J. Phys. Chem. 1995, 99, 4679-4686
4679
Silver Agglomeration in SAPO-5 and SAPO-11 Molecular Sieves Jacek MichalikJ Naoto Azuma,f Jaroslaw Sadlo: and Larry Kevan*3' Institute of Nuclear Chemistry and Technology, Dorodna 16, 03-195 Warsaw, Poland, and Department of Chemistry, University of Houston, Houston, Texas 77204-5641 Received: October 11, 1994; In Final Form: January 5, 1995@ Electron spin resonance and electron spin echo modulation (ESEM) spectroscopies have been used to study the location and coordination of silver ionic clusters and silver-alcohol adducts stabilized in y-irradiated Ag-SAPO-5 and Ag-SAPO- 11 molecular sieves. It was found that in dehydrated samples silver ionic clusters are not stabilized in contrast to zeolites, but after exposure to methanol, ethanol, and propanol before irradiation Ag2+ and Ag32+ are efficiently stabilized. Besides silver clusters, the formation of organosilver radicals is observed, i.e., Ag'CH20H+. By analyzing the ESEM spectra it was shown that organosilver radicals are located in the 10-ring channels of Ag-SAPO-11 and the 12-ring channels of Ag-SAPO-5 and are coordinated by two nonequivalent methanol molecules. Ag2+ is located in the 6-ring channels and in SAPO-5 interacts with three equivalent CH30H groups, whereas in SAPO-1 1 two methanol molecules are distinctly closer to Ag2+ than the third one. The higher yield and stability of Ag2+ in SAPO-1 1 is associated with the preferential location of Ag+ cations in two adjacent 6-ring channels. This increases the probability of encounter between radiation-induced mobile Ago and AgS
Introduction
SAPO-5
Silicoaluminophosphate (SAPO) molecular sieves form a new class of microporous crystalline materials comparable to the well-known zeolites, or aluminosilicatemolecular sieves. Zeolites, which have been widely used for adsorption and catalysis, have pores and channels formed by aluminum and silicon tetrahedra linked by oxygen bridges. Substitution of other elements for A1 and/or Si in the molecular sieve framework can yield various kinds of new materials. In 1982, Wilson et al. reported the synthesis of aluminophosphate (AlP04) molecular sieves.',2 The structure of Alp04 molecular sieves includes novel structure types, such as AlPO4-5, as well as structure types analogous to certain zeolites, such as A1P04-37 (faujasite structure). These materials have a neutral framework and no ion exchange capacity. In 1984, Lok et al. reported the synthesis of SAPO molecular sieve^,^.^ which can be viewed as silicon-substituted APo4. The numbering of structure types of SAPO follows that of so that SAPO-5 denotes the SAPO molecular sieve that possesses the same framwork structure as A1P04-5. The addition of silica into the aluminophosphate structure introduces both ion exchange capacity and catalytic a~tivity.~ More silicon substitutes for phosphorous than for aluminum so there is a net negative charge for the SAPO framework. Here we compare the radiation-induced silver agglomeration process in the SAPO-5 and SAPO-11 structures which do not have analogs in zeolite structures. These structures are shown in Figure 1 in which it can be seen that SAPO-5 is composed of 4-ring, 6-ring, and 12-ring straight channels which are interconnected by 6-ring windows, while SAPO-11 consists of 4-ring, 6-ring, and 10-ring channels. The important structural difference between SAPO-5 and SAPO-11 is that SAPO-5 has a circular 12-ring channel (7.3 A) while SAPO-11 has an elliptical 10-ring channel (6.4 x 4.0 A). The framework negative charges are balanced by H+ ions and the cationic form of the templating agent in the assynthesized SAPO and by H+ ions only in H-SAPO, the calcined form of SAPO molecular sieve. H+ ions can be exchanged by
' Institute of Nuclear Chemistry and Technology.
* University of Houston. @
Abstract published in Advance ACS Abstracrs, March 1, 1995.
P-l 12-ring channel
channel
SAPO-I 1
\
6-ring channel
10-ring channel
Figure 1. Framework of SAPO-5 and SAPO-11 viewed along [OOl], where the solid circles are Al, P, or Si (T atoms) and the open circles are 0 atoms: oxygens above the plane of the T atoms in a given ring are represented by 0, and those below the plane by 8.
Ag+ ions, and the ion-exchanged products are denoted AgSAPO-5 and Ag-SAPO- 11.
