J. Phys. Chem. 1994, 98, 2008-201 1
2008
Isolation of an ESR-Active Metallofullerene of La @ Cgz Kazunori Yamamoto,. Hideyuki Funasaka, and Takeshi Takahashi Nuclear Fuel Technology Development Division, Tokai Works, Power Reactor & Nuclear Fuel Development Corporation, Tokai, Ibaraki, 319-1 1 , Japan
Takeshi Akasaka Department of Chemistry, University of Tsukuba, Tsukuba, Ibaraki, 305, Japan Received: October 1 1 , 1993; In Final Form: January 3, 1994'
The ESR-active lanthanofullerene L a @ & has been isolated from various hollow fullerenes by an efficient single-stage chromatographic separation process. Pyrenylethyl-silica was used as a stationary phase with toluene as an eluent. Lanthanum-containing fullerenes were prepared by the arc heating of metal-impregnated graphite rods in a H e atmosphere. Fast atom bombardment mass analysis of the isolated sample confirmed the isolation of La@&. ESR absorption spectra of the present sample prove the separation of a La@(& isomer with a 1.15-G hyperfine splitting. The UV-vis-near-IR absorption spectra of the sample reveal near-IR absorption peaks which are characteristic of the fullerene radical anions.
Introduction Since the first macroscopic preparation of a lanthanum fullerene,' several metallofullerenes, including La @ C ~ 2 , l - ~ Y @ C c ~ 2 , 3and ~ , sc@c82?*12'13 ~~~ have been produced, extracted, and characterized by many researchers. Because all of the species considered as metallofullerene have been obtained in mixtures together with empty all-carbon fullerenes, species-specific detection techniques such as ESR, XPS, EXAFS, and mass spectrometry have been used for characterizing the metallofullerenes. Moreover, due to the scarcity of metal-containing fullerenes in soot? ultrasensitive detection techniques, especially ESR and mass spectrometry,have commonly been used in previous reports. In the case of soluble lanthanum-containing fullerenes, mass spectrometry revealed that the available samples were mixtures of metallofullerenes with different numbers of carbons and/or metals, such as La @ c76, La @ c80, La@c82,La @ c84, La@ CW, and
[email protected]~6~7J4J5 Moreover, ESR measurements revealed that there coexist isomers of metallofullerenes, such as La@Cs2, Y@C82, and Sc@Cs2,with different gvalues as well as different hyperfine coupling constants.596 This may correspond to the fact that empty fullerenes such as (273, C82, and c 8 4 consist of the corresponding mixtures of structural isomers.'6J7 Because pure metallofullerenesamples were not available at that time, previous metallofullerene studies simply depended on the assumption that metal-containing species appearing both in the mass spectra and in the other species-specific measurements corresponded to each other. For this reason, purified samples are indispensable for proving this assumption and, moreover, for further metallofullerene studies. The separation and isolation of the metallofullerenes from various hollow fullerenes were accomplished for the first time for the discandium fullerenes Sc2@C74, Sc2@C82, and Sc2@C84 using two-stage high-performance liquid chromatography (HPLC). l 8 However, because these isolated discandium fullerenes are ESR silent, laser desorption time-of-flight mass analyses alone were used for confirming their isolation.18 Although thesediscandium fullerenes are also thought to have more than two structural isomers, isomeric purification of the discandium fullerenes is not clear. In this Letter, we report the isolation of La@&, a groundstate doublet, paramagnetic lanthanofullerene, using an efficient *Abstract published in Advance ACS Abstracts, February 1, 1994.
