3588
J . Phys. Chem. 1987, 91, 3588-3592
the exceptionally large C-H bond strength of the corresponding alkanes, but when bond strength differences are more similar, as in the C3 and C4 alkenes, it becomes more difficult to predict the occurrence of surface-generated gas-phase radicals.
Acknowledgment. We express appreciation to Drs. J. C. Evans and C. C. Rowlands of Univeristy College, Cardiff, for the EPR simulation program. The research was supported by the Division of Basic Energy Sciences, Department of Energy.
A Study by Solid-State NMR of 13'Cs and 'H of a Hydrated and Dehydrated Cesium Mordenite Po-Jen Chu, B. C. Gerstein,* Department of Chemistry and Energy and Mineral Resources Research Institute,t Iowa State University, Ames, Iowa 50011
John Nunan, and Kamil Klier Department of Chemistry, Lehigh University, Bethlehem, Pennsylvania 1801 5 (Received: December 29, 1986)
The solid-state NMR of 133Csand 'H in Cs-exchanged mordenite has been monitored as a function of dehydration of the zeolite. In the fully hydrated mordenite, '33Cs(I=7/2)exhibits a single line 64 ppm upfield of aqueous saturated CsCI. The anhydrous sample exhibits two major lines of intensities 3:1 with center of mass at -57 and -190 ppm, respectively, for Cs under magic-angle spinning. The major intensity is upfield of the line observed in the hydrated sample. The electric field gradient parameters for Cs of the anhydrous sample are eZqQ= 3.1 MHz and 9 = 0.65, which reduced to eZqQ= 210 kHz and 7 = 0 for the hydrated sample. Assignment for the three sites occupied by Cs in the anhydrous sample and the corresponding chemical shifts are site 11, -157 ppm; site IV, -186 ppm; and site VI, -24 ppm after correction for the second-orderquadrupolar shift. The static proton spectra decrease in intensity by a factor of 35 between the fully hydrated and anhydrous sample. While maintaining a roughly constant width of 6.6 kHz, the shape of the proton line changes with dehydration and exhibits an anisotropy in the anhydrous sample.
Introduction
TABLE I
Solid-state N M R has been shown to be a useful tool for studying both the static chemical environments and the dynamic behavior of nuclei in zeolites.' The nuclei studied in these systems have been predominantly 29Si,27Al,and 'H. 29Siand 'H are both spin and amenable to quantitative and qualitative detection by standard pulse techniques. 27Al is a quadrupolar nucleus with spin 5 / 2 and thus exhibits residual broadening under rapid sample spinning and a difficulty with quantitative detection associated with the magnitude of the quadrupolar splitting of the outer transitions relative to the bandwidth of the rf pulse. Nevertheless, its relatively small quadrupole moment allows NMR to be a useful technique for studying aluminum in and for the dealumination of zeolites. Cesium is an important promoter in catalysts used among other reactions for the fixation of CO and the production of higher alcohols. 13Csis therefore a nucleus that would be useful as a monitor of the chemistry of these systems. The nucleus has a natural abundance of loo%, a nuclear spin I = 7 / 2 , and a relatively weak quadrupole moment of -3.0 X 10-3/10-28 m2. However, its relatively low gyromagnetic ratio, and the relatively low mole ratios of Cs to zeolite in standard exchanged catalysts, raise questions about the applicability of N M R of I 3 T s for monitoring processes taking place in zeolitic catalysts. The present work was undertaken to determine the utility of '33Csas a nucleus to monitor local chemistry in zeolites and more generally the local structures around this ion in surface sites. Specifically the dehydration of Cs-doped mordenite has been followed by tracking the high-resolution solid-state N M R of 133Cs. The broad line N M R of 'H has been used as ancillary information to verify the model used in explaining the changes in Cs-doped mordenite with dehydration. The crystallographic sites of Cs in mordenite have been characterized by Schlenker, Pluth, and Smith.2 The sites for large Operated for the U S . Department of Energy by Iowa State University under contract No. W-7405-Eng-82.