0022-3654/95/2099-4679$09.Q0/0 0 1995 American Chemical Society
4680 J. Phys. Chem., Vol. 99, No. 13, 1995
Michalik et al.
Ag/SAPO-I I (synthetic) dehydrated with flowing 0 2
-
Samples gradually annealed to 200 K .)
lESEMl Annealing to AT
.+ Figure 2. Diagram illustrating the experimental treatment and measurements of SAPO-5 and SAPO-l l.
AgISAPO-11
cation exchanged
dehydrated
200 G
Figure 4. ESR spectra of synthetic AG-SAPO-11 y-irradiated at 77 K and measured during thermal annealing at the temperatures shown.
--
The silver agglomeration process in these two materials is followed by electron spin resonance (ESR), whereas the local environment of the silver clusters of different nuclearity is studied by electron spin echo modulation (ESEM) spectroscopy. ESEM is particularly useful in determining the distances to deuterium in specifically deuterated adsorbate molecules, which leads to the coordination geometry and the location of the silver cluster. In this work we focus on the differences in the coordination of dimeric and trimeric silver clusters between the SAPO-5 and SAPO-1 1 structures which can have relevance for their different catalytic activities. Experimental Section
Figure 3. ESR spectra of cation-exchanged and synthetic Ag-SAPO11 y-irradiated at 77 K and measured during thermal annealing at the temperatures shown.
The molecular sieves H-SAPO-5 and H-SAPO-11 were synthesized according to the Union Carbide patent3 as slightly modified in our l a b ~ r a t o r y . ~Silver . ~ ion was exchanged into these materials with an aqueous solution of silver nitrate to less than one silver ion per three unit cells. The amount of silver in AgH-SAPO-5 and AgH-SAPO-11 was determined by commercial atomic absorption analysis as 1.72 wt % and 0.82 wt %, respectively. Silver was also successfully incorporated into
Silver Agglomeration in Molecular Sieves
Ag/SAPO-11 (synthetic) high field lines
J. Phys. Chem., Vol. 99, No. 13, 1995 4681
AS-SAPO-5 I CD3OH
Figure 7. ESR spectra of Ag-SAPO-YCD3OH showing the formation of the Ag3*+ cluster (species D) measured after thermal annealing to 280 K.
Figure 5. High-field ESR lines in AG-SAPO-11 measured during thermal annealing at the temperatures shown showing transformation of Ago atoms to Ag2'.
200 G
1
Figure 6. ESR spectra of Ag-SAPO- 11KH30H y irradiated at 77 K and measured during thermal annealing at the temperatures shown: (A) Ago, (B) Ag'CHZOH', and (C) Ag2+.
SAPO-11 material by the addition of AgN03 to the synthesis mixture for SAPO-11 using diisopropylamine as a templating
agent. The as-synthesized material was yellow-green and characterized as the SAPO-1 1 structure by X-ray diffraction. The templating agent was removed by heating the as-synthesized Ag-SAPO-11 to 823 K in flowing oxygen. After drying, the ion-exchanged SAPO materials were loaded into 3 mm 0.d. x 2 mm i.d. Suprasil quartz ESR tubes and dehydrated under vacuum for 12 h at 200 "C. The samples were then heated at 300 "C under 500 Torr of dry oxygen for 3 h to reoxidize any reduced silver cations. After removal of the oxygen some samples were sealed off, and others were exposed at room temperature to specifically deuterated alcohol adsorbates including methanol, ethanol, and propanol from Stohler Isotopes and Aldrich Chemicals. All samples were y-irradiated in a Gammacell 220 6oCo source with a dose rate of 5 kGy h-I. A diagram illustrating the sample treatment before ESR measurements is shown in Figure 2. ESR spectra were recorded on a Bruker ESP-300 X-band spectrometer at 77 K or in the temperature range 120290 K by using a variable-temperature Bruker unit. ESEM signals were measured on a Bruker ESP-380 pulsed ESR spectrometer at temperatures in the range 4.4-5.4 K by using a helium flow cryostat. Usually a three-pulse experiment with a pulse sequence of 90"-z-90"-T-90" was employed and T was swept. The deuterium modulation was recorded for z = 0.28 ps to suppress modulation from zeolitic *'Al.To record 27Almodulation z was set to 0.40 ps. The ESEM data were analyzed by using an HCP 386 IBM PC compatible computer. Simulations of the experimental deuterium modulations were performed with the analytical expressions derived by Dikanov et aL8 The best fits were found by varying the number of interacting nuclei N , the interaction distance R, and the isotropic hyperfine interaction A,so,until the sum of the squared residuals was minimized. Results and Discussion We showed earlier in AgNa-A zeolites that cationic silver clusters are easily formed by y-irradiation at 77 K and subsequent thermal annealing regardless of the dehydration condition^.^ In contrast, in Ag-SAPOJ and Ag-SAPO-11 small silver clusters are not stabilized in hydrated samples or in
Michalik et al.
