J . Phys. Chem. 1990, 94, 2619-2623 exhibit less dipolar interaction with the network PEO matrix than the ionic ferrocenes, yet in fact it diffuses more slowly. We surmise that all of the ferrocenes have sufficiently large molecular size (>>vr)and/or reasonably strong interactions with the PEO matrix (hence their slow, non-Arrhenius diffusion), so that the free volume requirements for diffusive displacement are, within the group of ferrocenes, influenced more by molecular size than by variation in dipolar or nonpolar interactions. Relation of Ferrocene Do,, Values to the Ionic Conductivity of Network PEO. We finally consider the comparison in Figures 4 and 5 of the ionic conductivity of network PEO polymer electrolyte with ferrocene Dappvalues measured at low concentrations (where there is negligible electron hopping) as a way to better understand ionic diffusivity in this polymer. Ionic conductivity in polymer/electrolyte solutions is thought to be determined by the product of carrier ion population and ion diffusivity. Plots of ionic conductivity vs electrolyte concentration typically exhibit maxima5.lh (see Figure 4,top curve) which have been interpreted as two opposing contributions at increasing electrolyte concentration: an increase of carrier ion population and a decrease of the ionic diffusivity. There have been few r e p ~ r t ~ however, , ~ ~ ~ ,of~actual - ~ ~detection ~ of a decrease of ion diffusivity. In the ferrocene diffusion measurements, the (electron) carrier population is given without ambiguity by the total ferrocene concentration, since the impetus for transport in the measurement is a concentration and not a voltage gradient and ion pairing should be of little account. The decrease in Dappshown in Figure 4 and expressible by eq 5, accordingly a clear decrease in ferrocene diffusivity with increasing electrolyte concentration, is thus of interest since it supports the parallel proposal of decleasing ionic diffusivity with increasing electrolyte concentration.
2619
Continuously curved Arrhenius plots for ionic conductivity, as shown in Figure 5, are also widely observed in amorphous polymer electrolytes5 and have been fit to the Vogel-Tamman-Fulcher (VTF)27and Williams-Landel-Ferry equations.28 Both equations represent how diffusivity changes with temperature but leave unanswered questions such as whether the non-Arrhenius profile of ionic conductivity (a) represents how ion diffusion coefficients change with temperature or instead (b) represents a temperature dependence of ion pair and multiplet dissociations that control the actual number of ionic carriers present. We take it as significant, then, that Dappand ionic conductivity in the network PEO polymer exhibit quite similar (Figure 5) non-Arrhenius temperature dependencies in both magnitude and extent of curvature. These similarities can be taken to mean that the temperature dependence of ionic conductivity reflects the temperature dependence of ion diffusivity. This suggestion agrees with experimental comparisons of ionic diffusion coefficient and ionic conductivity in linear PEO electrolytes.2k Acknowledgment. This research was supported in part by grants from the Department of Energy (DE-FG05-87ER13675) and the National Science Foundation. M.W. acknowledges a sabbatical leave from Sophia University. (27) (a) Vogel, H. Phys. 2. 1921, 22,645. (b) Tamman, G.;Hesse, W. 2. Anorg. Allg. Chem. 1926,156,245. (c) Fulcher, G.S. J . Am. Ceram. SOC. 1925,8, 339. (d) The VTF equation of the form of a(T) = ( A / T ' / * )exp[ - B / ( T - T o ) ] where , To is ideal glass transition temperature and A and B are constants, is generally used for the expression of the temperature dependence. (28) (a) Williams, M. L.; Landel, R. F.; Ferry, J. D. J . Am. Chem. SOC. 1955, 77, 3701. (b) Reference 14.
Multlfrequency Electron Spin-Echo Envelope Modulation Spectroscopy of m-Dinltrobenzene Adsorbed on Alumina Sarah A. Cosgrove and David J. Singel* Department of Chemistry, Harvard University, Cambridge, Massachusetts 02138 (Received: July 31, 1989)
We report electron spin-echo spectroscopy of a radical species formed upon the adsorption of m-DNB (m-dinitrobenzene) from benzene solution on activated y-alumina. Two- and three-pulse echo modulation patterns were obtained at sample temperatures of 110 K and electron spin excitation frequencies of 6.2,7.4,8.2,9.1, 10.3, and 11.8 GHz. Modulation frequencies are assigned to 27Al,I4N,and 'Hnuclei, on the basis of suppression effects, the values of the modulation frequencies, and, particularly, the dependence of the modulation frequencies on external field strength. Aluminum modulations indicate that the radical species formed by interfacial electron transfer is bound to the alumina surface. Through frequency-tracking experiments, we assign the I4Nmodulation componentsas double-quantum frequenciesand determine hyperfine and quadrupole coupling constants of the observed I4N. The values of these parameters indicate that the adsorbed radical is an ion-pair complex.