0022-3654/94/2098-2008$04.50/0
single-stage HPLC system. Its isolation from the other fullerene species was confirmed by fast atom bombardment (FAB) mass spectrometry as well as electron spin resonance (ESR) measurements. These measurements indicated that the isolated sample was a lanthanofullerene, La@C82, with a hyperfine coupling constant of 1.15 G.586 Absorption spectra of the isolated La@C82 revealed wide absorption bands ranging over the nearIR region, which were not seen for the corresponding empty c82 fullerene and consistent with its open-shell electronic structure, described formally as
[email protected],3
Experimental Section Soot containing metallofullereneswas generated by evaporating metal-impregnated graphite rods using methods that have been described in detail.lJ+7J4 Graphite rods (Nippon Carbon Co., 13-mm diameter, EG-38H, 99.998%) were drilled out (7-mm diameter) and filled with a mixture of La203 (Chori Co.,99.9%), graphitepowder (Soekawa Chemical Co., 99.9%),and prepowder asphalt (Showa Shell Co.) with a ratio of 3:5:2 by weight. Before use, the rod was cured at 300 OC for 2 h and carbonized at lo00 OC for 1 h in flowing nitrogen. The resulting rod was 1-2 atom % lanthanum. The metal-impregnated graphite rod was used as the anode in a contact arc reactor1J4J9(Vacuum Metallurgical Co., UFP/SDl) at 250 A in a He (45 Torr) static atmosphere. The resulting soot was collected and extracted for 48 h in 1,2,4trichlorobenzene (TCB, Kanto Chemical Co., 99.0%) using a Soxhlet extraction apparatus. The extracts were dissolved in toluene prior to HPLC separation. The toluene solution of the extracts (5-mL injection volume, 2.5 mg/mL typical concentration) was separated using an HPLC system (Tosoh Co., HLC8070) with a 2-( 1-pyreny1)ethylsilylated silica column20 (COSMOSIL SPYE, 20-mm i.d., 250-mm length, Nacalai Tesque Co.) with toluene eluent (flow rate 10 mL/min). In this HPLC separation process (330-nm UV detection), the peak due to L a @ C a was observed between the c 8 4 + C86 and c 8 8 peaks. By collecting the corresponding fraction, we obtained the pure La @ C82 species. The positive and negative ion fast atom bombardment (FAB) mass spectra of the isolated product were measured in a m-nitrobenzyl alcohol (m-NBA) matrix on a JMS-SXlO2A mass spectrometer (JEOL Co., Ltd.). A xenon beam with an energy of 6 kV was used. Some of the mass spectra were measured for W-enriched ("C, 1.8%) samples. 0 1994 American Chemical Society
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The Journal of Physical Chemistry, Vol. 98, No.8, 1994 2009
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Figure 1. (A) An HPLC trace for the redissolved crude extract of the metallofullerene soot. Peaks due to empty fullerenes such as Cm, C70, C76, (278, and C82 + C84 are prominent and out of scale. The La@Cgl peak (retention time 40 min) is seen between peaks of cg4 + c.36and cgg. (B) A repeatedly isolated HPLC trace for La@(&. Besides solvent impurities near 6 min, only one peak is shown in (B).
The isomeric purification of lanthanofullerene was confirmed by ESR spectra. Both crude extracts and isolated La@Cs;! were dissolved in TCB and degassed. ESR measurements were performed using a conventional X-band ESR spectrometer (Bruker ESP 300) a t room temperature. Absorption spectra for the La@cg2were obtained by a 10-mm quartzcell with a Hitachi U-3410 UV-vis-near-IR spectrometer. Carbon disulfide (CS2) was used as a solvent.
