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origin and we measure it, in the laser-induced fluorescence experiment, to be 21 614 f 3 cm-l, in good agreement with the free jet experiment. Overall, this experiment has demonstrated that it is possible to observe the spectra of organic molecular ions cooled to a few degrees Kelvin by a supersonic expansion and to obtain significant new information. The spectra of parent ions produced by electron impact are very simple and well resolved. It is likely that similar ions produced by other means, e.g., photoionization, would exhibit similar spectra. Such experiments will be particularly attractive in conjuction with synchrotron radiation sources. Other transient molecular species such as free radicals might well be studied by similar techniques. However, in processes involving the breaking of ci chemical bond, significant vibrational and rotational excitation might occur. It may, in turn, be possible to probe just such dynamical processes themselves by the presently described techniques.
References and Notes (1) See, for example, D. H. Levy, L. Wharton, and R. E. Smalley in "Chemical and Biochemical Application of Lasers", Vol. 2, Academic Press. New York. 1977. D 1: Acc. Cbem. Res.. 10. 139 11977). (2) R. E. Smalley, L. Wharton: D:H. Levy, and D. W. Chandler, j . Moi. Spectrosc., 66, 375 (1977).
J. B. Hopkins, D. E. Powers, and R. E. Smalley, J. Chem. Pbys., 72, 5039 (1980). A. Amirav, U. Even, and J. Jortner, Opt. Commun.,32, 266 (1980). J. H. Callomon, Can. J. Pbys., 34, 1046 (1956). M. Albn, J. P. Maier, and 0. Marthaler, Cbem. phys., 26, 131 (1977). J. P. Maier, Chimia, 34, 219 (1980), and references therein. C. Cossart-Magos, D. Cossart, and S . Leach, Mol. Pbys., 37, 793 (1979). C. Cossart-Magos, D. Cossart, and S.Leach, Cbem. phys., 41,345, 363 (1979). T. A. Miller and V. E. Bondybey, Cbem. Pbys. Left.,58, 454 (1978). V. E. Bondybey and T. A. Miller, J. Chem. Pbys., 70, 138 (1979). T. J. Sears, T. A. Miller, and V. E. Bondybey, J . Am. Cbem. Soc., 102, 4864 (1980). T. J. Sears, T. A. Miller, and V. E. Bondybey, J. Am. Cbem. Soc., in Dress. V. 'E. Bondybey, T. J. Sears, J. H. English, and T. A. Mlller, J. Chem. Pbys., 73, 2063 (1980). V. E. Bondybey, T. A. Miller, and J. H. English, J. Cbem. Phys., 71, 1088 (1979). V. E. Bondybey, J. H. English, and T. A. Miller, J . Mol. Spectrosc., 81, 455 (1980). V. E. Bondybey and T. A. Miller, J . Chem. Pbys., 73,3053 (1980). T. A. Miller, V. E. Bondybey, and J. H. English, J . Cbem. Pbys., 70, 2919 (1979). T. J. Sears,'T. A. Miller, and V. E. Bondybey, J. Chem. Phys., 72, 6070 (1980). T. J. Sears, T. A. Miller, and V. E. Bondybey, J. Cbem. Phys., in Dress. V. E. Bondybey, T. A. Miller, and J. H. English, Pbys. Rev. Lett., 44, 1344 (1980).
EPR Evidence for the Formation of the Hexamethylethane Radical Cation by Charge Transfer in a Freon Matrix Jih Tzong Wang and Ffrancon Williams" Department of Chemistry, University of Tennessee, Knoxville, Tennessee 379 16 (Received: September 8, 1960)
The hexamethylethaneradical cation [Me3CCMe3]+has been generated through positive charge transfer by y irradiation of both glassy and polycrystalline Freon solutions, its EPR spectrum consisting of seven lines with binomial intensities and having the parameters 'A(6) = 29.0 f 0.2 G and g = 2.0031 f 0.0003. In contrast, only neutral alkyl radicals are produced from the hydrocarbon by y irradiation of the hexamethylethane matrix, even when powerful electron scavengers are present. It is suggested that the formation of the radical cation by positive charge transfer in a Freon matrix is a more relaxed process than that of vertical ionization by electron impact in a hexamethylethanematrix.
