350
J. Phys. Chem. 1981, 85,350-354
Acknowledgment. We thank Jackson Harrar, Lloyd Steinmetz, and Charles Stevens for their contributions to this study, and Mark Wrighton for his critical reading of the manuscript. This work was performed under the
auspices of the U.S. Department of Energy by the Lawrence Livermore Laboratory under contract No. W-7405Eng-48 and No. DE-AC02-77ER04263.AO02, Purdue University.
Vibrational Spectra, Jahn-Teller Distortlon, and the Structure of the Benzene Radical Anion Jesse C. Moore, Cynthla Thornton, Wllliam B. Collier, and J. Paul Devlln' Department of Chemktty, Oklahoma State lJn;vers& Stillwater, Oklahoma 74078 (Received: August 25, 1080)
Dilute thin-film codeposits of potassium or rubidium with benzene are light green or blue-green and, for the organic-rich samples, have an electronic absorption spectrum that can be identified with the ion pair M+B;. These films have a characteristic infrared spectrum but in a laser beam produce no intense discrete Raman spectra until bleached for several minutes. The resonant Raman spectrum that emerges during bleaching has many properties anticipated for that of the isolated benzene radical anion so the dominant features have been assigned to the stretching modes for that species. This assignment requires a Jahn-Teller splitting of the ea ring stretching mode of 111and 136 cm-I for C6H6-and C&-, respectively,values that are shown to be within a very few wavenumbers of the expected splitting based on the best theoretical estimate of the magnitude of the static Jahn-Teller distortion. A weaker subset of resonant Raman bands has been assigned to the ion pair M'B;, and it is suggested that, within the laser radiation field, the samples experience a steady-state dynamic distribution of electrons between isolated B; and M'B; and a mobile electron state.
Introduction The benzene anion radical (B;) has been the sourke of theoretical interest and speculation for many years.' In recent decades considerable structural information has been won from ESR2 and UV-~isible~?~ measurements. The ESR data for the ion-paired anion indicate that, on that particular time scale, the cation oscillates about a symmetric position over the ring with the ring carbon atoms assuming magnetically equivalent positions. The UV-visible measurements each show two principal broad anion absorptions of comparable intensity centered near 420 and 290 nm. However, Gardner's study, wherein the anion was generated by reduction in dimethoxyethane solution at -80 "C, also revealed an absorption band in the 600-700-nm range that was attributed to solvated potassium atoms.3 The solution studies have further shown that the more electropositive potassium or rubidium, rather than sodium, is required for benzene reductions2 This observation is consistent with the report by McCollough and Duly that a benzene-sodium complex, but no ion pair, is formed in a dilute low-temperature deposit of sodium in b e n ~ e n e . ~ As part of a program to elucidate the structures of conjugated radical anions, such as the anions of TCNE,6 TCNQ, and anthracene, we were interested in the preparation of the benzene anion in a form convenient for in(1)See, for example: (a) A. L. Hinde, D. Poppinger, and L. Radom,
J.Am. Chem. SOC.,100,4681(1978);(b) A. D. Liehr, Rev. Mod. Phys.,
32,436 (1960). (2)M. T.Jones and J. C. Kuechler, J. Phys. Chem., 81,360 (1977). (3)C. L.Gardiner, J . Chem. Phys., 45,572 (1966). (4)T.Shida and S. Iwata, J. Am. Chem. SOC.,95,3473 (1973). (5)J. D. McCollough and W. W. Duly, Chem. Phys. Lett., 15, 240 (1972). (6)See, for example, J. J. Hinkel and J. P. Devlin, J. Chem. Phys., 58, 4750 (1973). 0022-3654/81l2085-0350$01 .OO/O
