© Copyright 1996 by the American Chemical Society
VOLUME 100, NUMBER 22, MAY 30, 1996
LETTERS Enrichment and Characterization of a Noble Gas Fullerene: Ar@C60 Barbara A. DiCamillo,†,‡ Robert L. Hettich,† Georges Guiochon,†,§ Robert N. Compton,*,†,§ Martin Saunders,⊥ Hugo A. Jime´ nez-Va´ zquez,⊥ Anthony Khong,⊥ and R. James Cross⊥ Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6125; UniVersity of California, Los Angeles, Department of Chemistry, Los Angeles, California 90024; UniVersity of Tennessee, Department of Chemistry, KnoxVille, Tennessee 37996; and Yale UniVersity, Department of Chemistry, New HaVen, Connecticut 06520 ReceiVed: January 2, 1996; In Final Form: April 17, 1996X
Ar@C60, synthesized by heating primarily C60 (with trace amounts of C70) under a high pressure of argon gas, was purified by high-performance liquid chromatography (HPLC) and examined by Fourier transform mass spectrometry (FTMS). The sample, which consisted of 0.1% Ar@C60, was dissolved in toluene and separated with HPLC using a COSMOSIL 5PYE column as the stationary phase and toluene as the eluent (UV detection). Ar@C60 was enriched by a factor of about 400 relative to the starting material. Ar@C60 was also examined by X-ray photoelectron spectroscopy (XPS) but was found to be sensitive to radiation damage and subsequent depletion. Low-energy collisional dissociation of Ar@C60+ in the FTMS yields primarily C58+, generated by loss of the argon atom and C2. The ionization potential of Ar@C60 was bracketed by charge-exchange reactions and was found to be between 7.53 and 7.8 eV, which is indistinguishable from that of C60 (7.65 eV).
Introduction Fullerene molecules are capable of encapsulating atoms inside the hollow carbon cage through endohedral chemical bonding or physical trapping. The observation1 of “shrink-wrapping” La@C60 to the point of cage disruption played an important role in the early proof of the carbon cage structure of fullerenes. Rare-gas atoms can also be physically trapped within the carbon cage or even bound into a negative energy state as a result of the van der Waals interaction. The strength of the van der Waals attraction depends upon how well the rare-gas atom “fits” into the electron-deficient cavity of the carbon cage. Figure 1 illustrates this point by showing the 6-12 Lennard-Jones model of exohedral and endohedral van der Waals attraction of the †
Oak Ridge National Laboratory. University of California. § University of Tennessee. ⊥ Yale University. X Abstract published in AdVance ACS Abstracts, May 15, 1996. ‡
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rare gases within C602 and C84. One notes that endohedral bonding for Xe@C60 is approximately thermoneutral in Figure 1a, but Xe@C84 is bound by approximately 0.7 eV in Figure 1b. Other calculations indicate that Xe@C60 is unbound (positive energy site).3 The exohedral van der Waals attraction is about the same for the two cases. The prospect of studying the properties of rare-gas atoms trapped inside such microcavities has generated considerable excitement in the fullerene community. The preparation and purification of chemically bound endohedral species such as La@C82, Ym@C82, Scm@Cn, and Gd@C82 have been carried out in a number of laboratories.4-7 The isolation of milligram quantities of the metal endohedral fullerenes has permitted the study of many unique properties of these species. Rare-gas endohedral fullerenes were first observed8 in collisions between fast fullerene cations and stationary rare-gas atoms leading to rare-gas Cn cations. The preparation of rare-gas endohedral fullerenes began with the © 1996 American Chemical Society
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observation of Saunders and Cross,9 who showed that soot generated in the standard carbon arc discharge method using He buffer gas results in He@Cn at the part per million level. It was postulated that the He atom enters an open “window” in the carbon cage as a result of the high-temperature environment of the arc discharge. Later, this group succeeded in the incorporation of He, Ne, Ar, Kr, and Xe into the fullerenes using high-pressure and high-temperature conditions.10 This “brute force” method has yielded rare-gas endohedral fullerenes at the level of 0.03-0.1% relative to empty fullerenes. Despite the paucity of these materials, the Yale group has employed 3He NMR to clearly identify structural isomers of the fullerenes.11 In addition, the 3He NMR method has been demonstrated to provide a powerful new tool for the study of the functionalizational fullerene chemistry.12 Further progress in the examination of endohedral rare-gas fullerenes would be greatly enhanced by isolation from their parent empty fullerenes. A preliminary account of a partial separation of krypton-containing fullerenes on an alumina column has appeared.13 Inspection of Figure 1 along with simple calculations of the van der Waals size for endohedral fullerenes suggests that the inner π-electron distribution would be slightly pushed outward for Ar@C60. This observation along with the increased mass indicates that HPLC might be used to separate endohedral fullerenes. Herein we report the significant enrichment of Ar@C60 from a mixture with C60 using HPLC. Experimental Section Samples of endohedral Ar@C60 were generated using the method of exposing C60 to argon at high pressure (15 