Fullerene Research in the Cross Group

Yale University, Department of Chemistry

In collaboration with Prof. Martin Saunders we make fullerenes with atoms and small molecules trapped inside and study their properties. These are van der Waals molecules in that there is no chemical bond between the trapped atom or molecule and the carbon cage. Yet, they are very stable, since the atom cannot escape unless several bonds are broken. Numbers refer to publications listed below. So far we have put He, Ne, Ar, Kr, Xe and tritium and nitrogen atoms, as well as He2, Ne2, CO, and N2 inside a variety of fullerenes. The tritium atoms are inserted by generating them at high energies in a nuclear reaction. The noble gases are put in by heating the fullerenes in the presence of the gas at high temperatures and pressures or by shooting them in as ions or metastable atoms. The noble gas compounds can be detected by mass spectroscopy either as intact molecules or by decomposing them and detecting the noble gas. In the case of 3He we can see the 3He NMR signal. When a molecule is put inside a fullerene, the vibration-rotation spectroscopy can have some unusual features. [52]

The chief method used to make the noble-gas fullerene compounds is to heat fullerene in the presence of the gas at 650oC and 3000 atm.[3] We make an ampoule from a tube of OFHC copper by crimping one end, filling it, and crimping off the top. The ampoule is placed in a high-pressure bomb which is then filled with water, closed and heated to 650o. The pressure rises to 3000 atm, and the copper ampoule is squashed flat, compressing the gas within. After about 8 hrs., the bomb is cooled, and the ampoule is opened. The fullerene is extracted in CS2. About 85% of the fullerene is soluble, and about 0.1% of the molecules contain a noble gas atom. In the cases of He and Ne, we find small amounts of C70 containing two helium atoms[20] or two neon atoms [18]. We have found even smaller amounts of C60 containing two helium atoms[37].

Tritium atoms with high kinetic energy are generated using a nuclear reaction.[6][26] We prepare either a lithium salt of C60 or use 3He@C60 or a mixture of C60 and 3He gas. 6Li and 3He each absorb thermal neutrons in a reactor to give tritium. The tritium then loses energy by ionizing the fullerene until it eventually stops. Some of the time it stops inside a fullerene molecule which then remains stable. We can isolate tritium labeled C60. In the case of the T@C60 generated from lithium, we obtain trace amounts of 3He@C60 formed by the radioactive decay of the tritium. If the tritium were on the outside, the 3He would be on the outside and would be lost.

We have constructed a beam machine to put atoms inside fullerenes.[15] In the center is a cylindrical target, rotating slowly. On one side is an oven which produces a continuous beam of fullerene. Thus we have a freshly deposited surface of fullerene on the target. On the other side is a source of noble gas ions or metastable neutrals which hit the surface. The ions and mtastables are made in an electric discharge. Ions are extracted by an electric field and bent by 90o. The amount of incorporation for He+ is small at 30eV and rises to a maximum near 100eV and then decreases. Above 100eV the fullerene is partially destroyed. For Ne+, the yield is smaller, and the threshold is about 100eV. In the metastable mode, the ions are bent away from the target, while the metastables hit it. We find incorporation of both He* and Ne*. The method also works for nitrogen atoms. N @C60 consists of a free nitrogen atom with three unpaired electrons unbound to the carbon cage. It gives a clean atomic-like ESR signal. We have used the beam method to put He into dodecahedrane, C20H20, a hydrocarbon cage.[24]

3He labeled fullerenes and their derivatives can be studied by NMR spectroscopy.[2][10] The pi electrons around the fullerene molecules cause large diamagnetic shielding and an upfield shift of the 3He line relative to disolved 3He gas. C60 has an upfield shift of 6.4 ppm and C70 28 ppm. Higher fullerenes fall between these limits.[8][25] Adding groups to the outside changes the pi electron structure and the chemical shift of the 3He.[4][5][7][10][47] The most common adduct is across one of the 6,6 double bond joining two hexagons. Single addition usually causes an upfield shift of about 3 ppm from C60, the exact amount depends on the group being added. Addition across a 5,6 single bond joining a pentagon and a hexagon gives a much smaller shift. Multiple additions give a more complicated picture.[14] Each fullerene molecule and each fullerene adduct gives a different, unique NMR line. Adding six electrons to C60 and to C70 gives another closed-shell species. The anions have very different chemical shifts from the neutrals.[21] We also see the NMR signal for 129Xe in Xe@C60. The NMR of 129Xe@C60 is different from that of 3He@C60.[35]

We have constructed a mass spectrometer to analyse the noble gas inside the fullerenes.[12] We find that very pure C60 is extraordinarily stable. At 630oC the half life for decomposition is greater than one month, but even trace quantities of solvent or air absorbed in it will catalyze its decomposition. At 900oC the half life is 10 hours. In both cases the gas is largely or completely released by the decomposition of the C60. Using a different mass spectrometer, we can directly see the peaks for the various compounds.