Introduction The first electron-deficient ~7radical to be derived from nontransition elements and characterized by EPR spectroscopy was the boron-centered species, [ (MeO),B-B(0Me)J.l This radical resembles the prototype species Hzt and the ethane positive ion CzH6+in possessing a formal one-electron bond between two atoms or groups which are i d e n t i d 2 Another potential radical in this class with the required symmetry is the radical cation of 2,2,3,3-tetramethylbutane or hexamethylethane (HME), [Me3C.CMe3]+,so the recent report3 which attributed a multiplet EPR spectrum in y-irradiated HME to this species attracted our interest. Unfortunately, this assignment? was soon disproved, a thorough reinvestigation showing that the spectral analysis which had been proposed in terms of an expected binomial set of 19 lines3was erroneous,* and that the EPR spectrum of y-irradiated HME can be interpreted solely on the basis of the line components originating from neutral radical^.^,^ While the original claim3 was withdrawn: it was then reported that a new seven-line spectrum tentatively identified as that of the same radical cation was produced on y irradiation of the parent compound containing an
excess of electron scavengers such as CC14and CBr4,6athe conclusion being that the prevention of electron return allowed the radical cation to be detected. In a parallel development, we have shown that HME is a suitable matrix for the observation of narrow-line isotropic EPR spectra from fluorocarbon radical anions produced by the y irradiation of solid ~ o l u t i o n s .How~~~ ever, the routine examination of the central features in these spectra produced no obvious evidence for any unusual matrix radicals, in apparent conflict with the above interpretation6 which would suggest that the radical cation of HME should be produced in these experiments. We decided, therefore, to carry out further work in an attempt to resolve this discrepancy. In particular, we have sought to establish whether the radical cation of HME can be formed by a mechanism of positive charge transferQ?l0 from a halogen-containingcompound when the latter functions as the matrix and HME is present as the solute. Accordingly, we now report the results of two independent sets of y-irradiation and EPR experiments at 77 K: first, HME has been used as a matrix with various electron scavengers as solutes and, secondly, dilute solutions of HME in a Freon mixturell have been studied, the latter
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being a suitable matrix for the generation of solute cations.12J3
Experimental Section Solid solutions containing a nominal concentration of ca. 5 mol % of sulfur hexafluoride (Matheson, Inc.) or other electron scavengers such as hexafluorobenzene (PCR, Inc.) in polycrystalline HME (Aldrich Chemical Co.) were prepared as described previously.8 The Freon mixture (FM)11J2consisted of equal volumes of trichlorofluoromethane (Freon-11) and 1,2-dibromotetrafluoroethane (Freon-l14B2), both materials being obtained from PCR, Inc. Solutions containing up to ca. 3 mol % HME in FM or sec-butyl chloride (Aldrich) were prepared in Spectrosil or Suprasil EPR tubes on a vacuum line and cooled to 77 K before y irradiation and subsequent EPR measurements. When the FM solutions were inserted directly into liquid nitrogen from room temperature, clear glasses were obtained.'l-13 On the other hand, slow cooling to 77 K usually produced a polycrystalline matrix. Experiments were carried out with both glassy and polycrystalline samples of these frozen FM solutions. y irradiation at 77 K and subsequent EPR rneasurements were carried out as described previously,l'*the typical irradiation dose being ca. 1 Mrd.
Figure 1. Firstderivative EPR spectrum of a solid solution of 5 mol % SF6 in hexamethylethane recorded at 80 K after y irradiation at 77 K. A similar spectrum was obtained when the sample temperature did not exceed 77 IC. The stick diagram shows the components from SFB-.