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Introduction Radicals generated by interfacial electron transfer on dispersed metal oxides have recently been studied via ESEEM (electron spin-echo envelope modulation) and CW-ENDOR (electron nuclear double resonance) techniques.' The measurement of nuclear spin interactions via such techniques provides a means toward the elucidation of chemical, structural, and dynamical characteristics of the adsorbed species-information important for advancing the understanding of the electron-transfer process. Electron-nuclear
magnetic resonance techniques are of special utility in the study of complex systems because the nuclear spins in the vicinity of the paramagnetic center are selectively probed. Accordingly, they enable the investigation of specific sites and of specific chemical processes-an important advantage in the study of dispersed, catalytic materials. EPR (electron paramagnetic resonance) has long been used to monitor the formation of organic radicals on metal oxide surf a c e ~ . ~These - ~ studies have been particularly fruitful in estab-
(1) Snetsinger, P. A.; Cornelius, J. B.; Clarkson, R. B.; Bowman, M. K.; Belford, R. L. J . Phys. Chem. 1988, 92, 3696. Clarkson, R. B.; Belford, R. L.: Rothenberger, K . S.; Crookham, H . C. J . C a r d 1987, 106, 500. Rothenberger, K. S.; Crookham, H. C.; Belford, R. L.; Clarkson, R. B. J . Carol. 1989, 115, 430.
(2) Flockhart, B. D.; Leith, I. R.; Pink, R. C. Trans. Faraday SOC.1969, 65, 542. Muha, G. M. J . Catal. 1979, 58, 470. (3) Flockhart, B. D.; Scott, J. N. A,; Pink, R. C. Trans. Faraday SOC. 1966, 62, 730. (4) Flockhart, B. D.; Leith, I. R.; Pink, R. C. Trans. Faraday SOC.1970, 66, 469.
0022-3654/90/2094-2619$02.50/0
0 1990 American Chemical Society
2620 The Journal of Physical Chemistry, Vol. 94, No. 6, I990 lishing the dependence of radical yield on composition and chemical pretreatments of the surface and thus in providing insight into the underlying mechanism of the electron-transfer process. Analyses of the EPR spectra have also been undertaken to obtain information regarding the structure (and dynamics) of the adsorbed radical and of surface active sites. The depth of such analyses has been limited, however, because the EPR powder patterns of the surface adsorbed species are typically lacking in well-defined features: in some cases even the chemical identity of the radical product could not be unequivocally determined. An example of this situation that we find particularly intriguing concerns the radical formed on alumina when the electron acceptor m-DNB (m-dinitrobenzene) is a d ~ o r b e d .EPR ~ measurements in fluid solutions show that m-DNB forms both naked anion radicals5 and ion-pair complexes (adducts of the anion with, for example, Na+ or H+ counter ion^).^^^ In the anion, the two nitrogens are equivalent, with hyperfine couplings of 13 MHz, whereas in the ion pair, the electron spin becomes localized on one of the now inequivalent nitro groups; one large nitrogen hyperfine coupling (-21 MHz) and one small coupling (-0.6 MHz) are observed. On the alumina surface, the radical generated from m-DNB exhibits a broad EPR powder spectrum; the hyperfine structure from weakly coupled nuclei is obscured! Detailed simulation of the EPR spectrum-given the number of independent parameters involved-cannot be regarded as an ideal method for determining the spin couplings, even for the strongly coupled nuclei.8 It. has nonetheless been suggested, on the basis of EPR ~pectroscopy,~ that the radical formed on alumina is an ion pair. This dilemma can be definitively resolved by the use of electron-nuclear magnetic resonance methods to measure directly the interactions of the weakly coupled nitrogen. ESEEM is particularly well-suited for such an application. It has been employed in numerous determinations of 14N hyperfine and quadrupole coupling constants in a wide variety of chemical systems. In particular, multifrequency ESEEM has been successfully applied to 14N in the nitro groups of DPPH (diphenylpicrylhydrazyl).'o-ll Here we report the results of a multifrequency ESEEM study of m-DNB adsorbed on alumina. The assignment and the analysis of the ESEEM frequencies are discussed. Our results indicate that the radical species observed is present as an ion pair on the alumina surface.
