ESROF AROMATICHYDROCARBONS ADSORBEDON SILICA-ALUMINA
633
Electron Spin Resonance Studies of Aromatic Hydrocarbons Adsorbed on Silica-Alumina.
I.
Perylene
by G.M. Muha Esso Research and Engineering Company, Linden, New JeTSey 07036 (Received August 9.2,1066)
An analysis is presented of the electron spin resonance (esr) spectrum of the perylene radical cation generated on a silica-alumina catalyst surface. The magnitude of the larger proton hyperfine-coupling constants are shifted by 4% from the corresponding values obtained when the radical cation is prepared in sulfuric acid solution. The shift is discussed in terms of an electrostatic perturbation of the radical-cation ?r-electrondistribution due to the presence of the counterion. The severe line-width effects which are also observed are discussed in terms of a modulation of the anisotropic hyperfine and g-tensor interactions. It is found that the line-width variation can be satisfactorily described by a polynomial equation in the nuclear spin quantum number of the equivalent proton groups provided that the value of the correlation time for the radical-cation motion is chosen to be comparable to that found in solution. From this result it is argued that the radical cation is relatively unrestricted in its motion and by implication, perhaps the counterion, Le. , electrophilic agent, is also somewhat unrestricted in its movements on the surface.
Examples of the generation and stabilization of radical ions on the surface of solids have been known for some time.’ Recently, this technique has been used to study the nature of the catalytically active sites on the surface of silica-alumina catalysts. These surface sites are thought to function, in some broad sense, ~ 1 san acid2and induce a carbonium ion type of reaction in an adsorbed hydrocarbon. When aromatic hydrocarbons are used, optical s t ~ d i e s ~indicate -~ the presence of a radical cation in addition to the “classical” carbonium ion. Electron spin resonance (esr) s t u d i e ~ confirm ~ ~ ~ * ~the presence of a paramagnetic material adsorbed on the surface. To date, the principal use of these esr results has been to demonstrate certain features of the electrophilic sites responsible for the oxidation of the aromatic hydrocarbons. For example, it has been found that the intensity of the esr signal from the adsorbed species is decreased by a hydrogen reduction treatment of the catalystaJO-la and also (to a varying degree) by base exchange.7,QJl In addition, the number of paramagnetic entities per unit surface area is approximately equal to the number of surface sites active in the generation of carbonium ions.16
Little attention has been given to some of the aberrant features observed in the esr spectra of these (1) D. Bijl, H. Kainer, and A. C. Rose-Innes, Nature, 174, 830 (1954). (2) G. A. Olah in “Friedel-Crafts and Related Reactions,” Vol. 1, G. A. Olah, Ed., Interscience Publishers, Inc., New York, N. Y., 1963,p 201. (3) Cf. H. H. Voge in “Catalysis,” Vol. 6, P. P. Emmett, Ed., Reinhold Publishing Corp., New York, N. Y., 1958,p 407. (4) M. Okuda and T. Tachibana, Bull. Chem. SOC.Japan, 3 3 , 863 (1960). (5) R. M. Roberts, C. Barter, and H. Stone, J . Phys. Chem., 63, 2077 (1959). (6) W.K. Hall, J . Catalysis, 1, 53 (1962). (7) A. Terenin, V. Barachevsy, E. Kotov, and V. Kalmogarov, Spectrochim. A d a , 19, 1797 (1963). (8) J. J. Rooney and R. C. Pink, Proc. Chem. Soc., 70 (1961); Trans. Faraday Soc., 58, 1632 (1962). (9) D. M. Brouwer, Chem. I d . (London), 177 (1961); J . cat&& 1,372 (1962). (10) J. K.Fogo, J. Phya. Chem., 65, 1919 (1961). (11) H. Imai, Y. Ono, and T. Keii, ibid., 69, 1082 (1965). (12)F. R. Dollish and W. K. Hall, ibid., 69, 4402 (1965). (13) B. D. Flockhart and R. C. Pink, J . Catalysis, 4, 90 (1965). (14) R. P. Porter and W. K. Hall, ibid., 5, 366 (1966). (15) See ref 8 and 9; also see A. E. Hirschler and J. 0. Hudson, ibid., 3, 239 (1964); A. E.Hirschler, ibid., 5, 390 (1966).
