ESR Spectrum of a Radical Cation Adsorbed on
a Surface
The Journal of Physical Chemistry, Vol. 82, No. 16, 1978
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Evidence for Electron-Transfer Effects in the Electron Spin Resonance Spectrum of a Radical Cation Adsorbed on a Surface G. M. Muha Department of Chemistry, Rutgers, The State University of New Jersey, New Brunswick, New Jersey 08903 (Received August 22, 1977; Revised Manuscript Received March 24, 1978) Publication costs assisted by University College, Rutgers University
The concentration dependence of the line width variations observed in the ESR hyperfine spectrum of the radical cation of 9,lO-dimethylanthracenegenerated by adsorption on an acidic surface (silica-alumina) is reported. The dependence is found to be unlike that observed in other similar systems in that the line width variation is independent of hydrocarbon concentration provided the oxidation capacity of the surface is not saturated. If saturated, only small changes in line widths are observed; full exchange narrowing is not observed at any concentration level. The results are explained in terms of a model involving electron transfer reactions within “clusters” composed of hydrocarbon molecules and chemisorbed oxygen. Exchange reactions within the clusters are thought to be rapid compared to the transfer of species between clusters. The implications of the model as concerns the dynamic structure of the surface are discussed.
A topic of current interest in our laboratory is that of developing an understanding of the details of the nature of the electrophilic site present on the surface of aluminosilicates and responsible for the oxidation of polynuclear aromatic hydrocarbon molecules. Thus in a recent report’ details of the anisotropic rotational motion of the oxidation products of perylene and anthracene on this surface as observed in the ESR line width effects was reported. Yet in that report it was noted that a parallel line width analysis applied to the case of tetracene failed. Thus it was argued’ that an additional relaxation mechanism, not of significance in the case of perylene and anthracene, must be operative in the tetracene system. The form of this mechanism was clear; it must involve an exchange interaction, for the usual 85-line spectrum of the tetracene species when prepared in sulfuric acid degenerates to essentially a four-line spectrum (plus weak “background” lines) when the radical cation is prepared on an aluminosilicate surface. Yet further study (outlined below) gave the unexpected result that the general form of the spectrum, Le., four lines plus background, appeared essentially unchanged over the concentration range experimentally accessible. Thus if the collapse of the hyperfine spectrum to four lines is taken as evidence of an exchange interaction, the insensitivity to concentration effects could be taken to imply a particular if not peculiar structural and chemical relation among the exchanging species. It is not only with regards to the collapse of the hyperfine spectrum that tetracene gives results different than perylene and anthracene, for previous work2 had demonstrated that in general those aromatic molecules with half-wave potentials lower than that of perylene (specifically 9,10-dimethylanthracene,tetracene, and pentacene), exhibit a lower integrated ESR intensity than that exhibited by perylene and other molecules of higher halfwave potentials (e.g., anthracene, pyrene, chrysene, etc.). The lower ESR intensity of the first-named group was attributed to a side reaction depleting the radical cation concentration by the further oxidation of this species to a diamagnetic dispositive ion. The dipositive ion has been directly observed by its optical spectrum as among the oxidation products generated on this surfaces3 Possibly the exchange effects observed in tetracene might thus be related to the presence of a high concen0022-3654/78/2082-1843$0 1.OO/O
tration of dipositive ions, involving electron transfer intercon~ersion~ of diamagnetic and paramagnetic species similar to that observed in homogeneous systemsa5To test this hypothesis three systems were compared: tetracene and 9,10-dimethylanthracene (DMA) as representative of one class of behavior and anthracene as representative of the other. Both the tetracene and anthracene spectra have already been reported,12hence most of the examples given in the figures are concerning DMA results. Also DMA provides a greater number of hyperfine lines thereby facilitating the detection of changes in line widths. In all regards DMA and tetracene are found to exhibit parallel behavior (see below).
