J . Phys. Chem. 1990, 94, 4151-4754 employed to confirm the results.
Summary The present calculations indicate that Monte Carlo simulations are capable of providing useful estimates of confidence intervals for optimized values of parameters derived from nonlinear regression problems. Customary methods of variance analysis based on calculation using the covariance matrix may lead to incorrect
4751
results in problems where parametric covariances are important. Even in cases where the covariances may be safely neglected, non-Gaussian distributions of calculated parameters preclude probabilistic interpretations of standard error estimates derived in the usual manner. Acknowledgment. We are indebted to Prof. L. M. Schwartz for many helpful discussions during the preparation of this work.
Radiolytic Generation of Organic Radical Cations in Zeolite Na-Y X.-Z. Qin and A. D. Trifunac* Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439 (Received: October 20, 1989; In Final Form: December 15, 1989)
Several examples of radiolytically generated organic radical cations in zeolite Na-Y are illustrated. EPR studies of organic radical cations can be carried out in a wide range of temperatures up to room temperature. In every case, monomeric radical cations were observed. Comparison to previous work in freon and xenon matrices is made, illustrating that in the zeolite Na-Y there is considerably weaker radical cation-host interaction. A mechanism of radiolytic generation of radical cations in zeolite Na-Y is proposed.
Introduction The study of organic radical cations is closely tied to the methods that allow their preparation and stabilization. In the past decade, many organic radical cations have been examined by EPR and optical absorption spectroscopy using low-temperature halocarbon so1vents.l With smaller organic cations, the more inert neon matrix was successfully utilized.2 Recently, we have found that xenon matrices can be used to study organic radical cations by EPR over a considerable temperature range.3 A recent report has illustrated how superacidic membranes such as Nafion can be used to stabilize and study radical cations, even at room
temperature^.^ In order to stabilize organic radical cations, two dominant processes must be prevented. First, we must prevent the reverse electron transfer when the electron, ejected from the neutral to give the radical cation (eq I ) , returns to neutralize the radical cation (eq 2). RH
+ hv or y
RH'+
RH'+
e-
+ e-
RH*
(1)
(2)
This is accomplished by scavenging the electron and preventing the possibility of back electron transfer as in halocarbons, where dissociative electron capture (eqs 3 and 4) takes care of the electron permanently. e-
+ RCI
RCP-
-
-
R'
RCI'-
(3)
+ CI-
(4)
The second reaction we must prevent is an ion-molecule reaction of the radical cation. Several dominant radical cation reactions ( I ) (a) Shiotani, M. Magn. Reson. Reu. 1987, 12, 333. (b) Symons, M. Chem. Soc. Reu. 1984, 393. (c) Shida, T.; Haselbach, E.; Bally, T.Acc. Chem. Res. 1984,17, 180. (d) Lund, A.; Lindgren, M.; Lunnel, S.;Maruani, J. In Topics in Molecular Organization and Engineering, Acad. Publ.: Dordrecht, 1988; pp 259-300. (2) Knight, L. B. Acc. Chem. Res. 1986, 19, 313. Trifunac, A. D. J. Phys. Chem., in press. (3) Qin, X.-Z.; (4) Jacob, S. L.; Craw, M. T.; Depew, M. C.; Wan, J . K. S . Res. Chem. Inrermed. 1989, I I . 271.
0022-3654/90/2094-415 1 S02.50/0
have been examined by the time-resolved fluorescence detected magnetic resonance (FDMR) technique.s The main reaction that alkane radical cations undergo in hydrocarbons is proton transfer to the neighboring neutral molecules. This reaction can be prevented by segregating the radical cation from its parent neutrals as is done in halocarbons or inert gas matrices. In some cases, as in neon matrices, the rigidity of the matrix and the very low temperatures used, 4-1 0 K, effectively freeze any molecular motion. However, restricted temperature range is a serious limitation of neon or any inert gas matrix for the study of radical cations since well-resolved EPR spectra of larger organic cations and other temperature-dependent effects cannot generally be observed. Halocarbon matrices have had the most widespread use due to a considerable temperature range available up to 160 K. But, there is often a very strong cation-matrix interaction. The structure and chemistry exhibited by the radical cation can be a consequence of matrix interaction and may not reflect an intrinsic behavior of radical cations. The xenon matrix also has a wide temperature range, up to 110 K, but electron scavengers must be used, so there is still a possibility of interaction between the cation and anions produced, albeit far less than in the halocarbon matrix. Here we describe yet another way of generating and stablizing organic radical cations. As this preliminary report will illustrate, zeolite Na-Y satisfies two main criteria for stabilizing radical cations. A variety of organic radical cations can be radiolytically generated in zeolite Na-Y where no electron scavengers are used, and the tremendous flexibilityand temperature range make zeolite Na-Y one of the most promising means of stabilizing and studying organic radical cations with minimal matrix and counterion complications. Radical cations of a few aromatic compounds were studied on y-irradiated silica gel or the H-Y molecular sieve,6 but we are aware of only one previous report in which alkane radical cations were observed at 4 K in a synthetic zeolite (ZSM-5).'