Results and Discussion Figure 1A shows an HPLC profile of the crude extract of the fullerene soot redissolved in toluene. The most abundant fullerene in the mixture is c 6 0 (75% relative integrated intensity at 330 nm), followed by c70(IS%), C76 (1.5%), c 7 g (2.5%), c82 + c84 + c&5(4%), La@Cs2(OS%), and c~g(0.5%). The scarcity of the metallofullerenes in the extracts was seen in the chromatogram. The estimated amount of La@Cgz in each run (5-mL injection) was 0.1 mg. In this fullerene composition (major component of theextracts was C60), the obtained amount of the metallofullerene was restricted by a solubility of c 6 0 in toluene. The distribution of the hollow all-carbon fullerenes such as c60, c70, c76, and c78 resembles that in the extracts of pure carbon soot. When pure carbon electrodes were used for producing fullerenes, no peaks were observed between the Cg4 + c86 and cg8 peaks. All of the peak assignments have been performed by mass spectroscopic analyses of the corresponding fraction. It should be noted that our HPLC system could easily separate the new stable fullerenes, C86and c88. Their separation and identification will be published elsewhere.21 Figure 1B shows an HPLC trace of the purified metallofullerene,
[email protected] From the HPLC profile of the crude extract shown in Figure lA, the possible slight contaminants were c 8 4 and Cg6. An additional purification procedure resulted in emptyfullerene-free La@C82 (99% maximum purity from relative integrated intensity of the HPLC trace) as shown in Figure 1B. This fraction is yellow in a dilute solution and dark-yellow in a dense solution of toluene and TCB. Figure 2 shows the positive ion FAB mass spectrum for this rechromatographed fraction. Molecular ion peaks ascribable to LaQ.82 for the product are observed at 1123-1 126 amu. The obtained ion intensity ratio agrees well with the l3C isotope distribution,23 which was also verified for 13C-enriched samples. For every mass spectral signal the distribution is used as a criterion for the peak assignment. Besides La@CgZ peaks, some series of peaks arising from the
m-NBA matrix are seen in Figure 2. N o protonated fullerene species is seen in the mass spectra, which was also the case in the FAB mass spectra for the all-carbon fullerenesZ4and scandiumcontaining fullerenes.12 The all-carbon species such as C a r C70, c76, c78, Cg2, and c 8 4 are not clearly observed in the mass spectrum, although the background contains a continuous distribution of peaks every amu at several percent of the La@Cs2 signal (the signal-to-noise ratio of the mass spectral data is 5.2), which makes it impossible to analyze for trace impurities. Nevertheless, the predominance of La@cg2 among all-carbon fullerenes in Figure 2 is consistent with the HPLC profile shown in Figure 1B. Moreover, the lanthanum-containing fullerene shown in Figure 2 is La@C82. In contrast, preliminary negative ion FAB mass spectra for this fraction (not shown) failed to detect signals ascribable to fullerenes. Mass spectra of the crude extract showed peaks ascribable to La@C, (n = 74,76,80-112); among them La@C82 was the most abundant. These results mean that the isolation of La@C82 from both empty fullerenes and other metallofullerenes is successful in the present HPLC system. It should be emphasized that there are no c82 ions fragmented from La@Cg2in the mass spectra, suggesting that the La atom is strongly bound to the cage, although very weak peaks such as La@Cgo and La@C78 by successive C2 losses are accompanied. Essentially the same stability of endohedral fullerenes with respect to fragmentation has been reported extensively with FT-ICR mass spectrometry.25 Moreover, exohedral metallofullerenes are more reactive than endohedral fullerenes.1° Therefore, observed stability for metal elimination in the FAB mass measurement is also ascribable to the intrinsic stability of theendohedral metal-containing fullerenes. The FAB mass analyses of the exohedral fullerene derivatives, such as c 7 0 0 , Cm(C6H4)4,27 ~~ or fullerene silirane derivatives,28 usually show strong fragmentation peaks such as Cm and C70, which arise from a loss of exohedral functional group(s). It should be emphasized that there are lanthanofullerene oxide peaks of La@CgzO in the mass spectrum. Freshly separated samples did not show such peaks, but their intensity seems to increase as the sample exposure to the air (Le., oxygen) increases. The X-band ESR spectrum of the crude mixture of the lanthanum-containing fullerenes in TCB is shown in Figure 3A, which is the same sample shown in Figure 1A. The two dominant ESR hyperfine patterns for the lanthanofullerenes were essentially the same as those previously r e p ~ r t e d . ~The . ~ eight main equally spaced lines with a hyperfine coupling constant (HFCC) of 1.15 G (denoted as I) are followed by another set of ESR lines (HFCC = 0.83 G, denoted as 11) of weaker intensity, as denoted by the arrows in Figure 3A. From the similarity in spectral features between the main and the second octets, I and 11, both of them werelinked to thespecialabundant massspectral peak,
[email protected] Suzuki et al.s have assigned both of the octets to La@Cg2 with different cage structures. Hoinkis et a1.6 pointed out the second possibility that there is only one cg2 cage structure with different La atom positions. In either of the two interpretations, octet I1 corresponds to a molecular isomer of octet 1. The third explanation proposed by Hoinkis et a1.6 is that adducts are formed involving La@c82, solvent molecules, or hollow fullerenes.3J.6 However, there is a fourth explanation that octet I1 originates from another lanthanofullerene with a different carbon numbera7 Figure 3B shows the ESR spectrum of the isolated La@(& fraction. Interestingly, octet I1 is completely absent, and only octet I is seen in the spectrum. This indicates that the separation of a molecule exhibiting the octet I signal is complete in the present HPLC process. From our mass spectrometric measurement of the isolated sample described above, this species has proved to be La@Cgz. From now on we denote this species as La@Csz (octet I). For octet I, thegvalue and HFCC areidentical with those in Figure 3A. No dependence of the signal on solvents (such as toluene,5*6carbon disulfide: and TCB) has been observed.