Experimental Section The equipment, the method used to measure the integrated ESR intensities (ruby standard), and the reagents used are essentially as previously described.2 Benzene, carbon disulfide, or carbon tetrachloride, each suitably purified,2were used as solvents. No effects dependent on the choice of solvent were observed. The hyperfine coupling constant for a proton in a DMA methyl group (a,) is readily and unambiguously determined by generating the DMA radical cation on a deuterated surface6 since, under the acidic conditions prevailing on the aluminosilicate surface, the ring protons are expected to exchange with the surface deuterium while the methyl protons remain ~ n a f f e c t e d .From ~ the resulting spectrum (Figure l),the value a, = 7.25 f 0.12 G is obtained. The determination of the remaining coupling constants is difficult for the line width effects in the “normal” spectrum (Figure 2A) are exaggerated. Selective exchange of deuterium into the ring by varying the DMA/oxide concentration ratio as practiced in the case of anthracene6 proved of no avail. The ring exchange was always completed in the 3-5-min period required to transfer the sample from the preparation rack to the spectrometer. Since the value of a, is within 10% of the value obtained when the radical is generated in sulfuric acid (8.00 G),8as a first approximation we assume the same values for aa (=2.54 G) and a y (=1.19 G ) also in this work. Here the subscripts P and y refer to the groups of protons in the 1,4,5,8 and 2,3,6,7 ring positions, respectively. The stick spectrum calculated with the listed values of the coupling 0 1978 American Chemical Society
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The Journal of Physical Chemistry, Vol. 82, No. 16, 1978
G. M. Muha
Figure 1. First derivative, X-band ESR spectrum of the partially deuterated DMA radical cation prepared by adsorbing the hydrocarbon on a deuterated surface (S= 162). ^^ '.o t -
Figure 3. Observed line width variations in the DMA radical cation spectrum as a function of concentration: (A) S = 164; (B) S = 320; (C) S = 409; (D) S = 550; (E) anthracene radical cations at S = 10 included for comparison. Aside from small differences due to changes in the spectrometer gain used in recording the different spectra the relative intensities of a given line in different spectra A through D is essentially as shown.
Flgure 2. First derivative (A) and stick spectrum (B) of the DMA radical cation (S = 0.34). The triplet of numbers identifying some of the lines in (A) is written in the notation ( M a , Mp, My).
constants is shown in Figure 2B. Concentration Studies For the discussion that follows, it is convenient to introduce a concentration variable S defined as the ratio of the total number of molecules of DMA initially added to the sample to the total number of electrophilic sites available on the sample's surface. The quantity in the numerator is readily determined from the weight of the solution of DMA (of known molality) added when preparing the sample. Likewise the quantity in the denominator can be ascertained by simply weighing the catalyst sample prior to the introduction of the DMA, the number of electrophilic sites per unit weight of catalysts being determined as described previously2using perylene as a r e f e r e n ~ e . ~ Thus the spectra shown in Figures 1and 2 were obtained from samples with S = 162 and 0.34, respectively. In all, a set of 14 samples covering the range S = 0.16 to S = 550 were studiedel" The results of these experiments can be summarized as follows. In the range below S 1.0, (a) there is no DMA remaining in the supernatant 1iquid;l' (b) the measured integrated ESR intensity varies from 63 (at S = 0.16) to 72% (at S = 0.96) of that which would be obtained if all of the adsorbed DMA was oxidized to the radical cation; (c) the line width effects, as determined by measuring the ratio of the peak-to-peak high of various prominent lines to that of the central (O,O,O) line, remain sensibly constant ( f 5 % ) . That is, in this concentration range, all ESR spectra appear as shown in Figure 2, except of course for improved signal/noise characteristics as the value of S
-
Flgure 4. Changes in the ESR spectra due to manipulation of the concentrations of anthracene and DMA on the surface: (A) anthracene alone, S = IO; (B) addition of DMA at S = 310 to the preceding sample; (C) addition of DMA at S = 0.21 to previously prepared anthracene sample with S = 179; (D) both DMA (at S = 0.24) and anthracene (at S = 0.30) present; (E)anthracene (at S = 0.30) alone. The spectrometer gain varies considerably among the different samples; hence direct comparison of integrated intensities of different samples Is not appropriate.
