5228 J . Phys. Chem. 1992, 96, 5228-5231

(14) (a) EerNisse, E. P. J . Appl. Phys. 1973, 44, 4482-4485. (b) Eer- Nisse, E. P. J . Appl. Phys. 1972,43, 1330-1337. (c) Cheek, G. T.; OGrady, W. E. J . Elecrroanal. Chem. 1990, 277, 341.

(15) Impedance analysis was performed with a Hewlett-Packard 4194A impedancc/gain-phase analyzer capable of performing measurements over a frequency range of 100 Hz-40 MHz in the impedance mode. Data collection was accomplished via an HPIB interface with a Macintosh personal computer.

(16) Colacicco, G.; Buckelew, A. R., Jr.; Scarpelli, E. M. J . Colloid In- terface Sci. 1974, 46, 147.

(17) Colacicco, G.; Basu, M. K.; Littman, J.; Scarpelli, E. M. Adu. Chem. Ser. 1975, No. 144, 239.

(18) (a) Kanazawa, K. K.; Gordon, J. G., I1 Anal. Chem. 1985, 57, 1770-1771. (b) Kanazawa, K. K.; Gordon, J. G., I1 Anal. Chim. Acta 1985,

(19) (a) Thompson, M.; Arthur, C. L.; Dhaliwal, G. K. Anal. Chem. 1986, 58, 1206-1209. (b) Rajakovic, L. V.; Cavic-Vlasak, B. A.; Ghaemmaghami, V.; Kallury, K. M. R.; Kipling, A. L.; Thompson, M. Anal. Chem. 1991.63, 615-621. (c) Kipling, A . L.; Thompson, M. Anal. Chem. 1990, 62, 1514-1519. (d) Khurana, A. Phys. Today 1988,41, :7. (e) Krim, J.; Widom, A. Phys. Rev. 1988, 838, 12184.

175, 99-105.

(20) Deamer, D. W.; Cornwell, D. G. Biochim. Biophys. Acra 1966,116,

(21) Derjaguin, B. V.; Green-Kelly, D. Trans. Faraday Soc. 1%4,60,449, (22) Krim, J.; Watts, E. T.; Digel, J. J. Vac. Sei. Technol. 1990, A8,3417. (23) McCafferty, E.; Pravdic, V.; Zcttlemoyer, A. C. Trans. Faraday Soc.

555.

1970. 66. 1720. ~ (24) Kiseleva, 0. A.; Sobolev, V. D.; Starov, V. M.; Churaev, N. V. Kolloidn. Zh. 1979, 41, 245.

(25) Palmer, L. S.; Cunliffe, A.; Houeh, J. M. Nature 1952. 1970. 796. (26) (a) Israelachvilli, J. Acc. Chem. R&. 1987, 20,415. (b) Israelachvilli,

J. N.; McGuiggan, P. M.; Homola, A. M. Science 1988, 240, 189. (c) Israelachvilli, J. N.; Kott, S. J. J . Colloid Interface Sci. 1989, 129, 461.

(27) (a) Ulman, A. Ado. Mater. 1991, 3,298. (b) Hautman, J.; Bareman, J. P.; Mar, W.; Klein, M. L. J . Chem. Soc., Faraday Trans. 1991,87,2031. (c) Barton, S. W.; Goudot, A.; Bouloussa, 0.; Rondelez, F.; Lin. B.; Novak, F.; Acero, A,; Rice, S. J . Chem. Phys. 1992, 96, 1343. (d) Shih, M. C.; Bohanon, T. M.; Mikrut, J. M.; Zshack, P.; Dutta, P. J . Chem. Phys. 1992, 96, 1556.

(28) Okahata, Y.; Kimura, K.; Ariga, K. J . Am. Chem. SOC. 1989, 1 1 1 , 9190.