0022-3654/87/2091-3588$01.50/0
sample A
B C D E
F
calcin temp, 'C/time, h fully hydrated 100/8
320/2 (deep bed in NMR tube) 320/4 (deep bed in NMR tube) 450/4 (deep bed in NMR tube) 450/10 (shallow bed in 10-mm bulbs)
univalent cations are labeled 11, IV, and VI and have relative occupancies for Cs of 3.78:1.86:1.75. Sites I1 and IV place Cs near the center of an eight-ring of oxygen, and site VI places the Cs off center of a six-ring. Although the 8-ring sites I1 and IV could accommodate all the cations, occupation is also found for the one-sided coordination site VI. This site indication for 11, IV, and VI are equivalent to A, D, and E, respectively, used by M~rtier.~ Experimental Section N M R experiments on 133Cs(I=7/2)and hydrogen were performed at 28.877 and 220 MHz, respectively, in a home-built pulsed NMR spectrometer which has been previously d e s ~ r i b e d . ~ (1) Fyfe, C. A. Solid State N M R f o r Chemists; CFC Press: Guelph, Ontario,-Canada, 1983. (2) Schlenker, J. L.; Pluth, J. J.; Smith, J. V. Mater. Res. Bull. 1979, 14, 751. Schlenker, J. L.; Pluth, J. J.; Smith, J. V. Mater. Res. Bull. 1978, 13, 901. (3) Mortier, W. J. Compilation of Extra Framework Sites in Zeolites; Butterworth: New York, 1982. (4) Cheung, T. T. P.; Worthington, L. E.; DuBois Murphy, P.; Gerstein, B. C. J . Magn. Reson. 1980, 41, 158. (5) Kundla, E.; Samoson, A,; Lippmaa, E. Chem. Phys. Lett. 1981, 83, 229. ( 6 ) Torgeson, D. R.; Barnes, R. G. J . Chem. Phys. 1974, 62, 3968. (7) Taylor, P. C.; Bray, P. J. J . Magn. Reson. 1970, 2, 305. (8) Samoson, A. Chem. Phys. Lett. 1985, 119, 29. Equation 2 has been written in a different but identical form from eq 3 of this reference. (9) Maiwald, W.; Basler, W. D.Magnetic Resonance in Colloid and Interface Science; Fraissard, J. P., Reesing, H . A,, Eds.; Reidel: Dordrecht, 1979; p 629.
0 1987 American Chemical Society
N M R of 133Csand IH in Mordenite
The Journal of Physical Chemistry. Vol. 91, No. 13, 1987 3589
E LYR/CCO.II Y E 2 STATIC P O 1 D l R
200
00
Figure 1. NMR spectra of 'Hin Cs-exchanged mordenite as a function of degree of hydration. Bottom, spectrum A, is the fully hydrated sample. Top, spectrum F, is the anhydrous sample.
-200 -400 -600 Shlfl ( p p m l
SRTFT ( P P Y )
Figure 2. Static NMR spectrum of 133Cs in Cs-exchanged mordenite as a function of degree of hydration. Bottom, spectrum A, fully hydrated sample. Top, spectrum F, anhydrous sample. Note the satellite transitions visibile in spectra A and B.
The Cs-exchanged mordenite was prepared by repeated contact of 20% of N a mordenite with 250 mL of 1 M C s N 0 3 at 90 OC, until essentially complete removal of N a was effected. The N a content of the zeolite was monitored by atomic absorption analysis for N a after zeolite dissolution using HF. X-ray diffraction analysis before and after ion exchange confirmed that no loss in crystallinity occurred during sample preparation. Samples with varying degrees of hydration were prepared as indicated in Table I. The frequency of the sample spinning during NMR experiments on 133Cs was varied from 3.6 to 5.2 kHz to distinguish sideband structure from the central transitions. N M R measurements on Cs were taken with the sample static and spinning. All spectra of protons were taken under static conditions. N M R spectra of static and spinning samples were all taken at room temperature. Spin temperature inversion of the preparation pulses was used to minimize base-line artifacts in the Fouriertransformed spectra. All data were taken with fixed gain of the receiver-A/D chain. The spectra were normalized to constant intensity for graphical presentation. The normalization constant was then divided by the ratio of the weight of the sample compared to the fully hydrated sample in order to obtain relative amplification factors for each spectrum. For example, the relative amplification factors of the proton spectra shown in Figure 1 (vide infra) indicate that the proton signal intensity in the anhydrous sample was 35 times less than that of the fully hydrated sample. Longitudinal relaxation times for Cs in all samples were determined to be approximately 10 ms, so that a recycle rate of 0.1 s was used for accumulating N M R of Cs; 6 5 536 scans were accumulated for signal averaging on all samples. The longitudinal relaxation of hydrogen in the samples varies with the degree of hydration, the fully hydrated sample having a TI of less than 0.05 s and the anhydrous sample having a T , of less than 1 s. The trend of decreasing of T , at room temperature as degree of hydration increases is consistent with other measurements reported for univalent-cation-exchanged zeolite A."