increases. This last point is of central concern in the discussion given below. In the range above S 1.0, (d) a gradual change in the line width effects (again as defined in (c) above) is observed as shown in Figure 3; (e) although at the highest S value accessible1" no collapse of the hyperfine spectrum characteristic of the onset of exchange narrowing: and as observed in the case of perylene and anthracene (cf. Figure 3E),12is observed; (f) in this concentration range the integrated ESR intensity remains constant within the estimated experimental error of the measurement (&I570) and corresponds to a radical cation concentration equivalent to -75% that of the concentration of electrophilic sites determined by the perylene ~ t a n d a r d . ~ The absence of exchange narrowing in the DMA case is an important point in the discussion below and hence to further check this point an additional set of samples was prepared. Thus, for example, a sample containing anthracene with S = 10 and exhibiting exchange narrowing (Figure 4a) was prepared and a known quantity of solid DMA added, chosen such that if DMA was present alone the corresponding S value would be S = 310. It was observed (Figure 4b) that DMA simply replaced anthracene on the surface, an expected result given that
-
ESR Spectrum of a Radical Cation Adsorbed on a Surface
DMA has a much lower oxidation potential than anthraceneq2 The same results were obtained when solid anthracene was added to a DMA-containing sample and when a solution containing both anthracene and DMA was added to a freshly prepared catalyst sample. Thus the result is independent of the method of sample preparation. This type of experiment was repeated to cover a wide range of S values. The results can be summarized as follows: (g) if sufficient DMA is present to interact with all of the available electrophilic sites, only the DMA spectrum is observed and the line width effects are those corresponding to the appropriate S value for DMA (e.g., compare Figures 3b and 4b). That is, the presence of a large excess of anthracene does not appear to affect the DMA spectrum. Further, (h) if an insufficient amount of DMA is present to interact with all of the electrophilic sites but anthracene is present in large excess, the resulting spectrum (Figure 4c) is that of DMA superimposed on an exchanged-narrowed anthracene spectrum (cf. Figure 4a). Finally, (i) if DMA and anthracene are both present but the total aromatic concentration is less than that required to "use" all of the electrophilic sites, the resulting spectrum (Figure 4d) is simply that of the DMA and anthracene radical cations superimposed. To facilitate a comparison, a spectrum of the anthracene radical cation ( S = 0.30) is shown in Figure 4e. These experiments were repeated with six samples of tetracene and covering the range S = 0.6 to S = 200. Except that the integrated ESR intensity was lower than that of DMA, as expected,2parallel results were obtained. Discussion The formation of a DMA radical cation (DMA') by oxidation at an electrophilic site (E) presumably occurs according to the reaction DMA + E = DMA' + E(1) Based on electrode potential data previously reported: the equilibrium constant (K,) for this reason is estimated13to be of the order of log K1 = 10. Such a high value readily accounts for the absence of any DMA in the supernatant liquid for samples for which S < 1, an experimental observation noted ab0ve.l' Yet also noted above, for these same samples, the integrated ESR intensity corresponds to only -75% that expected if the formation of DMAt was the sole reaction occurring. The remaining quarter of the material presumably is further oxidized to the diamagnetic dipositive cation (DMA2'): DMA' + E = DMA2+ E(2) Now we reason that the equilibrium constant (K,) must be of the order of magnitude of unity for otherwise for those samples with low values of S , where the concentration of E is in great excess of that required to stoichiometrically convert DMA' to DMA2', the ESR intensity would be much lower than that observed. We find the value K2 0.4 gives a reasonable fit to the small variation in the DMA' concentration (63% at S = 0.16 to 72% a t S = 0.96) quoted above. However in view of the uncertainty in these values due to the difficulties in precise measurements of the integrated ESR intensities, quantitative arguments based on the value of K z cited are probably unwarranted. The same considerations of stoichiometry and ESR intensities rules out the possibility of a single electrophilic site accepting both of the electrons in the DMA2+ formation step to form E". Thus there is no need to postulate
+
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The Journal