Using multiple passes through an HPLC column gave us nearly pure Kr@C60.[22] We could see the small shift in the 13C NMR due to the presence of the Kr atom. There were small shifts in the IR, visible, and UV lines and a 10% decrease in the life time of the lowest triplet state as well. A similar separation has been achieved with Xe@C60.[35]

Using classical statistical mechanics, we can claculate the equilibrium constant for the incorporation of a noble gas atom into C60.[11] We start with a potential function V(R) for the gas atom as a function of the distance from the center. V(R) is obtained by using one of several literature potentials between the gas atom and each carbon atom. We can then calculate the equilibrium constant using classical statistical mechanicsq* is the the usual partition function for X@C60 not including the gas motion, qint is the internal partition function for C60, and pX. The values for He and Ne seem to be relatively independent of the potential model used, but the values for the higher noble gases are much more sensitive to the choice of potential. For He and Ne the equilibrium values are much higher than the amounts that we can get from our experiments, so that the experiments are far from reaching equilibrium.

9,10 dimethyl anthracene (DMA) reacts reversibly with C60. By measuring the 3He NMR peak heights as a function of DMA concentration, we can get the equilibrium constants for the addition of successive DMA molecules to C60 and C70.[28] By doing this as a function of temperature, we can get the enthalpy changes as well. Using mixtures of 3He@C60 and 129Xe@C60, we measured the ratio of the equilibrium constants for the two species. 3He is favored at low T and 129Xe at high T. Thus, putting Xe inside C60 changes both ΔH and ΔS for the DMA addition.[51] We found that the various isomers of C84 have very different equilibrium constants for the addition of DMA, and this can be used as a basis for separation of the isomers.[25] We used H2@C60 to measure the rate using an NMR T-jump mmethod. [54].

By adding suitable groups to the outside of fullerenes, it is possible to open a hole in the cage. We have measured both equilibrium constants and kinetics for noble gases entering and leaving chemically opened fullerenes. [31][44][50][56] We have put ammonia [53] and methane [55] into one of the opened fullerenes.The analysis required some unusual NMR spectroscopy.

Research support for this research from the US National Science Foundation is gratefully appreciated.

Recent papers. References 10 and 38 are review articles.