-
( 5 ) Werst, D. W.; Bakker, M. G.; Trifunac, A. D. J . Am. Chem. Soc., in press. (6) Komatsu, T.; Lund, A. J . Phys. Chem. 1972, 76, 172'7. (7) Toriyama, K.; Nunome, K.; Iwasaki, M. J . Am. Chem. Soc. 1987, 109, 4496.
0 1990 American Chemical Society
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Qin and Trifunac
The Journal of Physical Chemistry, Vol. 94,No. I I, 1990
Experimental Section The zeolite Na-Y sample (Princeton Standard Catalyst, 1975) was used without further purification. 1, I ,2,2-Tetramethylcyclopropane from Wiley, hexamethyl(Dewar benzene) ( 1,2,3,4,5,6-hexamethyIbicyclo[2.2.0]hexa-2,5-diene),tetramethylethylene, and trans-2,3-dimethylethyleneoxide from Aldrich were all used as received. The zeolite samples were prepared by the following way: 0.05 g of zeolite Na-Y was dehydrated in a 2.5-mm-o.d., 25-cm-long Suprasil EPR cell under a vacuum of Torr at 400 OC for 6 h. After being completely degassed by the freeze-pump-thaw method, 0.02 g of tetramethylcyclopropane or hexamethyl(Dewar benzene) was transferred into the EPR cell under the vacuum by freezing the zeolite at 77 K. Samples of tetramethylethylene and trans-2,3-dimethylethyleneoxide in zeolite were prepared by exposing the zeolite at room temperature in the EPR cell to the organic vapor at a pressure of 200 Torr. In each case about 0.008 mol of compound was adsorbed by the zeolite. The EPR tube was sealed off and kept at room temperature overnight in order for the sample to reach complete equilibrium. Samples before y-irradiation did not show EPR signals of the organic compounds. After samples were irradiated by 6oCoy-rays at 77 K to a dose of 0.5 Mrad, EPR measurements were made on a X-band (9.3GHz) Varian E109 spectrometer with 100-kHz field modulation. Data collection was assisted by the use of a Macintosh I1 computer. An Air Products LTR-3 liquid-transfer Heli-Tran refrigerator was used for temperature control between 60 and 250 K. For photolysis of y-irradiated zeolite samples, xenon UV and visible illuminators from ILC Technology were used.
50
c
Figure 1. (a) An EPR spectrum observed at 180 K for the tetra-
methylcyclopropane radical cation in the zeolite Na-Y and (b) its simulation by using the EPR parameters in Table I and a Lorentzian line width of 3.5 G.
An.
Results The zeolite we used has a formula of Na57(A102)57(Si02),35*240H20.The pretreatment of the sample has removed the water. The framework is composed of SO4and A104 tetrahedra that are linked to each other by shared oxygen atoms to form intracrystalline interconnected cavitiess as depicted by 1 (oxygen atoms are not shown). Each aluminum atom of the framework contributes a negative charge. These negative charges are balanced by exchangeable cations. Most (99%) cations are Na+. About 1% of cations, such as Fe3+ and Mn2+, exist as impurities.
Figure 2. An EPR spectrum observed at 100 K for the hexamethyl-
(Dewar benzene) radical cation in the zeolite Na-Y. this temperature it undergoes ring opening to form a paramagnetic species (3) with the unpaired electron localized at one dimethyl-substituted carbon. H
'I,
1
The open 12-membered rings form nearly circular apertures of diameter about 0.7 nm, through which organic molecules can be adsorbed into roughly spherical supercages with a diameter of 1.25 nm. In this study we selected four organic molecules (RH), each having a size about 0.7 nm, and we observed the corresponding monomeric radical cations in each case. Figure l a shows the EPR spectrum recorded at 180 K after y-irradiation of the tetramethylcyclopropanezeoliteNa-Y system at 77 K. The radical cation of tetramethylcyclopropane was first studied in freon mat rice^.^ In CFCl3 the radical cation is a ring-closed species (2) with the unpaired electron located at the four methyl-substituted carbon-carbon bonds. In CFzClCFC12 the ring-closed radical cation was observed below 110 K. Above (8) (a) Breck, D. W. Zeolite Moleculnr Sieues; Wiley-lnterscience: New York, 1974. (b) Smith, J. V. In Zeolite Chemistry and Catalysis; Rabo, J. A., Ed.;ACS Monograph 171; American Chemical Society: Washington, DC, 1976; p 3 . (9) Qin, X.-Z.; Snow, L. D.; Williams, F. J . Am. Chem. SOC.1984, 106, 7640.