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Theory The general principles that underlie the ESEEM effect, as well as the specific applications of these principles to particular spin systems, have been discussed in numerous publication^.^^'^-^^ We recapitulate only selected results from this work that are important to the assignment and analysis of ESEEM results presented here. With the external magnetic field directed along a specific direction in the molecular frame, the three-pulse echo (5) Maki, A. H.; Geske, D. H. J. Chem. Phys. 1960,33,825. Rieger, P. H.: Fraenkel. G. K. J. Chem. Phvs. 1963. 39. 609. (6) Ward, R L. J. Chem. Phis. 1960,' 32,' 410. Ward, R. L. J. Chem. Phys. 1962, 36, 1405. (7) Ward, R. L. J. Chem. Phys. 1963, 38, 2588. Fox, W. M.; Gross,J. M.; Symons, M. C. R. J. Chem. Soc. A 1966, 448. (8)The overall width of the pattern would be expected to be roughly similar whether derived from one large "N hyperfine coupling or two, equivalent "N nuclei with approximately half as large a coupling. Inhomogeneous broadening could lead, in either case, to the 'three-line" derivarive spectrum observed when mono- and dinitrobenzenes are adsorbed onto the metal oxides. (9) Flanagan, H. L.; Singel, D. J. J. Chem. Phys. 1987, 87, 5606, and references therein. (10) Flanagan, H. L.; Singel, D. J. Chem. Phys. Lerr. 1987, 137, 391. (1 1 ) Flanagan, H. L.; Gerfen, G. J.; Singel, D. J. J . Chem. Phys. 1987, 88, 20. (12) Mims, W. B. Phys. Rev. E 1972, 5 , 2409. (13) Mims, W. B. Phys. Rev. E 1972,6, 3543. (14) Mims, W. B. In Electron Paramagnetic Resonances; Geschwind, S., Ed.; Plenum: New York, 1972; p 263. Barkhuijsen, H.; de Beer, R.; Pronk, B. J.; van Ormondt, D. J. Magn. Reson. 1985, 61, 284. (15) Flanagan, H. L.; Singel, D. J. J. Chem. Phys. 1988, 89, 2585. (16) Lai. A,: Flanagan, H. L.; Singel, D. J. J. Chem. Phys. 1988,89,7161.
Cosgrove and Singel envelope-modulated by interaction with a single nuclear spinassumes the form13
L.
ij
L.
k.n
In (1) 7 and T refer to the time respectively between the first and second and between the second and third microwave pulses; a and @ designate the two electron spin levels on resonance; i j and kn index pairs of nuclear spin levels within the a and @ manifolds, respectively; and the x factors are amplitudes of the modulation frequencies, designated by w , the resonance frequencies of the coupled nuclei. The amplitude factors depend on the composition of the nuclear spin eigenstates within the a and /3 manifolds. Maximal amplitudes are expected when the external field matches the local field exerted by the electron on a nuclear pin.^,'^ When more than a single nucleus contributes to the modulation, the ESEEM pattern is calculated as the product of the individual modulation functions.I2 Consequently, additional, "multinuclear" combinations may appear. Typically in three-pulse ESEEM experiments the time 7 is set and Tis incrementally scanned; the modulation of such three-pulse echo envelopes derives from the cosine terms in (1) with arguments proportional to T + T. Noteworthy in (1) is the dependence of the amplitude of the modulation components on the value of 7 selected. If, for example, at the selected value of 7 1
ktn
then the amplitude of the w i frequency component is zero. This suppression effect12J3 (together with the usual slower damping of three-pulse versus two-pulse echo envelopes) makes three-pulse ESEEM particularly useful for resolving closely spaced peaks in ESEEM spectra. Moreover, 7-suppression bel-avior can be used to correlate ESEEM frequencies as belonging to the same nucleus, but arising from opposite ( a vs 0) manifolds. For an I = nucleus, each summation in (1) encompasses only a single term; hence we may drop the eigenstate indices and, omitting the zero-frequency terms, write
in which
