11575
J . Phys. Chem. 1993,97, 11575-11577
Odd-Numbered Fullerene Fragment Ions from C a Oxides Jin-Pei Deng, Dar-Der Ju, Guor-Rong Her, and Chung-Yuan Mou Department of Chemistry, National Taiwan University 1 , Sec. 4 , Roosevelt Rd., Taipei, Taiwan, R.O.C.
Chun-Jen Chen and Yann-Yow Lin Department of Chemistry, National Chung Hsing University, Taichung, 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: August 4, 1993; In Final Form: September 22, 1993'
Odd-numbered carbon-only anion clusters, C53-, CSS-,C57-, and C59-, are observed in laser desorption ionization of CwO obtained in reactions of Cw with 0 3 . These odd-numbered carbon clusters are suggestive of the presence of the ether form monoxide resulting from oxygen insertion into the C-C bond shared by adjacent five- and six-membered rings.
We report in this Letter our observation of anionic oddnumbered fullerenes, Cn- where n = 53, 55, 57, and 59, in laser desorption ionization (LDI) of products generated in reactions of Cm with O3 carried out in toluene at room temperature. McElvany et al. recently reported the observation of C119-, c129-9 and Cl39- in a similar ozone reaction by thermal desorption and the absenceof these ions when COz laser desorption and secondary ion mass spectrometry were attempted.' While CI19-must be derived from the coupling of two C ~ Ohow , and when during this coupling process one carbon atom is lost remains an open question. Among many conceivable reaction routes that may lead to the loss of a carbon atom in this coupling event under the action of ozone is the initial formation of C600, which then couples to another Cm with the loss of a CO fragment occurring before or concertedly with this coupling. Our LDI-MS data show that decarbonylation of CmO-, resulting in odd-numbered all-carbon fragments, can occur under reasonably mild conditions, which further suggests the possible contribution from the stepwise oxidation, decarbonylation, and coupling route to the stable, oddnumbered molecules C119, C129, and c139 observed by McElvany. 03was generated with a 5% 0 2 (balanced with Ar) gas mixture from a discharge generator and slowly bubbled through the reaction solution.* Control runs indicated that toluene can be considered as inert toward O3 under our reaction conditions. Reactions were monitored at constant time intervals by HPLC, optical spectroscopy, and mass ~pectrometry.~In Figure 1 mass peaks, obtained with fast-atom-bombardment (FAB) ionization using 3-nitrobenzylalcohol as the matrix, correspondingto CmOi, where i = 1-3, are observed, confirming the incorporation of oxygen atoms into the Cm framework. In Figure 2, a typical high-performance liquid chromatography (HPLC) chromatogram4 of the reaction mixture, three products are clearly discernible, and they most likely reflect the three oxygencontaining species shown in Figure 1. For reactions stopped at about 30%conversi~n,~ the peak of visible absorption was observed to have shifted from 533 to 501 nm, whereas McElvany reported no visible change in color.' The monoxide CmO is a particularly interesting species. Anderson and co-workers produced the metastable ion CmO+ and many interesting fragments by reacting O+ with CW in v a ~ u u m .Cox ~ et al. obtained CmO from photooxygenation of Cm and assigned to it, based on NMR chemical shifts, an epoxide structure with the oxygen atom added to a C-C bond between *Abstract published in Aduance ACS Abstracts, November 1, 1993.
0022-3654/93/2097-11575%04.00/0
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Figure 1. FAB mass spectrum of ozonolysis reaction mixture showing the existence of CaO, CaO2, and CaO3.