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the existence of a new, previously unobserved, species on the surface to explain the observations concerning concentration effects as reported here. It is generally thought that the gegenion in these systems is the superoxide ion, Og,the electrophilic site being some form of chemisorbed oxygen.14 Our present work then is concerned with these chemisorbed molecules, apparently a pair of which is required for the formation of each DMA2' species. Recall that in the range S < 1, the line widths are sensibly constant, small changes are observed when S > 1, but no collapse of the hyperfine spectrum over the range of S experimentally accessible. It might be possible to explain these results in terms of a model requiring the presence of three species (i.e., neutral molecules, dipositive ions, and radical cations) for exchange relaxation effects to be manifest in the ESR spectrum. Thus in the range S < 1, the species are sufficiently isolated that exchange effects are negligible while then S > 1 the necessary proximity of the reactions facilitates electron transfer effects.15 Such a model does indeed explain the behavior of perylene and anthracene adsorbed on this surfacee6 Yet the extension of the model to the case of DMA and tetracene leads to serious difficulties for there seems to be no direct way to explain the lack of complete collapse of the hyperfine structure at high S values as observed in the case of perylene and anthracene. It is true that in the case of tetracene only four rather than 85 lines are observed, but this four-line spectrum does not change in its essentials over the range of S values accessible. Indeed, judging from the tetracene results, it appears that exchange interactions are indeed present but their characteristic is that they are little affected by concentration. Since the DMA results parallel that of tetracene, the implication is that exchange interactions are to be found in DMA spectra also, and little changed over the range of S values. It is this essentially concentrationindependent behavior, observed when dipositive ion formation is possible, that appears to be the test of any proposed model. In our view a plausible model to explain the effect involves the assumption that electron transfer effects resulting from the interconversion of the radical and the dipositive ion are to be found in the DMA spectra over the full range of S values. Thus for S < 1, the ratio DMAt/DMA2+ is approximately constant and hence the spectra should be sensibly unchanged (as observed), provided that due to restraints introduced presumably by the surface structure, these two species as they form are always localized so as to facilitate electron transfer. (We consider the question of stoichiometry below.) Further, for S > 1, neutral molecules are also present and hence become involved in the exchange process but since their effect on the line widths is small (Figure 3),the model would also require that the rate constants for interconversion of DMA with DMA+ or DMA2+ is small. That is, electron transfers resulting in interconversion of DMA' and DMA2+ proceeds through many cycles before a neutral DMA molecule exchanges with one of these species. This last-mentioned exchange must occur, of course, for such is the mechanism established6 for deuterium exchange with ring protons as shown in Figure 1. The model we have in mind is simply an extension of the "tight" ion-pair scheme thought to be found in certain cases in ~olution.~ There triple and quadruplet structures6 have been postulated. In the present case the formation of such multiple structures is thought to be facilitated by the necessary requirement that the gegenion be restricted
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to the vicinity of the surface since the electrophilic site is chemisorbed oxygen. To observe the corresponding situation in solutions, e.g., in the alkali metal/ ketyl s y ~ t e m ssome , ~ form of constraint acting on the alkali metals would be necessary and causing them to remain in clusters rather than being “free” to diffuse throughout the bulk of solution. In the present case it is the function of the surface to supply this constraint. Specifically we propose a structure involving a “cluster” of hydrocarbon molecules and chemisorbed oxygen, within which cluster the interconversion reactions of the form specified by chemical eq 1and 2 are occurring. In the case of DMA, the stoichiometry is such that on average it is expected that the cluster contains three DMA+, one DMA2+,and four 02-ions, or some multiple thereof. The absence of evidence for exchange narrowing of the ESR lines indicates that within the cluster, the DMA+ species are sufficiently distant to render the effect minimal. In the case of perylene and anthracene the absence of