Results Solutions of Sulfur Hexafluoride in Hexamethylethane. The results obtained with these solutions were similar to those for HME solutions of other electron scavengers such as CCl4? perfluorocycloalkanes,' and C6F6,8the radiation-produced radicals consisting of the species generated from electron capture by the solute together with an assortment of matrix radicals (vide infra). The reason why we have chosen to illustrate the EPR results for the solution containing SF6 is that the spectrum of SF, in HME is isotropic, even a t 77 K. As a result, it is easy to differentiate the four central line components (MI = 0) of SF, from the signals due to the matrix radicals in the center of the spectrum, and this allows an analysis of the spectrum obtained at law temperatures where it is more likely that an unstable or reactive radical cation would be detected. The pertinent EPR spectra are presented in Figures 1 and 2, the lalter showing an expanded view of the central region. First.,it is clear from Figure 1that the overall signal intensity of the SFC spectrum15 (19a(6) = 190.5 G) is comparable to that of the matrix radicals, suggesting that electron capture by SF, has occurred efficiently in the HME matrix. Secondly, examination of Figure 2 shows that all the resolvable line components in the center of the spectrum can be assigned to SF6- and the three matrix radicals, .CH2CMe2CMe3,CMe2CMe3,and CMe3 These neutral alkyl radicals are also produced by the irradiation of pure HME.475We must conclude either that the radical cation is not produced in the HME matrix, irrespective of the presence of electron scavengers, or that the EPR spectrum from such a species possesses no discernible hyperfine structure and is too broad to be detected under these conditions. In view of the results to be described below for thie FM matrix, we believe that the former conclusion is much more likely to be correct. Solutions of Hexamethylethane in the Freon Mixture. In Figure 3, the EPR spectrum of a y-irradiated glassy solution of HME in FM is compared with the "blank" spectrum of the FM glass obtained for the same irradiation and recording conditions. The difference between these two spectra is a clearly defined septet with binomial intensity ratios resulting from hyperfine interaction with six
li Figure 2. Expansion of the central region of the spectrum in Figure 1 showing the line components from SFC (*), CH,CMe,CMe, (J), CMe,, and CMe2CMe,.
equivalent spin 1 / 2 nuclei, measurements on several spectra yielding a hyperfine coupling constant of 29.0 f 0.2 G and a g factor of 2.0031 f 0.0003 for this species. Spectra similar to those presented in Figure 3 were obtained for the corresponding polycrystalline samples, and the EPR parameters for the septet were indistinguishable from the above values for the species prepared in the glass. On annealing the glassy samples, the septet spectrum disappeared completely in the narrow range between 77 and 82 K suggenting that the softening point of the FM glass lies in this region. In any event, no product radical was seen to grow in. By contrast, the decay of the septet spectrum occurred more gradually in the polycrystalline sample up to 105 K, but again no other spectral changes
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Me M e
11
^:
I/
H H
H H
'1
Me M e
Flgure 4. Newman (a) and W-pbn (b) projections showing the proposed conformation of the hexamethylethane radlcal cation with DBdsymmetry and six axial hydrogens.
The interpretation of the septet pattern in terms of six strongly coupled protons6 is undoubtedly correct, and the preferred conformation we strongly favor is depicted in structures a and b of Figure 4. In this arrangement, all conformations about carbon-carbon bonds are staggered, and the six strongly coupled protons are identified with Flgure 3. Firstderivative EPR spectra of a y-irradiated glassy solution the six axial hydrogens, one from each methyl group. The of hexamethylethane in a Freon mixture (top) and of the y-irradiated Freon mixture (bottom) at 77 K. The outer portions of the spectra are relatively large coupling to these axial hydrogens is easily also shown at higher gain to demonstrate that the main spectrum (top) rationalized via the McConnell cod 8 relation~hip'~ or the consists of only seven components as represented in the stick diagram. W-plan,l* each hydrogen and the orbitals containing the unpaired electron being coplanar in a 11/2V arrangement. were observed on annealing. Also, the conformation for each half of the molecule is An attempt was also made to generate the septet species essentially the same as that recently deduced for the from a solution of HME in a sec-butyl chloride g l a s ~ , ~ J ~ somewhat analogous (CFJ3CI- radical anion.l9 The inbut since the EPR spectrum was virtually the same as that teresting question concerning the extent of planarity at obtained from the pure sec-butyl chloride, no evidence was each of the two spin-bearing carbon atoms cannot be anobtained for the formation of the unique paramagnetic swered directly without a knowledge of the 13Chyperfine center in this case. couplings at these