J. Phys. Chem. 1992,96,7 159-7 161 (17) Badger, J.;Caspar, D. L. D. Proc. Narl. Acad. Sci. U S A . 1991,88,
622.
(18) Cheng, X.; Schoenborn, B. P. Acta Crysraflogr. 1990, 816, 195. (19) Blake, C. C. F.; hlfd,W. C. A.; Artymiuk P. J. J. Mol. Biol. 1983, 167, 693.
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(20)Gilson, P.J.; Honig, B. Proteins 1!M, 4, 7-18. Tanford, C.; Kirkwood, J. G. J . Am. Chem. Soc. 1957,79, 5333-5339. (21) Davis, M. E.; McCammon, J. A. J. Compur. Chem. 1991, 12, 909-912. (22)Hagler, A.; Moult, J. Nature 1978, 272, 222.
Isomers and ''C Hyperfine Structures of MetaEEncapsulated Fullerenes M@Ca2(M = Sc, Y, and La) Shinzo S t u d , * Satoghi Kawata, Haruo Shiromaru, Kotaro Yamauchi, Koichi Kikuchi, T a t " Kat02 and Yohji Achiba* Department of Chemistry, Tokyo Metropolitan University, Hachioji, Tokyo 192-03, Japan, and Institute for Molecular Science, Myodaiji, Okazaki 444, Japan (Received: June 1, 1992; In Final Form: July 20, 1992)
The formation of two isomers of C& with a metal inside, M a c E 2(M = Sc,Y,and La) was fmt identified by ESR spectroscopy. The production ratio of two isomers found for M a c E 2suggests that the cage structure of the isomers of M a c E 2is closely associated with those of the empty Cs2. However, contrary to the existence of four or more distinct isomers of empty Cs2, the observation of only two metallofullerene isomers strongly suggests that a metal is selectively encapsulated in the CE2 cages with particular structures. An endohedral form of metal-containing CE2fullerene was established by observation of well-resolved ESR spectra of hyperfine coupling to I3C in natural abundance on the carbon cages.
Introduction Recent discovery of macroscopic preparations of metal-encapsulated fullerene M@Cs2(M = Sc, Y, and La) enabled us to study the nature of such an exotic molecular form in detail. Among the many fascinating issues concerning the nature of these metal-fullerene complexes,1* a central question is the following: why is cs2 so special? M@Cs2has been indicated to be the only metallofullereneto be extracted in solution except for the case of La2Cw2 Mass spectra of Sc-containingcrude extract indicated the existence of diatomic species Sc2C2,(2n = 80, 82,84, ...).s,6 In this sense, structural information on these endohedral complexes must be unambiguously important to understand the peculiar properties of these fullerenes. On the other hand, the cage structures of the empty Cs2have recently been successfully determined by Kikuchi et al., using 13C NMR in solution for the chromatographically separated C8z.7 According to their results, the major isomer of Cs2 has a C, symmetry, and at least three other minor isomers with C,, C,,, and C2(or C,) symmetries coexist. Therefore, it is interesting to investigate whether or not isomers exist even for the fullerene Cs2 with a metal inside. In the case of M a c s 2 (M = Sc, Y, and La), if odd numbers of electrm transfer from the metal embedded inside to the carbon cage outside, the cage would have an electron spin and show ESR signals. Furthermore, if there coexists isomers of metallofullerenes, each isomer would have a different g value as well as a different hyperfine coupling constant, which should be, in principle, distinguhhed from each other. The first expectation mentioned above was indeed the case in La@CS2for which the ESR spectrum caused by a half-spin on the carbon cage was actually observed by Johnson et ala3for the first time. For La@Cs2,they have shown eight equally spaced lines with an equal intensity centered at g = 2.0010. The eight ESR lines were well interpreted in terms of a hyperfine structure due to the '39Lanuclei with I = '/> They Collcluded that the ekctronicstructure Of La@%2 is well dacribed by La3+@Csf. Recently, Weaver et a1.4 have observed the ESR spectrum of Y@CS2,consisting of an I = l / 2 system. Shinohara et al.s and Yannoni et a1.6 more recently reported the ESR spectrum of the Sc,-Cs2 system (x = 1 and 3). To whom corre6pondence should be a d d r d .
tInstitute for Molecular Science.