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Figure 2. HPLC chromatogram of Ca + 03 reaction mixture. The numbers represent the elution time, in minutes, of the pcaks. The first peak is Ca and the second CaO. The third and fourth pcaks, while we have not isolated them for identification, we tentatively ascribe-to CaO2 and Ca03, respectively.
two adjacent six-membered rings.6 However, semiempirical (MNDO) ab initio calculations and density functional studies have suggested a more stable ether form (oxidoannulene, CmO) where the oxygen atom had inserted into the C-C bond originally shared by adjacent five- and six-membered Addition reactions occurring at 6,6- and 5,6-ring junctions have both been identifiedSq Samples used in LDI-TOF experiments were CaO-enriched reaction mixtures'O similar to that shown in Figure 2. Theanalyte mixture was applied to a stainless steel probe tip and air-dried before mass spectrometric analysis on a home-built reflectron time-of-flight mass spectrometer and an FT-ICR mass spectrometer. Mass spectra of similarly prepared c 6 0 samples 0 1993 American Chemical Society
11576 The Journal of Physical Chemistry, Vol. 97, No. 45, 1993
Letters
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F i p e 4. The 266-nm MALDI-FT-ICR negative ion mass spectrum of CWO-enriched reaction mixture. The matrix is an underlayer of nicotinic ,
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Figure 3. (a) The 266-nm LDI-TOF negative ion mass spectrum of CmO-enrichedreaction mixture. (b) The 266-nm MALDI-TOFnegative ion mass spectrum of CmO-enriched reaction mixture. The matrix is an underlayer of nicotinic acid.
contained almost exclusively the CU- peak and lacked the special features of CUO-containing samples. Therefore, we processed all samples transferring in ambient air without worrying about possible interference introduced by air. Figure 3a is a typical 266-nm LDI-TOF negative ion mass spectrum of product-enriched reaction mixture. In addition to even-numbered fullerene and fullerene monoxide anions, mass peaks corresponding to Cs3-, CSS-,Cs7-, and (less clearly) Csgcan be identified. In an attempt to try to identify the origin of these odd-numbered all-carbon ions, we applied the matrixassisted LDI (MALDI) technique" to the same product-enriched mixture. Figure 3b shows the result of a typical MALDI-TOF mass spectrum with an underlayer of nicotinic acid as the matrix material. It is apparent that the signal intensity of C&- has significantly increased while those of the fragment ions have all decreased, and it is difficult to positively identify the presence of any of the odd-numbered species. Considering the fact that under similar desorption laser conditions Ca- always appeared as the base peak in LDI and MALDI mass spectra of CU, we conclude from the results presented in Figure 3 that the observed fragment ions originated mainly from CaO. Also, these results suggest that even with our unconventional way of preparing samples," MALDI is a softer ionization technique relative to LDI. The CmO-enriched reaction mixturelo from a reaction that had been stopped at approximately 30%conversion2 was analyzed by both LDI and MALDI techniques on an FT-ICR mass spectrometer. LDI-ICR mass spectra were qualitatively similar to those obtained with LDI-TOF-MS, and we present just the MALDI-ICR result in Figure 4. In order to minimize fragmentation of the monoxide, we reduced irradiation photon
intensity to near the desorption threshold, and we obtained 1: 1 ratio of Cm-:Cs8-. All ICR experiments were performed with multiple desorption laser pulses and unquenched multiple remeasurements, as had been demonstrated by McLafferty,12 so as to enhance signal-to-noise (S/N) ratio. However, this experimental condition might have induced secondary gas-phase photofragmentation of desorbed C&, and this may account for the difference in fragmentation patterns seen in Figures 3b and 4. While S/Nis not as good as we wished, we still have enough resolving power to observe isotopic peaks in the mass range of interest. HereweclearlyseeCh-, withn= 1-6. Underidentical experimental conditions, MALDI of CU yielded no clearly detectable fragment ions (mass spectra not shown). This observation suggests that C a O is more prone to fragmentation than Cm. Particularly noteworthy is the presence of Css-, C57-, and less prominentlyCsg-. These odd-numberedcarbon-only ions were similarly observed in LDI-ICR experiments at 266, 355, and 532 nm; their relative intensities with respect to each other and with respect to CsgO- were observed to be insensitive to desorption photon wavelength and were also similar to those seen in Figure 4. In Anderson's ion-neutral collisional