Volume 71, Number 9 February 1067
634
surface-generated radicals. I n this and in the following papers of this series,I6 this aspect of the problem will be considered. It will be seen that by disentangling some of the physical and chemical interactions that manifest themselves in the line-width variations, exchange effects, and line shifts observed in these spectra, information concerning the nature of the surface interactions in this catalyst system can be obtained. Thus, in this paper, attention is directed to perylene and an analysis of the effects of the counterion on its esr spectrum. I n 11, a study is made of chemical- and electronexchange effects as exhibited in the anthracene spectrum. I n 111, a measure of the oxidation strength of the electrophilic sites on this catalyst surface is given along with a discussion of the significance of the results of I and I1 to the theory of the action of this catalyst. Finally in IV, a discussion of the line-width effects in the spectra of methylsubstituted anthracenes is given which further illusstrates some aspects of the nature of adsorption on this surface.
Experimental Section Esr Spectrometer. X-Band spectra were obtained with a Strand Model 600 spectrometer equipped with a Varian V-4531 variable-temperature cavity. The modulation amplitude and microwave power level were set below the point a t which line broadening or saturation effects could be observed in the recorded spectrum. A lock-in reference frequency of 6 kHz and a field modulation frequency of either 6 or 3 kHz were used. The latter frequency was used for second-derivative presentation, the former for first-derivative presentation, When a second-derivative presentation was used, a multisection filter was placed between the modula tion amplifier and the modulation coils to reduce the line distortion from a small admixture of the firstderivative signal.” ‘T,ine-sharpened” spectrals were recorded by using a modulation wave form consisting of a superposition of three odd-harmonic frequencie~.’~I n our instrument, these frequencies were 6,2, and 1.2 kHz. Spectrum Calibration. The magnetic field was calibrated with a Harvey Wells F502 nmr gaussmeter, the frequency of which was monitored by a HewlettPackard 524C frequency counter. The g values were obtained by comparison with a diphenylpicrylhydrazil (DPPH) sample (g = 2.0036) sealed in a capillary and placed concentric with and internal to the samole tube. The spin concentration was measured relative to the signal from a Cr3+-doped ruby sample mounted inside the microwave cavity and oriented so a8 to give a single line in a region (ca. g = The Journal of P h y a i d Chemistry
G. M. MUHA
4) not overlapping the sample line.2o The comparison of the microwave susceptibility per gram of sample and the ruby standard was made on the basis of the integrated area obtained by doubly integrating the first-derivative curve. The reproducibility of this type of measurement is estimated as =klO%. The conversion of the relative spin concentration to absolute units was made by calibrating the ruby standard in terms of known weights of DPPH and CuSOI.5Hz0. The absolute measurements are estimated to be correct within f60%. For quantitative measurements on those adsorbed aromatics exhibiting hyperfine structure, the modulation amplitude was deliberately raised to a level such that a single modulation-broadened line was obtained in the recorded spectrum.21 Chemicals. The aromatics, deuterated aromatics, and solvents used in this study were obtained from commercial suppliers. Some were repurified before use and all were checked for decomposition or other impurities by suitable physical techniques. In addition, in those aromatic systems permitting, the esr spectrum of the sulfuric acid generated radical cation was examined for spurious lines. All deuterated aromatics were found to be at least 96 atom % deuterium as determined by mass spectroscopy. 22 Solvents were stored for 24 hr over “activated” silicaalumina (see below) prior to use. Catalyst. The catalyst samples used in this study were taken from the same lot of an American Cyanamid Aerocat AAA silica-alumina cracking catalyst Organic contaminants containing ca. 22% were removed by heating in air for 24 hr at 500”. The catalyst samples were “activated” by heating the decontaminated material in the esr tube at 500” in either an air or a hydrogen atmosphere. When activated in air, the samples were heated for 2 hr in an openended tube and then sealed (while still hot) with a tightfitting rubber septum cap. A solution of the aromatic was introduced through the septum by means of a hypodermic syringe. Samples heated in a hydrogen atmosphere were similarly treated except the cell was equipped in addition with a stopcock and the (16) G. M. Muha, J. Phya. Chem., 71, 640 (1967). In the text, these papers will be referred to as 11, 111, and IV, respectively. (17)J. Gendell, J. H. Freed, and G. K. Fraenkel, J. Chem. Phya., 41, 949 (1964). (18) L. C. Allen, H. M. Gladney, and S. H. Glarum, ibid., 40,3136 (1964). (19) 8. H. Glarum, Rev. Sci. Znstr., 36, 771 (1965). (20) L. S.Singer, J. A p p l . Phye., 30, 1463 (1959). (21) K. Halbach, Phys. Rev., 119, 1230 (1960); J. S. Hyde and H. W. Brown, J. Chem. Phya., 37, 368 (1962). (22) The maas spectral analyses of these compounds were kindly performed by Dr. Theresa Mao.