000 psi and above) and high temperature (600 -620 °C) for several hours. (See ref 10 for general details.) Figure 2a shows the positive ion laser desorption mass spectrum of the starting material with an estimated 0.1% Ar@C60 concentration. Ejection of the C60+ peak was necessary before Ar@C60+ was observable, as shown in the insert of Figure 2a. X-ray photoelectron spectroscopy (XPS) was performed on a Perkin-Elmer PHI Model 5602-LS (5602 refers to the twoconsole version of the 5600 model and LS stands for large sample), using the Al KR X-ray source (1486.6 eV). The starting material (500 µL; concentration of about 2 mg/ mL) was injected into the HPLC system (Hewlett-Packard HP 1090 liquid chromatograph with diode array detector and autoinjection system) in 5 µL increments. A 2-(1-pyrenyl)ethylsilylated silica column (COSMOSIL 5PYE, 4.6 mm i.d., 250-mm length, Nacalai Tesque Co.) served as the stationary phase and toluene (flow rate 1 mL/min) as the mobile phase. HPLC detection of the eluting material was accomplished using UV absorption at 330 nm as a function of elution time. Fractions were collected at 0.4 min intervals starting briefly before the elution time for C60 (5.4 min). These fractions were collected with a Gibson Fraction collector and analyzed with FTMS to compare their Ar@C60 content. All mass spectra were obtained with a Finnigan-FTMS Fourier transform mass spectrometer (3 T magnet) equipped with a Spectra Physics DCR-11 Nd:YAG laser.14 A small amount of sample (few micrograms) was placed onto a stainless steel solids probe, which was then inserted into the vacuum chamber of the FTMS. The third harmonic of the Nd:YAG laser (355 nm) was focused onto this probe tip (∼106 W/cm2) to simultaneously desorb and ionize the sample. The laserdesorbed ions were trapped in the FTMS cell for a few milliseconds, excited into coherent cyclotron motion, and detected under medium resolution conditions. A complete mass spectrum (m/z 25-4000) can be obtained from a single laser
Figure 1. Translational potential energy for R@C60 (a, top) and R@C84 (b, bottom) as a noble-gas atom approaches C60 and C84, respectively. Note that argon has the greatest well depth in the case of C60, and xenon is very stable in C84. Also notice the smaller atoms (He and Ne) are most stable slightly displaced from the center of C84. The calculation for R@C60 and R@C84 were performed using the method and potential given by Pang and Brisse.2
pulse, although several spectra were usually signal-averaged in order to provide higher quality mass spectra. In some cases, the abundant C60+ ion was ejected from the FTMS cell prior to ion detection to enhance detection of minor components such as Ar@C60+. Collisional dissociation experiments were performed by isolating an ion of interest and then accelerating that ion via sustained off-resonance irradiation into argon as a collision gas.15 The center-of-mass energy for the collisions between Ar@C60+ and argon gas was varied between 0 and 10
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Figure 2. Positive ion laser desorption FTMS spectra of the initial material (Figure 2a) and Ar@C60-enriched material (Figure 2b) after passing through the HPLC twice. Although the initial material appears to contain primarily C60, the inset in Figure 2a illustrates that Ar@C60+ can be seen upon ejecting both C60 and C70.
eV. Under these conditions, multiple collisions occur prior to ion dissociation. Thus, internal energy is added by small discrete steps, in contrast to a high-energy collision where dissociation is induced by a single collision. Charge-exchange bracketing reactions were studied by isolating an ion of interest and then trapping that ion in the presence of a reagent gas for a period of time to determine whether charge transfer occurs. By careful choice of the reagent gas, it is possible to observe whether or not electron-transfer reactions occur with the ion of interest and thus “bracket” its ionization potential or electron affinity. Results and Discussion The concentration of Ar@C60 relative to other fullerenes was monitored using laser desorption Fourier transform mass spectroscopy (LD-FTMS). The Ar@C60+ peak could be observed in the original unseparated sample only by ejecting the dominant C60+ ion, as shown in the insert of Figure 2a. The other fullerenes present in the spectra in the region from C56 to C66 are generated by laser-induced fragmentation of C60 as well as C70 impurities in the sample. The estimated concentration of Ar@C60 relative to that of C60 is about 0.1% and is consistent with previous estimates for this method of producing Ar@C60. X-ray photoelectron spectroscopy (XPS) of this sample also provided information on Ar@C60. Figure 3b shows the photoelectron signals resulting from ejection from the 2p3/2 and 2p1/2 orbitals of Ar from Ar@C60 in the original sample. XPS of argon inserted into graphite by ion implantation is shown in Figure 3a for comparison. The signal due to Ar@C60 in both the original and separated sample was seen to slowly decrease after exposure to the Al KR (1486.6 eV) X-rays. In fact, exposure of the enriched sample to the X-rays resulted in total
Figure 3. XPS spectra illustrating the presence of argon in the sample. Figure 3a illustrates the Ar 2p multiplet of graphite impregnated by Ar, and Figure 3b indicates that Ar is also present in the sample.