CHj
2
3
We have recently studied the tetramethylcyclopropane radical cation in the xenon m a t r i ~ and , ~ we found evidence that 3 in CF2CICFCI2is a neutral radical formed from the ion-molecule reactions between the radical cation and parent molecules. The spectrum shown in Figure l a can be simulated by the known EPR parameters ( 4 2 HB) = 18.7 G and 4 1 2 HB) = 14.9 G) for 2, except for the much stronger central line as shown in Figure 1b. The ring-closed radical cation is stable up to 200 K in zeolite Na-Y without undergoing ion-molecule reactions with parent molecules. The much stronger central line may be attributed to the overlapping of the central line with the EPR signals of ionic paramagnetic species in the zeolite (see Discussion). Figure 2 shows the EPR spectrum observed at 100 K by yirradiation of the hexamethyl(Dewar benzene)-zeolite Na-Y system at 77 K. The hexamethyl(Dewar benzene) radical cation
Organic Radical Cations in Zeolite Na-Y
The Journal of Physical Chemistry, Vol. 94, No. I I , 1990 4753
TABLE I: ‘H Hyperfine Coupling Constants of Organic Radical Cations radical cation or radical 2
3 2 4
matrix
T, K 180 145 109 117 77 100 77 77 205 120 120 100 80 77 190
or solvent zeolite Y
CFCI3 CFZClCFClz CFzClCFClz xenon zeolite Y
CFCl3 CF2ClCFCI2 cyclopentane zeolite Y
CFC13 CF2ClCFCl2 zeolite Y
CFCI3 C6H 1 lCH3
a,
G
18.7 (2 HB),14.9 (12 HB) 18.7 (2 HB),15.0 (12 HB) 18.7 (2 HB),14.9 (12 HB) 23.3 (6 HB), 11.7 (2 HB) 18.7 (2 HB),14.9 (12 HB) 9.3 (12 HB) 9.5 (12 HB) 14.0 (6 HB) 9.2 (12 HB) 22.5 (1 H,, 3 HB) 14.0 (2 H,), 16 (6 HB) 23.0 (1 H,, 3 HB) 17.1 (12 HB) 17.2 (12 HB) 17.2 (12 HB)
ref this work
8 8 8 3 this work 1 Oa 10b .
11 this work
12a 12b this work
14 16
Figure 3. An EPR spectrum observed at 120 K for the trans-2,3-dimethylethylene oxide radical cation in the zeolite Na-Y.
was studied in freon matrices. In CFCI, the radical cation of this compound was assigned to the 2B2 state (4)1°”while in CF2ClCFC12 it was assigned to the 2Al state (5).Iob Recently, the assignment of the 2Al state was shown to be in error.lOc*d
4
5
We have recently determined that the hexamethyl(Dewar benzene) radical cation has a 2B2ground state by using the FDMR observations of the cation in alkane solvents.ll Figure 2 consist of at least nine lines which belong to a binomial 13-line pattern as described by the stick spectrum. The single coupling constant is 9.3 G. These are in good agreement with the EPR data of the 2B2 ground cation. This radical cation was stable up to 120 K in zeolite Na-Y. Figure 3 shows the EPR spectrum observed at 120 K after y-irradiation of the trum-2,3dimethylethyleneoxide-zeolite Na-Y system at 77 K. The spectrum is a quintet with a coupling constant of 22.5 G. The trans-2,3-dimethylethyleneoxide radical cation was also studied in freon matrices. In CFCl, the radical cation is a ring-opened planar allylic form (CH(Me)=O’+=CH(Me), 6)12*while in CF2ClCFC12the planar radical cation undergoes transformation at 95 K to yield a localized cation with the unpaired electron at one side of the molecule and the positive charge at the (10) (a) Rhodes, C. J. J . Am. Chem. SOC.1988,110,4446. (b) Rhodes, C. J. J . Am. Chem. SOC.1988, 110, 8567. (c) Williams, F.; Guo, Q.-X.; Nelsen, S. F. Submitted to J . Am. Chem. SOC.(d) Arnold, A.; Gerson, F. Submitted to J . Am. Chem. SOC. (1 1) Qin, X.-Z.; Werst, D. W.; Trifunac, A. D. J . Am. Chem. SOC.,in press. (1 2) (a) Ushida, K.; Shida, T.; Shimokoshi, K. J . Phys. Chem. 1989, 93, 5388. (b) Qin, X.-Z.; Snow, L. D.; Williams, F. J . Phys. Chem. 1985, 89, 3602.