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The Journal of Physical Chemistry, Vol. 98, No. 8, 1994
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Figure 2. FAB mass spectra of in m-NBA matrix from 500 to 1160 amu and expanded views of the 1000-1 160 and 11 15-1 135 amu regions. 139La@lZC82 (1123 amu), 139La@13C12Csl (1 124 amu), and 139La@13C212C80 (1 125 amu) are the main spectral peaks. Peaks around 613,766, and 919 amu are matrix (m-NBA) tetramer, pentamer, and hexamer. Trace amount of peaks ascribable to La@Cso and La@C82O are also observed. See text. k
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Figure 3. ESR spectra of (A) extracted lanthanofullerene mixture (which corresponds to Figure 1A) and (B) isolated La@C82 fraction of sample (A) (which corresponds to Figure 1B). Both spectra were measured at room tempoeraturein 1,2,4-trichlorobenzene. In (A), twooctetsdenoted as I and I1 (denoted by arrows) are seen. The isolation of La@& was performed on a 2 4 1-pyreny1)ethylsilylated silica column20with toluene as an eluent. It is noteworthy that Octet I1 is absent in (B).
Moreover, by mixing hollow fullerenes such as C ~with O La@Cs2 (octet I), no change was observed in the ESR spectra. On the other hand, octet I1 was absent in all the fractions eluted from the HPLC column. A similar disappearance of the octet I1 signal was observed for the column chromatographed ~ample.~JIIn order to further investigate this phenomenon, a preliminary anaerobic HPLC separation was performed for the crude fullerene mixtures including octets I and 11. By using degassed toluene as an eluent, we found that octet I1 eluted after hac82 (octet I).29 This result means that, during our aerobic HPLC separation, the octet I1 species reacted with oxygen, forming an ESR-silent species. This interpretation is consistent
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Figure 4. UV-vis-near-IR absorption spectrum from 400 to 2500 nm for the isolated La@C82 (octet I) in CS2 at room temperature. The feature at 800 nm corresponds to an instrument detector change. Characteristic absorption features appear in the near-IR region, which are not observed in the corresponding c 8 2 spectrum.