frared and Raman measurements. It seemed likely that potassium and rubidium would be sufficiently electropositive to produce the anion in a low-temperature codeposition with benzene. This paper reports the spectroscopic characterization of such codeposits with particular emphasis on photoionization processes induced in the matrices by 4880-A photons from an argon laser. The effect of the addition of a radical electron on the bonding of the conjugated molecules TCNE and TCNQ is best characterized as a strengthening of single bonds and a weakening of multiple bonds with a net reduction in molecular bond energie~.~ Both theory and an empirical force constant analysis indicate that the TCNQ quinonoid ring becomes progressively more benzenoid with the addition of each electron to the system, up to a maximum of three.7b,8 However, when static Jahn-Teller distortion is considered, quite an opposite effect is expected for benzene. The ground state of the undistorted anion is degenerate so the Jahn-Teller distortion, which in principle must occur, will produce either a quinonoidlike structure (I) or an elongated structure with two weakened
0 0 I I1 ring bonds and four that are relatively unaffected (11). Both of the possible structures are of D 2 h symmetry, and, for comparable distortions from D6h symmetry, one expects a comparable magnitude of splitting of the e2gand el, vibrational modes. (7)(a) M. S. Khatkale and J. P. Devlin, J. Phys. Chem. 83, 1636 (1979);(b) M. S. Khatkale and J. P. Devlin, J. Chem. Phys., 70,1851 (1979). (8)D. A. Dixon, H. Simmons, and W. N. Lipscomb, J. Mol. Struct., 50, 155 (1978).
0 1981 American Chemical Society
The Journal of Physical Chemistry, Vol. 85,No. 4, 1981 351
Structure of the Benzene Radical Anion
TABLE I : Observed and Calculated Planar Fundamental Raman-Active Frequencies for Isolated C,H; and C,D,Assigned for Structure I (Quinonoid) Vobsda
C,H,a1 g 1586 1152 957 616
C,D,-
b
C,H,-
C,D,-
3055 3053 1563 1603 1186 822 966 587 603
2280 2270 1569 841 920 582
3053 1507 1339 1184 597
2273 1477 1041 841 577
b3, 1475 1358 1303 648
calcd
1428 1190 615
assignment
ringstr C-Hbend ringstr ring deformation ringstr C-Hbend C-Hbend ring deformation
a Other observed Raman bands were as follows. C,H,-: 408 and 496 cm-'. C,D;: 408 cm-'. Calculated frequencies were obtained from Scherer's benzene force constants (J. R. Scherer, Spectrochim. Acta, 20, 345 (1964)) modified to include Pecile's TCNQ-, ring-stretching force constants. l4
The magnitude of the Jahn-Teller distortion for the cation of trifluorobenzene has been deduced from laserinduced-fluorecence studies and may be gauged from the 250-cm-l splitting determined for the e' ring-deformation mode.g A similar magnitude of distortion could be anticipated for the benzene anion and should, therefore, be apparent from a vibrational study. In fact it can be argued that the static ring distortion predicted for the benzene anion via a Gaussian orbital MO calculation'" leads to an expected splitting of 100-150 cm-l for the e2g ringstretching mode. The predicted distortion, whether to a quinonoid or elongated D2,,structure, was characterized by ring bond-length differences of 0.08 A. This is slightly greater than the measured bond differences in the TCNQ monoanion ring.1° Thus, to the degree the MO calculation successfully predicts the static Jahn-Teller distortion, the TCNQ- ring force constants become acceptable zero-order force constants for the benzene anion. The calculated splitting for the e2gring stretching mode, based on the available TCNQ- force constants, is 100 cm-l (see Table I).