Figure 4. An EPR spectrum observed at 80 K for the tetramethylethylene radical cation in the zeolite Na-Y.
other side of the molecule (CH(Me)OC+H(Me), 7).12bThe localized cation shows a quintet EPR spectrum with a coupling constant of 23 G due to the interaction of one a-proton and three &protons of one methyl group. Therefore, the trans-2,3-dimethylethylene oxide radical cation produced in zeolite Na-Y is a localized species, which was stable up to 200 K. In addition, the EPR spectrum shown in Figure 3 does not display any superhyperfine couplings from nuclei of the zeolite matrix while the EPR spectra in CF2ClCFC12show coupling from the matrix. The ring-opened planar allylic form of the radical cation of this compound was not observed between 77 and 200 K in the zeolite. Figure 4 shows the EPR spectrum observed at 80 K after y-irradiation of the tetramethylethylene-zeolite Na-Y system at 77 K. One observes 11 equally spaced lines with a coupling constant of 17.1 G. As indicated by the stick spectrum, these 11 lines are part of binomial 13 lines due to the interaction of the 12 protons of the four methyl groups of the molecule. The spectrum is undoubtedly attributed to the monomeric tetramethylethylene radical cation ((Me)2C=C(Me)2*+,8).I3-I6 The tetramethylethylene radical cation is known to easily react with parent molecules to form dimer,15J6trimer, and oligomer16radical cations. But in the present case only monomer radical cation was observed, which was stable up to 120 K. Furthermore, we have performed photolysis of tetramethylethylene radical cation in zeolite Na-Y at 80 K. Neither visible nor UV illumination for 10 min was found to cause any significant intensity change of the EPR spectrum of the cation.” (13) Ichikawa, T.; Ohta, N.; Kajioka, H. J. Phys. Chem. 1979,83, 284. (14) Shida, T.; Egawa, Y.; Kubodera, H.; Kato, T. J. Chem. Phys. 1980, 73, 5963. (15) Williams, F.; Qin, X.-Z. Radiut. Phys. Chem. 1988, 32, 299. (16) Desrosiers, M. F.; Trifunac, A. J . Phys. Chem. 1986, 90, 1560. (1 7) For photochemical studies on zerolites, see: (a) Suib, S. L.; Kostapapas, A.; Psaras, D. J. Am. Chem. SOC.1984,106, 1614. (b) D. Turro, N. J. Pure Appl. Chem. 1986, 58, 1219, and references therein.
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The EPR parameters of these organic radical cations in zeolite Na-Y are collected in Table I and are compared with the corresponding data from other sources.
Discussion We have demonstrated that a variety of organic radical cations can be easily generated and stabilized in the zeolite Na-Y by y-irradiation of the adsorbed parent compounds in the zeolite. We believe that this illustrates a most general method for the generation of simple organic radical cations in a wide temperature range in matrices other than halocarbon matrices. While the temperature range for studying radical cations in halocarbon matrices is determined by the softening point of the matrix, it is dependent upon the structure, relative stability, and, possibly, concentration of the individual cation in the zeolite Na-Y. Some radical cations, for example, the tetramethylallene radical cation, are found to be stable up to the room temperature in zeolite Na-Y.'* We expect that zeolite Na-Y is one of the most useful hosts for stabilizing radical cations. As shown in the Results section, for every organic compound studied, the monomeric radical cation was observed. Unimolecular dissociation of the radical cation and ion-molecule reactions between the radical cation and parent molecules were not observed in a wide temperature range. The size and shape relationship between the molecule and the zeolite in question must play an important role in stabilizing radical cation. The dimensions of these selected compounds match the size of the channels. It appears that the adsorbed molecules of these compounds are well-isolated from each other in the zeolite under our experimental conditions. We are now studying organic compounds with larger and/or smaller sizes, and we are trying different types of zeolites, such as zeolite Na-X, as hosts. y-irradiation of dehydrated zeolite Na-Y only gave a very broad and weak signal. Therefore, the background signals from the irradiated zeolite do not interfere with the strong EPR signals of the organic radical cations. The EPR spectra of organic radical cations are nearly isotropic especially at higher temperatures, indicating that the organic molecules have some freedom of motion in the cages or channels with restricted space. The apparent absence of superhyperfine coupling from the zeolite in the EPR spectra suggests that the interaction between organic radical cations and the zeolite framework is weak. However, weak interactions may still exist. For example, only the localized radical cation of trans-2,3-dimethylethyleneoxide was observed, and the separaton of the spin and the positive charge in this molecule can be considered to be caused by the interaction of the negative charge of the zeolite framework with the carbocation side of the cation, resulting in the localization of the spin at the other side of the cation. Zeolite Na-Y certainly satisfies the conditions of producing and stabilizing organic radical cations mentioned in the Introduction. A mechanism for generating radical cations in zeolite Na-Y adsorbing organic compounds by y-irradiation is proposed: (18) The coupling constant from the interaction of 12 protons of the four methyl groups of the tetramethylallene radical cation in zeolite Na-Y, in liquid alkane solvents, and in freon matrices is 14.1, 11.5, and 8.1 G, respectively, which points out a solvent-dependent Jahn-Teller distortion of the cation (Qin, X.-Z.; Trifunac, A. D. To be published).