Steady-State and Time-Resolved Direct Detection EPR Spectra of Fullerene Triplets in Liquid Solution and Glassy Matrices. Evidence for a Dynamic Jahn-Teller Effect in Triplet Ceot

Gerhard L. Gloss,*** Pennathur Gautam,*+s Daisy Zhang,*

Department of Chemistry, The University of Chicago, Chicago, Illinois 60637

Paul J. Krusic,* Steven A. Hill, and Edel Wasserman*

Central Research and Development, E . I. du Pont de Nemours & Co., Wilmington, Delaware 19 (Received: April 7, 1992; In Final Form: May 4, 1992)

90-0328

UV irradiation of methylcyclohexane solutions of C, produces a very narrow, transient EPR absorption which is assigned to the first excited triplet state of Cm The line width of only 0.14 G, uncommon for motionally narrowed triplet EPR spectra, is attributed to a very rapid interchange of the magnetic axes by pseudorotation converting the degenerate Jahn-Teller states into each other. Time-resolved, direct-absorption EPR measurements with a time resolution of 0.5 ps support this conclusion. They indicate that in solution the triplet EPR absorption decays at rates comparable with those obtained by optical methods for )Cb0. A relaxation time TI of 8 ps was obtained from the oscillations observed in the early stages of the decay curve following laser excitation. This T, , and the line width in solution, require correlation times between and s, too short for rotation. Polarized, partially averaged powder triplet spectra were also observed in methylcyclohexane glasses at low temperatures. The pseudorotation proposal is supported by the distinctly different behavior of C70.

Among the many spectroscopic measurements made recently on CU and other fullerenes,l there has been the detection of the EPR spectra of the lowest triplet states of C, (3C,) and c70 (3C70) in rigid matrices at 5 K.2 The spectrum of 3C, had nonvanishing zero-field splitting parameters indicating the loss of spherical symmetry in the triplet state as is expected from the Jahn-Teller distortions in the excited states of C,.2c In this communication we wish to report the CW and time-resolved EPR spectra of the triplet states of CU and CT0 in liquid solution and in glasses at different temperatures.

When a degassed and saturated solution of C60 in methyl- cyclohexane is irradiated inside the cavity of an EPR spectrometer with a xenon arc lamp a t temperatures between 300 and 180 K, a very sharp (0.14 G) line is observed at g = 2.001 35 (Figure 1A). As shown in the inset, the line width does not change appreciably from room temperature to 200 K. Below 180 K the line begins to broaden and can no longer be detected in a con- ventional EPR experiment below 145 K. This signal decays rapidly

'du Pont Contribution No. 6222. 'Deceased, May 24, 1992. f Present address: Center for Fast Kinetics Research, University of Texas

at Austin, Austin, TX 78712.

0022-3654/92/2096-5228%03.00/0

when the light is extinguished and can be observed repeatedly without loss of intensity, indicating the absence of efficient photochemical changes. A possible candidate for the carrier of the spectrum is the lowest triplet state of Cm. Optical studies reported by Foote and collaborators3 and corroborated by others4 have determined lifetimes of 40 ps and longer for the triplet state. To obtain evidence that the EPR signal originates from the triplet state, time-resolved EPR experiments were carried out using the direct detection method with a time resolution of 0.5 ps.5 In these experiments the carrier of the EPR signal is generated by pulses from an excimer laser with a wavelength of 308 nm and width of 12 ns fwhm. The laser repetition rate is set at 80 Hz, and the magnetic field is swept a t 5 G/min. Using a boxcar integrator with a 100-ns gate width and a 5-ps delay between the laser pulse and the sampling gate, an absorption spectrum is obtained and is shown in Figure 1B. The line width and its gvalue are the same as in the steady-state experiment, assuring that the carrier is the same in the two different experiments. By changing the delay between the laser pulse and the sampling gate, it is possible to obtain the decay kinetics of the signal. They are displayed in Figure 2 and show complex behavior a t short times and an ex- ponential decay after 20 ps. The time evolution of the EPR signal can be simulated reasonably well by solving the Bloch equations to which a damping term has been added to account for the slower

