R. E. Haufler

Uranium Stabilization of C28: A Tetravalent Fullerene

Laser vaporization experiments with graphite in a supersonic cluster beam apparatus indicate that the smallest fullerene to form in substantial abundance is C28. Although ab initio quantum chemical calculations predict that this cluster will favor a tetrahedral cage structure, it is electronically open shell. Further calculations reveal that C28 in this structure should behave as a sort of hollow superatom with an effective valence of 4. This tetravalence should be exhibited toward chemical bonding both on the outside and on the inside of the cage. Thus, stable closed-shell derivatives of C28 with large highest occupied molecular orbital—lowest unoccupied molecular orbital gaps should be attainable either by reacting at the four tetrahedral vertices on the outside of the C28 cage to make, for example, C28H4, or by trapping a tetravalent atom inside the cage to make endothedral fullerenes such as Ti@C28. An example of this second, inside route to C28 stabilization is reported here: the laser and carbon-arc production of U@C28.

XPS probes of carbon-caged metals

Abstract X-ray photoemission spectral probes of endohedral lanthanum—fullerene complexes, La@C n , show that the central La atom is in a formal charge state close to +3, and has been effectively protected from reaction with water an oxygen by the enclosing fullerene cage. Similar results were obtained with endohedral yttrium complexes, Y @C n , and the first carbon-caged metal cluster, Y 2 @C 82 . EPR spectral results are also reported for Y @C 82 which is found to be an S = 1 2 spin system with a 0.48 G hyperfine splitting (at 9.216 GHz) centered at g =1.9999(1).

Fullerene triplet state production and decay: R2PI probes of C60 and C70 in a supersonic beam

Abstract Lifetimes of the lowest triplet state of the two most stable fullerenes, C 60 and C 70 , were measured in a supersonic beam by two-color resonant two-photon ionization. When prepared by intersystem crossing from the singlet manifold, excited at 4.03 eV, these triplet states were found to have lifetimes of 42 and 41 μs, respectively. The energies of these triplet states (1.7 and 1.6 eV, respectively) were measured by photoelectron spectroscopy of the corresponding negative ions.

Carbon ARC Generation of C60

Generation of C 60 at a rate of more than 10 grams per day has been accomplished by operation of a carbon arc in an atmosphere of helium. Optimum yield of 15% was found to occur near 100–200 torr, but yields greater than 3% were found throughout the range between 50 and 760 torr. A model is proposed to explain the observed behavior based on competition between annealing of graphitic sheets to curve so that they minimize dangling bonds, and further rapid growth of these sheets in the gas phase to form giant fullerenes. In agreement with predictions of this model, laser vaporization of graphite targets was found to produce macroscopic quantities of C 60 only when performed in an oven above 1000 C.

Cold molecular beam electronic spectrum of C60 and C70

The electronic spectra of supersonically cooled C60 and C70 have been observed in a molecular beam by two‐photon ionization spectroscopy in the regions 375 to 415 nm and 595 to 640 nm. Clear spectral features are observed in both regions for C60 and in the long wavelength region for C70. Neither molecule is responsible for the diffuse interstellar bands.

Thermionic emission from giant fullerenes

Large even-numbered carbon clusters in the size range from 100 to 600 atoms (giant fullerenes) were generated by laser vaporization and directly injected as positive ions via a supersonic beam into the magnetic trap of an ion cyclotron resonance apparatus. Intense laser excitation of the magnetically levitated fullerenes at 4.0 eV was found to result in production of multiply charged fragments in the trap. Details of the time scale and size dependence of this process suggest it is due to thermionic emission from the superheated gas phase clusters.

Rate of decomposition of C60 and C70 heated in air and the attempted characterization of the products

Solid C60 and C70 were decomposed by heating in air at temperatures ranging from 150 to 250 °C and times ranging from 1 to 743 h. The products dissolved increasingly poorly with increasing temperatures and longer times. The rates at which soluble fullerenes were extracted from products decreased also with longer time of heating. Extrapolations of decomposition rates to 25 °C suggested that a fullerene mix containing 15 mole% C70 became totally insoluble in toluene after about 450 y, while that time is about 2,000 y for C60. Analytical techniques for the characterization of products included X-ray diffraction, Raman spectroscopy, infrared spectroscopy, UV-visible spectroscopy, mass-spectrometry and electron microprobe analysis. The insoluble products contained no detectable amounts of fullerenes. The products were partially soluble in acetone, methanol, and water. The insoluble portions contained a fine-grained, black powder consisting of very small carbon particles. The products contained substances with aromatic bonds and with C  O, C-C and carboxylic groups.