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Metallofullerene and Fullerene Formation from Condensing Carbon Gas Under Conditions of Stellar Outflows and Implication to Stardust

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Metallofullerene and Fullerene Formation from Condensing Carbon Gas Under Conditions of Stellar Outflows and Implication to Stardust Metallofullerene and fullerene formation from condensing carbon gas under conditions of stellar outflows and implication to stardust Paul W. Dunka,b,1, Jean-Joseph Adjizianc, Nathan K. Kaiserb, John P. Quinnb, Gregory T. Blakneyb, Christopher P. Ewelsc,1, Alan G. Marshalla,b,1, and Harold W. Krotoa,1 aDepartment of Chemistry and Biochemistry, Florida State University, Tallahassee, FL 32306; bIon Cyclotron Resonance Program, National High Magnetic Field Laboratory, Florida State University, Tallahassee, FL 32310; and cInstitut des Matériaux Jean Rouxel, Centre National de la Recherche Scientifique, Unité Mixte de Recherche 6502, Université de Nantes, BP 32229 Nantes, France Contributed by Harold W. Kroto, August 29, 2013 (sent for review June 13, 2013) Carbonaceous presolar grains of supernovae origin have long confirmed to exist in circumstellar and interstellar environments. been isolated and are determined to be the carrier of anomalous C60 and C70 were first unequivocally detected in a planetary 22Ne in ancient meteorites. That exotic 22Ne is, in fact, the decay nebula in 2010, which was thought to be hydrogen deficient (11). isotope of relatively short-lived 22Na formed by explosive nucleo- Thereafter, Buckminsterfullerene was detected in hydrogen- synthesis, and therefore, a selective and rapid Na physical trapping rich [including the least H-deficient R Coronae Borealis stars] mechanism must take place during carbon condensation in super- (12, 13) and oxygen-rich environments (14), as well as the ISM nova ejecta. Elucidation of the processes that trap Na and produce (15) and a protoplanetary nebula (16). Moreover, fullerenes large carbon molecules should yield insight into carbon stardust have been detected in a host of other circumstellar and in- enrichment and formation. Herein, we demonstrate that Na effec- terstellar sources, with new reports of cosmic fullerene de- tection constantly emerging. tively nucleates formation of Na@C60 and other metallofullerenes during carbon condensation under highly energetic conditions in Fullerenes have recently been experimentally shown to self- assemble in condensing carbon through a closed network growth oxygen- and hydrogen-rich environments. Thus, fundamental car- (CNG) mechanism (17, 18), in which small fullerenes form ini- bon chemistry that leads to trapping of Na is revealed, and should tially and then progress into larger species, such as C60, by in- be directly applicable to gas-phase chemistry involving stellar envi- corporation of atomic and diatomic carbon into growing cages. ronments, such as supernova ejecta. The results indicate that, in That process resolves how fullerenes form spontaneously under addition to empty fullerenes, metallofullerenes should be constit- highly energetic conditions from carbon vapor, as well as how uents of stellar/circumstellar and interstellar space. In addition, C60 forms as the most abundant species. The CNG mechanism gas-phase reactions of fullerenes with polycyclic aromatic hydro- for fullerene formation should occur in condensing carbon vapor carbons are investigated to probe “build-up” and formation of of stellar ejecta; although, formation by photochemical pro- carbon stardust, and provide insight into fullerene astrochemistry. cessing of existing carbonaceous materials, such as hydrogenated amorphous carbon (HACs), polycyclic aromatic hydrocarbons ne of the most profound findings in cosmochemistry has (PAHs), or PAH-like structures, could also be important for- Obeen the isolation and study of presolar grains recovered mation routes (12, 19, 20). Indeed, such photochemical pro- from carbonaceous chondrites, which are ancient meteorites (1– cessing could work in tandem with CNG formation to produce 3). A particularly striking example is the micrometer-sized low- density graphite grains of supernovae origin (1, 4). This discovery Significance was due to identification of noble gas isotopic anomalies iden- ASTRONOMY tified by mass spectrometry; upon stepwise heating of bulk samples We experimentally study the processes that result in fullerene 22 of carbonaceous chondrite, highly enriched Ne is released (3, 4). formation in oxygen- and hydrogen-rich carbon gas. Metal- 22 That observation led to the possibility that exotic Ne was a decay lofullerenes are found to form as readily as empty cages and 22 product of the extinct nuclide Na, produced from a supernova thus, like fullerenes, should be important constituents of (cir- 22 explosion (5). Subsequently, Ne was used as a guide to pinpoint cum)stellar/interstellar space. Element trapping by metal- the