The Journal of
Physical Chemistry
0 Copyright I995 by the American Chemical Society
VOLUME 99, NUMBER 41, OCTOBER 12,1995
LETTERS Electrospray and Laser Desorption Ionization Studies of
C60O
and Isomers of C~OOZ
Jin-Pei Deng and Chung-Yuan Mou* Department of Chemistry, National Taiwan Univeristy, 1, Sec. 4, Roosevelt Rd., Taipei, Taiwan, R.O.C.
Chau-Chung Han Institute of Atomic and Molecular Sciences, Academia Sinica, P.O.Box 23-166, Taipei, Taiwan, R.O.C. Received: June 23, 1995; In Final Form: August 11, 1995@
The reaction of Cm with ozone in toluene results in the formation of Cm oxides. CmO and two isomers of Cm02 were isolated; they both have the 423 nm absorption peak characteristic of an epoxide structure between adjacent 6,6-rings in c60. Intact molecular ions are generated by electrospray ionization (ESI) without fragmentation. The enhanced ESI sensitivity of the higher oxides reflects the increase in electron affinity upon oxidation. In contrast to ESI, odd-numbered fragments are observed in laser desorption ionization (LDI) of CmO. The two isomers of (26002 show different intensity distributions of the same fragment ions in LDI, suggesting different distances between the two oxygen atoms in them. Mechanisms are proposed for the formation of the two isomeric forms of (26002 and for mass spectrometric fragmentation processes.
Introduction Macroscopic quantities of C a have become available since the advent of the electric arc method in 1990.' Beside fullerenes, Cm oxides are also present in the soot originating from oxygen contamination. The presence of CmOn,n = 1-5, in the soot was first detected in a mass spectrometric study.* The reaction of oxygen with Cm anion first results in the production of C600n, n = 1-4, and prolonged electrolytic reduction in the presence of oxygen and water yields C50 products at the loss of a Cto fragment.3 Alternatively, CmO can be synthesized by W-induced oxidation of C M . ~The reaction of Cm with dimethyldioxirane also gives CmO as a minor p r o d ~ c t . ~Here, we report our isolation and mass spectrometric study of CmO and two Cm02 isomers obtained in the reaction of Cm with ozone.6
Experimental Section Reaction with Ozone. Cm used in this study was purchased from the Texas Fullerene Corp. (purity 99.9%) without further @
Abstract published in Advance ACS Abstracts, October 1, 1995.
0022-365419512099-14907$09.0010
treatment. Ozone was generated from a discharge generator with a 5% 0 2 (balanced with Ar) gas mixture and slowly bubbled through the room-temperature toluene solution in which Cw dissolved. Under our reaction conditions, the reaction rate of Cm with ozone is so fast that toluene can be considered as inert toward ozone. Reactions were monitored by reversedphase high-performance liquid chromatography (HPLC). The detection wavelength was 340 nm. Acetonitrile and toluene in 1: 1 ratio was used as the elute solvent. The 533 nm absorption peak of Cm was observed to undergo blue-shift over the entire course of the reaction. Laser Desorption Ionization Mass Spectrometry (LDIMS). Samples used in LDI-MS experiments were HPLCseparated components of the ozonolysis mixture. The analyte was applied to a stainless steel probe tip and air-dried before mass spectrometric analysis on a FT-ICR mass spectrometer. The mass spectra were recorded by using 266-nm irradiation (pulse width 5-8 ns). The pressure in the chamber was in the low Torr range and the magnetic field was 3 T. Electrospray Ionization Mass Spectrometry (ESI-MS). Samples used in ESI-MS (Fison Quattro Bio-Q) studies were
0 1995 American Chemical Society
Letters
14908 J. Phys. Chem., Vol. 99, No. 41, 1995 Caoz
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,
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Figure 1. Reversed-phase HPLC chromatograms taken at increasing reaction times.