ESROF AROMATIC HYDROCARBONS ADSORBEDON SILICA-ALUMINA
heating period extended to 24 hr with frequent changes of the hydrogen atmosphere. Deuterated catalyst surfaces were prepared by substituting deuterium in the activation step abovez3 or by refluxing the sample with a large excess of deuterium oxide for 2 hr.24 Both techniques gave identical results as judged from the esr results. The latter technique, being simpler, was used to prepare the deuterated samples used in all experiments except where specifically noted. The completeness of the deuterations obtained by each technique was checked by following (with esr) the chemical-exchange effects of certain aromatic radical cations adsorbed on the surface. These exchange effects are discussed in other sections of this and subsequent papers in this series.ls Catalyst samples heated in a hydrogen or deuterium atmosphere exhibited a weak esr signal, several hundred gauss wide and centered at g = 2.0. This signal could be detected only by using high microwave power and large modulation amplitudes; thus it caused no difficulties in interpreting the spectrum of the radical cations. The signal probably arises from paramagnetic26or ferromagneticzsimpurities. Catalyst samples heated in air did not exhibit a resonance.
Results g Value and Hyper$ne-Coupling Constants. The esr spectrum of a benzene solution of perylene adsorbed on silica-alumina is given in Figure la. The perylene : catalyst concentration ratio for this spectrum corresponds to -95% surface ~aturation.~’ The odd shapes of the various hyperfine lines is particularly striking. This asymmetry has not previously been reported and is easily missed if the field is overmodulated. From the second-derivative presentation (Figure lB), it is evident that the asymmetry arises because of the overlapping of individual hyperfine lines. The line-sharpened first-derivative spectrum18 (Figure IC) allows most of the lines to be resolved ; however, serious discrepancies in the relative intensities are apparent. These discrepancies are not unexpected, for the linesharpening technique works best when all the lines have the same width,18 a condition not obtained in the present case. For the spectrum shown in Figure lC, the spectrometer settings were arbitrarily chosen’9 to give the narrowest width to the line in the center of the spectrum. Because of the asymmetry of the lines, it is difficult to measure the g value of the spectrum; also, care must be exercised in superimposing the three presentations shown in Figure 1. To circumvent these difficulties, each presentation was independently calibrated with a
635
U Figure 1. First-derivative (A), second-derivative (B), and line-sharpened (C)spectra of the perylene radical cation generated on a silica-alumina catalyst. In (A), the horizontal scale is 40% that of the other two spectra. The arrow in (B) corresponds to a value of g = 2.0025.
g scale. The figure is then drawn to align these scales, The spectrum g value (= 2.0025) is taken to correspond to the center peak in the second-derivative presentation. The poor resolution coupled with the variation in the widths of the individual hyperfine lines precludes an unambiguous identification of the radical present. Thus in this paper we reverse the procedure and assume that a perylene radical cation gives rise to the spectrum. An analysis of the spectrum then will serve to define any special features of the radical cation when prepared on this surface. There can be little doubt concerning the validity of this assumption, for the optical spectrum of perylene adsorbed on silicaalumina correlates well with that obtained from perylene dissolved in sulfuric acid.z8 The measured g value quoted above corresponds to that measured for the perylene radical cation in sulfuric acid:z9 thus this result is consistent with the assumption. (23) The hydrogen and deuterium treatments of these samples were kindly performed by Dr. D. J. C. Yates. (24) S. G. Hindin, G. A. Mills, and A. G. Oblad, J. Am. Chem. SOC., 73, 278 (1951). (25) P. A. Berger and J. F. Roth, J. Catalysis,4, 717 (1965). (26) L. S. Singer and D. N. Stamires, J. Chem. Phys., 42, 3299 (1965). (27) We define 100% surface saturation as that point at which the
addition of further increments of the aromatic does not increase the integrated esr intensity. For a further discussion of concentration effects, see 11. (28) See ref 6 and 9. Both of these investigators agree that the radical cation is present; however there appears to be a disagreement a8 to whether a carbonium ion is also present. (29) B. G. Segal, M. Kaplan, and G. K. Frwnkel, J. Chem. Phys., 43,4191 (1965).