0 1992 American Chemical Society I ,

Letters The Journal of Physical Chemistry, Vol. 96, NO. 13, 1992 5229

TABLE I: Lifetime T (in ps) of C, Triplet State as a Function of Temperature and Laser Power temperature, K

181 193 203 213 228 243 253 258 273 293 EPR 41 24

opticalb 125 1 1 1 105 91 80 71 59 50 40 opticalC 125 1 1 1 105 95 91 88 80 71 61

opticala 62 56 50 41 20 10

a Flash photolysis method with laser power at 20 mJ/pulse. Flash photolysis method with laser power at 2.6 mJ/pulse. Flash photolysis method with laser power at 0.7 mJ/pulse.

0 t +

Figure 3. Direct detection EPR signals measured 1 ps after the laser flash for Cm in methylcyclohexane glass at the temperatures indicated. Total sweep width is 400 G.

Figure 1. (A) Field-modulated CW EPR signal of Cm in methylcyclo- hexane irradiated with a xenon lamp. (B) Direct detection signal 5 ps after the laser flash (308 nm) of the same solution used in (A). Sweep width in both experiments is 10 G. Inset: line width as a function of temperature.

I i - 2600 7 .

important at lower triplet concentration and with the slower diffusion rates at lower temperatures. Unfortunately, the mini- mum laser power required in the EPR experiments for acceptable signal-to-noise ratios is higher than that in flash photolysis. Accepting this fact, there is reasonable agreement between the two types of experiments.

To make the connection with the reported EPR spectrum of 3C60, the sample was cooled to the glass transition point of me- thylcyclohexane and below. As can be seen in Figure 3, new features begin to appear that spread out as the temperature is lowered. Qualitatively, these changes are understandable if axis averaging motions are slowed down to frequencies approaching the width of the powder spectrum of immobilized 3C60 (244 G,” 4 X lo9 rad/s). It should be noted that the spectra are strongly polarized as observed previously.” Only at 8 K, the spectrum becomes a broadened version of the reported spectrum with similar zero-field splitting parameters.

With the evidence presented above, there can be little doubt that the carrier of the spectra at all temperatures is 3C60. In the liquid phase the dipolar interactions are averaged by very rapid motion. It appears, but it is not certain, that the signal a short time after laser excitation contains some positive spin polarization.

The observation of a single sharp line in the EPR spectrum of a photoexcited triplet state is rare. The few cases where sharp spectra for triplets in liquids have been observed previously are characterized by very small zero-field splittings. The requirements for observing narrow spectra in solution are (1) small zero-field splittings and (2) fast orientational averaging of the magnetic axes. This averaging can be accomplished by fast rotation of the molecule or by internal interchange of the magnetic axes by pseudorotation associated with interconverting degenerate Jahn- Teller states. The fit of the fast decay pattern of the solution spectrum requires a T I relaxation time of 8 MS. This, together with the experimentally measured zero-field splitting parameters, yields a correlation time T for motional averaging of no longer than lo-” s as calculated by the expression for the dipoldipole

3 1400 c

1100 1 -

time (us) Figure 2. Decay of the direct detection signal as function of time. Solid line is the best fit for the experimental data to the equation y = Ae-&‘ + Be-‘/Tl sin (?HI - 8) where A = 2916, B = 1000, I/& = 41 ps, TI = 8 ps, yHI = 9.0 X lo5 radls, and 8 = T .

decay of the carrier.6 The fit is shown in Figure 2 and yields a TI of 8 MS and 41 ps for the slow decay constant. The oscillation frequency and amplitude are power dependent as expected.