1. Stable compounds of Helium and Neon: He@C60 a and Ne@C60, M. Saunders, H. A. Jiménez-Vázquez, R. J. Cross, and R. J. Poreda, Science 259, 1428 (1993).
2. Probing the Interior of Fullerenes by 3He NMR Spectroscopy of Endohedral 3He@C60 and 3He@C70, M. Saunders, H. A. Jiménez-Vázquez, R. J. Cross, S. Mroczkowski, D. I. Freedberg, and F. A. L. Anet, Nature 367, 256 (1994).
3. Incorportion of Helium, Neon, Argon, Krypton, and Xenon into Fullerenes using High Pressure, M. Saunders, H. A. Jiménez-Vázquez, R. J. Cross, S. Mroczkowski, M. L. Gross, D. E. Giblin, and R. J. Poreda, J. Am. Chem. Soc. 116, 2193 (1994).
4. 3He NMR: A Powerful New Tool for Following Fullerene Chemistry, M. Saunders, H. A. Jiménez-Vázquez, B. W. Bangerter, R. J. Cross, S. Mroczkowski, D. I. Freedberg, and F. A. L. Anet, J. Am. Chem. Soc. 116, 3621 (1994).
5. Reaction of Cycloprop[b]naphthalene with 3He@C60, M. Saunders, H. A. Jiménez-Vázquez, R. J. Cross, E. Billups, C. Gesenberg, and D. J. McCord, Tetrahed. Letts. 35, 3869 (1994).
6. Hot-Atom Incorporation of Tritium Atoms into Fullerenes, H. A. Jiménez-Vázquez, M. Saunders, and R. J. Poreda, Chem. Phys. Letts. 229, 111 (1994).
7. Synthesis and 3He NMR Studies of C60 and C70 Epoxide, Cyclopropane, and Annulene Derivatives Containing Endohedral Helium, A. B. Smith, R. M. Strongin, L. Brard, W. J. Romanow, M. Saunders, H. A. Jiménez-Vázquez, and R. J. Cross, J. Am. Chem. Soc. 116, 10831 (1994).
8. Analysis of Isomers of the Higher Fullerenes by 3He NMR Spectroscopy, M. Saunders, H. A. Jiménez-Vázquez, R. J. Cross, W. E. Billups, C. Gesenberg, A. Gonzalez, W. Luo, R. C. Haddon, F. Diederich, and Z. Herrmann, J. Am. Chem. Soc. 117, 9305 (1995).
9. Chromatographic Fractionation of Fullerenes Containing Noble Gas Atoms, M. Saunders, A. Khong, R. Shimshi, H. A. Jiménez-Vázquez,and R. J. Cross, Chem. Phys. Lett. 248, 127 (1996).
10. Noble Gas Atoms Inside Fullerenes, M. Saunders, R. J. Cross, H. A. Jiménez-Vázquez, R. Shimshi, and A. Khong, Science 271, 1693 (1996).
11. Equilibrium Constants for Noble-Gas Fullerene Compounds, H. A. Jiménez-Vázquez and R. J. Cross, J. Chem. Phys. 104, 5589 (1996).
12. Release of Noble Gas Atoms from Inside Fullerenes, R. Shimshi, A. Khong, H. A. Jiménez-Vázquez, R. J. Cross, and M. Saunders, Tetrahedron, 52, 5143 (1996).
13. Enrichment and Characterization of a Noble Gas Fullerene: Ar@C60, B. A. DiCamillo, R. L. Hettich, G. Guichon, R. N. Compton, M. Saunders, H. A. Jiménez-Vázquez, A. Khong, and R. J. Cross, J. Phys. Chem. 100, 9197 (1996).
14. Use of 3He NMR for Structural Assignment of Isomers Resulting from Bis-addition to C60, R. J. Cross, H. A. Jiménez-Vázquez, Q. Lu, M. Saunders, D. I. Schuster, S. R. Wilson, and H. Zhao, J. Am. Chem. Soc. 118, 11454 (1996).
15. Beam Implantation: A New Method for Preparing Cage Molecules Containing Atoms at High Incorporation Levels, R. Shimshi, R. J. Cross, and M. Saunders, J. Am. Chem. Soc. 119, 1163 (1997).
16. pi-Electron Ring Current Effects in Multiple Adducts of 3He@C60 and 3He@C70: A 3He NMR Study, M. Rüttimann, R. F. Haldimann, L. Isaacs, F. Diederich, A. Khong, H. A. Jiménez-Vázquez, R. J. Cross, and M. Saunders, Chem. Eur. J. 3, 1071 (1997).
17. Incorporation of Helium into Endohedral Complexes of C60 and C70 Containing Noble-Gas Atoms; A Tandom Mass Spectrometry Study, D. E. Giblin, M. L. Gross, M. Saunders, H. A. Jiménez-Vázquez, and R. J. Cross, J. Am. Chem. Soc. 119, 9883 (1997).
18. An Artificial Molecule of Ne2 inside C70, J. Laskin, T. Peres, C. Lifshitz, M. Saunders, R. J. Cross, and A. Khong, Chem. Phys. Lett. 285, 7 (1998).