Volume 71, Number S February 1967
636
G. M. MUHA
The hyperfine-coupling constants can be obtained by computing a theoretical spectrum to fit the experimental second-derivabive spectrum. This procedure is necessary because the overlapping of lines with different widths and relative intensities causes the apparent center of the line to shift from the true value by an unknown amount. For a Lorentzian line, the second derivative is given by d2X I t 4SN 6s' - 2
-- dH2
--
TAH (1
9 8 7
6 S 4 3
3' 4' 5' 6'
2 I 2'
1 8'
/ I 9'
lo' 11'
A
+ z2)3
where S is the instrumental sensitivity factor, x = (1/Ai7)-1,(H - Ho) is the reduced independent variable, and AH is the line width of the line centered at Ho with degeneracy N . Approximate values for the coupling constants in ring positions A and B (see Figure 1) can be obtained from the line-sharpened spectrum (Figure IC). Coupling in the C position is expected to be small and is neglected for the present. These values, together with an initial estimate of AH, were refined by a least-squares procedure for two groups of lines (numbered 4, 5 and 4', 5' in Figure 2B).30 From this fit, the values U A = 3.92 and U B = 3.16 gauss were obtained. In sulfuric acid solution, the corresponding values are 4.11 and 3.09, respecti~ely.~~ To test the reliability of this result, the theoretical fit to the lines at the center of the spectrum (numbered 2, 1, 2' in Figure 2B) were computed next. This procedure is not entirely arbitrary, for now only one parameter, AH, can be varied for each line; the instrumental sensitivity, line position, and perhaps the most critical factor, the zero value of the ordinate, have been fixed by the previous iteration. The result is shown in the center region of Figure 2C. The close agreement between the computed and experimental curves lends credence to the method. The stick spectrum corresponding to the values of the coupling constants determined above is plotted in Figure 2B. (Four lines of intensity 1/36 that of the center line are not shown.) A comparison of this spectrum with the experimental result (Figure 2A) shows discrepancies between the relative heights of corresponding lines. These discrepancies arise because the experimental spectrum exhibits a variation in the line widths of the individual hyperfine lines.33 Such variations are a manifestation of the interactions between the radical and the solvent, surface, other radicals, etc. It is possible to extend the iterative computation to these other lines and thus determine the line widths and their variation. However, to minimize computational labor we resort to another scheme. Under certain conditions, the line-width variation The Journal of Physical Chemistry
11 $3
Figure 2. Second-derivative (A) and stick spectrum (B) of the perylene radical cation generated on a silica-alumina catalyst. I n (B), the lines are numbered as in Table I. I n (C), a comparison of the theoretical spectrum (solid line) computed by eq 1 with the experimental result (dotted line) is given. I n (D), the individual lines (widths listed in Table I) making up (C) are shown. I n (B), (C), and (D), four lines of degeneracy 1/38 that of the center line are not included.