-
N
Experimental Section The alkali metal-benzene codeposits were prepared by 10-30 minute vapor codepositions onto a CsBr truncated prism mounted in a brass holder within a liquid-nitrogen-cooled glass Dewar optical cell. The metal vapor densities were controlled by using a resistance-heated Pyrex-glass Knudsen oven while the benzene flow rate was monitored via a Fischer-Porter glass flowmeter. The organic-metal ratio in the deposited thin films was varied over 2 orders of magnitude, centered around the composition at which metal aggregates first become common. The ratio was estimated to have ranged from 10 to lo3 in a manner that was visually apparent since samples ranged from the deep blue color, characteristic of metal aggregates, to a pale yellow-green representative of the potassiumbenzene ion pair. The CsBr truncated-prism substrate permitted measurement of infrared and visible spectra by the transmission of light through the sample film and prism. (9) T. Sears, T. A. Mmiller, and V. E. Bondybey, J. Chern. Phys., 72, 6759 (1980). (10)B. Goldsteen, K. Seff, and K. N. Trueblood, Acta Crystallogr., Sect. R,24, 778 (1968).
However, of much greater consequence to this study, the Raman spectra were obtained by the interference enhanced internal-reflection technique which depends on the impingement of the laser beam at the synchronous angle at the CsBr substrate-sample film interface.11J2 This approach, which requires precise incident-angle control, gives a sizeable Raman signal enhancement compared to external reflection techniques and, more importantly, permits useful depolarizationratio measurements for Raman bands of thin films.'l The Raman spectra were measured by using 4579-, 4880-, and 5145-A radiation from a Coherent Model 53 argon laser. All potassium and rubidium codeposits, regardless of original color, were markedly bleached by exposure to the nearly 1W of 4880-A radiation. Most of the reported data are for samples bleached for at least 1h after which time the spectra were invariant for several hours.
Results Though the organic-metal ratio was varied considerably throughout this study, and the UV-visible spectra were strongly affected by the degree to which metal aggregates were present in the sample films, the basic Raman and infrared spectra of the reaction products of potassium and rubidium with benzene were highly reproducible. Nevertheless, there are several observations relative to sample preparation that should be kept in mind when considering the discussion in this section: (1)No reaction of sodium with benzene was evident, with or without laser excitation, in either the infrared or Raman spectra. (2) The organic-rich light-green samples showed product (anion) bands in the infrared and UV-visible spectra but yielded only the neutral benzene Raman spectrum. (3) In the more metal-rich samples, a dominant resonant Raman spectrum emerged slowly, apparently through laser photoionization, with a steady growth of the resonant product bands relative to the neutral benzene bands over periods ranging from a few minutes to 1h. (4) The infrared and UV-visible spectra reflect a metal-benzene reaction product formed as the sample films were deposited and not the product formed by laser photoexcitation. The observation for sodium-benzene films is not surprising since sodium does not reduce benzene in a liquid solution.2 Further, McCollough and Duly have reported that sodium forms a complex rather than an ion pair in solid b e n ~ e n e .The ~ failure of the green organic-rich films to evidence a resonance Raman spectrum of the benzene anion is also understandable since any film must contain traces of moisture which provide trapping sites for a limited number of electrons. The presence of mositure is confirmed by sharp hydroxide infrared bands in spectra for samples warmed to room temperature. The laser photons could bleach the green samples by promoting electrons from the anion into these deeper traps. The blue samples, containing some metal aggregates, have sufficient available electrons both to saturate the deep traps and to form a significant concentration of benzene anions. Finally, the fact that the primary Raman spectrum is produced by a species formed by photoexcitation requires that within this paper no correspondence can be presumed between the infrared and Raman data. Efforts are underway to bleach sufficiently large sample areas to allow infrared measurements of laser bleached films. Raman Spectra. The most interesting data obtained in the present study were the Raman spectra, including (11) P. C. Li and J. P. Devlin, J . Chern. Phys., 59, 547 (1973). (12) P. K. Tien, R. Ulrich, and R. J. Martin, Appl. Phys. Lett., 14, 291 (1969).
Moore et al.
The Journal of Physical Chemistry, Vol. 85, No. 4, 1981
352
I
f
I
I
I
I
I I
1
I
300
400
500
600
700
I I I
t I t
I
I
I
I
I
nrn
E 2
-
KrBZ.
