Anthony Khong

Noble Gas Atoms Inside Fullerenes

Heating fullerenes at 650°C under 3000 atmospheres of the noble gases helium, neon, argon, krypton, and xenon introduces these atoms into the fullerene cages in about one in 1000 molecules. A “window” mechanism in which one or more of the carbon-carbon bonds of the cage is broken has been proposed to explain the process. The amount of gas inside the fullerenes can be measured by heating to 1000°C to expel the gases, which can then be measured by mass spectroscopy. Information obtained from the nuclear magnetic resonance spectra of helium-3-labeled fullerenes indicates that the magnetic field inside the cage is altered by aromatic ring current effects. Each higher fullerene isomer and each chemical derivative of a fullerene that has been studied so far has given a distinct helium nuclear magnetic resonance peak.

Some new diatomic molecule containing endohedral fullerenes

Abstract Several new diatomic molecule-containing endohedral fullerenes were prepared by heating C 60 or C 70 under high pressures of the corresponding gases. The species prepared are N 2 @C 60 , N 2 @C 70 , 13 CO@C 60 , and 3 He 22 Ne@C 70 . Their existence was demonstrated through high sensitivity, wide dynamic range mass spectrometry. Out of two thousand C 60 molecules about one is observed to incorporate N 2 . The nitrogen molecule containing endohedral molecules are stable and the mass spectrometric signal is not lost even after several hours heating at 500 K. The corresponding endohedral ions undergo the Rice shrink-wrap mechanism; a mass-analyzed ion kinetic energy spectrum demonstrates the loss of a C 2 unit from the cage. The observation of 13 CO@C 60 by mass spectrometry opens up the possibility for future NMR studies of this molecule. The observation of 3 He 22 Ne@C 70 is in accordance with the “promoter” mechanism of Thiel and co-worker [J. Am. Chem. Soc. 118 (1996) 7164; Helv. Chim. Acta 80 (1997) 495], whereby the singly and doubly occupied fullerenes are in equilibrium but the empty and filled fullerenes are not.

An artificial molecule of Ne2 inside C70

Abstract C 70 was heated with 22 Ne at high pressure. Analysis by mass spectrometry unexpectedly showed the formation of Ne 2 @C 70 in addition to Ne@C 70 . The ratio of empty C 70 to mono-occupied to di-occupied was 1000:1:0.02. A mechanism where a small fraction of the fullerene breaks open and reaches equilibrium with the neon and then closes is suggested to explain these results.

3He NMR spectra of highly reduced C60

Two signals were observed in the 3He NMR spectrum of 3He@C60H36. The major signal corresponds with the 3He chemical shift calculated for a structure with D3d′ symmetry.

Release of noble gas atoms from inside fullerenes

Abstract Noble gas atoms can be introduced into the interior of fullerene molecules. A procedure, using a mass spectrometer system, is described for measuring the rates and amounts of noble gas released on heating these compounds. We find that the half life for Ne@C 60 reacting to Ne + C 60 is at least many weeks at 630°C. Additionally, the rate of thermal decomposition of fullerenes can be very substantially faster if traces of trapped solvent are not removed. Noble gas atoms can be introduced into the interior of fullerene molecules. A procedure, using a mass spectrometer system, is described for measuring the rates and amounts of noble gas release on heating these compounds. We find that the half life for Ne@C 60 reacting to Ne+C 60 is at least many weeks at 630°C. Additionally, the rate of thermal decomposition of fullerenes can be very substantially faster if traces of trapped solvent are not removed.

Reduction of C60 using anhydrous hydrazine

Abstract Buckminsterfullerene (C 60 ) can be reduced by anhydrous hydrazine to yield a mixture of C 60 H 2 , C 60 H 4 and other more highly reduced fullerenes.