dipositive ions and their attendant pair of 02-ions necessarily implies that the stoichiometry of the cluster, and thus presumably its structure, is different than in the DMA and tetracene cases. The structural difference can be taken as the reason that neutral molecules are able to be more readily involved in the exchange phenomena and lead to 1 a collapse of the hyperfine structure in the region S (Figure 4a) and exchange narrowing at higher S values.l2 The question of the stoichiometric composition of the cluster is a difficult one for in the case of tetracene the intensity data,2 using similar reasoning as for the DMA system, would lead to a (radical):(dipositive ion):(02-)ratio of -1:1:3 or multiples thereof. Yet although there are presumed to be fewer radical cations in the cluster than in the case of DMA, the collapse of the hyperfine structure to four lines in the case of tetracene indicates a larger effect. However the effect depends not only on the concentration but also on the rate constant for the reaction involved and there is considerable evidence that the latter parameter depends intimately on the structures in~ o l v e d . ~That J ~ is, the difference between DMA and tetracene behavior may reflect only a difference in rate constants. Our point is that, based on the evidence presently available, arguments based on details of the structure of the cluster are unwarranted. The notion of a cluster is only introduced to ensure that the radical and dispositive ion are localized so that the interconversion of the species can proceed without a “delay” dependent on the time required for diffusion of the reactants from the remote regions of the surface.15 If surface diffusion was an important factor, then the line widths would necessarily be expected to depend on concentration for the average distance between species would then be expected to depend on surface ~ 0 v e r a g e . l Such ~ is, of course, not the observed effect. We note that the structure of the cluster, whatever its form, must be such that parallel stacking of the aromatic rings sufficiently close together to permit dimer formation17 is unlikely. In such an instance, one would expect twice the number of hyperfine lines with a value of one-half the “monomer” splitting constant, a well-known result in certain radical cation systems in solution.17 We observe no indication of such an effect in the present work, although the phenomena has been observed on other oxide surfaces under far more stringent oxidizing conditions than used before. Specifically, the dimeric benzene radical cation has been observed on irradiated silica g e P and the strongly acidic hydrogen form of ze01on.l~
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G.M. Muha
The role of chemisorbed oxygen as the electron acceptor was assumed in the above and it is important to consider the implications thereof as concerns the interconversion process envisioned in chemical eq 1 and 2. The above discussion centers on changes in line width as a function of the S value. Analysis of the line width effect per se was not considered. Yet previous work1,zp6has shown that the radical cations possess a high degree of mobility (correlation time s) although admittedly anisotropic.l Is such a behavior consistent with chemisorbed oxygen as the electrophilic site? Oxygen tracer experimentsz0indicate that exchange of the surface oxygen is essentially instantaneous at 450 O F , with an exchange of all oxygens (i.e., including those within the bulk lattice) complete within 20 min. Thus the surface oxygens apparently can exhibit the required mobility for the surface is to be considered as a “dynamic structure”.20 Indeed after reviewing the evidence Oblad, Hindin, and Mills20observed that “the surface...behaves to a certain degree like a two-dimensional mobile acid.” The only addition to this observation needed to explain the results of present concern is the qualifier: two-dimensional oxidizing acid. The activity of this catalyst for hydrocarbon conversion reactions is thought to be due to its acidic nature, reactions proceeding through a series of carbonium ion intermediates.21 Yet we note the suggestion by a number of workers2zthat perhaps instead the rate-determining step involves the ease of electron transfer from adsorbed molecules to the surface site. This transfer is of course the central concern in this paper and the quantitative assessment of the rate constants involved was one of the reasons for undertaking the work. However the contribution in this regard must, at least for the present, remain qualitative in nature and as described above for the experimental difficulties in disentangling the various processes possible and obtaining reliable estimates of rate constants involved are formidable.