A C6H6
I
I
I
I
40
30
25
20
In BZ matrix at 9oK
I \
15
LK
Flgure 2. The electronic spectra for the benzene anion radical: upper
curve is from ref 4, while the bottom curve is for dilute codeposits of potassium in benzene. I
I I 1600
1
1400
I 1 1200
I
I
1 1000
cm.1
Flgure 1. Resonance Raman spectra of laser-bleached potassiumbenzene codeposits: (A) regular benzene and (B) deuterated benzene. The Xs denote bands produced primarily by neutral benzene, and the dashed vertical lines represent neutral benzene Raman band positions.
the time evolution of those spectra. Upon initial exposure of a sample to the laser beam, the alg u2 band at 992 cm-l dominated the Raman spectra. In a matter of minutes, as the sample bleached, the dominant set of Raman lines (1586, 1475,1358,1303,1152, and 957 cm-l) apparent in Figure 1,curve A, evolved. In most cases a second weaker subset of bands was also observed (1540,1260,1185, and 980 cm-l), the intensity of which decreased relative to the dominant resonant bands during the bleaching period. Eventually the intensity of the dominant resonant Raman lines, relative to the v2 benzene band, maximized and became stable in time. The resonant Raman pattern in Figure l A , obtained with 4880-A laser exitation, was insensitive to the metalorganic ratio. Further, no definite correlation with this ratio has been found between the intensities of the dominant set of resonant Raman bands and the second weaker set of Raman bands though the relative intensities of the two subsets varied from sample to sample, suggesting that they are produced by two different species. However, it has been firmly established that the dominant set of bands gains intensity rapidly with decreasing values of h excitation with a greater than 10-fold increase in the resonant enhancement upon changing from 5145- to 4579-A excitation. By contrast, the weak subset of bands was not observed with 4579-A excitation but rivaled the dominant band subset in intensity for 5145-A excitation. Since an apparently identical resonant Raman spectrum, compared to that for the potassium-benzene codeposit, has been obtained for rubidium-benzene deposits, that spectrum is not presented. However, the spectrum for the potassium codeposit with deuterated benzene is included as curve B in Figure 1. That spectrum shows the isotopic shifts expected for the vibrational modes of the radical anion, as projected from known shifts for benzene itself. In particular, the average deuteration shift of 35 cm-‘ noted for the 1586- and 1475-cm-l bands can be compared with
a value of 26 cm-l for benzene, uncorrected for Fermi resonance,13and 33 cm-l for the corresponding modes of TCNQ-l.14 Electronic Spectra. The Raman results, being resonant in nature, are best understood in the context of the wavelengths of major electronic absorption bands. Thus, the UV-visible spectra for the benzene radical anion are reproduced in Figure 2. Curve A is a reproduction of the spectrum reported by Shida and Iwata for the anion formed in a benzene-doped 2-methyltetrahydrofuran glass by y irradiation so that ion pairing was improbable.4 Curve B, which is the spectrum for an unbleached green Kbenzene codeposit from the present study, is nearly identical with the ion-pair spectrum reported by Gardner for a dimethoxyethane solution at -80 OCe3The 650-nm band in curve B, which is either missing or concealed in curve A, has previously been attributed to solvated potassium atoms and is largely responsible for the green color of the organic-rich films. However, an argument can be made that this 650-nm band is produced by an inherent absorption of the ion-paired benzene radical. There is general agreement that the benzene anion, whether ion paired or not, has electronic absorption bands of comparable intensity near 280 and 400 nm. Thus, an intensification in the anion resonance Raman spectrum, such as noted above for the strong subset of Raman bands, is expected as X of the laser excitation is reduced from 5145 A. The intensification of the weaker subset of bands as h was increased was less predictable but is understandable if the 650-nm absorption is produced by the ion-paired radical anion and the weaker Raman subset is assigned to that species. Infrared Spectra. As already noted the infrared spectra recorded to data have been for the unbleached blue and green potassium-benzene codeposits. The results for numerous samples have proved to be highly reproducible and are, therefore, recorded in Table 11. An assignment to the modes of the ion-paired radical anion is presented as well. Interpretation of the Spectra The data presented in the previous section can be given the following interpretation. Depositions of benzene with (13) G. Herzberg, “Infrared and Raman Spectra of Polyatomic Molecules”, Van Nostrand, Princeton, NJ, 1945, p 364. (14) R. Bozio, A. Girlando, and C. Pecile, J. Chem. Soc., Faraday Trans. 2, 71, 1237 (1975).