For a comparison with the optical measurements, the flash photolysis experiments of Foote and *workers were repeated with a different and more favorable wavelength (745 nm) for obser- vation of the triplet-triplet absorption. The time constants ob- tained by flash photolysis at different temperatures are shown together with the slow decay constants obtained by EPR in Table I. As reported by others,4b it was found that the lifetime is strongly dependent on laser power and on the temperature. This is best explained by triplet-triplet annihilation that becomes less

5230 The Journal of Physical Chemistry, Vol. 96, No. 13, 1992

100 K

Letters

and 3C70 requires certain combinations of polarization axes and signs of D. They are consistent with either a Z-axis polarization and a positive D or X,Y-axes polarization with a negative D." Since the C70 molecule in its ground state is known to have a prolate shape, it is reasonable to assume that its triplet state will also have a prolate spin distribution resulting in a negative D value if the spin density is uniformly distributed over the surface. This in turn requires that the triplet state is X,Y polarized to interpret the spectrum. Polarization in the X,Y plane is the common type for *-systems. Although not required, it appears reasonable to assume that the anisotropy of the spin-orbit coupling in both molecules is qualitatively the same and that therefore the Jahn- Teller states of Cm are also represented by a prolate spin system. It is easy to see how that distortion can move around the molecule in a nearly activationless fashion so as to locate at one time or another any of the 60 carbon atoms at the apexes of the pseu- doellipsoid. A correlation time shorter than s for this process does not seem unreasonable. In the frozen viscous matrix, friction will slow down this geometry change until its frequency becomes smaller than the spectral width of the anisotropic spectrum (<lo9 SI). In this connection it is interesting to note that much lower temperatures are needed to observe the fully extended powder spectrum of 3Cm than for 3C70, fully in accord with two different averaging mechanisms for the two molecules.

At the lowest temperature (8 K) TI as measured by the dis- appearance of the spectrum is approximately 5 p . I 2 This ex- tremely short relaxation time is an indication that even in the glass at low temperatures there is still significant pseudorotation. The correlation time 7 required by eq 1 for this motion is between lo+' and lo-* s, sufficiently long to observe the dipole-dipole splitting but causing the relaxation time to be short compared to that of other aromatic triplet states a t that temperature.

An unresolved mystery at this time is the absence of 13C sat- ellites. A simple estimate of the magnitude of the expected splittingsI3 shows that it should be well within the resolution of the experiment. Either the 13C relaxation rate is unexpectedly high or some other averaging mechanism must be a t work.

Note Added in Proof. (a) I3C enrichment of Cm (-18%) produced only a broadening of the solution CW spectrum of 3Cm (0.28 G vs 0.16 G at 200 K). We are grateful to Dr. C. S. Yannoni of IBM Research Division for a sample. (b) Professor K.-P. Dinse (Darmstadt, Germany) kindly provided us with a preprint dealing with the Fourier-transform EPR of the same photogenerated transient absorption discussed here and containing some differences in the interpretation.14

Acknowledgment. The work in Chicago was supported by NSF Grant CHE-902 1-487,

References and Notes

The polarization of the low-temperature EPR spectra of

(1) Kroto, H. W.; Allaf, A. W.; Balm, S. P. Chem. Rev. 1991, 91, 1213. (2) (a) Wasielewski, M. R.; O'Neil, M. P.; Lykke, K. R.; Pellin, M. J.;

Gruen, D. M. J. Am. Chem. SOC. 1991, 113, 2774-2116. (b) Lane, P. A,; Swanson, I. S.; Ni, Q. X.; Shinar, J.; Engel, J. P.; Barton, T. J.; Jones, L. Phys. Reu. Lett. 1992,68, 887. (c) Negri, F.; Orlandi, G.; Zerbetto, F. Chem. Phys. Lett. 1988, 144, 31.

(3) Arbogast, J. W.; Darmanyan, A. P.; Foote, C. S.; Rubin, Y.; Diederich, F. N.; Alvarez, M. M.; Anz, S. J.; Whetten, R. L. J. Phys. Chem. 1991, 95, 11-12.