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AS-SAPO-5 / C ~ H S O D
Ag-SAPO-1 I/ C2H50D
225 K
290 K
200 G
Figure 8. ESR spectra of Ag-SAPO-ll/C*H50D irradiated at 77 K and measured during thermal annealing at the temperatures shown: (A) Ago, (B) Ag'CH(CH3)OD+, and (C) Ag2+.
samples dehydrated in vacuum and activated with static oxygen at 250 "C. The ESR spectra of dehydrated Ag-SAPO-11 prepared with Ag+ in the synthesis mixture (synthetic) or by cation exchange after y-irradiation at 77 K show (Figure 3) very weak lines representing Ago atoms and Ag2+ cations and a more intense, slightly anisotropic single line M at g = 2.007 with AHQQ= 31 G which is assigned to conduction electron spin resonance (CESR) in silver metallic clusters of 1.0 nm diameter as predicted theoretically by Kawabata.Io On thermal annealing, Ago and Ag2+ decay completely above 200 K and there is no buildup of any new ESR lines associated with that decay. The same was observed for dehydrated Ag-SAPO-5 samples. However, when samples are dehydrated in the presence of flowing oxygen at 300 "C, the intensity of the Ago doublet is distinctly higher and on annealing a new doublet split by 625 G (Figure 4)is formed. We assign this doublet tentatively to the Ag2+ species, although the central line near g = 2.00 is not observed because of overlap with the intense CESR line. The spectral changes due to decay of Ago and formation of Ag2+ are shown for the high-field region in Figure 5. The structure of the Ag2+ lines results form the presence of three dimers with different isotopic content: lo7Ag2+,lo7Ag'09Ag+,and lo9Ag2+, as well as from g or A anisotropy (Figure 5). This indicates that silver atoms produced by autoreduction during dehydration in vacuum are able to move more freely
D
-
300 K
200 G
D D
V
Figure 9. ESR spectra of A~-SAPO-S/C~HSOD irradiated at 77 K and measured during thermal annealing at the temperatures shown: (A) Ago, (B) Ag'CH(CH3)OD+, (C) Ag*+, and (D) Agj2+.
through the channels toward the surface of the SAPO crystallites where metallic agglomerates are formed. Oxidation with static 0 2 at 250 "C is not sufficient to redistribute silver metallic agglomerates back into the molecular sieve cages. This autoreduction process to form metallic silver is much less effective when dehydration is carried out in the presence of flowing 0 2 . It turns out, however, that the process of the formation and stabilization of small silver ionic clusters is even more effective in the presence of alcohol molecules adsorbed before irradiation. The ESR spectra of Ag-SAPO-lKH3OH are shown in Figure 6. After annealing at 125 K the major paramagnetic species are hydroxymethyl radicals ('CH20H) represented by an ESR triplet at g 2.00 and silver atoms Ago (A). After annealing at 170 K the intensities of the 'CH20H and Ago lines are much smaller and a new spectrum, doublet B with a 180 G hyperfine splitting at g = 2.003, appears. ESR doublet species with a much smaller hyperfine splitting than for silver atoms were reported earlier and associated with silver clusters or organosilver radicals."%'* In irradiated low-temperature methanol glasses Janes and co-workers'* observed a doublet with a 136 G splitting at g = 2.007. In Ag6Na-A zeolite exposed to methanol before irradation we recorded a doublet with a 106 G splitting at g = 2.003." In both cases, the doublet was assigned to silver-
-
J. Phys. Chem., Vol. 99,No. 13, 1995 4683
Silver Agglomeration in Molecular Sieves
L o n
1.o
R,m A,MHr 0.34 0.17 0.48 0
Shell N 1 1 2 1
Shell N 9
1
R,nm A.MHz 0.43 0
0.8
c 0.6
VI
I-
P Shell N 1 2
id
z
. 6
3
IJ
I
.