tiny carbonaceous grains embedded within the bulk meteorite, lofullerene formation is shown to be selective and rapid, which and the supernovae origin was further confirmed by other isotopic can explain long-standing astrophysical puzzles such as the “cosmic fingerprints” (2–4). Thus, these particular carbonaceous anomalous element enrichment of stardust. Infrared spectro- grains represent a direct chemical sample from a supernova. scopic signatures are simulated to provide an observational Despite many detailed experiments on presolar carbonaceous test for metallofullerenes in space. Further, energetic reactions 22 grains, the mechanism whereby Na is selectively trapped to between larger polycyclic aromatic hydrocarbons (PAHs) and enrich stardust remains a puzzle. The carbon chemistry of su- fullerenes are established form stable classes of complex mol- pernova ejecta, in part, determines the composition of carbon ecules that hold high astrochemical importance. Bottom-up stardust. Supernovae-originating dust is ejected into the in- fullerene growth is also demonstrated to result from PAH terstellar medium (ISM), directly impacting interstellar chemis- processing, another potentially important extraterrestrial for- try and subsequent stellar evolution. Furthermore, dust is mation mechanism. ubiquitous in the universe, and recent observational studies – suggest that supernovae are major dust contributors (6 8). Author contributions: P.W.D., C.P.E., A.G.M., and H.W.K. designed research; P.W.D. and Therefore, study of the gas-phase chemical processes that can J.-J.A. performed research; N.K.K., J.P.Q., and G.T.B. contributed new reagents/analytic operate under conditions of stellar outflows should provide in- tools; P.W.D., J.-J.A., C.P.E., A.G.M., and H.W.K. analyzed data; and P.W.D., J.-J.A., C.P.E., sight into a broad range of phenomena, including stellar and A.G.M., and H.W.K. wrote the paper. interstellar chemistry, distribution of carbon in the universe, and The authors declare no conflict of interest. supernova mixing. 1To whom correspondence may be addressed. E-mail: [email protected], chris.ewels@ Fullerenes have been principal astronomic molecular targets cnrs-imn.fr, [email protected], or [email protected]. since the discovery of Buckminsterfullerene, C60 (9, 10). The This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. closed-caged molecules, however, have only recently been 1073/pnas.1315928110/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1315928110 PNAS | November 5, 2013 | vol. 110 | no. 45 | 18081–18086 Downloaded by guest on September 23, 2021 C60 and other larger fullerenes in lower-energy conditions, out- spectrometer provides ultrahigh resolution and high mass accu- side of stellar environments. We note that because energetic racy, and overcomes that experimental challenge (22, 25). barriers are low or nonexistent for incorporation of C and C2 To probe the ability of Na to nucleate metallofullerene for- into closed cages and carbon catalyzed bond rearrangements mation in carbon plasma, a Na-doped carbon rod (1.0 atom % − (17), CNG formation should also be possible in circumstellar Na) was subjected to laser ablation (∼35 mJ∙cm 2 fluence or environments, such as planetary nebula. 5 mJ per pulse) under a 10-psi He flow. Because 22Na is not Many investigations into the astronomical origin of these hol- available for experimental study, 23Na is used. Both isotopes low caged molecules are currently under way. By contrast, little exhibit the same essential chemical reactivity and kinetics, and consideration has been given to endohedral metallofullerenes, thus 23Na can be used to accurately elucidate interaction of 22Na which are cages that encapsulate metals and other elements, as with carbon plasma. Fig. 1 shows the cluster cations generated relevant astrochemical species. Understanding how large carbon under conditions that yield empty-caged fullerenes. The empty- molecules can form in the hostile environments of stellar out- caged species are observed, as expected, but an entire family of flows is a central issue that should offer valuable information on carbon clusters containing a single Na atom is also present. the origin and cosmic role of fullerenes. Critical insight into Na@C60 forms as the most abundant species, followed by Na@C70. that problem can be achieved by study of the key processes All Na@C2n are unequivocally resolved from the “overlapping” and resulting molecular products formed through carbon con- empty cages. For example, as shown in Fig. 1, Na@C60 is clearly densation reactions in high-energy oxygen- and hydrogen-rich resolved from empty cage C62,definitively showing that Na@C60 environments. spontaneously forms in condensing carbon. Fragmentation ex- Herein, we explore carbon condensation and interaction with periments unambiguously confirm that Na@C60 and all other Na under highly energetic conditions by use of
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