with the previously reported properties that octet I1is very reactive and air-sensitive.' From both the FAB mass and ESR spectra of the La@C82 fraction (Figures 2 a n d 3B), we conclude that La@C82 (octet I) is surely isolated by our HPLC system. Figure 4 shows the electronic absorption spectrum for the isolated (octet I) in carbon disulfide. The spectral features are quite characteristic as compared, for example, with the corresponding empty c 8 2 f ~ l l e r e n e .The ~ ~ salient features are as follows: (1) the strong absorption peaks a t 637,1010, and 1428 nm (broad) areobserved in the LaQCs2 (octet I) spectrum; (2) the broad absorption profile continues down to 2300 nm. As previously reported,30 the absorption spectrum of c 8 2 continues down to 1100nm and shows absorption peaks at 580 and 680 nm. The near-IR region beyond 1100 nm for c82 was found to be featureless in our study. Independently, Kikuchi et al. have isolated the same La@CS* isomer31 and reported the same
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The Journal of Physical Chemistry, Vol. 98, No. 8, 1994 2011
(3) Weaver, J. H.;Chai, Y.; Kroll, G. H.; Jin, C.; Ohno, T. R.; Haufler, absorption spectra as in Figure 4. It should be emphasized that R. E.; Guo, T.; Alford, J. M.; Conceicao, J.; Chibante, L. P. F.; Jain, A.; Figure 4 is much different from the absorption spectra for another Palmer, G.; Smalley, R. E. Chem. Phys. Lett. 1992, 190, 460. reportedmetallofullerene,Sc2@Cu,lSwhich shows no ESR signals (4) Ross, M. M.; Nelson, H. H.; Callahan, J. H.; McElvany, S. W. J . or near-IR absorption at wavelengths above 800 nm. To Phys. Chem. 1992, 96, 5231. understand the absorption spectra of La@Cs2 (octet I) quali(5) Suzuki, S.; Kawata, S.; Shiromaru, H.; Yamauchi, K.; Kikuchi, K.; Kato, T.; Achiba, Y. J . Phys. Chem. 1992, 96,7159. tatively, one should consider the absorption bands for fullerene (6) Hoinkis, M.;Yannoni, C. S.;Bethune, D. S.; Salem, J. R.; Johnson, radical anions. The anionic forms of C ~ and O C70 (i.e., c60+ and R. D.; Crowder, M. S.;de Vries, M. S. Chem. Phys. Lett. 1992, 198,461. C70”- where n = 1-4) show characteristic near-IR absorption (7) Bandow, S.;Kitagawa, H.; Mitani, T.; Inokuchi, H.; Saito, Y. b a n d W 3 that are not observed for neutral c 6 0 and c 7 0 . For Yamaguchi, H.; Hayashi, N.; Sato, H.; Shinohara, H. J. Phys. Chem. 1992, 96, 9609. example, C603-shows absorption bands in the near-IR and visible (8) Funasaka, H.; Yamamoto, K.; Sakurai, K.; Ishiguro, T.; Sugiyama, regions with peaks at 780,850, 1000, and 1370 nm.32 Note that K.; Takahashi, T.; Kishimoto, Y. Fullerene Sci. Technol. 1993, 1, 437. the magnetic ground state of C603-is a doublet.34 C703- shows (9) Shinohara, H.; Sato, H.; Saito, Y.; Ohkohchi, M.; Ando, Y. J. Phys. absorption bands in the near-IR and visible regions with peaks Chem. 1992, 96, 3571. at 760 and 1170 nm.33 From the previous experimental finding (10) McElvany, S.W. J . Phys. Chem. 1992, 96,4935. that La@(& is paramagnetic and its open-shell electronic (11) Soderholm, L.; Wurz, P.;Lykke, K. R.; Parker, D. H.; Lytle, F. W. structure (i.e., doublet) is described formally as L ~ ~ + C S the ~ ~ - , ~ JJ . Phys. Chem. 1992, 96,7153. (12) Shinohara, H.;Sato, H.;Ohkohchi, M.; Ando,Y.; Kodama,T.; Shida, absorption features