In this situation, the T dependence of the amplitude of a given frequency component involves a simple sinusoidal oscillation at the frequency of its partner in the opposite manifold. When 7 = n / w a (n = 0 , 1 , 2, ...), the partner frequency, u p , is completely suppressed-and vice versa. For systems with I > this behavior could, in principle, be considerably more complicated: all I ( 2 I (17) Even though the branching is not completely restricted in any finite field, I = 1 double-quantum frequencies should still show simple sinusoidal variation with T (see ref 15). At large deviations from exact cancellation (for finite quadrupole couplings), the xa* factor (eq 1) that involves the two doublequantum coherence is precisely half, in magnitude, of the fundamental amplitude factor (e.g., xu) for the double-quantum coherence and is opposite in sign. Provided the influence of the xu* factors that jointly involve the double-quantum coherence and one of the two single-quantum coherences is small, this relationship suffices for simple sinusoidal T variation and complete suppression effects. These latter p . 8 factors are indeed substantially smaller-under the assumed conditions-than the former one (which involves the two double-quantum coherence). Moreover, they are equal and opposite. Their effects thus tend to cancel if the single-quantum peaks are narrow and (hence) largely overlapping, whereas their effects would rapidly dissipate (and likely not persist beyond experimental dead times) if the single-quantum peaks were broad.
Multifrequency ESEEM of m-Dinitrobenzene on Alumina
+ 1) fundamental frequencies of one manifold could influence the 7-dependent amplitudes of their partners. This potential complexity is alleviated when the branching of electron spin transition probability is restricted. In perhaps the simplest situation, the microwave field would exclusively couple a given pair of nuclear sublevels in one manifold with exactly one pair in the opposite manifold. This “isolated” four-level system would then exhibit 7-suppression behavior characteristic of an I = system. Far from exact cancellation, the double-”quantum” peaks of I = 1 systems are exclusively correlated in this manner.15J7 Mims has shown that strong suppression effects also obtain when couplings are weak and nearly no branching occurs.13 In an orientationally disordered solid, the powder average of the modulation function in (1) or its multinuclear product describes the experimentally observed ESEEM pattern. Although this powder averaging can cause substantial broadening of the ESEEM peaks, those peaks that have lent themselves to detailed analysis have, for the most part, retained a narrow spectral profile, despite the powder averaging. Narrow lines can clearly occur in ESEEM powder spectra if all anisotropic interactions are small. Even if anisotropic interactions are large, however, narrow lines may still p r e ~ a i I . ~ * ’ ~Cases ~ ’ * involving I4N have been widely d i s c ~ s s e d . ~ In particular, at large deviations from exact cancellation (in systems with nearly isotropic hyperfine interactions) sharp double-quantum peaks arising from both the a and 0 manifolds appear in the ESEEM spectrum. Experimental Procedures Preparation of Samples. We employed the procedure of Flockhart, Leith, and Pink4 to generate m-DNB radicals by adsorption from benzene solutions onto activated y-alumina. A 0.1-g powder sample of y-alumina (Alfa, surface area 150 m2/g) was activated, in an EPR sample tube, by heating in air at 650 OC for 4 h. The sample tube was then sealed with a rubber septum and cooled. A pale yellow, lo-’ M solution of m-DNB (Aldrich, recrystallized from ethanol before use) in degassed benzene (Mallinkrodt, reagent grade) was then injected into the sample tube. An orange coloration of the sample was immediately observable; the concommitant presence of free radicals was confirmed by EPR spectroscopy. The concentration of the radicals, monitored by the intensity of their EPR signal, increased over a 24-h period. By analogous adsorption of other precursors, including perylene and nitrosobenzene, we have generated a number of other radical species on alumina. We have also prepared, following the procedure described by Ward? the proton adduct of m-DNB anion radicals by photolysis of a tetrahydrofuran solution of m-DNB. Electron Spin-Echo Spectroscopy. ESE experiments were performed at excitation frequencies of 6.2, 7.4, 8.2, 9.1, 10.3, and 11.8 GHz. The spectrometer