can be expressed as a simple polynomial in the total nuclear spin quantum numbers of the equivalent groups of p r o t o n ~ , viz. ~~J~
H
=
A
+ B M A + CMB +
+ EMA' + FMB'
DMAMB
(1) where the coefficients A , B , . . ., can be expressed in terms of certain spectral densities describing the interaction. We use this expression to determine the widths of the remaining lines. To determine the six unknowns in eq 1, the values of AH for six of the lines computed above are used. Thus the values A = 1.6, B = -0.05, C = 0.05, D = -0.2, E = 0.65, and F = 0.05 are obtained. With these coefficients, the value of AH for the remaining (30) In the actual computation, the iteration was limited to negative values of the dependent variable since significant contributions are made to the wings from adjacent lines. Also small contributions from lines 6,3 and 6', 3' were included. (Note that Figure 2B is inverted with respect to the usual convention.) (31) Shifts of up to *35 mgauss in either jaAl or ICZB~ (but not both) could be tolerated in the parameters used in the iteration. However, the over-dl fit of the computed to the experimental curve including all lines (to be discussed next, 8ee Figure 2C) was much poorer. (32) A. Carrington, F. Dravnieks, and M. C. R. Symons, J. Chem. Soc., 947 (1959). (33) J. H.Freed and G. K. Fraenkel, J . Chem. Phys., 39,326 (1963). (34) D. Kivelson, {bid., 27, 1087 (1957); 33, 1094 (1960). (35) A. D.McLachlan, Proc. Roy. SOC.(London), A280, 271 (1964).
ESROF AROMATIC HYDROCARBONS ADSORBED ON SILICA-ALUMINA
lines can be computed. A summary of the results is given in Table I. The resulting calculated spectrum is given in Figure 2C and the individual line shapes are shown in Figure 2D.36
Table I: Results of the Line-Width Determination -Line LeastLine no.a
11 10 9 8 7 6 5 4 3 2 1 2’ 3’ 4’ 5’
MA
MB
-2 -1 -2 -1
-1 -2 0
0
-2 -1
n 1
-1 0 1 -1 0
-1 -2 1 0 -1 -2 1 0 -1 2 1
1
0
6’ 7’
a
8’
I 2
-1 2 1
9’ 10’ 11’
0
1
2
0
2 1
Degeneracy
4 4 6 16 6 4 24 24 4 16 36 16 4 24 24 4 6 16 6 4 4
square
fit
widthFrom eq 1
3.9 2.0 4.3 2.1 1.7 4.8 2.3 1.6 2.7 2.6 1.6 2.3 3.0 1.7 2.2 4.5 1.9 2.1 4.1 2.1 3.8
The numbering of the lines is shown in Figure 2B. Four lines of unit degeneracy are not included.
637
stants; the first-derivative presentation, however, still exhibits the asymmetry noted above. It is likely that even below 25% surface saturation, the solvents have little effect on the line widths or coupling constants, for it is possible to obtain resolved hyperfine spectra without the use of a solvent by distilling the aromatic, in vacuo, onto the surface of the cat% l y ~ t . ~This J ~ method of preparation was not used in the present study since it was found that the mixing between the aromatic and the catalyst was nonuniform at the higher concentration levels and interradicalexchange effects became important (see 11). A t the lower concentration levels, the signal-to-noise ratio was poor and the second-derivative presentation could not be used. However, the first-derivative spectrum again exhibited asymmetry. It is possible to remove the solvent by a vacuummanipulation technique after the radicals have been formed. In this case, the same coupling constants and line-width variation are obtained as reported above. However, it is not possible to demonstrate that all of the solvent has been removed, and for this reason arguments concerning solvent effects based on this technique are not entirely convincing. Temperature Efects. The appearance of the esr spectrum is essentially unchanged within the temperature interval defined by the boiling and freezing points of the particular solvent used. As the temperature is lowered below the freezing point of the solvent, the hyperfine lines quickly broaden, and at 77°K only a faint trace of the hyperfine structure remains. Anisotropy in the g tensor (as exhibited in a glass average) is too small an effect to account for this residual structure.
The over-all fit of the computed to the experimental Discussion curve is quite satisfactory except for the misalignment Hyperfine-Coupling Constants. In the preceding of the base line of the computed curve at high and low section, it was noted that the hyperfine-coupling confield and the overestimation of the region in the vicinity stants are shifted -4% from the values observed in of line 8‘. However, eq 1 certainly is an adequate sulfuric acid. I n the present section we wish to confirst approximation with which to discuss the nature sider the possibility that this shift is caused by a moduof the interactions giving rise to the line-width varialation of the isotropic magnetic interactions. This tions. type of modulation effect can arise from internal vibraEffect of Solvents and the Aromatic Concentration. tions and rotations as well as fluctuating solvent or Within the interval 25-95y0 surface s a t u r a t i ~ n , ~ ~ counterion complexes with the radical cation. 3 3 the spectral resolution, line-width variation, and It is difficult to get an accurate quantitative estimate coupling constants are not appreciably changed by the of the magnitude of the shift, for it is known that when perylene :catalyst concentration ratio nor the choice line-width effects are large, static and dynamic freof solvent. In the present experiments, carbon disulquency shiftsa7 must be considered. fide, carbon tetrachloride, hexane, benzene, and perdeuteriobenzene were used as solvent. (36) In summing the contributions from the individual lines that Below 25% surface saturation, the signal-to-noise combine to form Figure 2C, it is necessary to adopt eome cutoff procedure concerning the wings of the line. For this purpose, the ratio of the spectrum degrades to such an extent that wings were truncated at the point at which they contribute less then the second-derivative presentation is of marginal 10% to the De& of the next a&acent line.