shown in Figure 4 are considered as a spectral T.; Kato, T.; Saito, Y. Nature 1992, 357,52. superposition of a singlet La3+ and a doublet Csz3-to a first-order (13) Yannoni, C. S.;Hoinkis, M.; deVries, M. S.; Bethune, D. %;Salem, approximation. Note that some C60- salts in benzonitrile have J. R.; Crowder, M. S.;Johnson, R. D. Science 1992, 256, 1191. similar near-IR spectra ascribable to C ~ O -We . ~ may ~ attribute (14) Alvarez, M. M.; Gillan, E. G.; Holczer, K.; Kaner, R. B.; Min, K. S.; Whetten, R. L. J . Phys. Chem. 1991, 95, 10561. the broad absorption profile continuing down to 2300 nm to the (15) Yeretzian, C.;Hansen, K.; Alvarez, M. M.; Min, K. S.; Gillan, E. lower molecular symmetry (&/C,,CzU/C1,or CZ)of Cs217.36as G.; Holczer, K.; Kaner, R. B.; Whetten, R. L. Chem. Phys. Lett. 1992, 196, compared with c 6 0 (Zh) and C70 (D5h), although the carbon cage 337. symmetry of La@CS2(octet I) is not clear at the present time. (16) Diederich, F.; Whetten, R. L.;Thilgen, C.; Ettl, R.; Chao, I.; Alvarez, M. M. Science 1991, 254, 1768. There have been several theoretical investigations for both fullerene ani0ns3~J8and lanthanofullerenes, LaCm39and L ~ C S ~ . ~ ~(17) , ~Kikuchi, K.; Nakahara,N.; Wakabayashi,T.; Suzuki, S.;Shiromaru, H.; Miyake, Y.; Saito, K.; Ikemoto, I.; Kainosho, M.; Achiba, Y. Nature Early Huckel M O calculations for the electronic structure of 1992, 357,142. fullerene anions predicted the magic number stability of fullerene ( 1 8) Shinohara, H.; Yamaguchi, H.; Hayashi, N.; Sato, H.; Ohkohchi, dianions including Cs2’- from its estimated HOMO-LUMO M.; Ando, Y.; Saito, Y. J . Phys. Chem. 1993, 97,4259. (19) Haufler, R. E.; Chai, Y.; Chibante, L. P. F.;Conceicao, J.; Jin, C.; energy g a p ~ . 3 ~ Restricted J~ open-shell Hartree-Fock selfWang, L.; Maruyama, S.; Smalley, R. E. Mafer.Res. Soc. Symp. Proc. 1990, consistent-field M O calculations of C ~ containing O a central La 206,627. atom found that the two metal valence electrons transfer to the (20) Kimata, K.; Hosoya, K.; Araki, T.; Tanaka, N. J. Org. Chem. 1993, cage LUMO, but the electronic ground state is a quartet.39These 58,282. electron transfers to the fullerene cages are expected to show the (21)Yamamoto, K.; Funasaka, H.; Takahashi, T. Manuscript in preparation. charge-transfer bands in the near-IR region, which cause a red (22) Although its elution was slightly later than in the case of the fullerene shift of the absorption bands. More advanced electronic structure mixture as shown in Figure 1 A, both peaks corresponded to each other, which calculations on Laa.82 have been performed by two groups. was caused by the slight decrease of the column temperature. Laasonen et al. performed fully optimized ab initio calculations (23) Shinohara, H.; Sato, H.; Saito, Y.; Izuoka, A.; Sugawara, T.; Ito, H.; Sakurai, T.; Matsuo, T. Rapid Commun. Mass Spectrom. 