where Bq equals the splittings of the singularityof the first satellite m = 3/2, or m = from the center of mass. These singularities correspond to the orientation with the principal axis of the electrical field gradient tensor being perpendicular to the external magnetic field? Equation 1 implies that splitting B, depends also on the asymmetry parameter 1 of the electric field gradient (efg)
(10) Gerstein, B.; Dybosky, C . Transient Techniques in N M R of Solids: Academic: New York, 1985.
(1 1) Barrer, R. M. Zeolites and Clay Minerals as Sorbents and Molecular Sieues; Academic: New York, 1978.
Recycle rates of greater than five T , were used in accumulation of proton N M R , 10000 scans were taken in each set of accumulations in the proton measurements. All values of chemical shifts of 133Csare referenced to a saturated aqueous solution of CsCI. The chemical shift of ' H is referenced to water. The shift scales are expressed with increasing negative values being upfield.
Results The 'H spectra with increasing degree of hydration are shown in Figure 1. The N M R spectra of 133Cswith increasing hydration are shown in Figures 2 and 3 for samples under static and magic-angle spinning (MAS) conditions, respectively. The relative intensities of the 'H spectra give an approximate ratio of the water content in the dehydrated to that in the fully hydrated samples. The proton line width decreases with dehydration and gives indication of inhomogeneous dipolar broadening as in sample B. Upon further dehydration, the line width gradually increases and develops an asymmetry at full dehydration. This trend in the change of the 'H spectra with dehydration is consistent with that observed for different degrees of hydration for the cation-exchanged zeolite A.9 The quadrupole coupling constant e2qQ can be measured from the singularities of the first satellite transitions ( 3 / 2 , 1 / 2 ) and (-1/2,-3/2) by the following equation63l0
Bq =
3e29Q 41(21- 1)
The Journal of Physical Chemistry, Vol. I , No. 13, 1987
3590
I\
A 1
I
200
00
3.8 I
I
-200
I
-400
-600
Shift (ppm)
Figure 3. MAS spectra of 133Csin Cs-exchanged mordenite as a function of degree of hydration. Bottom, spectrum A, fully hydrated sample. Top, spectrum F, anhydrous sample. Sample rotation speed in kilohertz is indicated at the right of each spectrum. Starred (*) peaks are spinning side bands. TABLE I t Ouadrumlar Couoline Constant ~
F kHz MHz e2qQ; MHz B
VC
,a
58 4.6 3.1 0.65
E
D
C
B
A
46 2.6 2.1 0.5
39
32 0.9
12.8 0.36
7.5 0.21
0.0
0.0
0.0
1.8 1.7 0.4
“The first satellite splitting from the center of mass of the static sample. bCalculated from (1) and B, assuming the q value obtained from line-shape simulation for samples D, E, and F and zero for A, B, and C. ‘Obtained from the line-shape simulation of the MAS center band of the central transition. tensor. For each of the m = and m = transitions there will be two critical frequencies if TJ # 0. These will coincide when 7 = 0. The term (1 + TJ)corresponds to a shoulder of the satellite spectrum, which is not visible in the powder spectrum. The term (1 - 9 ) corresponds to an infinity in the unbroadened powder spectrum; it is this singularity which is measured in the experimental spectrum which we listed in the first row in Table 11. The negative sign is therefore chosen when utilizing the first satellite transition and a nonzero of TJ to calculate e2qQ. Both the static and M A S spectra show a consistent trend of increase in quadrupolar coupling constant with extent of dehydration. This is to be expected, as the electric field gradient would be expected to become more intense as the spherically symmetrical first coordination sphere of waters is removed. As the dehydration proceeds and the coupling constant becomes larger, the satellite transitions become more removed from the central transition, and less intense. Thus, the accuracy of determining the coupling constant from the position of the satellite transition decreases with increasing extent of dehydration.