depletion of the Ar@C60 as determined by FTMS. This degradation was so severe that an adequate XPS spectrum could not be accumulated for this limited sample. The only ion peak observed after irradiation was C60+ in the FTMS. The HPLC chromatographs are shown in Figure 4. Injection of the original sample into the HPLC resulted in a chromatogram consisting primarily of one major peak at tr ) 5.4 min, as shown in the insert of Figure 4a. By expansion of the Y axis, the appearance of other minor peaks could be observed, shown in the main portion of Figure 1a, which are thought to be trace levels of other fullerenes in this C60 sample. Twelve fractions, each one with a width of 0.4 min, were collected from this chromatogram in the tr ) 5.3-10.1 min. window. Visual inspection of these fractions provided some basic information. Fractions 1 and 2 (tr ) 5.3-6.1 min.) were intensely purplecolored, indicating that these samples consist primarily of C60, whereas fractions 8 and 9 (tr ) 8.1-8.9 min) were reddishorange colored, indicating that presence of C70, which is known to be in the sample at low concentrations. Laser-desorption FTMS was used to examine each of these fractions. Two types of mass spectra were acquired for each fraction: one experiment in which the entire mass range of m/z 25-3500 was examined, and a second experiment in which the major components (i.e., C60 and/or C70) were ejected prior to ion detection to search for trace components such as Ar@C60. Inspection of these 12 fractions verified the presence of C60, C70, and other fullerenes and indicated that while the amount of Ar@C60 observed was very low in any sample, this component was only observed in the fractions with tr ) 5.76.1 min (which is slightly displaced from the C60 peak at tr )
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Figure 4. HPLC chromatograph of sequential injections of the sample. Figure 4a is a chromatograph of the starting material dissolved in toluene and injected into the HPLC. Figure 4b is a chromatograph of the fraction eluting at 5.7-6.1 min from the first injection. Figure 4c is a chromatograph of the fraction eluting at 5.7-6.1 min from the second injection. The fraction containing the greatest concentration of Ar@C60 is that collected after the second injection (Figure 4b), and reinjection shows it is comprised of the peaks illustrated in Figure 4c.
5.4 min). This evidence confirmed our original suspicions that the purification of Ar@C60 from C60 was going to be difficult, due to the need to collect a small HPLC peak that elutes directly after a large “tailing” component. Because of the relatively poor chromatographic separation observed for Ar@C60 and C60, the experimental approach chosen to accomplish purification of Ar@C60 consisted of sequential fraction collection and reinjection. On the basis of this mass spectrometry evidence outlined above, an HPLC fraction was collected for the range of tr ) 5.7-6.1 min, and was reinjected into the HPLC. The insert in Figure 4b shows the HPLC of this collected fraction. Note that while C60 is still observed, the absolute amount has been reduced by a factor of about 20. The appearance of C60 is not surprising since the tailing observed in Figure 4a reveals that there is still a substantial amount of C60 present at tr > 5.7 min. Six discrete HPLC fractions were collected from Figure 4b in the range 5.37.7 min, with each fraction 0.4 min in width. Fraction 1 contained primarily C60, with a small amount of Ar@C60 also present (this component could be observed only when the abundant C60 ions were ejected prior to ion detection). Fraction
Letters 2 (5.7-6.1 min) contained a relatively large amount of Ar@C60 comparable to C60, as shown in Figure 2b. Note that ion ejection techniques are not used in this case, and the Ar@C60 ion is approximately 40% of the C60 abundance. This is the first case in which we have been able to easily detect the Ar@C60 component without first ejecting the more abundant C60 component. This mass spectrum shows an enrichment of Ar@C60 in the HPLC fraction by a factor of about 400 relative to its concentration in the starting material. The peak areas in the FT mass spectrum suggest a 40:60 ratio of Ar@C60:C60; however, the Ar@C60 may in fact be underrepresented if the laser desorption process in the mass spectrometer destroys some of the Ar@C60 to yield C60. We further assume that the laser desorption FTMS detection efficiency for C60 and Ar@C60 is about the same, which is justified based on the indistinguishable ionization potentials, as discussed below. Fraction 3 is similar to fraction 1 (i.e., large amount of C60, some Ar@C60 present); however, the total ion signal is lower due to lower concentrations. Fractions 4-6 were observed to be devoid of Ar@C60. Fraction 5 contained low mass components (m/z < 400) which were not examined in detail. Collection of the 5.7-6.1 min fraction from Figure 4b and reinjection