R.J. Cross

Stable Compounds of Helium and Neon: He@C60 and Ne@C60

It is demonstrated that fullerenes, prepared via the standard method (an arc between graphite electrodes in a partial pressure of helium), on heating to high temperatures release 4He and 3He. The amount corresponds to one 4He for every 880,000 fullerene molecules. The 3He/4He isotopic ratio is that of tank helium rather than that of atmospheric helium. These results convincingly show that the helium is inside and that there is no exchange with the atmosphere. The amount found corresponds with a prediction from a simple model based on the expected volume of the cavity. In addition, the temperature dependence for the release of helium implies a barrier about 80 kilocalories per mole. This is much lower than the barrier expected from theory for helium passing through one of the rings in the intact structure. Amechanism involving reversibly breaking one or more bonds to temporarily open a window in the cage is proposed. A predicted consequence of this mechanism is the incorporation of other gases while the window is open. This was demonstrated through the incorporation of 3He and neon by heating fullerene in their presence.

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.

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.

Hot-atom incorporation of tritium atoms into fullerenes

We have used classical hot-atom chemistry to put tritium atoms inside fullerene molecules. The tritium is generated in a nuclear reactor with the reaction 6Li(n,α)3H. The hot tritium atom slows down and can end up being thermalized inside a fullerene where it is trapped. The irradiated sample is dissolved, chromatographed, and counted in a scintillation counter, showing a small tritium activity. After some time, the sample is analyzed with a sensitive mass spectrometer, and 3He was found on heating above 400°C, showing that the tritium had decayed leaving the 3He trapped inside the fullerene.

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.

The sudden approximation as applied to vibrationally inelastic scattering

Abstract The sudden approximation is used to calculate transition probabilities for vibrationally inelastic scattering in the c.m. energy range of 0.17–17 eV. The system studied was He+H 2 using the potential of Krauss and Mies. Effects of concurrent rotationally inelastic scattering and the orientation dependence of the potential were included in the calculations. In general, colinear collisions were found to give the largest energy transfer, although there were large transition probabilities for other orientations of the molecule and for non-zero impact parameters.

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.

Crossed-beam studies of negative ion-molecule reactions: O− + D2O → OD− + OD

Abstract Product energy and angular distributions have been measured for the endoergic reaction O − + D 2 O → OD − + OD over the relative energy range 3.2–10.5 eV (5.7–18.8 eV lab). The reaction dynamics are found to be well approximated by the spectator-stripping model. Most of the available energy appears as translational energy of the products.

Vibrationally inelastic scattering of H+ + H2

Abstract Using an ab initio potential-energy surface and both classical trajectories and a semiclassical approximation, differential and total cross sections were calculated for H+ + H2 (ν = 0) → H+ + H2 (ν′ = 0,1,2) at an initial c.m. energy of 10 eV. The results compare well with the experiments of Udseth et al.

Collisional fragmentation of Ar@C60

Abstract The fragmentation patterns resulting from collisions between (Ar@C 60 ) + or (Ar@C 60 ) − ions and H 2 , He, CH 4 , Ne, Ar and Kr target gases have been measured. The ion-source material Ar@C 60 was synthesized by heating C 60 under 3000 atm of argon gas, leading to a 10 −3 concentration of endohedral fullerenes. The fragmentation spectra (charged molecules only) are dominated by positive ions both when positive or negative endohedrals break up. Endohedral fragment ions Ar@C n + (48⩽ n ⩽60) as well as all carbon fragments are observed. For collisions involving (Ar@C 60 ) − , ejection of the Ar atom together with two electrons, without permanently damaging the fullerene cage, is a prominent reaction channel, indicating that a `window or a deformation in the form of e.g. a large hole, through which the argon atoms can exit, is opened during the collision.