In this report, it is shown that a metal gets into two different CE2carbon cages with particular structures. The systematic measurements of ESR spectra for three metal-encapsulated fullerenes indicate that two isomers of M a c s 2 are commonly formed with almost the same fraction ratio. The possible cage candidates for these metalofullerenes are discussed by comparison with the results recently obtained from 13CNMR measurements on the empty Cg2fullerene. Exper&nenWSection The sample was prepared by a method essentially the same as those described already.'* Briefly, a mixture of graphite powder and metal oxide (MZOJ, to approximately 1 metal atom per 130 carbon atoms, was mixed with graphite cement (551-R, Aremco Products Inc.) to an approximately 1:3 volume ratio and pressed into a rod. After curing at 200 OC overnight, the rod was heated to 1200 OC for 10 h. The soot was produced by dc arc discharge of the rod under about 200-Torr He atmosphere. The soot was collected and extracted by CSPs Laser desorption timesf-flight mass spectra of crude extracts, taken by a negativeion mode, have shown that the fulkrene species associated with a single metal was YcS2 LacS2, and ScCs2.From the mass spectra,it was also found that there were no mass peaks due to impurity metals whose nuclear magnetic moments are accidentally the same as La, Y, or Sc. Therefore, it was confvmed that the species giving the ESR signals in the present work was due to MC82 (M = Sc, Y, and La), as was also indicated by Chai et a1.l and by Alvarez et aL2 and Johnson et aL3for h@c82, by Weaver et a1.4 for Y@Cg2,and by Shinohara et alq5and Yannoni et a1.6 for Sc@CE2. After each crude extract was dissolved into toluene or CS2and degassed, ESR measurements were performed using a conventional X-band ESR spectrometer (JEOL RE-3X).
Results and Discussion The X-band ESR spectra of the metallofullerenes MCS2(M = Sc, Y, La) in solution at room temperatureare shown in Figure 1, a,b, and c, respectively. The main features are essentially the same as those already reported for La@C82,3Y@Cs2,4and except the fact that there appears small, but wellresolved, additional lines for the present case. Therefore,we mostly focus on these small ESR lines in the present report.
0022-3654/92/2096-7 159$03.00/0 Q 1992 American Chemical Society
Letters
7160 The Journal of Physical Chemistry, Vol. 96, No. 18. 1992
I
I
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336.5
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B /mT
Ii I
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L - I - - - L
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Li / m T
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3370 0 /mT
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Figure 2. (a) ESR spectrum of ScCn2shown by an expanded scale. The arrows indicate the hyperfine structures due to isomer I1 (seetext). The signal designated by an asterisk was considered to be due to the unknown impurity. (b) Simulated ESR spectrum obtained by adding (c) and (d). (c) Simulated ESR spectrum attributable to the main lines (isomer I). Weak satellite lines were simulated by use of the pattern of experimental weak lines appearing in the lowest magnetic field in (a). (d) Simulated ESR spectrum due to isomer 11.
- L - i
i-_L--L
,-A_-.
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B /mT
Figure 1.
ESR spectra of metal-containing fullerenes: (a) ScCs2 in CS2
(9.4325 GHz, 0.1 mW), (b) YCs2in toluene (9.4296 GHz, 0.1 mW), and (c) Lacn2in toluene (9.4307 GHz, 0.1 mW).