19. Collisional Fragmentation of Ar@C60, C. Brink, P. Hvelplund, H. Shen, H. A. Jiménez-Vázquez, R. J. Cross, and M. Saunders, Chem. Phys. Lett. 286, 28 (1998).
20. An NMR Study of He2 Trapped inside C70, A. Khong, H. A. Jiménez-Vázquez, M. Saunders, R. J. Cross, J. Laskin, T. Peres, C. Lifshitz, R. Strongen, and A. B. Smith, J. Am. Chem. Soc. 120, 6380 (1998).
21. 3He NMR of He@C606- and He@C706-, New Records for the Most Shielded an the Most Deshielded 3He inside a Fullerene, A. Weitz, E. Shabtai, R. Haddon, R. E. Hoffman, M. Rabinovitz, A. Khong, R. J. Cross, M. Saunders, P. C. Cheng, and L. T. Scott, J. Am. Chem. Soc. 120, 6389 (1998).
22. Isolation and Spectral Properties of Kr@C60, a Stable van der Waals Molecule, K. Yamamoto, M. Saunders, A. Khong, R. J. Cross, M. Grayson, M. L. Gross, A. F. Benedetto, and R. B. Weisman, J. Am. Chem. Soc. 121, 1592 (1999).
23. Mass Spectrometric Study of Unimolecular Decompositions of Endohedral Fullerenes, J. Laskin, T. Peres, A. Khong, H. A. Jiménez-Vázquez, R. J. Cross, M. Saunders, D. S. Bethune, M. S. de Vries, and C. Lifshitz, Int. J. Mass Spectrom. 187, 61 (1999).
24. Putting Helium inside Dodecahedrane, R. J. Cross, M. Saunders, and H. Prinzbach, Org. Letts. 1, 1479 (1999).
25. A New Method for Separating the Isomeric C84 Fullerenes, G.-W. Wang, M. Saunders, A. Khong, and R. J. Cross, J. Am. Chem. Soc. 122, 3216 (2000).
26. From 3He@C60 to 3H@C60: Hot-Atom Incorporation of Tritium in C60, A. Khong, R. J. Cross, and M. Saunders, J. Phys. Chem. A 104, 3940 (2000).
27. 3He NMR Study of 3He@C60H6 and 3He@C70H2-10, G.-W. Wang, B. R. Weedon, M. S. Meier, M. Saunders, and R. J. Cross, Org. Letts. 2, 2241 (2000).
28. Reversible Diels-Alder Addition to Fullerenes: A Study of Equilibria using 3He NMR Spectroscopy, G.-W. Wang, M. Saunders, and R. J. Cross, J. Am. Chem. Soc. 123, 256 (2001).
29. Do Nitrogen-atom-containing Endohedral Fullerenes Undergo the Shrink-Wrap Mechanism?, B. P. Cao, T. Peres, R. J. Cross, M. Saunders, and C. Lifshitz, J. Phys. Chem. A 105, 2142 (2001).
30. Binding Energy in and Equilibrium Constant of Formation for the Dodecahedrane compounds He@C20H20 and Ne@C20H20, H. A. Jiménez-Vázquez, J. Tamariz, and R. J. Cross, J. Phys. Chem. A 105, 1315 (2001).
31. Insertion of Helium and Molecular Hydrogen through the Orifice of an Open Fullerene, Y. Rubin, T. Jarrosson, G.-W. Wang, M. D. Bartberger, K. N. Houk, G. Schick, M. Saunders, and R. J. Cross, Angew. Chem. Int. Ed. Engl. 40, 1543 (2001).
32. Does H2 Rotate Freely inside Fullerenes?, R. J. Cross, J. Phys. Chem. A 105, 6943 (2001).
33. Some New Diatomic Molecules Containing Endohedral Fullerenes, T. Peres, B. P. Cao, W. D. Cui, A. Khong, R. J. Cross, M. Saunders, and C. Lifshitz, Int. J. Mass Sprectrom. 210, 241 (2001).
34. Chromatographic Purification of Kr@C60, T. Suetsuna, N. Dragoe, H. Shimotani, A. Takada, S. Ito, R. J. Cross, M. Saunders, H. Takagi, and K. Kitazawa, Fullerene, Nanotubes, and Carbon Nanostructures 10, 15 (2002).
35. 129Xe NMR Spectrum of Xenon Inside C60, M. S. Syamala, R. J. Cross, and M. Saunders, J. Am. Chem. Soc. 124, 6216 (2002).
36. Crystallographic Characterization of Kr@C60 in (0.09Kr@C60/0.91C60)•{NiII(OEP)}•2C6H6, H. M. Lee, M. M. Olmstead, T. Suetsuna, H. Shimotani, N. Dragoe, R. J. Cross, K. Kitazawa, A. L. Balch, Chem. Comm. 2002, 1352 (2002).
37. Two Helium Atoms inside Fullerenes: Probing the Internal Magnetic Field in C606- and C706-, T. Sternfeld, R. E. Hoffman, M. Saunders, R. J. Cross, M. S. Syamala, M. Rabinovitz, J. Am. Chem. Soc. 124, 8786 (2002).
38. Putting Nonmetals into Fullerenes, M. Saunders and R. J. Cross, Endofullerenes, ed. T. Akasaka and S. Nagase, Kluwer (Dordrecht, 2002).