The Journal of Physical Chemistry, Vol. 85, No. 4, 1981 353
Structure of the Benzene Radical Anion
TABLE 11: Assignment of Observed Infrared and Raman Spectra for the Ion-Paired Benzene Anion Assuming C,, StructureC band frequency
activity
strengthb assignment
R S sw IR M IR W R W R W IR R MS IR S S IR M IR R S C,D; M IR M IR W IR W R M IR W IR M IR W R IR M IR S M IR a b, and b, modes are formally inactive under C6,,, W, M,and S represent weak, medium, and strong, respectively. Corresponding benzene frequencies are given in parentheses. C,H;
1540 (1595) 1350 (1479) 1275 (1309) 1260 (1309) 1185 (1178) 1080 (1146) 980 (992) 968 (1037) 927 (1037) 850 (850) 1520 (1559) 1235 (1333) 1172 1057 947 942 (945) 892 870 860 (867) 852 755 (813) 720 (813)
either potassium or rubidium at -100 K result in the formation of the green M+B; ion pair isolated in excess benzene which also contains some deep electron traps, probably water. The infrared and UV-visible data obtained from these deposits reflect primarily the spectra of neutral benzene and the ion pair. However, exposure of such films to 4880-A laser light excites the radical electrons, as well as electrons on any metal clusters, into mobile states from which they are either caught in colorless deep traps or localized on random benzene molecules, forming the nearly colorless isolated benzene ion (curve A, Figure 2). Thus, whether initially blue or green, the sampled spot becomes nearly colorless. However, if one assumes that the deep traps are eventually saturated, a steady state should develop in the field of the laser beam between isolated anion radicals, ionpaired radicals, and excited, and therefore mobile, electrons. hv
M+B,- Ze-
hv
B;
Extinguishing the laser beam eliminates the mobile electrons leaving the other species trapped in the steady-state concentration. The correctness of this interpretation depends most strongly on the validity of the assignment of the dominant resonant Raman subset of bands (Figure 1) to the isolated benzene anion radical. The circumstantial evidence in support of such a view is considerable but not conclusive. The ion-paired radical is clearly present in the initial deposits, but the dominant Raman bands are not observed until after several minutes of photoexcitation. Once developed the spectrum (which does not relax in the absence of the laser light) is cation independent, seemingly ruling out an ion-paired species since stretching vibrations in the TCNE anion salts have been shown to shift -30 cm-' for the same cation exchange.15 Further, the magnitude of the deuteration shifts, the magnitude of the (15) J. C. Moore, D. Smith, Y. Youhne, and J. P. Devlin, J. Phys. Chem., 75, 325 (1971).