used in these experiments differs in a few respects from that which we have previously described.’O First, we have added a I-kW traveling wave tube amplifier (Litton, Model 624, 8-18 GHz). Second, we have adopted a different microwave resonator design. Previously, at frequencies other than 9.1 GHz (where an overcoupled Varian TEIo2is used), we employed a three-loop, two-gap resonator with an 1 I-mm central (sample-containing) loop. This resonator was enclosed in a shield designed to mimic the exterior of the Varian cavity. The temperatures required for the observation of electron spin echoes were established with commercially available finger Dewar inserts or nitrogen gas flow cryostats designed for the Varian cavity. A similar system has been applied in CW-EPR spectroscopy and was described very recently by Hyde et aI.l9 Although this resonator assembly is convenient, the large central loop renders it inefficient in the conversion of incident microwave power to oscillating field. Therefore, we have implemented a design in which the central-loop diameter is just over 4 mm and the shielded resonator is suspended in a simple glass Dewar. Cooling gas is introduced directly into the resonator shield via a copper tube
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( 1 8) Singel, D. J. In Advanced EPR in Biology and Biochemistry; Hoff, A. J., Ed.; Elsevier: Amsterdam, 1989; p 119. (19) Hyde, J. S.;Froncisz, W.; Oles, T. J . Magn. Reson. 1989, 82, 223.
The Journal of Physical Chemistry, Vol. 94, No. 6, 1990 2621
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Figure 1. Two-pulse ESEEM patterns, obtained a t 110 K, of m-DNB adsorbed on activated y-alumina. The electron spin excitation frequencies, uEpR, are (from front to back) 11.8, 10.3, 9.1, 8.2, 7.4, and 6.2 GHz.
u
0.0
5.0
10.0
15.0
20.0
Frequency (MHz) Figure 2. Three-pulse ESEEM (magnitude) spectra of m-DNB adsorbed
-
on activated y-alumina at the electron spin excitation frequency 9.08 GHz, and sample temperature 110 K. The upper four panels (a-d) correspond to ESEEM spectra obtained a t specific time intervals (7) between the first and second microwave pulses; the bottom panel, (e), is a composite of the upper four. The values of 7 used in the upper panels are (a) 500, (b) 380, (c) 350, and (d) 290 ns. The peaks a t 1.8 and 2.8 MHz are assigned as I4N double-quantum frequencies. The peaks a t 3.6 and 13.8 M H z are assigned to matrix *’AI and matrix ‘H nuclei, respectively. The spectral estimates shown are computed via the linearprediction method, as described in detail in ref 10.
which follows a coil immersed in liquid nitrogen in the same Dewar. Various microwave resonators were employed within this apparatus including three-loop, two-gap resonators (7.3, 10.3, 11.8 GHz), and a tunable, two-loop, one-gap resonator (6.4-8.2 G H Z ) . ~ In the experiments reported here, the temperature of the resonator assembly was maintained at -110 K. Two- and three-pulse B E E M patterns were obtained at each frequency via irradiation in the center of the EPR line (no orientation selection). The typical duration of a ?r/2 pulse in the echo sequence was approximately 15 ns at X-band and 50 ns at C-band (Varian TWTA, 18 W). The echoes were repeated at a rate of -2 kHz. The typical data record in two-pulse ESEEM experiments consists of 128 points at 25-11s intervals; stimulated echo patterns consist of 256 points taken at 25-11s intervals. In three-pulse echo experiments, 7 was varied from 220 to 650 ns. We did not undertake full two-dimensional (7,7‘)analyses of 7-suppression behavior.21 Instead, we ascertained the successive values of T at which modulation components were completely suppressed. The spectral analysis of the modulation patterns was carried out by procedures that we have outlined elsewhere, involving linear-prediction and Fourier transformation methods.I0 The external field strength was (20) Tuning was achieved by varying the insertion of a metal stub into the resonator gap. (21) Merks, R. P. J.; de Beer, R. J . Phys. Chem. 1979, 83, 3319. Barkhuijsen, H.; de Beer, R.; de Wild, E. L.; van Ormondt, D. J . Magn. Reson. 1982, 50, 299.