0.8
n
R,nm A.MHz 0.47 0.09 0.36 0
Shell
N
1
3
R,nm A,MHz 0.34 0.1
I I
O 0.2 m4I
Oa2
U
0
0
1
2
5
3 4 T, ys
TABLE 1: ESEM Simulation Parameters for Silver Species in SAPO-11 silver species annealing temperature, K adsorbate shell N R, nm ~~
230
CH3OD
1 2 1 2 1 2 1 2 1 1
0.34 1 0.48 CD30H 3 0.47 3 0.36 2 0.31 Ag2+ 300 CH3OD 1 0.48 CDjOH 6 0.41 3 0.39 Ag'C 300 CzHsOD 2 0.37 Ag2+ 300 CzHsOD 2 0.37 1 0.41 1 0.36 Ag'C 240 C3H70D 1 a Ag'C stands for silver hydroxymethyl, silver hydroxyethyl, or silver hydroxypropyl radicals specifically deuterated. 1
TABLE 2: ESEM Simulation Parameters for Silver Species in SAPO-5 silver species annealing temperature, K adsorbate shell N R, nm ~
AgT"
220
2
4
3
5
T, ys Figure 11. Experimental and simulated three-pulse ESEM spectra
Figure 10. Experimental and simulated three-pulse ESEM spectra recorded at 6 K of the Ag'CH20H+ radical in SAPO-1 1 with adsorbed CH3OD and CD30H.
Ag'C"
1
CH30D
1 1 0.34 2 1 0.42 210 CD3OH 1 3 0.45 2 3 0.39 Agz+ 220 CH30D 1 3 0.34 CD3OH 1 9 0.43 Ag3" 240 CHjOD 1 2 0.29 CD30H 1 6 0.40 Ag'C stands for silver hydroxymethyl radicals specifically deuterated.
hydromethyl radicals Ag'CH20Ht. A similar doublet appears during thermal annealing of Ag-SAPO-5NeOH. Then however, the hyperfine splitting is 147 G. In both SAPO-11 and SAPO-5 the hyperfine splittings are substantially larger than in a A zeolite indicating a larger spin density on the silver nucleus. In A zeolite, methanol molecules are located in a-cages with a diameter of 11.6 A, whereas in SAPO molecular sieves they are in channels with much smaller cross sections: circular with
recorded a 6 K of the Ag2+cluster in SAPO-5 with adsorbed CH3OD and CD3OH.
a 7.3 8, diameter in SAPO-5 and an elliptical 6.4 8, x 4.0 8, channel in SAPO-11. Thus, it seems reasonable to postulate that steric hindrances can affect the structure of the silverhydroxymethyl radicals which modifies the singly occupied molecular orbital and leads to different spin densities on the silver nucleus. On annealing to temperatures higher than 200 K in Ag-SAPO11, the silver atom ESR lines transform completely into triplet C with the 308 G splitting of the Ag2+ cluster (Figure 6). This spectrum is unchanged up to room temperature. In Ag-SAPO-5 the spectral changes up to 250 K are similar to those in AgSAPO-1 1. However, the Ag2+ dimer in SAPO-5 is not stable above 250 K and is transformed to Ag32+ represented by an ESR quartet with a splitting of 200 G at g = 1.973 (species D in Figure 7). The silver clustering process in SAPO-5 and SAPO-11 in the presence of ethanol proceeds similarly (Figure 8 and 9); however, Ag2+ and Ag32+ are stabilized more efficiently. In contrast, the yield of silver-hydroxyethyl radicals is much less. More bulky ethanol molecules can completely pack the channels and block long-distance migration of Ago at higher temperature. Owing to that, the local concentration of silver atoms inside the channels does not decrease with the annealing temperature. This increases the number of encounters between Ago and Agf and leads to a several times higher cluster yield. Steric hindrance is suggested to be an important factor for the reaction between hydroxyethyl radicals ('CH3CHOH) and Ag+ in SAPO channels which decreases the yield of the AgVCH(CH3)0H+ radical by several times. The reorientation of hydroxyethyl radicals into a reactive configuration in comparison to hydroxymethyl radicals seems to require more energy because the steric barriers in the narrow SAPO channels play a more important role for larger species. The clustering process was also studied in the presence of n-propanol. It was found to be basically the same as for samples with methanol and ethanol. Three-pulse electron spin echo modulation (ESEM) spectra showing deuterium modulation have been recorded for silver-
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6-ring channel - /
AS-SAPO-1 1 1st shell
2nd shell
1MeOH
1 MeOH
\
/
6-ring window
1st shell 2nd shell 2MeOH 1MeOH
Figure 12. Schematic representation of the location of the Ag'CHZOH' radical denoted as Ag'C and the Ag2+ cluster coordinated to methanol in Ag-SAPO-11, showing two nonequivalent MeOH interacting with Ag'C and three MeOH groups located in different 10-ring channels interacting with Agz+.