(4) (a) Ebbesen, T. W.; Tanigaki, K.; Kuroshima, S. Chem. Phys. Lett. 1991, 181, Sol-504. (b) Kajii, Y.; Nakagawa, T.; Suzuki, S.; Achiba, Y.; Obi, K.; Shibuya, K. Chem. Phys. Lett. 1991, 181, 100-104. (c) Hung, R. R.; Grabowski, J. J. J. Phys. Chem. 1991,95,6073-6075. (d) Kroll, G. H.; Benning, P. J.; Chen, Y.; Ohno, T. R.; Weaver, J. H.; Chibante, L. P. F.; Smalley, R. E. Chem. Phys. Lett. 1991, 181, 112-116.

( 5 ) For the description of the apparatus, see: Closs, G. L.; Forbes, M. D. E. J. Phys. Chem. 1991, 95, 1924-1933.

(6) For a similar treatment of time-resolved EPR data, see: Verma, N. C.; Fessenden, R. W. J. Chem. Phys. 1973, 58, 2501.

(7) Carrington, A.; McLachlan, A. D. Introduction to Magnetic Reso- nance; Harper & Row: New York, 1967; p 201.

(8) Equations 1 and 2 are strictly valid only for systems that can be defined by a spin temperature. Since this condition is not met in our experiments, they give only approximate estimates for the relaxation times. However, for small polarizations the errors are estimated to be small.

(9) The rotational correlation time of ground-state Cm at rmm tempera- ture has been determined in tetrachloroethane to be - lo-'' s: Johnson, R. D.; Bethune, D. S.; Yannoni, C. S. Acc. Chem. Res. 1992, 25, 169.

145 K

3 1 4 0 160 1 8 0

T (K)

Figure 4. CW EPR spectra obtained upon UV irradiation of a me- thylcyclohexane solution of C70 at the indicated temperatures. The sharp line at the center of the spectrum a t 110 K may be originating from freely rotating molecules in cavities of the matrix. Spectral integration indicates that it represents -3% of the total intensity. Inset: the line width of the motionally narrowed spectrum of T7,, as a function of the temperature above 140 K.

induced relaxation rate (eqs 1 and 2).798 Even the observed line

1/Tl (2/15)*D27((1 + U2T2)-l + 4(1 + 4WZT2)-') (1)

*D = (D2 + 3E2)lI2 (rad/s), w = spectrometer frequency (rad/s)

1/T2 = (1/15)*D27{3 + 5(1 + w%~)-I + 2(1 + 4 ~ ~ 7 ~ ) - ' ) (2)

for T << w : T I = T2 width at room temperature requires a minimum value of Tz of 810 ns, corresponding to a correlation time of less than 10-l2 s (eq 2). These short correlation times cannot reasonably be at- tributed to molecular rotation in methylcyclohexane a t -70 O C 9

This leaves pseudorotation as the only other attractive explanation of the short correlation times.

The proposal of pseudorotation as the averaging mechanism for 3C60 is strongly supported by results obtained with solutions of C7,, UV irradiated in a conventional EPR experiment. Con- sidering that the ground state of C70 has the shape of a rugby ball or dumbbell,I0 it is clear that no pseudorotation is possible and that the orientation of the magnetic axes relative to the external magnetic field can only be switched by real rotation. Figure 4 shows the spectra obtained at four different temperatures for methylcyclohexane solution of C7@ At the lowest temperature, rotation has ceased and the zero-field splitting parameters agree with those reported previously.2 As the temperature is raised, the features of the triplet powder spectrum approach the center, indicating some motional averaging of the dipoldipole interaction as the glass softens. At 145 K this averaging is sufficiently rapid to produce a single absorption line of 8.8 G width and g = 2.001 85. The line continues to narrow as the temperature increases (Figure 4, inset) while losing intensity until it can no longer be observed above 180 K. The smallest observable width is 6.6 G compared with 0.14 G for 3C60. Also, the sharp line of 3C60 is much easier to power saturate, indicating a longer T I for )C6,, than for 3c70. These differences are all the more impressive as 3C60 has a D more than double that of 3c70. Evidently, the averaging of the di- pole-dipole interaction is much more effective in 3C60 than in 3C70, supporting the pseudorotation mechanism for the former.