The appearance of main eight lines of hC82 is diagnostic for isotropic hyperfine coupling to a nuclear magnetic moment of Sc with spin 7 / p The eight main lines are followed by several weak satellite lines on both sides with exactly the symmetric pattern. This pattem strongly suggests that the satellite lines are due to I3Chyperfine structure. Since, roughly speaking, the ratio of the C,, carbon cages which have only '2c to those which have at least one I F is about 2:3, and thus if the ESR lines due to "C hyperfiie splittings arc well distinguiphed from the main line of '2c,the total intensity of the I3C lines is expected to have the same order. In fact, the total intensity of the satellite lines otmerved here is about onehalf of the strong line. This experimental evidence, in addition to their symmetric patterns, supports that the satellite lines are assigned to natural abundant I3Chyperfine splittings. This fact, in turn, confirms that a metal is trap@ inside. In Figure la, we can notice the existence of another set of ESR linea with weak intensity, as denoted by arrows in Figure 2a. The weak second ESR system has also eight equally spaced lies with almost the same intensity. Similarity in spectral features between the main and the second system likely indicates that the latter with different cage structure (isomer 11). This is also Sc@Cg2 system has a smaller hyperfine coupling constant but a slightly larger g value than those of the main component (see also Figure 2d and Table I). The ESR spectra of Y @ C s 2and La@Cs2seem to be rather complex (Figures 3a and 4a). However, On the basis of the idea from sC@Cs2, it was easily deduced that the ESR spectra for these fullerenes can also be divided into two species, in addition to the I3Chyperfine lines beside the main lines. In order to clarify these fmdings more quantitatively, we carried out simulations of an ESR line profile as followings. (1) In order to simulate ESR spectra
I
1
3365
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3367 3360 3369 3370 6 /mT Figure 3. (a) ESR spectrum of YCsz shown by an expanded scale. (b) Simulated ESR spectrum obtained by adding (c) and (d). (c) Simulated ESR spectrum for isomer I obtained by the same procedure in Figure 2c. (d) Simulated ESR spectrum due to isomer 11.
TABLE I: Summary of g Vlllues and Hyperfiw Coupling Coartrats (HFCC, in mT) Due to Metals Determined from tbe ESR Spectra of M e t . l - h W t e d Fullerews M @ C a (M = SC, Y, h) M @ Cs2 SC@C82 isomer I isomer I1 Y@C82 isomer I
isomer I1 La@Cs2 isomer I isomer I1
g value
HFCC by M
re1 ratio
2.0002 2.0009
0.3799 0.1744
1.o 0.07
2.0006 2.0001
0.0490 0.0321"
0.15'
2.0012 2.0006
0.1151 0.0835'
1.o 1.o 0.27'
"These values are deduced based on the results of ESR simulations.
shown in Figures 3c and 4c,we used experimental satellite lines appearing at the lowest magnetic field as pure I3C hyperfine splitting lines. (2) Comparing the spectra thus obtained with the experimental ones, the ESR spectra of second isomers were simulated (Figures 3d and 4). Actually, the superposition of two simulated spectra, (c) and (d), well reproduce the whole experimental spectral features as shown in Figures 3b and 4b. The resulting g values and the hyperfine coupling constants for Y @CS2
J. Phys. Chem. 1992,96,7161-7164
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Y@Cs2,and 0.27 for La@Cs2. Considering the similar values seen in the fraction ratio of the major and minor isomers found in ESR and NMR studies, it might be possible to deduce that the main ESR signal is due to the metallofullerene with a C2 symmetry, just like as that the empty q2 is, and the sccond isomer is due to one of three minor isomers identitied for the empty Cg2 by 13CNMR. These considerations,in turn,strongly suggest that a metal is salectively encapsulated into the CS2cages with specific structures among four or more candidates. Furthermore,it is also expected that the 13Chyperfine structures, which were fvst distinguished in the present work, may provide very versatile and direct information on the molecular structure of M@Cg2. The analysis using semiempirical MO calculations is now in progress.
Note Added in proof. After submitting this report, we learned that Yannoni et al. have obtained results on Lacszsimilar to those reported here (ref 9). Acknowledgment. We thank Toyo Tanso Co. Ltd. for providing us La-containing carbon rods. We also thank Mr. Hitashi Kannari for the preparation of metal-containing carbon rods throughout this work. This work was partially supported by the grants from the Ministry of Education, Science and Culture of Japan. L
336.1
I
I
I
I
I
336.3 336.5 336.7 336.9 337.1 337.3 B /mT
Figwe 4. (a) ESR spectnun of Lacllzshown by an expanded scale. (b) Siulated ESR spectrum obtained by adding (c) and (d). (c) Simulated ESR spectrum for isomer I obtained by the same procedure in Figure 2c. (d) Simulated ESR spectrum due to isomer 11.