hydroxymethyl radicals as well as for Ag2+ and Ag3*+ formed during thermal annealing of Ag-SAPO- 11 and AG-SAPO-5 samples exposed to deuterated methanol, ethanol, and propanol vapor. Figure 10 illustrates ESEM signals in y-irradiated AgSAPO-11 with adsorbed CH30D and CD30H for silverhydroxymethyl radicals. Simulations of these signals indicate that these radicals interact with two nonequivalent methanol molecules. The distances to the methyl and hydroxyl deuterons for both methanol molecules are listed in Table 1. A configuration of Ag'CH20H+ with weakly coordinated methanol is found also for Ag-SAPO-5, although the distances are slightly different (Table 2). The ESEM spectra of Ag2+ in both SAPO-5 and SAPO-1 1 were recorded during the same experiment by setting the magnetic field on the high-field ESR line of Ag2+. The ESEM spectra together with simulations for Agz+ in Ag-SAPO-5 with adsorbed CH30D and CD30H are shown in Figure 11. For CH30D the best fit was obtained for three deuteriums located at 0.34 nm from Ag2+. For CD3OH the simulation gives 9 nuclei at a distance of 0.43 nm. For the simulation of Ag2+ in SAPO-11 it was necessary to use a two shell model (Table 1). Ag2+ in SAPO-11 is also coordinated with three methanol molecules, but two of them are located distinctly closer to the dimer than the third one. The ESEM results for the silver trimer Ag32+ in SAPO-5 (Table 2 ) indicate direct coordination with two methanol molecules. Such a configuration is not possible for Ag32f
located in a 6-ring channel and indicates the location of Ag32+ in the 12-ring channel. The locations and postulated coordination geometries for the Ag'CH20H+ and Ag2+ species in SAPO-1 1 and SAPO-5 are shown in Figures 12 and 13, respectively. The ESEM data indicate that silver-hydroxymethyl radicals are located in the large 10-ring or 12-ring channels, whereas the Ag2+ clusters are in the 6-ring channels in both molecular sieves. There are, however, some differences in the locations. In SAPO-11 the distance between the Ag'CH20H+ radical and the second-shell methanol is distinctly larger than in SAPO-5. This means that the Ag'CH20H' radicals occupy a more central position in the 10-ring channel of SAPO-1 1. In such a location there is less spin density on the framework nuclei, which explains the larger Ag splitting for Ag*CH20H+ in SAPO-1 1 compared to SAPO5. Dimeric silver is produced in Ag-SAPO-11 with a much higher yield than in Ag-SAPO-5. This can be related to the structure of the SAPO-11 molecular sieve which consists of two adjacent' 6-ring channels. It seems reasonable to assume that Ago atoms formed radiolytically in a hexagonal prism will preferentially migrate through a 6-ring channel until they meet a Ag+ cation in the same or an adjacent channel. The probability of such an encounter will be higher in SAPO-11 because two 6-ring channels are adjacent. The coordination geometry of Ag2+ in SAPO- 11 with two methanol molecules closer to the silver dimer than a third one suggest a displacement
J. Phys. Chem., Vol. 99, No. 13, 1995 4685
Silver Agglomeration in Molecular Sieves
Figure 13. Schematic representation of the location of the Ag'CH20H+ radical denoted as Ag'C and the Ag2+ cluster coordinated to methanol in Ag-SAPO-5, showing Ag'C interacting with two nonequivalent MeOH groups and Ag2+ interaction with three equivalent MeOH groups.