1992, 6,413. for La@Csz by the local density approximation of the density (24)Shinohara, H.; Sato, H.; Saito, Y.; Takayama, M.; Izuoka, A.; functional theory.4o They found that a certain off-center metal Sugawara, T. J . Phys. Chem. 1991, 95, 8449. position is more stable, and the three metal valence electrons (25) Wiess, F. D.; Elkind, J. L.; OBrien, S.C.; Curl, R. F.; Smalley, R. transfer to the cage, forming La3+@C8z3-, which is consistent E. J . Am. Chem. SOC.1988, 110,4464. with the experimental results.2.3 They also found that La does (26) Diederich, F.; Ettl, R.; Rubin, Y.; Whetten, R. L.; Beck, R.; Alvarez, M.; Ans, S.; Sensharma, D.; Wudl, F.; Khemani, K. C.; Koch, A. Science not simply behave as an electron donor but actually forms a strong 1991, 252,548. bond with the C atom of the cage. Their estimated HOMO(27) Tsuda, M.; Ishida, T.; Nogami, T.; Kurono, S.;Ohashi, M.Chem. LUMO gap at the configuration is about 0.4 eV. Since their Lett. 1992, 2333. calculations include excited states, it may correspond to our (28) Akasaka, T.; Ando, W.; Kobayashi, K.; Nagase, S. J. Am. Chem. absorption onset of 0.54 eV (2300 nm). Nagase et al. found SOC.1993, 115, 1605. (29) Due to the reaction with a trace amount of oxygen in solution, the similar results by unrestricted Hartree-Fock calculation^.^^ signal intensity of Octet I1 was not so strong; therefore, the main component In conclusion, by taking advantage of the well-separated of this fraction was hollow fullerenes such as Cas. The successful isolation retention times of the hollow fullerenes and lanthanofullerenes, of Octet I1 probably has more to do with its higher aerobic sensitivity. an ESR-active La@c82 isomer has been separated efficiently on (30) Kikuchi, K.; Nakahara, N.; Wakabayashi,T.; Honda, M.;Matsumiya, H.;Saito, K.;Yamauchi,K.; Ikemoto, H.; Moriwaki,T.;Suzuki,S.;Shiromaru, a 2 4 1-pyrenyl)ethylsilylated silica column. FAB mass analyses I.; Achiba, Y. Chem. Phys. Lett. 1992, 188,177. as well as ESR spectra prove its successful isolation. Its UV(31) Kikuchi, K.; Suzuki,S.;Nakao, Y.; Nakahara, N.; Wakabayashi,T.; vis-near-IR absorption spectra reveal near-IR absorption peaks Shiromaru, H.; Saito, K.; Ikemoto, I.; Achiba, Y. Chem. Phys. Letr., in press. down to 2300 nm, which is consistent with its open-shell electronic (32) Lawson, D. R.; Feldheim, D. L.; Foss C. A.; Dorhout, P. K.; Elliot, structure. C. M.; Martin, C. R.; Parkinson, B. J . Electrochem. SOC.1992, 139,L68. (33) Lawson, D.R.; Feldheim, D. L.;Foss, C. A.; Dorhout, P.K.; Elliot, C. M.; Martin, C. R.; Parkinson, B. J . Phys. Chem. 1992, 96, 7175. Acknowledgment. We express thanks to Mr. Kenji Sugiyama (34) Dubois, D.; Jones, M. T.; Kadish, K. M. J. Am. Chem. Soc. 1992, (PNC), Koji Sakurai (Zuiho Sangyo Co.), Toshiaki Ishiguro 114, 6446. (Genshiryoku Gijyutsu Co.), and Yoshiharu Kano (Genshiryoku (35) Stinchcombe, J.; Penicaud, A.; Bhyrappa, P.; Boyd, P. D. W.; Reed, Gijyutsu Co.) for their experimental help. We would also thank C. A. J . Am. Chem. SOC.1993, 115,5212. (36) Nagase, S.;Kobayashi, K.; Kato, T.; Achiba, Y. Chem. Phys. Lett. Dr. Achiba for sending us a preprint of ref 31. 1993, 201,475.
(37) Manolopoulos, D. E.; Fowler, P. W. Chem. Phys. Lett. 1991, 187,
References and Notes
1.
(1) Chai, Y.; Guo, T.; Jin, C.; Haufler, R. E.; Chibante, L. P. F.; Fure, J.: Wann. L.: Alford. J. M.: Smallev. R. E. J . Phvs. Chem. 1991. 95. 7564. (2)-johnson, R. D.; de Vries, M.S.; Salem, J.; Bethune, D. S.; Yannoni, C. S. Nature 1992, 355,239.
(38) Fowler, P. W.; Manolopoulos, D. E. Nature 1991, 355,428. (39) Chang, A. H. H.; Ermler, W. C.;Pitzer, R. M. J. Chem. Phys. 1991, 94, 5004. (40) Laasonen, K.; Andreoni, W.; Parrinello, M. Science 1992,258,1916.