zeolite 2 zeolite+ + ee- + Na+ Na e- + Fe3+ Fe2+
+ Fe3+ zeolite+ + R H Na
-- + + Fe2+
Na+
(5)
(6)
(7) (8)
zeolite RH+ (9) y-irradiation first produces a positive hole (zeolite') and an electron (eq 5). Since no electron scavenger was added, the ejected electron is assumed to be captured by the cationic species in the zeolite (Na+, Fe3+, Mn2+) (eqs 6 and 7). The reduced sodium is known to e x i ~ t 'as~ ionic , ~ ~ clusters such as Nad3+with an EPR spectrum consisting of 13 lines and a coupling constant of 35 G in the zeolite Na-Y,20 which was not detected in our studies. It is thus assumed that sodium or sodium clusters can be oxidized by Fe3+to form more stable cation Fe2+ (eq 8). This is consistent with the implication that electrons are deeply trapped by reduced cationic species, as demonstrated by the insensitivity of the tetramethylethylene radical catin EPR intensity in zeolite Na-Y to UV and visible light irradiation. The positive hole of zeolite is mobile and can be transferred to the adsorbed organic molecules to form the corresponding radical cation (eq 9). The present work established that positive charge transfer from the zeolite to species possessing (gas phase) ionization potentials as high as 9.88 eV (trans-2,3-dimethylene oxide2') is feasible.
Conclusions We can conclude that radiolytic generation and stabilization of a wide variety of organic radical cations in zeolite Na-Y are feasible. EPR studies of radical cations can be carried out at remarkably high temperatures, in some cases even at room temperature. Well-resolved, isotropic EPR spectra of radical cations are observed. The electron ejected in the ionization event is trapped by the zeolite, presumably by cationic species. The absence of any appreciable superhyperfine splittings of the radical cation EPR spectral lines suggests that the interaction between the cation and the zeolite is weak in the above-mentioned systems. While many details and possibilities for the study of radical cations in zeolite Na-Y are yet to be established, this work illustrates the wide range of possibilities for generation and stabilization of a considerable variety of organic radical cations in zeolites. Acknowledgment. We thank Dr. Lennox Iton for providing us with samples of zeolites and for advice on the use of and properties of zeolites. We also thank Drs. David Werst and Martin Bakker for discussions. This work was performed under the auspices of the Office of Basic Energy Sciences, Division of Chemical Science, US-DOE, under Contract No. W-3 1-109ENG- 38. Registry No. 2, 56324-44-8;4, 85293-78-3;7, 126294-95-9;8, 34512-36-2;1,1,2,2-tetramethylcyclopropane,4127-47-3;1,2,3,4,5,6hexamethylbicyclo[2.2.0] hexa-2,5-diene, 7641-77-2; tetramethylethylene, 563-79-1;truns-2,3-dimethylethyleneoxide, 21490-63-1. (19) Harrison, M. R.;Edwards, P. P.; Klinowski, J.; Thomas, J. M.; Johnson, D. C.; Page, C. J. J . Solid Stare Chem. 1984,54, 330. (20) Yoon, K. B.; Kwhi, J. K. J . Cfiem.Soc., Chem. Commun. 1988,510. (21) McAlduff, E. J.; Houk, K. N. Can J. Cfiem. 1977,55, 318.