2622
The Journal of Physical Chemistry, Vol. 94, No. 6, 1990
Cosgrove and Singel
measured with a Micro-Now Instruments Model 515B-1 gaussmeter. Results and Analysis In Figure 1 we show the two-pulse ESEEM patterns exhibited by m-DNB adsorbed on alumina at the surveyed excitation frequencies. This set of patterns reveals a general decrease in modulation depth with increasing excitation frequency. In Figure 2, we show a series of three-pulse ESEEM spectra at different 7 values, obtained at the excitation frequency, 9.08 GHz. The bottom panel in Figure 2 is a composite of the upper four. The gross features exhibited in this spectrum are typical of the spectra observed at all excitation frequencies (up to 11.8 GHz, vide infra). The fundamental ESEEM frequencies at 13.8, 3.6, 2.8, and 1.8 MHz in Figure 2 are attributable to 'H, 27Al,and I4N nuclei. These assignments are based primarily on the frequencies of the peaks, the tracking of their frequencies with external field strength, and 7-suppression effects in three-pulse echo experiments. In the three-pulse ESEEM spectra, all four of the peaks exhibit a simple sinusoidal variation in amplitude as a function of 7. We find that the peaks at 13.8 and 3.6 MHz are modulated in the 7 dimension at their own frequencies. Both therefore belong to nuclei for which w0 = ab, that is, to nuclei weakly coupled to the electron spin. We also observe them to track linearly in external field strength at a rate equivalent to the gyromagnetic ratios of 'H and 27Al,respectively. Accordingly, we assign these peaks to 'H and 27Alnuclei.22 The two lower frequency peaks-at 1.8 and 2.8 MHz-are modulated in the 7 dimension at frequencies of 2.8 and 1.8 MHz, respectively. (We also observe modulation components at the sum and difference of these frequencies in two-pulse echo envelopes, with the phase of these combination peaks shifted by K relative to the fundamentals.) The behavior of the low-frequency pair of peaks demonstrates their exclusive correlation and is consistent with their assignment as doublequantum peaks of an '*N n ~ c l e u s . ' ~ This assignment is substantiated by the field dependence of the frequencies. To the extent that the hyperfine coupling to the relevant I4N may be taken as i s o t r ~ p i c , ~ - "and ~ ' ~the mixing of the electron Zeeman eigenstates by all of the hyperfine interactions may be neglected,23the frequencies of the double-quantum peaks are given by9 (3) in which K (e2Qq/4) and q are the quadrupole coupling constant and asymmetry parameters; uef, = u, 7 a/2; a is the isotropic hyperfine coupling constant; u, is the nitrogen Larmor frequency; and f indexes the two electron spin levels ( a and 6 ) on resonance. It is convenient to rewrite (3) as vdsa2/4 - u,2 = Tuna + K2(3 q2) + a 2 / 4 (4)
+
in which a linear relationship between the - u: and external field strength is expressed. This relationship tests the assignment of I = 1 double-quantum peaks and additionally provides a convenient means to evaluate the hyperfine coupling constant and G ( 3 q 2 ) through linear regression. In Figure 3, we plot our multifrequency ESEEM data (the I4N,low-frequency pair) according to (4).24 The filled circles are the experimental data; the error bars are estimated as half the ESEEM frequency times the digital resolution of the spectrum. The solid line is a graph of (4) with the parameters (standard deviations in parentheses) Jal = 0.57 (0.02) MHz
+
p(3
+ a2) = 0.029 (0.02) MHz2
obtained from the slope and intercept of the straight line deter(22) An 27AILarmor frequency peak might arise from coherences between the nominal nuclear spin eigenstates, even if a substantial quadrupole coupling were present. (23) Gerfen, G. J.; Singel, D. J. J . Chem. Phys., submitted for publication. (24) The assignment of particular frequencies to particular electron spin levels, i,is arbitrary; we determine only the magnitude of the hyperfine coupling constant.
