S. Stevenson,t H. C. Dorn,’J P. Burbank,t K. Harich,? J. Haynes, Jr.,? C. H. Kiang,* J. R. Salem,§ M. S. DeVries,s P. H. M. van Loosdrecht,§ R. D. Johnson,§ C. S. Yannoni,§ and D. S. B e t h u d Department of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 2406 1, Materials and Molecular Simulation Center, Beckman Institute, California Institute of Technology, Pasadena, California 9 1 125, and IBM Research Division, Almaden Research Center, San Jose, California 95120

In this study, we report production, isolation, and characterization for the relatively small endohedral metallofullerene, La2@C72. As described, La2@C72 is readily isolated from conventional electric-arc-generated carbon/metal soot. This new species was purified by HPLC chromatography and characterized by laser desorption mass spectrometry and UV-vis spectroscopy. The mass spectrum also demonstrates the presence of the monometal species, La@C72, but the absence of empty-cage C72. Since empty-cage C72 has not been successfully isolated to date, the results of the present study are in agreement with the argument for metalmediated stabilization of the C72 carbon cage by lanthanum ions. The chromatographic retention data suggest that the electronic structure of La2@C72 is consistent with a (La)2@C72 species and the prediction of a relatively small dipole moment.

In this study, we report production, isolation, and characterization for the relatively small endohedral metallofullerene, La-2@C-72. As described, La-2@C-72 is readily isolated from conventional electric-arc-generated carbon/metal soot. This new species was purified by HPLC chromatography and characterized by laser desorption mass spectrometry and UV-vis spectroscopy. The mass spectrum also demonstrates the presence of the monometal species, La@C-72, but the absence of empty-cage C-72. Since empty-cage C-72 has not been successfully isolated to date, the results of the present study are in agreement with the argument for metal-mediated stabilization of the C-72 carbon cage by lanthanum ions. The chromatographic retention data suggest that the electronic structure of La-2@C-72 is consistent with a (La3+)(2)@C-72(6-) species and the prediction of a relatively small dipole moment.

Many polymers with short chains and molecular weights up to several thousand atomic mass units can be transferred to the vapor phase whole by pulsed laser desorption, in contrast with the well-known, destructive ablation process encountered with long-chain, high-molecular-weight polymers. However, the vaporized whole polymers are hot and thus fragment extensively when photoionized. For their detection as molecular ions, it is necessary first to cool the vaporized polymers, which is accomplished by entraining them in an Ar jet expansion. Near-threshold, single-photon ionization at 125 nm (9.9 eV) was used in all cases and compared with 193 nm, two-photon ionization for polystyrene. Mass spectra showing oligomeric distributions for samples of poly(dimethylsiloxane), poly(ethylene oxide), poly(isoprene), poly(perfluorotrimethylene oxide) and polystyrene are reported. Application of the technique to samples of increasing average molecular weight shows that, as the chain length increases, thermal decomposition eventually predominates over vaporization. © 1997 Elsevier Science S.A.

We have measured the far infrared transmittance of Sc2@C84 and Er2@C82 at 1.5 K between 30 and 200 cm−1. Both materials are observed to have a large primary absorption feature centered at 95 cm−1 with a width of approximately 50 cm−1, as well as a number of secondary absorption features which are different in the two materials. This is the first study of the far infrared properties of metallofullerenes and may help in the determination of the structural and electronic properties of these materials.

We have used a combination of laser desorption, separate multiphoton ionization, and mass spectrometry to characterize films of perfluorinated polyethers. Photoionization is achieved through end group chromophores, which are either present in the polymer or chemically attached. The technique provides parent molecular weight distributions without fragmentation. Examples are given of end group analysis, wavelength dependent spectroscopy, analysis of repeat unit distributions in copolymers, and polymer degradation analysis.

Electron paramagnetic resonance experiments on endohedral ${\mathrm{Sc}}_{3}$ in ${\mathrm{C}}_{82}$ show 22 hyperfine coupling split transitions with unusually large linewidths. Both the nuclear hyperfine coupling and the linewidths are found to be strongly temperature dependent. The data show that the three Sc ions are equivalent, and strongly suggest that they form a trimer which rapidly reorients within the ${\mathrm{C}}_{82}$ cage. A simple model is proposed which is in good agreement with the data.