treated as in LDI-MS experiments. The nebulization gas was dry nitrogen. The sample solution was eluted with toluene at flow rates of 5-20 puWmin, following the experimental conditions recently reported by Her et aL7
Results and Discussion In Figure 1, six HPLC chromatograms taken at increasing reaction times are shown, and five products are clearly discemible at retention times of 10.65, 7.73, 6.90,5.23, and 4.70 min, respectively. Dioxides became detectable in the reaction mixture at a conversion ratio, defined as [CmO]/[Cm], of approximately 0.3 under slow-bubbling conditions, and it took more reaction time to form the more polar products. In general, higher ozone bubbling rates resulted in the quick formation of brownish suspended particles and reduced the yields of soluble Cm oxides. According to spectroscopic data (see below), the product peak at 10.65 minutes is CmO, and the product peaks at 7.73 and 6.90 min are two isomers of Cm02. We designate the dioxide molecule having higher polarity isomer I and the other isomer 11. The following two product peaks at shorter elution times, corresponding to higher polarities, are CmO3. And the other smaller peaks at still shorter elution times are even higher oxides (C6004, C6005, and possibly CmO6). From the rate of change in the concentrations of the products, we estimate that the reactivity of each product toward ozone remained constant throughout the reaction. If the ozone generator is turned off in the middle of the reaction, while allowing the gas mixture to flow through the reaction flask, then no further increase in the concentrations of the products were observed for an extended period of time. Therefore, the reaction rate of c 6 0 with dioxygen in the absence of additional irradiation can be considered negligible, and we conclude that ozone is the only oxidizing agent in the reaction. The molecular weights of the reaction products are first determined by ESI-MS. For Cm, a very intense Cm- peak was observed without any sign of fragmentation. In contrast, the C60+ signal was much weaker and fragmentation persisted for all viable source conditions we tried. The difference can be ascribed to the mild source conditions used in the anion mode, which in tum, has to do with the high electron affinity of C a . For this reason, the ESI-MS experiments described below are limited to negative ion mode. Figure 2 is a typical negative ion mass spectrum of crudely separated reaction mixture containing samples eluted between 4.3 and 12 min. By comparing ion signal intensities with integrated HPLC peak
areas, the ESI-MS sensitivities of fullerene oxides are enhanced relative to that of Cm. This can be perceived by considering the effect of incorporating oxygen atoms in Cm. The electron affinities of the Cm oxides are expected to increase with the number of oxygen atoms incorporated, and thus the higher oxides compete more favorably for electrons. This effect levels off quickly with further incorporation of oxygen atoms. But we do not know whether electron-transfer reaction occurs at the spraying needle or in the gas phase. Generally speaking, the relative concentrations of the Cm oxides in the reaction mixture were more accurately determined by HPLC than by ESI-MS. As only intact negatively charged molecular ions are generated without fragmentation in electrospray ionization, ESIMS seems to be the method of choice for the determination of molecular weights of c 6 0 oxides. The molecular weights and chemical compositions of the six HPLC peaks labeled in Figure 1 were unambiguously determined by submitting each isolated components to ESI-MS analyses. The C60O sample we synthesized by ozonolysis reaction was identified by I3C NMR spectroscopy to have CzV symmetry as previously reported? Figure 3 shows typical LDIFTMS negative ion mass spectrum of CmO. In contrast to the ESI-MS results, the parent ion is barely observable in LDIFTMS. In addition to even-numbered fullerenes, mass peaks corresponding to C55-,C57-, C59-, and C580- can be clearly identified. In the epoxide form of CmO, the oxygen atom is loosely bound and will most likely be lost upon LDI leading to the even-numbered fullerene family Cm, c58,etc. On the other hand, cage shrinkage by emitting C2 fragments can also compete, perhaps less favorably, against losing the oxygen atom and CS8O is thus generated from C600. It is possible that bond reorganization induced by photon irradiation makes decarbonylation facile in C ~ O Oso, it gives C59 by CO extrusion, followed by subsequent C2 losses to give C57 and C55. Subsequent coalescence of C59 and c 6 0 may account for the formation of C119, first observed by McElvany et al.,8 via a reasonable mechanism proposed by Deng et ala6and Tay10r.