and La@CS2are summaflzcd in Table I, together with Sc@Cs2. As can be seen from the table, the general tendency of the c m in a g value and a hyperfine coupling constant from the isomer I to I1 is quite similar to each other, except the g value for Sc@C& Recent quantitative analysis of 13C NMR lines obtained for the empty Cg2strongly indicated that at least four isomers with C2,C, C, and another C2 (or C,) symmetries are formed with the fraction ratio of about 8:1 :1 :1, respectively.' On the other hand, from the prcptnt ESR spectra, we found two isomers (I and I1 in Table I) with the ratio of about 0.07 for Sc@CS2,0.15 for
References and Notes (1) Chai, Y.; Guo, T.; Jin, C.; Haufler, R. E.; Chibante, L. P. F.; Fure, J.; Wang, L.; Alford, M.; Smalley, R. E. J. Phys. Chem. 1991, 95, 7564. (2) Alvarez, M. M.; Gillan, E. G.; Holczer, K.;Kaner, R. E.; Min, K.S.; Whetten, R. L. 1. Phys. Chem. 1991,95, 10561. (3) Johnson, R. D.; de Vries, M. S.;Salem, J.; Bethune, D. S.; Yannoni, S.Nature 1992, 355,239. (4) Weaver, J. H.; Chai, Y.;Kroll, G. H.; Jin, C.; Ohno, T. R.;Haufler, R. E.; Guo, T.; Alford, J. M.; Conceicao, J.; Chibante, L. P. F.; Jain, A.; Palmer, G.; Smalley, R. E. Chem. Phys. Lett. 1992, 190,460. ( 5 ) Shinohara, H.; Sato, H.; Ohkohchi, M.;Ando, Y.; Kodama, T.; Shida,
T.; Kat0 T.; Saito, Y. Nature 1992, 357, 52. (6) Yannoni, C. S.;Holnkis, M.; de Vries, M. S.;Bethune, D. S.;Salem, J. R.;Crowder, M. S.;Johnson, R.D. Science 1992,256, 1191. (7) Kikuchi, K.;Nakahara, N.;Wakabayashi, T.; Suzuki, S.;Shuomaru, H.; Miyake, Y.; Saito, K.;Ikcmoto, I.; Kaiiosho, M.; Achiba, Y .Nature 1992, 357, 142. (8) Kikuchi, K.;Nakahara, N.;Wakabayashi, T.; Honda, M.;Matsumiya, H.; Moriwaki, T.; Suzuki, S.;Shiromaru, H.; Saito, K.;Yamauchi, K.;Ikemoto, I.; Achiba, Y . Chem. Phys. Lett. 1992, 188, 177. (9) Yannoni, C. S.;Wendt, H. R.;de Vries, M. S.;Siemens, R. L.; Salem, J. R.;Lyerla, J.; Johnson, R.D.; Hoinkis, M.; Crowder, M. S.;Brown, C. A.; Bcthune, D. S.Synth. Met., in press.
EWect of Confinement on the Resonant Intermolecular Vlbratlonai Coupling of the v2 Mode of Methyl Iodlde in Porous Silica Glasses
Y. T.Lee, S.L.Wallen, and J. Jonas* Department of Chemistry, School of Chemical Sciences, University of Illinois, Urbana, Illinois 61801 (Received: June 2, 1992; In Final Form: July 20, 1992) Using Raman scattering, the resonant transfer of vibrational energy was examined for methyl iodide confined in sol-gel prepared silica glasses with pore radii ranging from 12 to 70 A. The resonant intermolecularvibrational coupling (RIVC) of the v2 vibrational mode of methyl iodide reflects clearly the effect of geometric confinement. The RIVC induced line broadening and frequency shifts are in qualitative agreement with the Logan theory and the model of geometrically restricted energy relaxation as developed by Klafter et al.
hlroduch Although the dynamics of liquids in porous media have been studied by various techniques,'V2 attempts to unravel the effects of pure geometric confinement have proven to be experimentally difficult. Fkst of all, it is important to make sure that the observed signal arises only from the confined liquid and is not affected by the bulk liquid outside the pores. Another problem is related to the specific d a c e effects due to interactiom between the confined liquids and the confining surface; i.e., there is a significant dif-
ference between wetting and nonwetting liquids. The latter is especially important for studying polar wetting liquids since both the geometric confimement and surface interaction effects increase as pore size decreases. Optical t e c h n i q ~ e ssuch , ~ ~ as Raman scattering, avoid the fmt problem. Simple optical focusing within the transparent glass sample allows one to record signals directly from the confined liquid. Problems related to surface interactions can be partially Overcome by using porous glasses prepared by the sol-gel process
0022-3654/92/2096-7 161$03.00/0 Q 1992 American Chemical Society
