Marcos M. Alvarez

The Higher Fullerenes: Isolation and Characterization of C76, C84, C90, C94, and C70O, an Oxide of D5h-C70.

The toluene extract of the fluffy carbon material produced by resistive heating of graphite contains a variety of molecules larger than C60 and C70 in a total amount of 3 to 4% by weight. Repeated chromatography of this material on neutral alumina has led to the isolation of stable solid samples of C76, C84, C90, and C94. The characterization, which includes mass spectrometry, 13C nuclear magnetic resonance, electronic absorption (ultraviolet/visible) and vibrational (infrared) spectroscopy identifies these all-carbon molecules as higher fullerenes. In addition, C70O, a stable oxide, has been isolated that is structurally and electronically closely related to D5h-C70. This compound forms during the resistive heating process and probably has an oxygen atom inserted between two carbon atoms on the convex external surface of the C70 skeleton.

Fullerene isomerism : Isolation of C2v-C78 and D3-C78

Early reports on the formation of the higher fullerenes C76, C78, C84, C90, and C94 by resistive heating of graphite stimulated theoretical calculations of possible cage structures for these all-carbon molecules. Among the five fullerene structures with isolated pentagons found for C78, a closed-shell D3h-isomer was predicted to form preferentially. Two distinct C78-isomers were formed in a ratio of ∼5:1 and could be separated by high-performance liquid chromatography. The carbon-13 nuclear magnetic resonance (NMR) spectrum of the major isomer is uniquely consistent with a C2v-structure. The NMR data also support a chiral D3-structure for the minor isomer. The isolation of specifically these two isomers of C78 provides insight into the stability of higher fullerene structures and into the mechanism for fullerene formation in general.

Critical sizes in the growth of Au clusters

Abstract A systematic exploration of the growth of Au clusters has revealed a critical size for the formation of stable assemblies. Mass-spectrometric measurements of clusters, as grown in the presence of an excess of weakly binding long-chain n-alkylthiol(ate) groups, show high yields of new compounds with core masses near the 15k minimum (≈ 75 Au atoms; k = 10 3 amu), and also at 23k, and 29k. Structural analyse establish their cores as compact metal nanocrystals of the corresponding effective diameters (1.4 to 1.7 nm), probably of decahedral close-packing and morphology. The precise identities of this sequence of special structures can now be sought.

Atomic Force Microscope Studies of Fullerene Films: Highly Stable C60 fcc (311) Free Surfaces

Atomic force microscopy and x-ray diffractometry were used to study 1500 �-thick films of pure C60 grown by sublimation in ultrahigh vacuum onto a CaF2 (111) substrate. Topographs of the films did not reveal the expected close-packed structures, but they showed instead large regions that correspond to a face-centered cubic (311) surface and distortions of this surface. The open (311) structure may have a relatively low free energy because the low packing density contributes to a high entropy of the exposed surface.

Collisional probes and possible structures of La2C80

Abstract The soluble molecule La2C80 recently reported by Alvarez et al. exhibits a robustness similar to other fullerenes. It survives intact for ≈ 10−6s following impact against solid surfaces Si(100) at up to 200 eV, yielding an estimate Ef>4 eV for fragmentation processes. The high threshold for electron emission suggests that the electron affinity in La2C−80 is significantly larger than in C60. These facts seem to rule out an external site for either La atom and disfavor a network-site interpretation, and thus favor endohedral (encapsulation) sites. Among the possible cages, the Ih-C80 fullerene appears to be highly favorable for the oxidation state (La3+)2C6−80.

On-line sampling and intact mass analysis of nanometer-size aerosols via time-of-flight high-mass spectrometry

Abstract A device capable of continuously sampling and mass analyzing aerosols in the 1–10 nm diameter size range (masses up to 1 MDa) at part per billion (ppb, 10 10 cm −3 ) concentrations is described. Small aliquots of an aerosol flowing at atmospheric pressure are sampled into a time-of-flight mass spectrometer (TOFMS) via a pulsed molecular beam valve at 10–50 Hz. The aerosol molecular beam is singly and nondestructively ionized with light from an ultraviolet excimer laser and accelerated across a 20 kV electrostatic field. Ionized particle packets are separated in mass during a free flight and re-accelerated across an additional 30 kV into a stainless steel conversion dynode plate. Signals from ejected electrons and negative ion fragments resulting from the particle-dynode collisions are amplified in a dual microchannel plate detector, digitized, and stored in a fast transient recorder. Sampling of He flow streams bearing benzene, fullerenes, as well as Na, Mg, and CsI particles (nanocrystals) is demonstrated. In addition, the gas-phase reaction between a pre-formed Na nanocrystal and alkane thiols is monitored in real-time.

Laser-desorption mass spectrometry and induced coalescence of fullerenes

This article reviews the evolution of the UCLA fullerene research over the past two years from the perspective of laser desorption mass spectrometry (LDMS). As a fast and sensitive analytical method, it has critically contributed to a wide range of fullerene related research projects. Immediately after the discovery of a method for making C60, C70 and selected higher fullerenes in macroscopic amounts, a concerted effort in different laboratories led to the isolation and structure determination of C76, C78, C82 and tentatively C84. Simultaneously with the work on pure fullerene, important progress was achieved in the synthesis of metallofullerenes and derivatized fullerenes. LDMS continually documented progress or failure in extraction, isolation and synthesis and helped to plan future research. Recently, synthesis of C60 and larger fullerenes from precursors, e.g., C18(CO)6, and coalescence reactions of fullerenes, induced by high fluence of the desorption laser, introduced a new aspect of this otherwise exclusively analytical method.