Volume 71 Number 3 February 1967 ~
G. M. MUHA
638
For /UBI, the measured shift is -0.12 gauss,al a value two orders of magnitude too large as estimated from a simple second-order calculation,3* The complete theory of the shift3’ is too complex to be applied to the present problem, and since the contribution from this type of correction appears to be small, it will be neglected. In this connection we note that in the case of anthracene (see 11) and 9,lO-dimethylanthracene (see IV), shifts in the values of the larger coupling constants are again observed, but corrections due to static and dynamic effects are shown to be small. This point can be established with reasonable precision from the spectra of partially deuterated molecules. Coupling-constant shifts of a similar magnitude have been observed for the protons in the anthracene radical anion generated by electrolytic39 and alkali metal reduction40 and in various radical cations prepared in antimony t r i ~ h l o r i d e ~and ~ ~entahalide~ solvents. ~ Much larger effects are observed in radicals having polar substituent groups. 43 , 4 4 The effect is ascribed (in the case of alternant aromatic hydrocarbons) to a redistribution of the aelectron density caused by fluctuation solvent comp l e x e ~or~ ~an electrostatic perturbation due to the counteri~n.*~An explanation based on the latter mechanism is the more plausible for the present system since, as noted above, it is possible to prepare the radical cation in the absence of solvent. Thus the surface ( i e . , the counterion) interaction must be the major cause of the isotropic modulation effects. In principle, it should be possible to demonstrate a dependence of the magnitude of the coupling constant on temperature, concentration, and solvent.‘O In the case of a solid catalyst system this is not practical, for in addition to the handicap of poor resolution, the “counterion concentration’’ is fixed by the choice of catalysts45 and the surface heterogeneities lead to local concentration fluctuations (see 11). Indeed, one of the more striking observations concerning the experiments with any of the aromatics studiedI6 is that the line-width variation and coupling constants are sensibly invariant over the range 2595% surface saturation. This result suggests that the radical-counterion interactions are a major ej’ect in these catalyst systems. Shifts in the magnitude of a coupling constant can be understood in terms of a model involving a dissociative equilibrium between the radical-counterion complex and the individual component^.^^ I n the complex the value of the coupling constant will differ from that in the uncomplexed “free” radical. The experimental value is the average of these two values each weighted by the lifetime of the respective species. The Journal of Physical Chemistry
Unfortunately, data are not available to allow an estimate of the lifetimes nor the coupling constant for the complexed and “free” perylene radical cation. However consider the electrostatic perturbation approa~h.~OAs determined experimentally, the magnitude of a.4 decreases while that of a B increases. Thus the a-electron distribution is modified to increase the average charge density in the center of the molecule, (We use the Huckel approximation and thus the electronic density is proportional to the spin density.) Presumably then, the time-average location of the counterions is over the center of the molecule as would be required for the electrostatic perturbation to be effective.40 Therefore, when compared to the coupling constants measured in sulfuric acid, the silica-alumina results indicate that either the counterion has a closer distance of approach or, more probably, the complex has a longer lifetime. If the counterion were relatively immobile compared to its counterpart in solution, linewidth effects would be more pronounced than experimentally observed. The question of the counterion mobility is considered again briefly in the next section and more fully in 111. If the counterion is paramagnetic, the lifetime of the complex must be shorter than the inverse of the hyperfine splitting to preclude the possibility of quantum mechanical exchange broadening. 