References and Notes G. M. Muha, J . Chem. Phys., 67, 4840 (1977). G. M. Muha, J . Phys. Chem., 74, 2939 (1970). A. Terenin, V. Barachevsky, E. Kotov, and V. Kolmogorov, Spectrochim. Acta, 19, 1797 (1963);also see footnote 6 in ref 2. C. S. Johnson, Jr., Adv. Magn. Reson., 1, 33 (1965). N. Hirota in “Radical Ions”, E. T. Kaiser and L. Kevan, Ed., Interscience, New York, N.Y., 1968,p 35. G. M. Muha, J . Phys. Chem., 71, 640 (1967). A. I. Shatenshtein, “Isotope Exchange and Replacementof Hydrogen in Organic Compounds”, Consultants Bureau, New York, N.Y., 1962. J. R. Bolton, A. Carrington, and A. D. McLachlan, Mol. Phys. 5,31 (1962);A. D. McLachlan, /bid., 1, 233 (1958). Perylene is chosen as a reference in this scheme since careful studies (W. K. Hall, J. Catal., 1, 53 (1962);R. P. Porter and W. K. Hall, ibid., 5,366 (1966))have demonstrated that for this aromatic, only neutral molecules and radical cations are to be found on the surface. The value S = 550 represents the practical limit as defined by two experimental factors: the maximum solubllity of DMA in carbon disulfide and the total volume available in the ESR sample tube. The point is readily established with excellent sensitivity by simply transferring the supernatant liquid to a freshly prepared catalyst sample. The absence of any ESR signal then demonstrates the point. In the case of anthracene, additional examples at much higher concentration levels are shown in Figure 6 of ref 6. Perylene gives similar spectra (cf. G. M. Muha, J. Phys. Chem. 71, 633 (1967)). Based on the method outlined In ref 2,the equilibrium constant for reaction 1 may be estimated from the ratio of the ESR intensity of that for DMA’ to that for the perylene radical cation. Thus the value K , = 6.9X lo9 is obtained. For purposes of the present work we disregard the effect of the acidic nature of the surface in influencing this equilibrium constant (cf. ref 1)for our requirement here is only an order of magnitude estimate of K,. F. R. Dollish and W. K. Hall, J . Phys. Chem., 71, 1005 (1967);J. Colloid Interface Sci., 26, 261 (1968). I f a uniform distribution of adsorbed species on the surface is assumed, then at S 0.1,the interradical distance is -400 A; for S = 2, the distance is -26 A. See G.M. Muha, J . Phys. Chem., 74,787 N
(1970).
Undetected Cu(I1) Ions in Y Zeolite (16) W. Brunning and S. I. Weissman, J. Am. Chem. SOC.,88,373 (1966); T. J. Katz and H. L. Strauss, J . Chem. Phys., 32, 1873 (1960). (17) 0. W. Howarth and G. K. Fraenkel, J . Am. Chern. SOC.,86, 4514 (1966). (18) 0. Edlund, P. Kinell, A. Lund, and A. Shinizu, J. Chem. Phys., 46, 3679 (1967).
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(19) P. L. Corio and S. Shik, J . Catal., 16, 126 (1970). (20) A. G. Oblad, S. G. Hindin, and G. A. Mills, J . Am. Chem. SOC.,75, 4096 (1953). (21) Cf. H. H. Voge in "Catalysis", Vol. 6, P. P. Emmett, Ed., Reinhold, New York, N.Y., 1958, p 407. (22) Cf. J. L. Franklin and D. E. Nicholson, J. Phys. Chem., 60, 59 (1956).
Electron Spin Resonance of Undetected Copper(I1) Ions in Y Zeolite J. C. Conesa" and J. Soria Instituto de Catiilisis y Petroleoqdmica, C.S.I.C., Serrano, 119, Madrid (6),Spain (Received February 21, 1978) Publication costs assisted by Instituto de Catalisis y Petroieoquimica, C.S.I.C.
Double integration is carried out for the ESR spectra of copper-exchanged zeolites with varying degrees of dehydration. An intensity minimum is found after treatment at 100 "C, while magnetic susceptibility measurements show no relevant change in the paramagnetism of the sample under these conditions. This effect is attributed to line broadening of Cu(I1) in trigonal symmetry. Another decrease in integrated intensity is observed above 300 "C, together with a similar variation in susceptibility; in this case, a different mechanism involving a reduction of copper to the Cu(1) state is proposed.