Chu et al. Since calculating quadrupolar coupling constant, e2qQ, from the first satellite singularity depends also upon the asymmetry parameter, TJ,a value cannot be determined from the satellite splitting alone; in general a line-shape fitting is required to determine both e2qQ and TJ. In principle, this can be performed for both the static satellite singularity or the central transition under MAS. For the highly hydrated samples, Le., those with well-defined satellite splittings, the central transition is narrower than the dipolar broadening, and furthermore the transitions from Cs in sites 11, IV, and VI are too closely superimposed. To perform a line-shape fitting to a superposition of central transition powder patterns under M A S is not practical in this case. Recalling the fact that TJ was found to decrease with increased water content due to the spherically symmetrical first coordination sphere of water, the values of e2qQ for samples A, B, and C were determined from the first satellite transition alone, assuming a value of zero for TJ. As the dehydration proceeds, the splitting from these three sites separates, and it is possible to perform a meaningful fit of the central transition M A S spectra to a superposition of theoretical powder patterns. The fitting of the central transition in samples D, E, and F (discussed further in the Discussion section) yielded values of both 2 q Q and TJ. As a comparison, the 7 value was again used to determine e29Q by using the observed first satellite transitions from (1) for these samples as well. The larger deviation observed for sample F is due to the larger uncertainty present in estimating the first satellite splitting as mentioned previously. The values of the quadrupolar coupling constant, e2qQ, determined both from fit of the central transition and from the splitting of the first satellite, are listed in Table 11. Also listed are the experimentally observed values of the first satellite splitting, B,, and of TJ inferred from the fit to the central transition of samples D, E, and F. The second satellite transition (5/2,3/2) and (-3/2,-5/2) cannot be observed for Cs in the anhydrous sample and is just observable in the fully hydrated sample where the quadrupolar coupling is the weakest. The central transition line width (600 Hz) for the fully hydrated sample A in the M A S spectrum turns out to be 3-4 times larger than that calculated by using the quadrupolar coupling constant estimated from the position of the satellites. This residual experimental broadening for sample A may be accounted for in three ways: (1) The homogeneous broadening of the Cs nucleus due to heteronuclear dipolar coupling to 27Al;(2) an inhomogeneous distribution of the efg or the isotropic chemical shift throughout the sample;’ (3) a nonzero value of the asymmetry parameter 7, yielding a magnitude of e29Q larger than 210 kHz. The center of mass of a quadrupolar nuclei obtained by zeroing the first moment is a combined effect of the shift interaction and the true the second-order quadrupolar i n t e r a c t i ~ n . ~Hence ,~ isotropic chemical shift, C T ~does , not coincide with the center of gravity of the spectrum. A correction for the second-order quadrupolar shift, a,,, should be made once the quadrupole coupling constant e2qQ is determined by the relation8 ucm aqs
=
+
~ c s
cqs
(m) =
Z(Z + 1) - 9m(m - 1) - 3 40
-(
= -3 40
Je2qQ -2
(1
+ ;)f
where ucm is the center of mass of the central transition. The multiplication factorsf of the quadrupolar shifts are summarized in Table 111 for different transitions of half integer spin up to Z =
9/2.
The centers of mass of each peak in the 133CsMAS spectra are obtained through zeroing the first moment of the center band of the half-half central transition. These values with different
The Journal of Physical Chemistry, Vol. 91, No. 13, 1987
N M R of 133Csand IH in Mordenite
3591
TABLE 111: Multiplication Factor, f, of the Quadrupolar Shift (4 2)"
m 312 512 712 912
113 81100
15/212 24/942
-213 -1/100 6/212 15/942
(m,m - 1) and (-m
-281100 -21/212 -12/942
-66/212 -57/942
-120/942
+ l,-m) yield identical uqs.