~A recent report illustrated the formation of Cl19 by thermal decomposition of C~OO.~O The spectrum shown in Figure 3 is reminiscent of the results we reported earlier with a mixture of c 6 0 oxides6 In cation LDIFTMS studies, c 6 0 + was the most intense peak and only evennumbered fullerene fragment ions were observed. The two CmO2 isomers were identified by ESI-MS to have the same molecular weight. Figure 4 shows typical 266-nm LDI-FTMS negative ion mass spectra of Cm02 isomers taken under the same experimental conditions. In addition to evennumbered fullerenes (c60, c58, and c56), there appear c 5 9 0 2 , CmO, C58O2, C590, and c580. However, the dioxide molecular ions are not observed in our LDI experiments. It may stem from the instability of the parent molecular ion under our LDI conditions. C6002 can undergo sequential elimination of the oxygen atoms to give C600and Cm. Alternatively, loss of 0 2 may directly form c 6 0 . C5802 is formed upon the exclusion of a CZfragment from CmO2. As in CmO, CmO2 yields C590 by CO extrusion and c 5 9 0 would then give c58 in the same way. It is worth pointing out that the same fragments were observed for both isomers. The prominent difference is the relative intensities displayed in the fragments between mlz 720 and 730. While Cm is the base peak in Figure 4a, it shows up only as a minor peak in Figure 4b. On the other hand, C590 is apparently more prominent in Figure 4b than in Figure 4a. The observation of C5902 in the LDI of both C60O2 isomers indicates the opening of an unprecedented fragmentation channel that involves the loss of a bare carbon atom from the parent molecule, and this new channel is apparently induced by the
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Figure 2. ESI-MS negative ion mass spectrum of crudely separated reaction mixture. II
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Figure 3. Typical 266-nm LDI-FTMS negative ion mass spectrum of Cm isolated from the ozonolysis reaction.
simultaneous existence of two oxygens atoms via a mechanism that is not known at the present time. This subject deserves more in-depth studies, and we will look for this channel in continuing LDI-MS studies of higher oxides. The reaction of c 6 0 with ozone results in the formation of CmO which reacts further to produce CmO2 and so on. Detailed HPLC and mass spectrometric analyses suggest that there are only two dioxide isomers in the reaction solution and that both isomers are more polar than that of CmO, we thus conclude that these two oxygen atoms reside close to each other on the same side of Cm cage. CmO and both CmO2 isomers display the characteristic absorption peak near 423 nm (spectra not shown) of adducts formed across a double bond shared by adjacent 6,6-rings in C ~ OA . maximum of eight C60O2 isomers are expected from symmetry considerations. Why did we isolate just two isomers, instead of all eight? This unexpected result must have come from the effect of the oxygen atom in CmO. The inductive effect of the oxygen atom in CmO enhances the chemical reactivity of the double bonds nearby, and this effect decreases with increasing distance. If we further assume that this reactivity enhancement effect becomes negligible for x
760
"i Figure 4. Typical 266-nm LDI-FTMS negative ion mass spectra of Cm02: (a) isomer I and (b) isomer 11, isolated from the ozonolysis
reaction. bonds separated by more than two bonds from the epoxide functional group, then only two CmO2 structures, as shown in Figure 5, can be formed under our kinetically controlled reaction conditions. This activation effect of the epoxide functional group on adjacent n bonds had previously been observed by Balch et al." They isolated (r2-C,0)Ir(CO)Cl(P(C6H&)2 formed in the reaction of cmo with Ir(CO)C1(P(C6H5)3)2, and showed that the Ir atom had inserted into the C-C bond shared by adjacent 6,6-rings right next to the oxygen atom. If we
14910 J. Phys. Chem., Vol. 99, No. 41, 1995
Isomer - 1
Isomer - I1
Figure 5. Proposed structures for C~002isomers I and 11. The solid circles represent oxygen atoms.