I. Introduction The ESR spectra of Cu(I1) ions in Y zeolites have been investigated during the past years by several authors,l-1° with the aim of ellucidating the location of cations within the aluminosilicate framework. In samples with varying degrees of dehydration, such spectra frequently show the occurrence of several Cu(I1) ESR bands with isotropic or axial symmetry, which are assigned to different cationic sites in the lattice. In this way, models are drawn about the distribution and environments of the copper in the zeolite, which account for the observed spectra. In many of these studies, however, little or no attention is devoted to the possible occurrence of cupric ions which would give no ESR signal; this could lead to spectra reflecting only a part of the Cu(I1) present, while the rest would be overlooked in the assignment of the spectral bands. Experimental evidence of this fact must be apparent when double integration of the spectra is carried out. We report here an analysis of changes in the integrated intensity of the Cu(I1) ESR spectra, which are not always accompanied by a similar behavior of the magnetic susceptibility, indicating thus the existence of paramagnetic copper ions not observed by ESR. 11. Experimental Section Several portions of Linde Na-Y zeolite (56 cation equivalents/unit cell) from Union Carbide Co. were exchanged with varying concentrations of CuClz aqueous solutions, dried at 100 "C, and equilibrated a t room temperature with saturated aqueous NH,Cl solution. Two of the samples had undergone previous exchange with Ce(II1) following a similar procedure. The degree of exchange was determined in the samples through acid dissolution and analysis of the cations in the filtered liquid (atomic absorption for Cu, X-ray fluorescence for Ce). Copper zeolites (C samples) had 2.2,8, and 16 cations/unit cell, while cerium-containing ones (C/C samples, with about 11.5 cerium ions/u.c.) had 2.6 and 4.6 copper ions/u.c. The exchange conditions are presented in Table I. Thermal treatments were performed, unless otherwise stated, in a greaseless high-vacuum manifold; initial
TABLE I sample
zeolite, g
solution, g/L
volume,
C 2.2 C 8.1 C 16 Ce 11.5 C / C 2.6 C / C 4.6
20 Na-Y 20 Na-Y 20 Na-Y 60 N a - Y 10 Ce 11.5 10 Ce 11.5
0.04 CuC1,.2HZ0 1.70 CuCl,.2H,O
1 1 1 1 1 1
8.52 C u C 1 2 ~ 2 H , 0 5.00 Ce(N0,),.6H20 1.04 CuCl,.2H20 4.00 C u C l Z . 2 H Z O
L
evacuation at room temperature was carried out over 4 h, and was followed by heating in vacuo at increasing temperatures for intervals of 30 min. Oxidation of the samples was performed under an oxygen pressure of 400 Torr. The amount of sample used was in each case of about 50 mg. The spectra were obtained at room temperature with a JEOL JES-PE-3X spectrometer at 9.55 GHz. The integration was performed with a DCC computer Model D116E, connected on-line with the spectrometer and working in Basic language. Some problems with the baseline arose because of the superimposition of the copper spectra with a broad, symmetric line (g N 2.64, AH,, N 1500 G) due to ferromagnetic iron oxide impurities.l' A parabolic baseline was thus chosen, fitted to two points at the beginning and at the end of the spectrum, while the third point was determined by trial and error so that the difference between the areas above and under the baseline was less than 1% of the total area. Absolute measurements were obtained through comparison with the peak height of a DPPH standard fixed outside the quartz sample tube in a reproducible position. The standard was calibrated against a sample of bis(diethyldithiocarbamate)copper(II) diluted in the same Ni(I1) compound; the dependence of the absorption probability on the g valued2 was taken into account. An accuracy of 15% in the final results is estimated. Magnetic susceptibilties were measured at room temperature with a Bruker Minisusc system following Faraday's method. These measurements were made using the same quartz sample tubes as for ESR. They were fitted with small-size rubber stopcocks of special design, so that both ESR spectra and susceptibility measurements could be obtained on the same specimen after each step of the dehydration treatment.
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