Mass spectrometric study of unimolecular decompositions of endohedral fullerenes

Abstract Unimolecular decompositions of noble gas containing endohedral fullerenes as well as metallofullerenes were studied using tandem mass spectrometry techniques. Endohedral fullerenes do not lose the endohedral atom unimolecularly but fragment via the loss of C 2 units. Kinetic energy release distributions were measured for the emission of C 2 units from the positive ions of C 60 , Ne@C 60 , Ar@C 60 , Kr@C 60 , C 82 , La@C 82 , Tb@C 82 , C 84 , and Sc 2 @C 84 . These distributions were analyzed using both a model free approach, and a formalism developed by Klots, based on decomposition in a spherically symmetric potential. The C 2 binding energies were deduced from the models. Noble gas atoms are shown to stabilize the fullerene cage. The C 2 binding energies increase in the order: ΔE vap (C 60 + ) vap (Ne@C 60 + ) vap (Ar@C 60 + ) vap (Kr@C 60 + ). Endohedral metal atoms have a strong effect on the cage binding. The C 2 binding energy in La@C 82 + is about 1.5 eV higher than that in C 82 + . The Tb atom has an even stronger effect with a binding energy of about 3 eV higher than for C 82 + . The emission of a C 2 unit from the dimetallofullerenes Sc 2 @C 84 + and Tb 2 @C 84 + was studied as well. Two Sc atoms have a slight destabilizing effect on C 84 , whereas two Tb atoms stabilize the cage.

CHROMATOGRAPHIC FRACTIONATION OF FULLERENES CONTAINING NOBLE GAS ATOMS

Abstract Buckminsterfullerence containing krypton atoms inside the cage was partially separated from empty fullerene via column chromatography. The krypton content of portions of the peak emerging from the column was determined by the pyrolytic release of the krypton followed by mass spectrometry. It was found that material emerging more slowly is about 30% enriched over a faster fraction.

Synthesis, photochemistry and photophysics of stilbene-derivatized fullerenes

The photochemistry and photophysics of two sets of stilbene-derivatized fullerene isomers, in which stilbene is covalently linked to C60, are described. Synthesis and characterization of cis- and trans-stilbene substituted methanofullerenes 1 and 2, and cis- and trans-stilbene substituted fulleropyrrolidines 7 and 8 are described. While UV irradiation of the stilbene-substituted ketal precursors to 1 and 2 lacking the fullerene moiety afforded a photostationary state with 90:10 cis:trans ratio, similar to that of other model stilbene systems, direct and fluorenone-sensitized irradiation of 1 and 2 led to complete conversion to the trans isomer 2, as determined by HPLC analysis. The same results were obtained using cis-trans isomers 7 and 8, namely, the photostationary state on excitation below 350 nm is essentially 100% trans. No isomerisation in either system was obtained on excitation above 400 nm, where all the light is absorbed by the fullerene moiety. By analogy to previous studies of quenching of stilbene excited states, these results suggest that both singlet and triplet excited states of the trans-stilbene moiety in 2 and 8 are being quenched by intramolecular energy transfer to the attached C60, while the much shorter lived cis-stilbene excited states are not similarly quenched. Fluorescence studies on compound 8 support this hypothesis, since the characteristic fluorescence emission of trans-stilbene and trans-stilbene derivatives is not observed in the case of adduct 8. Because of the well established fact that trans-stilbene S1 states are longer lived than the S1 states of the corresponding cis isomers, rapid intramolecular singlet-singlet energy transfer to the appended C60 moiety, ket approximately 10(12) s(-1), is able to compete effectively with radiative and radiationless deactivation of the trans-stilbene S1 states in 2 and 8, but not in the corresponding cis isomers 1 and 7.

The 3He NMR spectra of C60F18 and C60F36; the parallel between hydrogenation and fluorination

Both i 3HeC60F18 and i 3HeC60F36 have been prepared by fluorinating i 3HeC60. The 3He NMR spectrum of i 3HeC60F18 shows a single line at –16.66 ppm, very close to the value of –16.45 ppm, recorded previously for the isostructural i 3HeC60H18. Density functional calculations afford values of –15.0 and –16.2 ppm for the hydrogenated and fluorinated compounds, respectively. The 3He NMR spectrum of i 3HeC60F36 consists of two almost coincident lines (intensity ratio of ca. 3∶1), at –10.49 and –10.52 ppm, attributable respectively to the C3 and T isomers shown previously to be the components (also in ca. 3∶1 ratio) of C60F36. The spectrum is similar to that (lines at –7.8 and –7.9 ppm in a similar ratio) recorded previously for i 3HeC60H36, and provides compelling evidence that C60H36 also consists of a mixture of C3 (major) and T (minor) isomers. The observed ca. 2–3 ppm upfield shift of the spectral lines for the C60F36 isomers compared to those for the C60H36 isomers is reproduced by both density functional and SCF calculations. The C3 isomer involved has been identified by comparison of its 2D 19F NMR spectrum with that for the T isomer. It is not the isomer calculated to be the most stable one, and its formation is believed to be favoured by contiguous activation of double bonds adjacent to those that have already undergone addition.