centers. It is doubtful, however, if the In view of the above results, it is worth noting that FM configurations of the two Me& groups can become comis considered to be superior to the butyl chlorides as a pletely planar in view of the steric interactions implied by matrix for positive charge transfer on account of the higher the lack of free rotation in the methyl groups. vertical ionization potentials (I) for the FM components, Mechanism of Cation Formation. Our results show that the pertinent values being I(CC13F) = 11.78 eV,I3I(CF2whereas the HME radical cation can be formed by y irBrCF2Br) = 11.40 eV,13and I(n-BuC1) = 10.67 eV.9 radiation of a rigid solution of HME in the FM matrix, it is not produced by the y irradiation of solid solutions Discussion containing electron scavengers in HME. We also suspect Identification of the Septet Spectrum. It is evident that that the previous work6 on the formation of the cation was the seven-line spectrum produced in the FM matrix oricarried out on dilute solutions of HME in polycrystalline ginates from HME. Moreover, the considerable experience C C 4 and CBr4 matrices. Indeed, the former matrix has which has been gained with this matrix1°J2J3suggests that been widely employed for studies of positive charge solute cations can be generated by positive charge transfer transfersg Hence, we feel there is probably no serious from the solvent in y-irradiated experiments. There is, conflict between our results and those obtained in the therefore, a strong prima facie case based on established preceding work.6 What is at issue, however, is the conradiation chemistry that the species of interest is the HME clusion that the incorporation of electron scavengers in radical cation. HME allows the cation to be detected.6 On the basis of We now turn to a discussion of the EPR spectrum. As our findings, this is clearly not the case. mentioned in the Introduction, a septet pattern attributed The mechanism of formation of solute cations in haloto the HME radical cation was previously reported from gen-containing solvents has been d i s c ~ s s e d ~ J and ~~'~J~ experiments in the presence of C C 4and CBr4.6 It should needs no elaboration here except to point out that the role be noted that the spectra obtained previously6differ from of the halide molecule is essentially twofold. Not only are the septet described here in having a reported hyperfine the electrons scavenged efficiently in these matrices, but coupling which is apparently larger (32 G vs. 29.0 f 0.2 the positive charge can migrate through the matrix until G) as well as an apparent substructure to each of the seven it is localized at a solute molecule well removed from the main components. Actually, the substructure is much site of the initial ionization caused by a fast electron. In more pronounced on the low-field side of the two spectra other words, the mechanism of solute cation formation in shown,6so that this asymmetry may affect the precision these matrices by the positive charge transfer reaction of the measured hyperfine coupling. It is curious that no (reaction 1) is fundamentally different from that which substructure was revealed in our experiments, in either the glassy or polycrystalline samples, and it is conceivable that RX+ + M -,RX + M+ (1) the reported fine structure6 arises from site splitting16in obtains by the (e-, 2e-) process caused by direct electron the CCI4 and CBr, matrices rather than from hyperfine impact (reaction 2 ) when the solute molecule is itself used interaction.
Ivl + e-
-
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M+
+ 2e-
(2)
as a matrix. In particular, Franck-Condon restrictions are expected to be more relaxed in the former case so that electron transfer could take place with a slight change of geometry. Thus, cations generated by reaction 1are likely to be formed in a less reactive condition (i.e., with less vibrational energy) than those produced in reaction 2. It is suggested that the above considerations can plausibly account for our results showing the formation of the HME radical cation in the FM matrix but not in the HME matrix, even when electron scavengers are incorporated in the latter. Some support for this interpretation comes from the detection of the neutral radicals CMe3 and .CMezCMe3 after irradiation of the HME matrix containing SF6(Figure 2), since these two radicals may well be produced by the dissociation of vibrationally excited HME cations. Also, the radical .CHzCMe2CMe3which is also present could be formed by proton transfer from the parent cation to a neighboring HME molecule. Acknowledgment. This work was supported by the Division of Chemical Sciences, Office of Basic Energy Sciences, U.S. Department of Energy (Document No. 0R0-2968-130).
References amid Notes (1)R. L. Hudson and F. Williams, J. Am. Chem. Soc., 99,7714 (1977). (2) L. Pauling, "The Chemical Bond", Cornell University Press, Ithaca, N.Y., 1967,p 188. (3) M. C. R. Symons, J. Chern. SOC.,Chern. Commun., 686 (1978). (4)J. T. Wang and F. Williams, Chem. Phys. Lett., 87, 202 (1979). (5) H. Shiraishi, H. Kadoi, K. Hasegawa, Y. Tabata, and K. Oshima, Bull.