J . Phys. Chem. 1992, 96, 5231-5234 5231

rate constant for the latter process is much smaller (Table I) so that the disappearance of the spectrum is determined almost entirely by T I .

(13) For a uniform spin distribution and pure p-orbitals the estimated hyperfine coupling is 11 5/60 = 1.9 MHz = 0.68 G. This is a minimum value because of the curved surface the orbitals must contain some s-character.

(14) Riibsam, M.; Dinse, K.-P.; Pliischau, M.; Fink, J.; Kratschmer, W.; Fostiropoulos, K.; Taliani, C. Submitted for publication.

(10) Mckenzie, D. R.; Davis, C. A.; Cockayne, D. J. H.; Muller, D. A.; Vassallo, A. M. Nature 1991, 355, 622-624.

(11) Thurnauer, M. C.; Katz, J. J.; Norris, J. R. Proc. Narl. Acad. Sci. U.S.A. 1975, 72, 3270.

(12) Since the spectrum is polarized to the extent that the integral over the entire spectrum is practically zero, the disappearance of the individual parts of the spectrum are governed by TI and the decay of the carrier. The

Production and Characterization of Metallofullerenes Mark M. ROSS,* H. H. Nelson, John H. Callahan, and Stephen W. McElvany Code 61 lO/Chemistry Division, Naval Research Laboratory, Washington, D.C. 20375-5000 (Received: April 8, 1992; In Final Form: May 12, 1992)

Negative ion/desorption chemical ionization mass spectrometry was used to characterize fullerenes with an encapsulated metal atom@), metallofullerenes (M,@C,), in arc-generated soot, pyridine extracts, and the extract residue. In agreement with results from other laboratories, the pyridine extracts of La203/graphite soot contain mostly La@Cg2 and La2@CCB0, in addition to the “pure” (empty) fullerenes. However, the raw soot and the extract residue contain a broader range of metallofullerenes with relative abundances different from those observed from the extract (e.g., abundant La@Cm, La@CTO, and L ~ @ J C , ~ ) . Arcing mixed-metal oxide-impregnated graphite rods yielded a mixed-metal dimetallofullerene (YLa@Cgo) and higher relative abundances of metallofullerenes with the lower ionization potential metal atom. The thermal desorption behavior and solubility in different solvents of the doped and undoped fullerenes indicate an interaction between the C, and La,@C, species. Finally, analysis of aqueous solutions of dried pyridine extracts of lanthana/graphite soot shows C, and La,@C,, which is consistent with the possible presence of metallofullerene/fullerene ionic complexes, (La,@C,)+C,,-.

Introduction One of the many interesting directions of fullerene research

involves encapsulation of an atom(s) inside the fullerene cage to form metal/fullerene endohedral complexes, or metallofullerenes. This idea dates back to the original experiments by Smalley, Kroto, and co-workers’ in which laser photodissociation (“shrinkwrapping”) studies of metal-carbon cluster adducts provided strong evidence for the postulated cage fullerene structures. The availability of large quantities of fullerenes2 and improved fullerene production methods3 have advanced this field significantly. In addition to representing another new and un- precedented set of chemical species, the metallofullerenes have given rise to a unique chemical nomenclature, M@C,, where M is a metal atom (or M, for multiple atoms) encapsulated in a fullerene consisting of n carbon atoms.