We describe an automated HPLC separation of the endohedral metallofullerenes such as Sc@C-2n and Y@C-2n from empty-cage fullerenes utilizing two polystyrene chromatographic columns (500 and 1000 Angstrom) in series. Rapid separation of the metallofullerene fraction from the empty-cage fullerenes (e.g., C-60) under anaerobic conditions is achieved. For the isolated Sc@C-2n fraction, all even-carbon-membered species from Sc-2@C-74 to Sc-2@C-104 were identified by negative-ion chemical ionization mass spectrometry. In addition, Sc-3@C-82 was a prominent component of this fraction. For the separated Y@C-2n sample, the mass spectral data indicate the presence of Y@C-82 and all even-carbon-nurnbered diyttrium species from Y-2@C-82 to Y-2@C-104.

Perfluorinated polyethers of the type Ra(OCF2CF2CF2)nF, where Ra is the end group C6H6HCH2CH2O(CO)CF2CF−2-, were laser-vaporized and entrained in a pulsed jet expansion. Two photon ionization of the jet-cooled polymers via the phenoxy chromophore was combined with time of flight (TOF) mass spectrometry. Mass spectra were obtained for polymer distributions extending to 7000 Da, with minimal fragmentation. Under the appropriate expansion and desorption conditions parent masses of van der Waals dimers of these polymers were also observed. By scanning the ionization laser and monitoring particular mass-to-charge ratios, resonance-enhanced two photon ionization (R2PI) spectra were obtained for the jet-cooled polymers and their dimers near the electronic origin. Polymers with the two end groups, present as an impurity in the samples, were detected exclusively in an internally dimerized form. In both the internal and external cases, the dimerization occurs only at the phenoxy chromophore. The R2PI spectra of a series of model compounds were measured and used to characterize the evolution of the spectra from phenol toward the polymer. The model compound spectra revealed the role of multiple conformations and molecular size in the polymer spectra, which are ultimately broadened by low frequency motions of multiple conformers. The results are discussed relative to teh general problem of the photoionization of large molecules.

The direct coupling of high-performance liquid chromatography (HPLC) with on-line electron paramagnetic resonance (EPR) detection is demonstrated for monitoring separations of endohedral metallofullerenes (M@C-2n). The HPLC-EPR approach readily permits detection of the paramagnetic species, such as Y@C-82 and Sc-3@C-82, in the presence of the dominant empty-cage fullerenes (C-60, C-70) and diamagnetic metallofullerenes (e.g., M(2)@C-2n). The results indicate that on-line EPR provides a noninvasive, selective detector for HPLC metallofullerene separations that is readily adaptable to air-sensitive and/or labile compounds. Specifically, the ''EPR-active'' metallofullerenes, Y@C-82 and Sc-3@C-82, are selectively monitored on-line for an initial separation of the metallofullerene fraction from the dominant empty-cage fullerenes utilizing a combination of polystyrene columns. This preparative ''cleanup'' procedure is followed by HPLC-EPR separation and monitoring of Y@C-82 and SC3@C-82 species using a selective tripodal pi-acidic-phase column (Trident-Tri-DNP) for the final stages of isolation.

Encapsulating atoms or molecules inside fullerene cages could give rise to a myriad of novel molecules and materials. The existence of such species is now strongly supported by a growing body of experimental evidence. Fullerene-metal complexes generally thought to be endohedral are being produced and purified in milligram quantities, and their structure and properties are beginning to be explored.

CARBON exhibits a unique ability to form a wide range of structures. In an inert atmosphere it condenses to form hollow, spheroidal fullerenes1-4. Carbon deposited on the hot tip of the cathode of the arc-discharge apparatus used for bulk fullerene synthesis will form nested graphitic tubes and polyhedral particles5-8. Electron irradiation of these nanotubes and polyhedra transforms them into nearly spherical carbon 'onions'9. We now report that covaporizing carbon and cobalt in an arc generator leads to the formation of carbon nanotubes which all have very small diameters (about 1.2 nm) and walls only a single atomic layer thick. The tubes form a web-like deposit woven through the fullerene-containing soot, giving it a rubbery texture. The uniformity and single-layer structure of these nanotubes should make it possible to test their properties against theoretical predictions10-13.

Analysis of interior samples of the Murchison meteorite by two routes yielded an upper limit of 2 ppb for its C60 content, as compared to parts per million levels for individual polycyclic aromatic hydrocarbons (PAHs). Provided the samples contain an interstellar component, which is probable since Murchison hydrocarbons contain excess deuterium, this result argues against the ubiquitous presence of C60 in the interstellar medium. A possible explanation for the absence of C60 was found in experiments showing how PAHs replace fullerenes as stable end products when hydrogen is present during carbon condensation. As a secondary result we found high molecular weight PAHs in the Murchison and Allende meteorites. Coronene and its methyl derivatives are especially interesting since features in the coronene spectrum have been shown to match some of the unidentified interstellar infrared emission bands.