Chem. Sac. Jpn. 47, 1400 (1974). (6) (a) M. C. R. Symons, Chern. Phys. Lett., 89, 198 (1980); (b) M. C. R. Symons and I. G. Smith, J. Chem. Res. (S), 382 (1979);(c) M. C. R. Symons, corrigendum, J. Chem. Soc., Chem. Commun.,
640 (1979). (7)J. T. Wang and F. Williams, Abstracts of the International Symposium on Magnetic Plesonance in Chemistry, Biology, and physics, Argonne National Laboratory, 1979,p 48. (8)J. T. Wang aind F. Williams, Chem. Phys. Lett., 71, 471 (1980). (9) W. H. Hamill in "Radical Ions", E. T. Kaiser and L. Kevan, Ed., Wiley, New York, 1968,Chapter 9,p 321,and especially pp 387-408. (10) T. Shida and S. Iwata, J. Am. Chem. SOC.,95, 3473 (1973). (11) C.Sandorfy, Can. Spectrosc., 10, 85 (1965). (12) A. Grimison and G. A. Simpson, J. Phys. Chem., 72, 1176 (1968). (13) T. Shida, Y. Nosaka, and T. Kato, J. Phys. Chem., 82,695 (1978). (14) A. Hasegawa, M. Shiotani, and F. Williams, Faraday DISCUSS. Cbern. SOC.,63, 157 (1977). (15)J. R. Morton, C:. F. Preston, and J. C. Tait, J. Chem. Phys., 82,2029 11975). (16) See,-e.g., R. .L Hudson and F. Williams, J. Chem. Soc., Chem. Commun., 1125 (1979). (17)C. Heller and H. M. McConnell. J. Chem. Phys.. 32, 1535 (1960). (18) G. A. Russell, Science, 161, 423 (1968). (19) J. T. Wang anid F. Williams, J . Am. Chem. SOC.,102, 2860 (1980). ~
ARTICLES Modified Electronic Structure and Enhanced Catalytic Activity of Cobalt Tetraphenylporphyrin Supported by Titanium Dioxide Isaa Mochida, * Kazuhiko Tsuji, Katsuya Suetsugu, Hiroshl Fujitsui, and Kenjiro Takeshita Research Institute of Industrial Science, Kyushu Universiv 86, Fukuoka, Japan 8 12' (Received: February 11, 1980; In Final Form: June 24, 1980)
The electronic structure and the catalytic activity of cobalt tetraphenylporphyrin supported on titanium dioxide (CoTPP/TiO2)were studied in order to reveal the electronic interaction between the oxide and the planar complex, which can modify the nature of the latter substance. CoTPP/Ti02 showed a sharp isotropic ESR signal at a g value of 2.003 and a UV band around 590 nm, values which were completely different from those of the unsupported CoTPP. The formation of an anionic radical, which has an odd electron in the porphyrin ring, is suggested. CoTPP/Ti02 showed remarkable catalytic activity for the reduction of nitric oxide to nitrous oxide and molecular nitrogen even at 50 OC with hydrogen, which was found to be adsorbed on CoTPP/Ti02. The activity was much accelerated at 150 "C,where a successive reduction of nitric oxide in the sequence NO NzO Nzwas clearly indicated.
- -
Introduction It has been recognized for a long time that certain supports play essential roles in heterogeneous catalytic and photoelectric reactions. Metal porphyrin molecules, square planar complexes which are extensively studied as oxygen carriers,l can offer one of their axial sites to be adsorbed on some catalyst supports such as alumina. Although the adsorption states of organic molecules on oxides have been extensively studied by means of various spectroscopy,2 there have been few reports on the electronic perturbation of metal porphyrin molecules supported on oxides, a process which is expected to modify their electronic and 0022-3654/80/2084-3159$01 .OO/O
catalytic properties. Very recently, Fan and Bard3 described the spectral sensitization and the catalytic activity of phthalocyanine-coated titanium dioxide. In the present study, we report on the modified electronic structure and the enhanced catalytic activity of cobalt tetraphenylporphyrin supported on titanium dioxide (CoTPP/'TiO,) for the reduction of nitric oxide, which was reported to be catalyzed by COTPP.~This oxide is expected to modify the nature of the adsorbed molecule by transferring its electron to the CoTPP molecule as a n-type semicond~ctor.~ Recently, some interesting behavior of the oxide as the catalyst support was reported? 0 1980 American Chemical Society