1.0
k,
-2.0 L
'
-1.0
I
,
I
1
-0.5
0.0
0.5
1.0
fv, Figure 3. Frequency tracking of the 14N double-quantum peaks. The quantity udq+2/4 - u,Z is graphed versus Y,. The filled circles correspond to the pairs of I4N double-quantum frequencies measured at the electron spin excitation frequencies 6.2, 7.4, 8.2, 9.1, and 10.3 GHz. The larger
"+"
spin manifold frequency of each pair is arbitrarily assigned to the (only the magnitude of a is determined in the present experiments). The solid curve is a plot of the straight line (slope = -0.57 MHz; intercept = 0.37 MHz2) that best fits the data in the (weighted) least-squares sense. This line corresponds to a graph of (4) with the parameter values 101 = 0.57 MHz and @(3 az) = 0.29 MHz2.
+
mined, according to the (weighted) least-squares criterion, by the data (weighted root-mean-squared deviation 0.1 MHz2). The standard errors (error bars) were used for weighting the data in the least-squares fitting procedure and propagated to obtain the standard deviations of the fitted parameter^.^^ Note that the observed hyperfine coupling constant signals that exact cancellation would occur at an excitation frequency of -2.6 GHz. Thus, large deviations from exact cancellation-absolute differences between Y, and a / 2 that are -2-4 times greater than K-obtain at each of the excitation frequencies at which we registered ESEEM patterns. Our observation of only the double-quantum peaks for the 14N nucleus, in both the a and (3 manifolds, together with the exclusive correlation of these peaks is entirely consistent with such large deviations from exact can~ e l l a t i o n . ~Additionally, J~ we note that the amplitudes of these modulation components are not large at an:! of the surveyed excitation frequencies. They are in fact absent at 11.8 GHz and reach their greatest amplitudes at the lowest excitation frequencies surveyed. This behavior is again consistent with the determined coupling constant^.^ Discussion In the preceding section, we detailed the spectroscopic assignment and analysis of our ESEEM data. We begin the discussion of our results by comparing them with ESEEM results obtained in studies of chemically related systems. These comparisons lend additional support for our spectroscopic assignments. In the study of the radical formed by adsorption of perylene on alumina, modulation frequency components corresponding to those that we have assigned to 'H and 27Al were observed and were similarly suggested to arise from 'H and 27Alnuclei that sustain weak hyperfine c o ~ p l i n g s .The ~ ~ ~low-frequency ~ peaks that we observe and assign to I4N are not exhibited by the perylene radical. They are, moreover, not observed in the ESEEM patterns of the radical formed by adsorption of nitrosobenzene on alumina.z6 (This latter species has an EPR spectrum that is qualitatively similar to that of m-DNB.) We have also examined the multifrequency ESEEM spectrum of the proton adduct of the m-DNB (25) A preliminary version of Figure 4 appears in ref 18. Refinement of experimental methods, in particular, spectral analysis, has led to our achieving more precise measures of the ESEEM frequencies. Moreover, our present estimates of the standard errors of the data are more appropriate, according to tests based on the x2 probability function. The results of the fitting procedure are not changed significantly if equal weights are, used and the rootmean-squared deviation is employed as the standard error in the data in the propagation of errors. (26) Cosgrove, S . A,; Singel, D. J. Unpublished results. (27) Dikanov, S. A,; Yudanov, V. F.; Samoilova, R. I.; Tsevtkov, Yu. D. Chem. Phys. Lett. 1977.52, 520. Ichikawa, T.; Kevan, L. J . Am. Chem. Soc. 1981, 103, 5355. Ichikawa, T.; Kevan, L. J . Am. Chem. SOC.1983, 105,402.