study of O+ Cm,5 Css+ and CseaO+ were observed. The different compositions of the smaller odd-numbered species observed by Anderson and by us may arise either from different charge or structure in the same stoichiometricspeciesC@O+/-or simply from different excitation modes. In the epoxide form of CmO, the oxygen atom is loosely bound and will likely be lost upon LDI leading to the even-numbered fullerene family CU, CSS..., and so on. In their LD-TOF study of this isomer, Whetten et al.13 observed CU+ as well as CwO+. In addition to even-numbered fullerenes,which apparentlyresulted from further loss of C2 fragments from C,originating from CWO, we also observed odd-numbered carbon clusters under LDI-MS conditions. We infer that the odd-numbered series are produced from decarbonylation of a different isomer of CmO. This could be the theoreticallypredicted lowest-energyether form of C60O.7.8 The calculatedlarge energy barrier for the interconversionbetween the ether and the epoxide forms of CU is suggestive of their distinguished identities.8 It has been reported that oxygen can be more firmly bound in the ether form upon heating the corresponding oxide." Presumably, the oxidation of CU by O3 produces two isomeric forms of CmO, the epoxide form resulting in even-numbered and the ether form in odd-numbered carbon cluster ions under LDI-MS conditions. In addition, mass peaks corresponding to CnO- and CnO2-9 some with odd-numbered carbon atoms, and Csl-, C~Z-, and less obviously C63- can be assigned. We note that while the isotopic abundance patterns of many of the weaker groups of peaks varied from one run to the next, the group of peaks assigned to C~802consistently had the M 1 peak as the most abundant one. As the neighboring peaks are separated by 1 amu, we believe that this group of peaks is not due to C12002-;instead, the M + 1 peak
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The Journal of Physical Chemistry, Vol. 97, No. 45, 1993
Letters most likely contains contributions from C ~ ~ O Z HIt- .is impossible for us to further comment on these delicate features before we understand more about the role of the matrix molecules. During our ICR experiments, we observed that the oddnumbered carbon cluster ions are lost somewhat faster than the even-numberedones at trapping times greater than 2-3 min under our working pressures of (1-5) X Torr and 3 T magnetic field. This seems to be in line with our general prediction that the odd-numbered ions, if they are stable at all, are more reactive than the even-numbered fullerenes. However, we have not been able to identify their decay channels by MS/MS due to difficulties encountered with isolating one of them by double-resonance ion ejection. Finally, we wish to mention briefly our other observations made in the cation monitoring mode under LDI and MALDI conditions with either TOF-MS or FT-ICR detection. We observed significantlyweaker signals of C a O and no odd-numbered carbononly clusters. For coalescenceproducts, only even-numbered ions in the neighborhood of C120+were observed when the desorption photon intensity is significantly above threshold. Summarizing our observations, we have demonstrated the formation of C a O simply by bubbling O3through C a solutions, and C a O undergoes decarbonyl reaction and further breaking up upon laser desorption ionization to give the odd-numbered C59-,C57-rC55-, and C53-. An analogous odd-numbered neutral decarbonylproduct, e.g., C59, may thus account for the production of bulk quantities of C119 and other odd-numbered coalescence carbon clusters earlier observed by McElvany et al.’ by the total reaction scheme C,,
- + c,, - c,, + co + 0,
C6,0
0,
hv
The ozone reaction of C a suggests a very promising new approach to fullerene chemistry. For example, step-by-step oxidation and loss of carbon atoms in the form of CO from fullereneoxides may enable chemiststo practice controlledopening of fullerene cages.1s-17 Since five-membered rings are located only at the ends of buckytubes, selective cleavage of C-C bonds adjoining five- and six-membered rings by O3 would be an attractive means of opening such tubes at their ends. CaO2 with a dicarbonylstructure, photosynthesizedin solvent-freethin films, may have similar selectivity toward cleaving such C-C bonds in fullerenes.18
Acknowledgment. We thank Professor Tien-Yau Luh, Professor Jin-Ming Fang, Mr. Jynn-Huei Hwang, Mr. Kuang-Ruang
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Ho, Mr. Ching-Long Huang, Mr. Shui-Yuan Wu, and Mr. HsiuHsin Hung for their illuminating discussion and technical assistance. Generous funding support from the National Science Council of the Republic of China is gratefully acknowledged.