TABLE IV: NMR Parameters of 133Csin Cs-Exchanged Mordenite F E D C B A
u,,,"ppm uqsrb ppm ucs,c ppm
-57 -190.0 -33 -24 -157
-61
-195.0 -15 -46 -179
-63 -198
-65
-10
-2.5 -62.5
-53 -188
-65
0 -65
-64
0 -64
"The center of mass of the central transition. *The second-order quadrupole shift calculated from (2). CTheisotropic chemical shift. degrees of hydration are listed in the first two rows in Table IV. The second-order quadrupolar shift uqsfor the central transition of 133Cs( I = 7/2) is calculated from (2) by using vo = 28.87 MHz and the best measured value of e2qQ (Table 11). The values of ucsare then obtained from ucs= u,, - ass.These values characterizing the N M R spectra of 'j3Csin the samples of the present work are listed in Table IV. All shift values are tabulated with increasing negative values being upfield.
Discussion In the fully hydrated Cs mordenite the Cs' ions can be considered as floating in the zeolite water, the coordination sphere of Cs+ thus being occupied by water molecules. This has the effect of placing all the Cs+ ions in the same symmetrical nearestneighbor environment and thus of all Cs' ions having the same isotropic chemical shifts. Dehydration results in the loss of the hydration water and the subsequent migration of Cs' ions to sites 11, IV, and VI of the zeolite lattice where coordination is now provided by the framework oxygen. Because of the differences in geometry, coordination number, and because of the replacement of water oxygens by lattice oxygens, different chemical shifts are expected for 133Csin the different cation positions. In the present study a detailed analysis of the 133CsN M R spectra for the anhydrous sample is greatly facilitated by the X-ray diffraction work of Schlenker, Pluth, and Smith2 who have described in detail the geometry of the Cs' ions located at the eight-ring sites I1 and IV, and the six-ring site VI with the site occupancy in fully exchanged Cs mordenite. This picture fits with the observed single sharp peak in the fully hydrated sample while the anhydrous sample exhibits individual chemical shifts at different sites. As the degree of hydration increases the quadrupolar constant e2Qq gradually decreases from 3.1 MHz to 210 kHz for the fully hydrated sample. This trend is seen in Figure 2, where the first satellites for samples A and B can be observed, and the satellite splitting gradually increases with dehydration. The spectrum under MAS further confirms that the residual broadening is mainly due to the second-order quadrupolar interactions. The broadening changes the observed chemical shift from 1.2 kHz for the anhydrous sample to 600 H z for the fully hydrated sample. In the fully anhydrous sample, there appear two main peaks from the C s spectrum under MAS. The peaks have a ratio of 3: 1, indicating that there are at least two different C s populations in the structure. The higher field peak (-1 57 ppm) under MAS appearing in the anhydrous compound has been fitted by a superposition of two central transitions with an intensity ratio of 2:l. The fit is shown in under the upfield peak in Figure 4. The parameters obtained in this fit are the asymmetry parameter, 9 = 0.6; the quadrupole coupling constant, e2qQ = 3.1 MHz; and the isotropic values of the two shifts in the upfield peak, u,, = -157 and -186 ppm.I2 The choice of single quadrupolar coupling constant and asymmetry parameters for Cs at the two sites is based
I'
I
2
I
0
I
I
- 2
-4
-6
I
-8
I
,
-10
-18
Shlfl I K " 3 I1XhZ = 3 4 54 .spunk
Figure 4. Fit of the MAS peaks of ")Cs in anhydrous Cs-exchanged mordenite to a superposition of three peaks. See Discussion for the fitting parameters. TABLE V Assignment of 133CsShift Value of Cs-Exchanged Mordenite" site I1 IV VI UcS, PPm -157.0 -186.0 -24.0 area (NMR) 2 1 1 population (X-ray) 3.78 1.86 1.75
uSites indicated are the same as in ref 2. upon the fact that the two Cs species resides in similar environment. This choice reduced the parameters and greatly simplifies the calculation. The downfield peak was fitted to a single central transition powder pattern under MAS. The parameters are 9 = 0.7, e2qQ = 3.2 MHz; and uCs= -32 ppm. The asymmetry parameter of the efg tensor is only a rough estimate. This cannot be determined unambiguously through line-shape analysis of the MAS spectra due to the signal-to-noise ratio limitations and the possible inhomogeneous distribution of these Although the quadrupolar coupling constant is expected to be much different from the -157 ppm species, this is not observed, however. This result may also explain why only one first satellite