Several groups have used different methods to generate soot from metal compound-impregnated graphite rods and have shown that the soot contains metallofullerenes as well as fullerenes. For example, Smalley and co-workers4 have shown that La- and Y-encapsulated fullerenes (La,@C,, Y,@C,), in addition to fullerenes (C,), can be produced by laser vaporization of graphite rods impregnated with the appropriate metal oxide. Laser de- sorption mass spectrometric (LD/MS) analysis of a film of sublimed material from the soot generated by laser vaporization of a La203/graphite rod showed mostly La@C60, La@C7,,

and h@c82 while extraction with toluene yielded mostly La@C,,. Similar results were obtained from studies of yttrium- fullerene complexes, with the additional observation of an anom- alous abundance of Y2@cg2 in the sublimed film.5 Johnson et a1.6 generated a large quantity of La@Cg2 and used electron paramagnetic resonance (EPR) spectroscopy to show that La has a 3+ formal charge and the CS2 a charge of 3-. Scandium-, yttrium-, and lanthanum-encapsulated fullerenes were also pro- duced, extracted, and characterized by Shinohara and c o - ~ o r k e n , ~ who noted that pyridine and carbon disulfide are much better solvents than toluene for the metallofullerenes. In agreement with the previous studies, some metallofullerenes such as Y@Cm and Y@C70 were not observed in the solvent extracts. A dilanthanum fullerene, La2@Cm was observed first by Whetten and co-workers8 to be very abundant in the laser desorption mass spectrum of the toluene extract of soot generated using the Kratschmer-Huffman method. This particular dimetallofullerene had not been observed using similar soot production and extraction methods, and it was speculated to be dependent on the amount of La203 loaded into

the graphite rod. Until recently, our work has focused on the mechanism of metallofullerene formation by laser vaporization in vacuum. In these studies evidence was obtained for laser-in- duced coalescence reactions between fullerenes and metal oxide compounds to form a wide size distribution of mono- and di- metallof~llerenes.~ These investigations showed that laser de- sorption/vaporization can produce species that are not initially present in the sample.

Despite the significant effort and achievements several questions remain. For instance, it is not clear why certain stoichiometries of M,@C, are more abundant than others and, specifically, why particular dimetallofullerenes (e.g., La.@,, and Y2@ CE2) are not uniformly observed. The dramatic differences in the abun- dances of the metallofullerenes observed in the soot extracts versus those in a film sublimed from the soot are not understood, although they are likely due to differences in solubility or reactivity. The above and other experimental observations have led to speculations that the metallofullerenes could exist as ionic complexes, possibly with “pure” (undoped) fullerenes.5J0 The chemical nature of these unusual species must be determined before any potential materials application can be approached. We report here some new insights obtained from characterization of metallofullerenes from soot extracts and directly from the soot using desorption chemical ionization mass spectrometry.

Experimental Methods Soot containing fullerenes and metallofullerenes was generated

by arcing metal oxide-impregnated graphite rods using methods that have been described in detail.3q4.8 In brief, 0.25-in.-diameter graphite rods (Wale Apparatus Co.) were drilled out (0.17- in.-diameter by 2 in. deep) and fiued with a mixture of metal oxide, La203 (Fisher Scientific, 99%), Y203 (Alfa Products, 99.99%), Sc203 (Reacton, 99.99%) or a 1/1 mixture (by weight), and graphite cement (GC-HS, Dylon Industries). The resulting rod was approximately 20% metal oxide by weight. Before use, the cement was cured at 140 OC for 4 h and the rods were degassed by heating to 1000 OC under vacuum overnight. Two metal oxide-impregnated rods were used as the two electrodes in a “Smalley-type” water-wold contact arc r e a ~ t o r . ~ The resultant soot was collected and extracted for 2-8 h in pyridine using a Soxhlet extraction apparatus.

All analyses of the soot, soot extracts, and extract residues were performed using negative ion/desorption chemical ionization mass spectrometry (NI/DCIMS) with a triple quadrupole mass

This article not subject to U S . Copyright. Published 1992 by the American Chemical Society