frequency shifts relative to neutral benzene, and the X dependence of the resonant Raman bands are all appropriate for the isolated benzene anion radical. Isolated Anion. If, as predicted from theory, isolated B; is distorted to a Dw structure, the ea. degenerate modes should each split into an agand a b, vibration. Further, assuming that the net bonding is reduced by the radical electron, one ag-b3, doublet is expected, centered below 1550 cm-l for the ring stretching modes, while the original 1178-cm-l benzene C-H e2gbending mode should appear as a doublet centered near 1180 cm-l. Further, the 992cm-l v2 benzene mode should shift to slightly lower frequencies, with retention of the totally symmetric character, while the 1338-cm-l inactive a2 bending mode becomes a Raman active b, vibration. As !or other conjugated radical anions>14strong resonant enhancement of the symmetric stretching modes is expected. An examination of curve A of Figure 1for C6H6-shows that the resonance Raman spectrum from 880 to 1600 cm-l mirrors these expectations, as indicated by the lines drawn to represent the presumed effect of the radical electron on the modes of the parent benzene molecule. A similar splitting and shifting pattern, consistent with expectations, was found for C6D6-, as is clear from curve B of Figure 1 and the frequencies listed in Table I. The Raman data thus suggest that the vibrations of B; resemble those of benzene except for the slightly reduced stretching frequencies and the split degeneracies. It can also be noted that the splittings of the ring stretching mode degeneracy (111cm-' for C6H6-and 136 cm-I for C6D6-) are precisely the corresponding splittings for TCNQ-.14 From these two observations it was deduced that a reasonable zero-order force field for B; could be obtained by using the published benzene force constants with a single modification; namely, the TCNQ- stretching force constants (&+ = 5.9 and K c 4 = 7.2 mdyn/A) were substituted for the benzene stretching constant. The results for the frequency calculation based on this modified benzene force field and using a quinonoidlike structure are presented in Table 11. This calculation revealed, primarily, that a distortion of the benzene ring greater than, but comparable to, the distortion in TCNQ- is necessary to give the magnitude of the degeneracy splittings observed for C6H; and C6D;. As noted earlier, this is also the magnitude of the static Jahn-Teller distortion predicted by the best theoretical treatment.la Although the above calculation suggests that the observed frequencies are compatible with a quinonoidlike B; structure (I), an equally good fit to the observed frequencies is obtained if the alternate D2,,structure (11) is presumed and force-constant values Kc, = 5.9 and K C d = 7.2 mdyn/A are used in a normal model calculation. The fit of observed vibrational frequencies to calculated vibrational frequencies clearly does not result in the identification of a particular stable D2hstructure. If a single stable B; structure of Dul symmetry is favored at 100 K in a benzene matrix, Raman depolarization measurements should identify the precise structure. For structure I the 1586- and 957-cm-I bands would be of ag symmetry with the 1475-cm-' band belonging to the bSg class. The 1586- and 1475-cm-' vibrational mode symmetries would be reversed for structure 11. Significant depolarization data for thin films can be obtained by the synchronous-angle,internal-reflectiontechnique employed in this study.'l Witness the fact that the 992-cm-I benzene band was routinely reduced to 10% intensity by the use of an analyzer to sample only the perpendicularly polarized scattered light. However, within the same spectral scans
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354
J. Phys. Chem. 1981, 85,354-360
depolarization ratios of 0.65 f 0.1 were found for each of the three strong anion bands at 1586,1475, and 957 cm-l. The origin of the comparable depolarization ratios for the three modes is tentatively identified as the rapid switching of B; between two D2h structures of different form but nearly equal energies. It is presumed that, consistent with calculational results showing rather minor energy barriers between the Dul structures,l* the structure fluctuates at a rate approaching the vibrational time scale such that an assignment of the vibrational modes to simple symmetry classes is impossible. This tentative explanation will be pursued in subsequent studies in the light of theoretical analyses that are applicable to this case.16 K'B; Ion Pair. The steady-state relationship proposed for the laser-irradiated sample requires the presence of the K'B; ion pair as well as the isolated anion radical. Further, it is clear that the ion pair also has absorption bands near the laser excitation frequencies, though the assignment of the 650-nm absorption is not without some doubt. Thus, a resonant scattering from the ion pair is anticipated, and the weak subset of Raman bands, which is enhanced by increasing the excitation wavelength, is tentatively assigned to the K+B; ion pair. There is a definite argument for such an assignment independent of this preresonance behavior. The spectrum more closely resembles that of neutral benzene than does the stronger Raman subset that has been assigned to the isolated B;. This is anticipated for the ion pair, particularly if the symmetry is not disrupted by the cation, Le., the cation is positioned on the molecular axisa2 Back-donation of charge to the cation, which is apparently complete for sodium, is expected to be significant for potassium and rubidium as well and thus should reduce the static JahnTeller distortion of the ring, along with the frequency shifts resulting from bond strength reduction from the radical electron. The infrared data for the unbleached films is attributed to M'Bz- and is therefore included in Table I1 where the (16) M. Pawlikowski and M. 2. Zgierski, Chem. Phys. Lett., 48 201 (1977).