Multifrequency ESEEM of m-Dinitrobenzene on Alumina produced by irradiation of THF solutions of m-DNB.26 The peaks assigned to weakly coupled protons, and the low-frequency peaks assigned to 14N,are observed in the ESEEM spectra of this radical in frozen solution, but the peak assigned to 27Al is absent. No other frequency components are observed, in particular (presumably owing to powder broadening) no modulations attributable to benzene ring protons. The quadrupole coupling constant that we obtain from our measurements compares very well with couplings determined for m-DNB by other methods and for other nitrobenzenes by ESEEM. The values of K and q determined in the multifrequency ESEEM study of DPPH, combine to give a value of 0.25 MHzZfor f?(3 + q2).10,11 Cheng and Brown2* have studied a series of nitrobenzene derivatives in the solid state by pure nuclear quadrupole resonance techniques. For m-DNB they obtain the values K = 0.31 MHz and q = 0.36 MHz, whence IP(3 s2)= 0.30 MHz2. The value that we obtain for the hyperfine coupling constant closely matches that found in fluid solution for the weakly coupled 14N in the proton adduct of the m-DNB radical anion (la1 = 0.56 M H z ) . ~This value-in fact the very observation of modulations attributable to I4N-indicates that the radical under study is indeed the ion pair rather than the naked anion. The localization of the spin density onto one of the nitro groups, as opposed to its equipartition among the two, is clearly demonstrated by the small hyperfine interaction sustained by the I4N that modulates the spin-echo envelope. On the basis of the observed amplitudes of the I4N modulation components, we are tempted to suggest that the ion pair is the only radical species present on the surface that significantly contributes to EPR spectrum. We note that the 14Nmodulations are, qualitatively, shallow-as expected for large deviations from exact cancellation and a single contributing nucleus. They do not appear, however, to be too shallow, that is, to demand the presence of a second radical (such as the naked anion) whose spin-echo envelope is unmodulated, but whose EPR spectrum broadly overlaps that of the ion pair. On the other hand, the overall width of the EPR spectrum has been reported to depend on the activation temperature of the alumina in a manner that does suggest the presence of more than one radical. Further, quantitative investigation of this issue is in progress. The presence of 27Almodulations indicates close contact between the alumina and the studied radical. This conclusion had been drawn in early EPR studies based on presence of radicals in the alumina sample after the removal of the solvent4 The observation of 27Al modulation provides positive, spectroscopic
+
(28) Cheng, C . P.; Brown,T. L. J . Magn. Reson. 1977, 28, 391.
The Journal of Physical Chemistry, Vol. 94, No. 6,1990 2623 evidence of the interaction of the radical with the surface. From further analysis of the 27Al ESEEM, it may prove possible to obtain information about the structure of the surface site at which the radical is lodged. Detailed investigation of the spectral features prevalent in ESEEM patterns of I = 5 / 2 nuclei such as 27Alis just beginning;29 the development of multifrequency ESEEM techniques specifically designed to simplify and clarify 27AlESEEM spectra will increase the prospects for tapping this source of information about surface-adsorbate interactions. Work toward this goal is in progress in our laboratory.
Conclusions We have obtained multifrequency ESEEM spectra of a radical species that is formed upon the adsorption of m-DNB on activated y-alumina. The ESEEM spectra are comprised of several narrow peaks. We have used the s-suppression behavior in three-pulse ESEEM to reveal interrelationships among these peaks and thereby to decompose the spectrum into contributions from different nuclear spin subsystems. The frequencies of the observed peaks and, most importantly, the variation of the frequencies with external field strength have enabled us to assign the peaks to IH and 27Alnuclei, with eigenfrequencies equal to the Larmor frequencies of the respective nuclei, and to I4N,with ESEEM at the double-quantum frequencies of the a and the /3 manifolds. Our assignment of particular ESEEM peaks to particular nuclei is corroborated by comparison with ESEEM patterns of radicals formed by the adsorption of perylene and of nitrosobenzene on alumina and of a frozen solution of the proton adduct of the radical anion of m-DNB. We have determined hyperfine and quadrupole coupling constants of the 14N that contributes to the ESEEM spectrum, through a simple analysis of the external field dependence of the observed double-quantum frequencies. The values of these parameters establish that the probed radical species cannot be the radical anion of m-DNB; the observed hyperfine coupling corresponds to that exhibited in fluid solution by adducts of the radical anion with available cations and thus provides a direct indication that the surface-bound radical is an ion pair. Acknowledgment. This work is supported by the National Science Foundation (DMR 86-14003) through the Harvard Material Research Laboratory. We thank Gary Gerfen and Russell Larsen for valuable discussions and assistance in the design and fabrication of the microwave resonators employed in this work. ~
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(29) Goldfarb, D.; Kevan, L. J . Chem. Phys. 1987, 1 1 , 6323. Goldfarb, D.: Kevan. L. J . M a ~ nReson. . 1989.82. 270. Naravana. M.: Kevan. L. J . Chem. Phys. 1981,73,3269. Heming; M.; Narayana, M.;Kevan, L. J . Chem. Phys. 1985,83, 1478.