The authors have investigated the rotational dynamics of C{sub 60} both in solution and the solid state, using {sup 13}C NMR. The rotational correlation times and activation energies of C{sub 60} in several solvents will be reported, and the results compared to measurements in the solid state. These results will then be used to interpret both EPR and NMR experiments on Scandium clusters encapsulated in fullerene cages. Measurement of {sup 45}Sc NMR relaxation rates are found to support conclusions from EPR experiments that the metal clusters form molecules inside the carbon cages.

Mass spectra of perfluorinated polyether (PFPE) films on metal substrates have been measured using a novel transition-metal cationization technique combined with FTMS. Full parent mass distributions were obtained with this technique. Two pulsed lasers were employed: a low fluence laser for PFPE desorption and a high fluence laser for metal ion formation. Cationization was observed to occur in the gas phase above the sample surface. The dynamics of the process are discussed. Applications to measure average molecular weights and study photochemistry of PFPE samples are presented. Electron attachment to PFPEs was also studied. Only negative ion fragments were observed from this process.

EXAFS experiments on a fullerene sample containing both YC82 and Y2C82 are reported, performed both at 10 K and at room temperature, to probe the structural environment of the yttrium atoms. The results are similar at both temperatures. The data can be fit with a model with two shells of 6 carbon atoms each, at 2.4 and 2.9 Å, respectively. This result supports the hypothesis that the metal atoms are trapped inside the fullerene cage, consistent with recent calculations on possible metallofullerene structures.

The results of a variety of experiments used to characterize fullerenes, metallofullerenes and alkali-metal intercalated C60 are reported. It is shown that differential scanning calorimetry and NMR characterization of the orientational phase transition in C60 provide sensitive means to assess the purity and crystallinity of fullerene samples. The results of a mass-spectrometric investigation of metallofullerene samples produced by co-vaporization of carbon and metals are described. An account is given of some electron paramagnetic resonance results obtained for LaC82 and solid-state NMR results obtained on an alkali-intercalated fullerite, Rb3C60, which show for the first time the presence of magnetically inequivalent carbons in underivatized C60. Together these experiments yield a great deal of information about the phase purity, molecular dynamics and structure of a variety of fullerene materials.

Both vibrationally and rotationally resolved spectra of the S1←S0 transition in jet-cooled triphenylamine (TPA) around 340-320 nm are reported. Medium resolution spectra (0.5–1.0 cm−1 resolution) are recorded using (1 + 1)-resonance enhanced multi photon ionization (REMPI) with mass selective time-of-flight (TOF) detection in a pulsed molecular beam apparatus. The origin of the S1←S0 transition is at 29520.7 cm−1, higher than halfway to the ionization potential (IP) found at 6.89 eV. A vibratioal progression in the symmetric torsion mode (114 cm−1) as well as in the symmetric CN stretching mode (280 cm−1) is observed in the electronic spectra. The spectrum of the most abundant isomer of the TPA-Ar (TPA-Kr) complexes is blue-shifted by 211 cm−1 (216 cm−1) with respect to the spectrum of the free TPA molecule. High-resolution (the resolution mainly being determined by the natural linewidth of the transition, i.e. 36 MHz) spectra are recorded using laser induced fluorescence (LIF) in a cw molecular beam apparatus. Individual rotational transitions are resolved and the spectrum shows unambiguously that TPA is a symmetric top molecule. The rotational constant B″ in the S0 state of TPA is equal to B″ = 403.7 ± 0.5 MHz. Upon S1←S0 excitation both B and C increase with 7.4 ± 0.1 MHz and 2.8 ± 0.1 MHz, respectively. The spectrum of the blue-shifted TPA-Ar isomer is the spectrum of a symmetric top molecule as well, and therefore the Ar atom has to be located on the C3 symmetry axis, either on top of or underneath the umbrella formed by the phenyl rings. It appears that when Ar or Kr forms a complex with TPA, the first Ar, Kr, atom goes preferentially in a position on the C3 symmetry axis of TPA, a position which causes an abnormal blue-shift of the spectrum. With the first rare-gas atom located in this special position, the second rare-gas atom is forced into a “normal” position, i.e. above one of the phenyl rings, causing a normal red-shift with respect to the TPA-Ar complex.

We have studied solid C60, measuring the shift of the optical absorption edge with pressure to 35 GPa. In the range of 0-17 GPa, extrapolation of the absorption edge indicates that metallization should occur by 33 GPa. However, from 17 to 25 GPa an irreversible transition to a "transparent phase" occurs. Raman scattering of the depressurized sample shows no trace of C60, diamond, or graphite, indicating that the transition involves the collapse of the C60 molecules into a new structure of carbon.