39. Direct Detection and Quantitation of He@C60 by Ultrahigh-Resolution Fourier Transform Ion Cyclotron Resonance Mass Spectrometry, H. J. Cooper, C. L. Hendrickson, A. G. Marshall, R. J.Cross, M. Saunders, J. Amer. Soc. Mass Spectr. 13, 1349 (2002).
40. The Inside Story of Fullerene Anions: A 3He NMR Aromaticity Probe. T. Sternfeld, M. Saunders, R. J. Cross, and M. Rabinovitz, Angew. Chem. Int. Ed. Engl. 42, 3136 (2003).
41. Using Cyanide to put Noble Gases inside C60. R. J. Cross, A. Khong, and M. Saunders, J. Org. Chem. 68, 8281 (2003).
42. Formation, isolation, and spectroscopic properties of some isomers of C60H38, C60H40, C60H42, and C60H44 - Analysis of the effect of the different shapes of various helium-containing hydrogenated fullerenes on their 3He chemical shifts. A. Peera, R. K. Saini, L. B. Alemany, W. E. Billups, M. Saunders, A. Khong, M. S. Syamala, and R. J. Cross, Eur. J. Org. Chem., 4140 (2003).
43. Kr Extended X-ray Absorption Fine Structure Study of Endohedral Kr@C60. S. Ito, A. Takeda, T. Miyazaki, Y. Yokoyama, M. Saunders, R. J. Cross, H. Takagi, P. Berthet, and N. Dragoe, J. Phys. Chem. B 108, 3191 (2004).
44. Helium Entry and Escape through a Chemically Opened Window in a Fullerene. C. M. Stanisky, R. J. Cross, M. Saunders, M. Murata, Y. Murata, and K. Komatsu, J. Am. Chem. Soc. 127, 299 (2005).
45. Transmutation of Fullerenes, R. J. Cross and M. Saunders, J. Am. Chem. Soc. 127, 3044 (2005).
46. Unimolecular Dissociations of C70+ and its Noble Gas Endohedral Cations Ne@C70+ and Ar@C70+: Cage Binding Energies for C2 Loss. B. P. Cao, R. J. Cross, M. Saunders, and C. Lifshitz, J. of Phys. Chem. A 109, 10257 (2005).
47. 3He NMR as a Sensitive Probe of Fullerene Reactivity: [2+2] Photocycloaddition of 3-Methyl-2-cyclohexenone to C70. J. Rosenthal, D. I. Schuster, R. J. Cross, and A. M. Khong, J. Org. Chem. 71, 1191 (2006).
48. Kinetic Energy Release of C70+ and its Endohedral Cation N@C70+: Activation Energy for N-Extrusion. B. Cao, T. Peres, C. Lifshitz, R. J. Cross, and M. Saunders, Chem., Euro. J. 12, 2213 (2006).
49. In Search of Microscopic Evidence for Molecular Level Negative Thermal Expansion in Fullerenes. S. Brown, J. Cao, J. L. Musfeldt, N. Dragoe, N., F. Cimpoesu, S. Ito, S., H. Takagi, H., and R. J. Cross, Phys. Rev. B 73, 125446 (2006).
50. Carbon Monoxide Inside an Open Cage Fullerene. S. I. Iwamatsu, C. M. Stanisky, R. J. Cross, M. Saunders, N. Mizorogi, S. Nagase, and S. Murata, Angew. Chem. Int. Ed. Engl. 45, 5337-5340 (2006).
51. The Effect of Xenon on Fullerene Reactions. M. Frunzi, R. J. Cross, and M. Saunders, J. Am. Chem. Soc. 129, 13343-13346 (2007).
52. Vibration-Rotation Spectroscopy of Molecules Trapped inside C60. R. J. Cross, J. Phys. Chem. A112, 7152-7156 (2008).
53. Putting Ammonia into a Chemically Opened Fullerene, K. E. Whitener, M. Frunzi, S. Iwamatsu, M. Murata, R. J. Cross, and M. Saunders, J. Am. Chem. Soc. 130, 13996-13999 (2008).
54. NMR Temperature-Jump Method for Measuring Reaction Rates: Reaction of Dimethylanthracene with H2@C60, M. Frunzi, H. Xu, R. J. Cross, and M. Saunders, J. Phys. Chem. A, 113, 4996-4999 (2009).
55. Methane in an Open-Cage [60] Fullerene, K. Whitener, R. J. Cross, M. Saunders, S.-I. Iwamatsu, S. Murata, N. Mizorogi, and S. Nagase, J. Am. Chem. Soc. 131, 6338-6329 (2009).
56. Putting Atoms and Molecules into Chemically Opened Fullerenes, C. M. Stanisky, R. J. Cross, and M. Saunders, J. Am. Chem. Soc. 131, 3392-3395 (2009).

From here you can go to:

Cross Group Home Page

Contact me at james.cross@yale.edu
Phone: (203) 432-5203, Fax: (203) 432-6144
Mail: Yale Chemistry Dept., PO Box 208107, New Haven, CT 06520-8107, USA

Last Updated: 7/26/12.