3 5 , 4 6 Thus the lifetime would be shorter than sec. If the lifetime of the complex could be lengthened by lowering the temperature, it might be possible to observe AM = 1,2 transitions4’ in a triplet complex similar to that observed in ketyl systems.48 As noted earlier, only the broad line due to the radical cation is observed and the fact that a faint trace of structure is still visible a t 77°K indicates that some surface mobility is retained even at this temperature. R. W. Fessenden, J. Chem. Phys., 37,747 (1962). J. R. Bolton and G. K. Fraenkel, ibid., 40, 3307 (1964). A. H. Reddoch, ibid., 43, 225 (1965). E. C. Baughan, T. P. Jones, and L. G. Stoodley, Proc. Chem. SOC.,274 (1963). (42) I. C. Lewis and L. S. Singer, J . Chem. Phys., 43, 2712 (1965). (43) J. G. Gendell, J. H. Freed, and G. K. Fraenkel, ibid., 37, 2832 (1962). (44) P. Ludwig, T. Layloff, and R. N. Adams, J . Am. Chem. SOC., 86, 4568 (1964). (45) The counterion concentration apparently depends on the oxidation potential of the aromatic used. A discussion of this effect is given in 111. (46) J. D. Currin, Phys. Rev.,126, 1995 (1962). (47) J. H. van der Waals and M. S. Groot, Mol. Phys., 2, 333 (1959). (48) N. Hirota and S. I. Weissman, J . Am. Chem. SOC.,86, 2538 (1964). (38) (39) (40) (41)
ESROF AROMATIC HYDROCARBONS ADSORBED ON SILICA-ALUMINA
If the counterion is diamagnetic, again a relative motion within the complex can give rise to a shift in the coupling constant,49but in this case the result of the present experiment does not permit an estimate of the lifetime of the complex. Line-Width Efects. I n the preceding discussion only the isotropic modulation effects introduced by the relative motion of the counterion with respect to the radical cation were considered. More generally, it is necessary to consider other perturbations in addition, e.g., internal vibrations, random molecular tumbling, exchange interactions, etc. Each of these introduces a fluctuating time-dependent force on the spin system and thus results in a modulation of both the isotropic and anisotropic magnetic parameters. These modulation effects manifest themselves in a line-width variation among the different hyperfine lines of the esr spectrum. The complete theory of the effecP is too complex to apply to the present data. Thus in the following, an approximate analysis35is used to estimate the effects arising from the anisotropic hyperfine interaction and the anisotropic part of the g tensor. Exchange and counterion effects are considered as an afterthought. The treatment is crude, for it assumes an arbitrary separation of the different contributions to the line width; nevertheless the agreement is quite remarkable, I n eq 1, an expression is given which describes the line widths in terms of the resultant nuclear spin quantum numbers of the individual groups of equivalent nuclei. A similar equation can be derived theoretially^^ in which the average line width (Tz-') of each hyperfine line is evaluated in terms of the inner product of the anisotropic hypefine and g tensors. The result isw-52
+ 3J1)(60 X lo-'*)-' K = 637O(7Jo + lOJ1)(4Jo + 3J1)-' - 2075 J
=
(4Jo
and J o and J , are the spectral densities of the interactions at zero and the microwave frequency, respectively. Equation 2 shows that terms involving M C make a minimal contribution to the line widths, a result which follows from the small coupling constant in this position. This result presumably accounts for the success in the attempted fit between the computed
639
and experimental spectrum (Figure 2C) even though M Cwas neglected. A comparison of eq 1 and 2 shows that the signs of the various terms53 are correctly predicted. The relative magnitude of the corresponding coefficients, however, exhibit considerable di~agreement.~~ Such a result is not surprising, for in addition to errors introduced through approximations inherent in the theory and in the calculation of the tensor components, the above treatment considers only the anisotropic effects. Line-width effects also arise from isotropic modulation43and exchange effects;46 to a first approximation both of these contributions give rise to a variation that is symmetrical about the center of the spectrum and thus contribute to K and quadratic terms in M . (Other relaxation mechanisms also contribute to K that are independent of These factors notwithstanding, it is possible to obtain an order of magnitude agreement between the larger coefficients in eq 1 and 2 if spectral densities corresponding to a correlation time of sec are chosen; this value is comparable to that found for radicals in solution.35 Such a result, coupled with the correct prediction of the signs of the coefficients, indicates that the theory gives adequate agreement with experiment and, therefore, at least the principal features of the interaction are correctly described. Such a result is quite remarkable, for on first viewing, the poor resolution and