester (lines 3 and 5). This is as expected if nuclear relaxation in .CH2COZ-competes with the scavenging reaction 9b. The ratio of these intensities thus contains information about rates both of nuclear relaxation and radical-radical reaction. The latter may be controlled by changing the dose, and therefore the radical concentration, thereby making it possible, in principle, to determine both (2) In contrast to the above, the net effects for the carbonyl carbons of the combination and scavenging products (lines 2 and 1,respectively) are of nearly equal amplitude. This suggests that the 13Cpolarization at this position in C H 2 C 0 2relaxes more slowly than the rate of radical-radical reaction at the radical concentrations em-
3448
The Journal of Physical Chemistry, Vol. 83, No. 26, 1979
ployed here. This is not too surprising since the unpaired electron density in the vicinity of this carbon is relatively low. (3) The absence of a I3C CIDNP line from the carbonate carbon of the ester combination product (predicted to be in emission between 150 and 160 ppm) is surprising. The similar magnitudes of ac for 13CO$- and CHz13C02-(Table 11) would lead one to expect detectable CIDNP produced in the pair in (11). This is a troubling gap in an otherwise [13C03-. CH2C02-] -0213C-OCHzC02- (11) consistent picture and merits more careful study by use of larger amounts of isotopically enriched carbonate. (4) The 'H NMR spectrum of the unpolarized sample obtained after radiolysis was carried out for several hours exhibited a weak line from succinate but no line at the position of the strong CIDNP from the carbonate ester (line 5). The sample shows instead a line from glycolate which is formed in an amount approximately equal to succinate. The carbonate ester is therefore a metastable product which hydrolyzes readily under the reaction conditions.
-
-
-02COCHzC02-+ DO- C032-+ DOCH2CO2- (12) We know, however, that in this case the hydrolysis must occur more slowly than the 1-2 s required for observation of the CIDNP signal.
Conclusions The examples presented here have illustrated that natural abundance 13C CIDNP can easily be detected under radiolysis conditions. The loss of sensitivity compared to lH NMR is not a serious limitation since signal averaging is easily employed by using a recirculating flow system. Sufficient signal/noise for most purposes may be achieved in a few minutes of spectrum accumulation. Carbon-13 CIDNP also has an obvious advantage for studying radiolysis of aqueous solutions since pure HzO may be used as the solvent (provided that an internal 2H field/frequency lock is not required) without interfering with the CIDNP signals. Except for those cases where unusually high sensitivity is required, there now seems little reason not to employ NMR routinely for CIDNP studies.
Acknowledgment. We gratefully acknowledge the assistance of the Van de Graaff operators, R. H. Lowers and A. Youngs. R.G.L. acknowledges the hospitality of Argonne National Laboratory during sabbatical leave from his position at Brown University.
References and Notes (1) A. D. Trlfunac, K. W. Johnson, and R. H. Lowers, J . Am. Chem. Soc., 98, 6067 (1976). (2) See, for example, J. W. T. Splnks and R. J. Woods, "An Introduction to Radiation Chemistry", 2nd ed,Wlley-Intersclence, New York, 1976. (3) B. Smaller, J. R. Remko, and E. C. Avery, J. Chem. Phys., 48,5174 (1968);A. D. Trlfunac, K. W. Johnson, B. E. Cllfft, and R. H. Lowers, Chem. Phys. Lett., 35, 566 (1975). (4) N. C. Verma and R. W. Fessenden, J. Chem. Phys., 85, 2139 (1976), and references therein.
(5) D. J. Nelson, A. D. Trlfunac, M. C. Thurnauer, and J. R. Norrls, Rev. Chem. Intermed., in press. (6) A. D. Trlfunac and D. J. Nelson, J. Am. Chem. Soc., 99, 1745 (1977); D. J. Nelson, C. Mottley, and A. D. Trifunac, Chem. Phys. Left., 55,
323 (1978).
Lawler, Nelson, and Trifunac
(7) D. J. Nelson, J . Phys. Chem., 82, 1400 (1978). (8) D. J. Nelson, J . Phys. Chem., 83,2186 (1979). (9) A. D. Trlfunac and D. J. Nelson, J. Am. Chem. Soc., 100, 5244 (1978). (10) R. Kapteln, R. Freeman, H. D. W. Hill, and J. Bargon, Chem. Commun., 953 (1973);R. Kaptein, R. Freeman, and H. D. W. Hill, Chem. Phys. Lett., 28,104 (1974); H. Iwamura, Y. Imahashi, and K. Kushlda, J. Am. Chem. Soc., 98, 921 (1974);W. B. Moniz, C. F. Poranski, Jr., and S. A. Sojka, J . Org. Chem., 40,2946 (1975);C. F. Poranskl, Jr., W. B. Moniz, and S. A. Sojka, J . Am. Chem. Soc., 97,4275 (1975);S. A. Sojka, C. F. Poranskl, Jr., and W. B. Moniz, lbld., 97, 5953 (1975);H. Iwamura, Y. Imahashi, K. Kushlda, K. Aokl, and S. Satoh, Chem. Lett., 357 (1976);R. Benn and H. Dreeskamp, Z. Phys. Chem. (Fra hrtamMaln), 101,11 (1976);S.A. Sojka, C. F. Poranki, Jr., and W,%, Monk J. Magn. Reson., 23,417(1976);F. J. J. de 2 . Sagdeev, and R. Kaptein, Chem. Phys. Lett., 58,334
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