References and Notes (1) McElvany, S. W.; Callahan, J. H.; Ross, M. M.; Lamb, L. D.; Huffman, D. R. Science 1993, 260, 1632. (2) Extended periods of reaction resulted in discoloration of the purple C a soltuion; mass spectrometric analysis of such overreacted mixture showed no mass peaks above 640 amu. On the other hand, high 03 bubbling rates resulted in the formation of brownish suspended particles. Either condition reduced the yield of CaO. At about 30% conversion under slow-bubbling conditions, the second product peak shown in Figure 1 was just becoming visible. (3) Jwl SX-102Areversedgwmetrydouble-focusingmasssptctrometer. TheXeFABgunwasoperatedat6 kV. InDCI/MSexperiments, theplatinum emitter was heated at a current-ramp rate of 10 mA/s until the maximum current of 0.9 A was reached, and NH3 was used as reagent gas. (4) Normal phasesilica gel column eluted with n-hexane containing0.596 toluene and 0.4% dichloromethane. (5) Christian, J. F.; Wan, 2.;Anderson, S. L. Chem. Phys. Lert. 1992, 199, 373. (6) Creegan, K. M.;Robbins, J. L.; Robbins, W. K.;Millar, J. M.; Sherwood, R. D.; Tindall, P. J.; Cox, D. M.;Smith, A. B., III; McCauley, J. P., Jr.; Jones, D. R.; Gallagher, R. T. J. Am. Chem. Soc. 1992, 114, 1103. (7) Raghavachari, K.Chem. Phys. Lert. 1992, 195, 221. (8) Raghavachari, K.;Sosa. C. Chem. Phys. Le??.1993,209, 223. (9) For a review of general fullerene chemistry, see: Taylor, R.; Walton, D. R. M.Nature 1993, 363, 685. (10) Reaction mixtures were removed mainly of unreacted Cm by column chromatography with normal phase silica gel and eluted with hexane/toluene mixedsolutions (99/1 to95/5v/v). While thissolution tremendouslyreduccd the separation time needed, it allowed only partial separation of the remaining reactant and the products, as evidenced by further HPLC analysis of the enriched sample. In this regard, CaO-enriched samples may contain as much as 50% Castarting material and small amounts of the presumed higher oxides. (1 1) Owing to the vastly different polarities of C a and nicotinic acid, we could not effectivelydissolve both in a common solvent and prepare the sample the way conventional MALDI experiments are performed. The spectrum shown in Figure 3 was obtained by first depositing an air-dried underlayer of nicotinic acid and then an overlayer of the reaction mixture applied. (12) Williams, E. R.;Henry, K.D.; McLafferty. F.W. J . Am. Chem.Soc. 1990, 112,6157. (13)
Elemes,Y.;Silverman,S.K.;Sheu,C.;Kao,M.;Foote,C.S.;Alvarez,
M. M.;Whetten, R. L. Angew. Chem., In?. Ed. Engl. 1992, 31, 351. (14) Werner, H.; Bublak, D.; Goebel, U.; Henschke, B.; Bensch, W.; Schlocgel, R. Angew. Chem., In?. Ed. Engl. 1992,31, 868. (15) Iijima, S. Narure 1991, 354, 56. (16) Tsang, S. C.; Harris, P. J. F.;Green, M. L. H. Nature 1993, 362, 520. (17) Ajayan, P. M.; Ebbesen, T. W.; Ichihashi, T.; Iijima, S.; Tanigaki, K.;Hiura, H. Nature 1992, 362, 522. (18) Taliani, C.; Ruani, G.; Zamboni, R.; Danieli, R.; Rossini, S.;Denisov, V. N.; Burlakov, V. M.;Negri, F.; Oriandi, G.; Zerbetto, F. J. Chem. S a . , Chem. Commun. 1993, 220.