vibrational assignments for the ion pair are indicated, based on a c6"molecular symmetry. Conclusions The resonant Raman spectrum for the benzene anion isolated in a benzene matrix can be interpreted by assuming that a major static Jahn-Teller distortion exists. The magnitude of this distortion, required to produce the observed splitting of the eZgvibrational modes, is similar to that predicted by Gaussian orbital-based quantumchemical calculations.'" Both the molecular orbital calculations and the vibrational data are consistent with ring bond-length differences of -0.08 A for the distorted structures. This means that one of the two probable DZh structures of similar energy has a ring structure that is nearly equivalent to that of the ring portion of the TCNQ monoanion. Though the Raman spectrum shows discrete bands, they are moderately broad (15-30 cm-l) and are tentatively interpreted as reflecting the rapid interconversion between D2h quinonoid and elongated structures on a time scale approaching that of the molecular excitation. In particular, some such viewpoint must be invoked to understand the comparable depolarization ratios for the three Raman active ring stretching modes. A second reaction product of benzene with metal is evident from the Raman spectra. Since it is reasonable that a steady state develops in the samples exposed to the laser beam, with the MfB; species participating as one component, the Raman bands that indicate the presence of a second product have been assigned to the ion-paired benzene anion. If this assignment is correct, then it follows that the 650-nm absorption band, reported by Gardiner and observed in our deposits as well, is produced by M+B; since the resonant character of the Raman scattering from this second species increases with increasing laser excitation wavelength. The assignment of such a low-energy, ion-pair electronic excitation is not firm but the chargetransfer excitation to the "no-bond" state is a likely candidate. Acknowledgment. The research was supported by NSF grants CHE 77-09653 and CHE 79-25567.
Electron Spin Resonance Study of the Reaction of Aromatic Hydrocarbons with Oxygen I. C. Lewis" and L. S. Slnger Union Carbide Corporation, Carbon Products Division, Parma Technical Center, Cleveland, Ohio 44 10 1 (Received: August 28, 1980)
Electron spin resonance was used to study the free-radicalintermediates formed during the reaction of polynuclear aromatic hydrocarbons with oxygen. The treatment of a variety of hydrocarbons with O2 at 275-320 "C in an inert solvent gave rise to colored solutions which generally exhibited intense ESR signals. Proton hyperfine spectra could be resolved for approximately half of the radicals. The high g values of between 2.0029 and 2.0040 confirmed the presence of chemically bound oxygen in the radical structure. The g values show a linear relation when plotted against either the energy of the lowest vacant molecular orbital or the polarographic half-wave reduction potential of the parent hydrocarbon. Preliminary analysis of some of the hyperfine spectra indicates that the radicals are stable aryloxy radicals produced by substitution of hydrogen by oxygen at a reactive site in the molecule. Introduction Polynuclear aromatic hydrocarbons are known to react directly with oxygen to form a variety of oxidation products. These reactions are usually catalyzed photochemi0022-365418112085-0354$01 .OO/O
cally,l by the addition of base2 or by strong acidsa3 Free radicals have often been postulated as intermediates in (1) C. DuFraisse, Bull SOC.Chim. Fr., 6,422 (1939).
0 1981 American Chemical Society