singularity is found for sample F. To exclude the possibility that the upfield peak of the fully anhydrous sample might include the overlap of the rotational side bands from the low-field peak (-24ppm), the spectrum was recorded for several different rotational frequencies. Invariance of the line shape with the varying rotation speed supported the idea that the upfield peak is a inhomogeneous superposition of two peaks but not overlapping of rotational side bands. Considering the information from X-ray data that a Cs ion at site VI is coordinated only on one side of the six-ring, while C s in sites I1 and IV are only slightly off center of an eight-ring, assignments for the three Cs nuclei are made as shown in Table V. The proton spectra and the 7', relaxation time for samples A-F also show interesting changes which are consistent with the above proposed mechanism of hydration. The trend of decreasing line width with decreasing water contents can be attributed to the chemical exchange motion of hydroxyl and water protons. As the water content decreases the hydroxyl group and water molecules tend to be less mobile and T , increase^'^,'^ The further broadening of the 'H spectra for the anhydrous sample can be (12) The program for simulating the center transition of the MAS spectra for arbitrary transition of half-integer quadrupolar nuclei is written according to the spatial dependent eigen energy expression from ref 5 . The transient decay goverened by the above eigen energy is first calculated and then Fourier transformed to obtain the spectrum. (13) Klier, K. J . Chem. Phys. 1973,58, 7 3 7 . (14) Shen, J. H.; Zettlemoyer, A. C . ; Klier, K. J . Phys. Chem. 1980, 84, 1453.
J. Phys. Chem. 1987, 91, 3592-3599
3592
due to shielding anisotropy plus an inhomogeneous distribution of the chemical shifts.
Conclusions 133Csis shown to be a useful nucleus for monitoring the local environments in mordenite by N M R . The N M R spectrum of 133Csin Cs-exchanged mordenite indicates that the efg tensor increases with decreasing water content. The quadrupole coupling constant increases from 2 10 kHz for the fully hydrated sample to 3.1 MHz for the anhydrous sample. The static spectra increase in line width from 1.2 kHz for the fully hydrated sample to 6 kHz for the anhydrous sample. Under MAS, the anhydrous sample shows two peaks, with relative intensities of roughly 1.3. Two different sites are clearly observed in the anhydrous sample with
center of mass of the peaks at -191.0 and -57 ppm. The assignment of the peaks to Cs locations is made on the basis of the structural difference of the six-ring coordination site VI from the eight-ring sites I1 and IV. After correction for the second-order quadrupolar shift, the downfield peak, -24 ppm, may be attributed to site VI while sites I1 and IV with similar structures yield similar chemical shifts at -157 and -186 ppm (see Table V). In the fully hydrated sample all three sites possess an identical isotropic value of -64 ppm.
Acknowledgment. This research was supported by the Assistant Secretary for Energy Research, Office of Energy Sciences, WPAS-KC-03-02-01. Registry No. '-'-'Cs,7440-46-2; Mordenite, 12173-98-7.
Spin-Polarized Electron Paramagnetic Resonance Spectra of Radical Pairs in Micelles. Observation of Electron Spin-Spin Interactions' Gerhard L. Gloss,*+* Malcolm D. E. Forbes,+ and James R. Norris, Jr.+* Department of Chemistry, The University of Chicago, Chicago, Illinois 60637, and Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439 (Received: January 9, 1987)
A model for polarized EPR spectra generated in radical pair reactions in micelles is being proposed. The model is based on electron spin-spin interactions which remain observablebecause of limited diffusion in micelles. The experimental observable is a doubling of hyperfine transitions, split by the magnitude of the interaction. The polarization is generated by the nonadiabatic generation of the radical pair with a triplet or singlet precursor. The time evolution of the wave function leads to a non-Boltzmann distribution of the populations among the four energy levels. The theory is tested by comparison with experiments, previously reported and repeated in this laboratory, obtained by laser flash photolysis of benzophenone in sodium dodecyl sulfate (SDS) micelles. Simulations of the shape of the spectra and their time dependence give excellent agreement with experiment. The model is further supported by experiments in micelles modified by different salt concentrations as well as different chain lengths of the micelle-forming molecules.
