Carbons Create a Curious Kind of Conducting Cord

Will Epothilone Write An Epitaph for Cancer?

by John Emsley

WHAT'S HOT IN CHEMISTRY...

Rank	Paper	Citations This Period Sep- Oct 97	Rank Last Period Jul- Aug 97
1	D.E. Woon, T.H. Dunning, "Gaussian basis sets for use in correlated molecular calculations. V. Core-valence basis sets for boron through neon," J. Chem. Phys., 103(11):4572-85, 15 September 1995. [Battelle Mem. Inst., Richland, WA] *RU110	15	2
2	R. Jimenez, et al., "Electronic excitation transfer in the LH2 complex of Rhodobacter sphaeroides," J. Phys. Chem., 100(16):6825-34, 18 April 1996. [U. Chicago, IL] *UF879	15	†
3	A. Thess, et al., "Crystalline ropes of metallic carbon nanotubes," Science, 273(5274):483-7, 26 July 1996. [Rice U., Houston, TX; U. Pennsylvania, Philadelphia; Inst. Charles Sadron, Strasbourg, France; Michigan St. U., E. Lansing] *UY983	14	†
4	A.P. Scott, L. Radom, "Harmonic vibrational frequencies: an evaluation of Hartree-Fock, Møller-Plesset, quadratic configuration interaction, density functional theory, and semiempirical scale factors," J. Phys. Chem., 100(41):16502-13, 10 October 1996. [Australian. Natl. U., Canberra] *VL579	14	5
5	M. Wilm, M. Mann, "Analytical properties of the nanoelectrospray ion source," Analyt. Chem., 68(1):1-8, 1 January 1996. [European Molec. Bio. Lab., Heidelberg, Germany] *TN244	13	1
6	T. Pullerits, M. Chachisvilis, V. Sundstöm, "Exciton delocalization length in the B850 antenna of Rhodobacter sphaeroides," J. Phys. Chem., 100(25):10787-92, 20 June 1996. [Lund U., Sweden] *UT213	13	†
7	P. Schwab, R.H. Grubbs, J.W. Ziller, "Synthesis and applications of RuCl₂(=CHR')(PR₃3)₂: the influence of the alkylidene moiety on metathesis activity," J. Amer. Chem. Soc., 118(1):100-10, 10 January 1996. [Caltech, Pasadena; U. Calif., Irvine] *TQ161	11	4
8	R. Koradi, M. Billeter, K. Wüthrich, "MOLMOL: a program for display and analysis of macromolecular structures," J. Molec. Graphics, 14(1):51-5, February 1996. [ETH-Hönggerberg, Zurich, Switzerland] *UH515	10	3
9	J. Gauss, J.F. Stanton, "Perturbative treatment of triple excitations in coupled-cluster calculations of nuclear magnetic shielding constants," J. Chem. Phys., 104(7):2574-83, 15 February 1996. [U. Karlsruhe, Germany; U. Texas, Austin] *TU985	10	†
10	J.P. Wolfe, S. Wagaw, S.L. Buchwald, "An improved catalyst system for aromatic carbon-nitrogen bond formation: the possible involvement of bis(phosphine) palladium complexes as key intermediates," J. Amer. Chem. Soc., 118(30):7215-6, 31 July 1996. [MIT, Cambridge, MA] *VA026	10	†

SOURCE: ISI's Hot Papers Database. Read the full legend.

Richard ("Rick") Smalley shared the Nobel Prize for chemistry in 1996, along with Harry Kroto and Robert Curl, for the discovery of a variety of carbon, the fullerenes, whose molecules consist of spheres of 60 atoms or more. Not only did the new compounds have fascinating chemical properties, but they were the forerunner of even more unusual forms of carbon, the so-called nanotubes.

Position #3 in the latest Hot Ten describes one of the more exceptional kinds, consisting of "ropes" formed from bundles of nanotubes. (This paper, with its 15 co-authors, also reached #3 in the July/August 1997 list.) The new material described in the paper is not only fascinating, but the authors offer a realistic theory of how the tubes come to have the same diameter. It is this which allows them to produce rope-like structures.

Some nanotubes are multi-layered, wrapped around one another like the sheathing of electric cables. Less common are the single-layered type of nanotubes, although these give rise to even more curious structures. For example, chemists at DuPont and SRI International made "sea urchins" in which such tubes radiated outwards in three dimensions from a gadolinium-carbon single-crystal core.

Two years ago Smalley's team at the Center for Nanoscale Science and Technology, of Rice University, Houston, Texas, came up with a fascinating new variant of single-layer nanotubes. As many as 500 such strands can organize themselves into a single rope, and this remarkable material is produced by using a laser to vaporize carbon from a graphite rod at 1,200° C, with a small amount of nickel and cobalt present to act as a catalyst.

Yields of the new material were in excess of 70%, and it was investigated by X-ray diffraction and transmission electron microscopy (TEM). Paper #3 contains some superb visuals of the ropes produced by TEM, showing the uniformity of the strands and how these pack together in a triangular array. In some samples the nanotubes appeared to have eaten their way through particles of apparently amorphous carbon. The Rice workers also investigated the electrical conductivity of a strand of rope by mounting a length of it under an optical microscope and applying two electrodes to it, then measuring the current that passed. Linear current voltages were recorded with good stability, and the values obtained showed that they were the most highly conductive carbon fibers known.

The ropes were up to 200 Å in diameter, and some were over 100 micrometers long; in experiments that produced these samples, it was impossible to find the ends of the ropes. However, by varying the experimental conditions for their formation it was possible to make shorter strands in which the ends were observed, and these appeared to be sealed with hemispherical caps of carbon, as with other nanotubes.

So how did Smalley's team manage to produce this extraordinary material? The sheer uniformity of the nanotubes, and the ropes formed from them, ruled out normal mechanisms that would account for their growth either as crystals, or from polyynes, which are chains of carbon atoms with triple bonds along their length. Instead Smalley proposes a "scooter" mechanism which involves atoms of the nickel or cobalt catalyst. It is these which scoot around the open end of the tube, encouraging the incoming carbon atoms to form the six-membered rings that are essential for its growth, and preventing the formation of five-membered rings that would curve the end of the tube and lead to a closed cap. Any large carbon fragments that also are picked up by the growing tube would be dismantled by the metal atoms, which would digest them into smaller radicals that can then be incorporated into the growing edge.

Dr. John Emsley is Science Writer in Residence at the
Department of Chemistry, University of Cambridge, U.K.