examine Figure 1 more carefully, we find that the more polar isomer of Cm02 is approximately twice more abundant than the less polar one, consistent with our proposed model. Our observation of just two (26002 isomers is supported by a recent AM 1 computations.I2 Finally, we can draw a picture accounting for the different behavior of the two (26002 isomers under LDI conditions shown in Figure 4 based on the model we have just established. Refemng to Figure 5, isomer I has two juxtaposed oxygen atoms and it may easily decompose into a Cm fragment at the loss of 0 2 in the process of laser desorption. On the other hand, the two oxygen atoms in isomer 11 are so far apart that the two epoxide functional groups exert chemical influence on the molecule more independently and they each behave like a CmO molecule. Therefore, isomer I1 is prone to expel CO and produce a C590 fragment. Thereafter, (2590 can expel a second CO to produce c5g. The structures proposed in Figure 5 are consistent with all available spectroscopic data. I3C-NMR of isomer I (spectrum not shown) supports a C, symmetry. Preliminary optical spectroscopic data show that isomer I has features ascribable to coupled vibrations of the epoxide functional groups, but these features are absent in isomer II in the IR region.I3 C6,,02 with the structure of isomer I has recently been isolated and identified by Balch et al. in the reaction of Cm with m-chloroperoxybenzoic acid.I4 Summarizing our observations, CmO and both Cm02 isomers, formed simply by bubbling ozone through a Cm solution, can undergo decarbonylation reactions under LDI conditions. We
Letters proposed structures for the two Cm02 isomers identified. The oxygen atoms are closer together in isomer I than in isomer II. All available data, including mass spectra, IR,visible spectra, and AM1 computations, are in agreement with our proposed structures and related reactivities. ESI-MS has high sensitivity and does not induce fragmentation, and it turns out to be the method of choice for determining the molecular weights of Cm oxides. More data are being collected toward final confiiation of the structures of the isomers of Cm02. Experiments on Cm03 are currently in progress. And finally, we are trying to find the best conditions for regiospecific control over ozonolysis of C60.
Acknowledgment. Generous funding support from the National Science Council of the Republic of China and assistance on ESI experiments from Ming-Fai Tam and LianYung Wang of the Institute of Molecular Biology of Academia Sinica are gratefully acknowledged. We thank Prof. G. R. Her for discussions on ESI techniques. References and Notes (1) Kratschmer, W.; Lamb, L. D.; Fostiropoulos, K.; Huffman, D.R. Nature 1990, 347, 354. ( 2 ) Wood, J. M.; Kahr, B.; Steve, H. H. 11.; Dejarme, L.; Graham, C. R.; Doc, B. A. J . Am. Chem. SOC.1991, 113, 5907. (3) Kalsbeck, W. A,; Throp, H. H. J. Electroanal. Chem. 1991, 314, 363. (4) Creegan, K. M.; Robbins, J. L.; Robbins, W. K.; Millar, J. M.; Shenvood, R. D.; Tindall, P. J.; Smith, A. B.; McCauley, J. P. Jr.; Jones, D.R.; Gallagher, R. T.; Cox, D. M. J . Am. Chem. SOC.1992, 114, 1103. (5) Elemes, Y . ; Silver", S. K.; Sheu, C.; Kao, M.; Foote, C. S.; Alvarez, M. M.; Whetten, R. L. Angew. Chem., Int. Ed. Engl. 1992, 32, 352. (6) Deng, J. P.; Ju, D.D.;Her, G. R.; Mou, C. Y . ;Chen, C. J.; Lin, Y . Y.; Han, C. C. J . Phys. Chem. 1993, 97, 11575. (7) Liu, T. Y.; Shiu, L. L.; Luh, T. Y . ;Her, G. R. Rapid Commun. Mass Spectrom. 1995, 9, 93. (8) McElvany, S. W.; Callahan, J. H.; Ross, M. M.; Lamb, L. D.; Huffman, D. R. Science 1993, 260, 1632. (9) Taylor, R. J . Chem. SOC.,Chem. Commun. 1994, 1629. (10) Beck, R. D.;Brauchle, G.; Stoermer, C.; Kappes, M. M. J . Chem. Phys. 1995, 102, 540. (1 1) Balch, A. L.; Costa, D.A.; Lee, J. W.; Noll, B. C.; Olmstead, M. M. Inorg. Chem. 1994, 33, 2071. (12) Sun, M. L.; Slanina, Z.; Lee, S. L.; Unlik, F.; Adamowicz, L. Recent Advances in the Chemistry and Physics of Fullerenes and Related Materials; Kadish, K. M., Ruoff, R. S., a s . ; The Electrochemical Society: Pennington, NJ, 1995. (13) Deng, J. P.; Mou, C. Y . ;Han, C. C., to be published. (14) Balch, A. L.; Costa, D. A.; Winkler, C.; Ginwala, A.; Olmstead, M. M. J . Phys. Chem., in press.
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