exaggerated line-midth effects would suggest that a relaxation theory designed for radicals in solution might require major modification before it was applied to the problem of radicals on a surface That such is not the case suggests that the radical cation is quite unrestricted in its movements (as opposed to being bound to a fixed catalytically active site on the surface). By implication then, the counterion may also be mobile since counterion effects are important to the description of the shift in the magnitude of the coupling constants. Such a picture (49) E. de Boerand E. L. Mackor, J.Am. Chem. Sac., 86,1513 (1964). (50) The anisotropic hyperfine tensor is obtained from the measured coupling constants' using Q = 23. The components of the anisotropic g tensor are obtained from theoretical considerations.62 Throughout the calculation, the microwave frequency is taken to be 9200 mHz. (51) H. M. McConnell and J. Strathdee, Mol. Phgs., 2, 129 (1959). (52) A. J. Stone, ibid., 7, 311 (1964). (53) The signs of the coupling constant in positions A and B are the same and opposite to that in position C. See E. de Boer and E. L. Mackor, ibid., 5, 493 (1962). (54) It has been reported that this theory also accounts for the linewidth effects observed in the perylene-sulfuric acid system;35 however a numerical evaluation of the terms was not given. Also the experimental data on the solution line widths have not been published; thus a comparison with the present results is not possible. (55)P.W. Atkins and D. Kivelson, J . Chem. Phgs., 44, 169 (1966).
Volume 71, N u m b e r 3 February 1967
640
is in good agreement with an older model of this catalyst surface,66but it has not been included in some of the more recent discussions. This question concerning counterion mobility as well as the nature of the counterion is discussed more fully in 111. Finally, we note that in the perylene-catalyst system discussed here, electronic and chemical-exchange effects can also be studied. The results so obtained for perylene parallel those for anthracene. Since partially deuterated compounds were available in the study of the latter aromatic hydrocarbon, the esr spectra are simpler to understand. Thus a brief discussion of the
perylene exchange results will be included with those of anthracene given in 11.
Acknowledgment. During the course of the work reported in this and the subsequent papers in this series, the writer benefited from the thoughtful criticisms and helpful suggestions of Drs. S. Bank, hl. T. Melchior, and D. J. C. Yates. Their assistance is gratefully acknowledged. (56) A. G. Oblad, S. G. Hindin, and G. A. Mills, J . Am. Chem. SOC., 7 5 , 4096 (1953).
Electron Spin Resonance Studies of Aromatic Hydrocarbons
Adsorbed on Silica-Alumina.
11. Anthracene
by G. M. Muha Esso Research and Engineering Company, Linden, New Jersey
07086 (Received August 98, 1966)
An analysis is presented of the line-width and exchange effects exhibited in the electron spin resonance (em) spectrum of the anthracene radical cation generated on a silicaalumina catalyst surface. The line-width effects and the magnitude of the proton hyperfine-coupling constants are discussed in terms of isotropic and anisotropic modulation of the magnetic parameters due to fluctuations in the radical-cation environment. The results parallel those obtained for perylene on this surface. When the aromatic concentration is in excess of that required to saturate the radical-forming ability of the catalyst, both chemical and electron-exchange effects are observed. The latter effect is discussed in terms of an electron-transfer mechanism with neutral aromatic molecules. Chemical exchange involves surface protons and occurs after the radical concentration has reached a limiting value; thus it is argued that proton transfer is not involved in radical-cation formation.
I n the preceding paper,' an analysis of the electron spin resonance (esr) spectrum of the perylene radical cation formed on a silica-alumina surface was presented. From the analysis, it was argued that the observed line-width effects and the shifts in the values of the hyperfine-coupling constants (relative to those in sulfuric acid) could be understood in terms of a combiThe Journal of Physical Chemistry
nation of effects arising from a random tumbling motion and a dissociative equilibrium of a complex between the radical cation and its counterion. Also, exchange effects were noted, but the system was not convenient to study the effect. (1) G. M. Muha, J. Phys. Chem., 71, 633 (1967), hereafter referred to as I.