into the HPLC gave the chromatogram shown in Figure 4c. Note that the C60 has been reduced by another factor of 10 and that a new “shoulder” appears at tr ) 5.7 min. Fractions from this chromatogram were collected; however, the low concentrations of components in the fractions made it difficult to get definitive information by mass spectrometry. Thus, is was not possible to unambiguously determine which fraction contained the Ar@C60 component; however, we feel that the shoulder at tr ) 5.7 min is probably due to the enriched Ar@C60. Note that this component is just barely beginning to separate from C60 after three injections. The dissociation of Ar@C60+ was studied in two different experiments using FTMS. Photodissociation processes were studied by increasing the fluence of the photodesorption/ ionization laser (107-108 W/cm2) to induce fragmentation. To examine the Ar@C60+ ion (m/z 760), the C60+ ion must be ejected prior to ion detection, as stated above. Because of this requirement, it is impossible under these experimental conditions to determine whether C60+ is a laser-induced fragment of Ar@C60+. Figure 5a shows that laser-induced fragmentation of Ar@C60+ results in sequential loss of C2 to generate Ar@C58+ and Ar@C56+. Empty fullerenes at C58+ and C56+ are also observed and may arise either by sequential loss of (C2 + Ar) from the parent Ar@C60+ ion or may be generated by C2 losses from the C60+ before it is ejected from the FTMS cell. The second fragmentation method consisted of first isolating the Ar@C60+ ion followed by accelerating that ion into argon (3 × 10-6 Torr) as a collision gas for 100 ms (Ecom ) 4.6 eV). This low-energy multiple-collision process reveals C58+ as the primary fragment product (Figure 5b). The difficulty in efficiently fragmenting this species under these low-energy collisional dissociation conditions is evident from the low signal/ noise observed in Figure 5b. Note that C60+, Ar@C58+, and Ar@C56+ are not observed (a small amount of these species may be present and would not be resolvable from the noise). For comparison, high-energy, single-collision experiments on a sector instrument reveal sequential C2 and Ar losses.16 Inspection of Figure 5 indicates that the laser-induced fragmentation is different than the collisional dissociation spectra. This is not too surprising, when one considers the manner in which the internal energy is added to Ar@Cn+ ion. The laser-induced fragmentation is a multiphoton absorption process in which the energy is added via electronic excitation followed by internal
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J. Phys. Chem., Vol. 100, No. 22, 1996 9201 IP of C60 is 7.65 eV,18 we conclude that the inclusion of an endohedral argon atom inside the C60 cage does not dramatically alter the ionization potential of the fullerene. This result is quite different from the bracketing results for La@C60, which shows that inclusion of a La atom inside C60 substantially decreases the IP from 7.65 eV for C60 to 6.3 ( 0.3 eV for
[email protected] A
Acknowledgment. Research at ORNL was sponsored in part by the Office of Naval Research through the Molecular Design Institute involving ORNL and Georgia Institute of Technology. Acknowledgment is also given to Dr. Changming Jin for assistance with the charge-exchange bracketing reactions and A. Lynn Glover for assistance with the XPS experiments. The Yale group acknowledges support from the National Science Foundation (CHE-9412768).
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References and Notes
Figure 5. FT mass spectra illustrating fragmentation of Ar@C60+ achieved with either (a) high fluence laser desorption or (b) collisional dissociation.
conversion into rotations and vibrations. For the FTMS collisional activation process, the energy is added via multiple low-energy collisions until a threshold for dissociation is achieved. This process of slowly adding energy in discrete steps allows substantial time for energy randomization and ion rearrangement to occur prior to fragmentation. The high-energy, single-collision dissociation resembles that of the laser-induced fragmentation since energy is added in one large step, rather than small, discrete steps. The ionization potential for Ar@C60 was determined by the following charge-exchange bracketing reaction:
Ar@C60+ + A f Ar@C60 + A+ Experimentally, this is accomplished by trapping Ar@C60+ in the presence of a neutral reagent gas A. Charge transfer is evidenced by a decrease in the abundance of Ar@C60+ and a concomitant increase in A+. By using various reagents with well-defined ionization potentials, this method allows the IP of Ar@C60 to be “bracketed”. Ar@C60+ was observed to charge exchange with 1,4-dimethoxybenzene (IP ) 7.53 eV) but is unreactive with 1-methylnaphthalene (IP ) 7.8 eV17). These results imply that 7.53 eV < Ar@C60 < 7.8 eV. Because the
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