Introduction In several recent papers Hayashi and collaborators reported time-resolved EPR spectra, obtained on photolyses of ketones in micelles, which showed highly unusual chemically induced dynamic electron polarization (CIDEP).Z,3 The authors attributed the unusual features of the spectra to fast intramolecular hydrogen migration in the alkyl groups of the micelle-forming molecules. If correct, this explanation requires drastic departure from conventional thinking in free-radical chemistry in which fast 1,2-shifts of hydrogen in straight-chain alkyl radicals have no p r e ~ e d e n t . ~ In this paper we wish to present evidence against the proposed mechanism and to suggest a different explanation which has as its foundation well-accepted principles of magnetic resonance and does not require any unusual chemistry. We believe the spectra to result from correlated radical pair states, the observation of which is made possible by the rather special diffusion processes associated with micelles. In fact, we believe these spectra represent the first examples where highly resolved EPR spectra, obtained from mobile radical pairs in which the two interacting electron spin carriers are not chemically linked or frozen into a matrix, show clear evidence for electron spin-spin interaction. However, spin-echo experiments, reported by Thurnauer and Meise15have revealed good evidence for electron spin-spin interactions in micelles by exhibiting unusual phase shifts. The authors interpreted their results qualitatively with a model which is very similar to the one offered here. Other work can be interpreted to show such interactions, although the explanations offered were very qualitative and did not address the important features of the model to be discussed here.6 'The University of Chicago. Argonne National Laboratory.
*
0022-3654/87/2091-3592$01.50/0
Experimental Section All spectra were recorded on a Varian E-9 X-band EPR spectrometer modified for direct detection in the following manner: the signal from the preamplifier of the microwave bridge was connected directly to both gates of a PAR Model 162 boxcar averager. The gate sizes were 250 ns for all experiments. The boxcar output, after amplification, was fed continuously to an IBM PC-AT computer equipped with a Data Translation DT2801 ADC board, where further data reduction was performed. All displayed spectra and simulations have a sweep width of 200 G. An optical transmission (V-line) microwave cavity was used with a Suprasil flat cell, 0.5 mm optical path length, through which the samples were pumped a t rates no slower than 1 L/h. The microwave power was 30 mW for all experiments. The laser, Lambda Physik Model 103-MSC, operating at 308 nm with a pulse of 15 ns fwhm, was fired at repetition rates of 80-120 Hz. The pulse energy exiting the laser was 120 mJ/pulse. Flowing (1) Work at the University of Chicago was supported by NSF Grant CHE-8520326 and work at Argonne by the Office of Basic Energy Sciences, US-DOE, under Contract No. W-31-109-ENG-38. (2) (a) Sakaguchi, Y.; Hayashi, H.; Murai, H.; I'Haya, Y. J. Chem. Phys. Lett. 1984, 110, 275. (b) Sakaguchi, Y.; Hayashi, H.; Murai, H.; I'Haya, Y. J.; Mochida, K. Chem. Phys. Lert. 1985, 1.20, 401. (c) Murai, H.; Sakagushi, Y.; Hayashi, H.; I'Haya, Y. J. J . Phys. Chem. 1986, 90, 113. (3) For general references on CIDEP see: (p) Spin Polarization and Magnetic Effects in Radical Reactions; Molin, Yu. N., Ed.; Elsevier: Amsterdam, 1984; Chapters 2, 4, 8. (b) Chemically Induced Magnetic Polarization; Muus, L. T., Atkins, P. W., McLauchlan, K. A,, Pedersen, J. B., Eds.; Reidel: Dordrecht, Holland, 1977; Chapters V-IX, XI, XIX. (4) Beckwith, A. L. J.; Ingold, K. U. In Rearrangements in Ground and Excited States: deMayo, P., Ed.; Academic: New York, 1980; p 252. (5) Thurnauer, M. C.; Meisel, D. J . A m . Chem. SOC.1983, 104, 3729. (6) (a) Turro, N. J.; Paczkowski, M. A,; Zimmt, M. B. Chem. Phys. Lett. 1985, 114, 561. (b) Trifunac, A . D.; Nelson, D.J . Chem. Phys. L e f t . 1977, 46, 346.
0 1987 American Chemical Society