of Agz' from the center of a 6-ring channel toward the border between two adjacent 6-ring channels in SAPO-11. This might indicate a more probable encounter of two silver species located in different adjacent 6-ring channels. The reason for the higher stability of Ag2+ in SAPO-11 is not clear and cannot be associated only with the presence of adjacent 6-ring windows. We tentatively associate that with the smaller dimensions of the 10-ring channel in SAPO-11, in which there is less free space between methanol molecules than in SAPO-5. Then the 6-ring windows to the large channels can be blocked and prevent migration of Ag2+ from a 6-ring to a large channel. The ESR results for Ag2+ stabilized in the presence of ethanol support this interpretation. In both molecular sieves the final products of silver agglomeration, Ag2+ in SAPO-1 1 and Ag32+ in SAPO-5, are observed to be stable for hours at room temperature in the presence of ethanol whereas in the presence of methanol they decay during minutes. The ESEM data indicate that Ag32f in SAPO-5 is located in the 12-ring channel, in which it is directly coordinated by two methanol molecules. This confirms that Ag2+ in SAPO-5 is able to migrate much more easily to the 12-ring channel, where it forms Ag32+by reaction with Ag+. It should be stressed that the silver agglomeration process in both molecular sieves proceeds in exactly the same way in the presence of ethanol and n-propanol as in the presence of methanol. The ESEM results for Ag-SAPO-11 exposed to different alcohols show that the distance between Ag2+ and the hydroxyl deuterons increases with the length of the alcohol alkyl chain. This can be explained by steric hindrances which prevent the larger molecules from
direct coordination with Ag2+ with their molecular dipole pointed toward the cation.
Conclusions In hydrated and dehydrated Ag-SAPO-11 and Ag-SAPO-5 Ago formed by y-irradiation does not form stable cationic silver clusters as it does in A zeolite. The only exception is Ag2+ which is formed with low yield in SAPO-11 when it is dehydrated in flowing oxygen. However, in SAPO samples exposed to methanol, ethanol, and propanol before irradiation, silver dimers in SAPO-11 and silver dimers and trimers in SAPO-5 are efficiently formed and stabilized. The role of alcohols in the silver agglomeration process is at least two-fold. First, by scavenging holes they prevent Ag2+ formation. Owing to that, during y-radiolysis the concentration of silver atoms and cations, the species active in the agglomeration process, is higher. Second, by blocking the large 10-ring or 12-ring channels, alcohols can decrease the silver atom and silver cluster mobility to reduce the formation of large metallic clusters. It was shown that the mobility of the silver species is more effciently reduced in SAPO- 11 than in SAPO-5, in which Ag32+clusters are formed on annealing above 220 K. In SAPO11, the dimeric silver cluster is unchanged even at room temperature. During y-radiolysis, silver-hydroxymethyl radicals are formed in addition to small silver clusters. The silverhydroxymethyl radicals are located in the large 10-ring or 12ring channels and interact with two nonequivalent methanol molecules located in two different channels.
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Acknowledgment. This research was supported by the U.S.-Poland Curie Fund (Grant PAA-NSF-92-91), the U.S. National Science Foundation, and the Robert A. Welch Foundation. References and Notes (1) Wilson, S. T.; Lok, B. M.; Flanigen, E. M. U.S.Patent 4310440, 1982. (2) Wilson, S. T.; Lok, B. M.; Messina, C. A.; Cannan, T. R.; Flanigen, E. M. J . Am. Chem. SOC. 1982, 104, 1116. (3) Lok, B. M.; Messina, C.A.; Patton, R. L.; Gajek, R. T.; Cannan, T. R.; Flanigen, E. M. U.S.Patent 4440 871, 1984. (4)Lok, B. M.; Messina, C. A,; Patton, R. L.; Gajek, R. T.; Cannan, T. R.; Flanigen, E. M. J . Am. Chem. SOC. 1984, 106, 6092.
Michalik et al. (5) Flanigen, E. M.; Lok, B. M.; Patton, R. L.; Wilson, S. T. Pure Appl. Chem. 1986, 58, 1351.
(6) Chen, X.; Kevan, L. J . Am. Chem. SOC. 1991, 113, 2861. (7) Lee, C. W.; Chen, X.; Kevan, L. J. Phys. Chem. 1991, 95, 8626. (8) Dikanov, S. A.; Shubin, A. A,; Parmon, V. N. J. Magn. Reson. 1981, 42, 474. (9) Michalik, J.; Kevan, L. J . Am. Chem. Soc 1986, 108, 4247. (10) Kawabata, A. J. Phys. SOC. Jpn. 1970, 29, 902. (1 1) Wasowicz, T.; Mikosz, J.; Sadlo, J.; Michalik, J. J . Chem. SOC., Perkin Trans. 2 1992, 1487. (12) Janes, R.; Stevens, A. D.; Symons, M. C. R. J. Chem. Soc., Faraday Trans. I 1989, 85, 3973.
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