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December 2, 2002
Volume 80, Number 48
CENEAR 80 48 p. 13
ISSN 0009-2347


MATERIALS SCIENCE

DIAMOND-LIKE HYDROCARBONS
Nanoscale molecules with diamond structure isolated from petroleum

REBECCA RAWLS

Diamondoids—Hydrocarbon molecules in which the carbon framework has the structure of a diamond lattice—can be isolated in great variety from natural gas condensates, according to a team of organic chemists at ChevronTexaco Energy Research & Technology Co. Rigid and durable, and with a variety of structures, these nanometer-sized molecules may be a promising new class of molecular building blocks, the researchers suggest.

The simplest diamondoid, the tricyclic molecule adamantane, was isolated from petroleum in the 1930s and synthesized half a century ago. Di- and triamantanes have also been made in the lab, but most of the larger and more structurally complex members of this class have not been available either synthetically or from natural sources.

Now, Jeremy E. Dahl, Shenggao Liu, and Robert M. K. Carlson have isolated and crystallized more than 20 higher order polymantanes [Science, published online Nov. 29, http://www.sciencemag.org/cgi/content/abstract/1078239]. These molecules contain from four to 11 of the 10-carbon adamantane units that make up the diamond crystal lattice.

TWINKLE, TWINKLE Hydrocarbon molecules, 1 to 2 nm in size, with characteristic diamond structure, are shown here both inside and outside a diamond crystal lattice.
The new molecules “are quite rigid and have well-defined structures,” Carlson says. “Some are rodlike, and we’ve isolated a whole series of these of increasing lengths. Some are helical, with different pitches, lengths, and diameters. Some are chiral, and we can separate the enantiomers.”

Liu adds: “We think they have real potential as molecular building blocks. We’ve shown that they can be connected to one another and have various functionalities attached.”

“These are fundamentally new materials that have probably never existed before except dissolved in petroleum,” Dahl points out. “We think their properties will be very interesting.”

Other researchers think so, too. The new work “may reawaken interest in this important class of compounds,” writes chemistry professor Alan P. Marchand of the University of North Texas in a perspective that accompanies the publication. Functionalized smaller diamondoid molecules have important pharmaceutical applications, Marchand points out, particularly 1-aminoadamantane, an antiviral drug that is also used to treat Parkinson’s disease.

“Increasing the tool chest of molecules with interesting properties is part of the game of nanotechnology,” says Wade Adams, director of the Center for Nanoscale Science & Technology at Rice University. “I don’t know whether this class of materials is ever going to amount to anything superspectacular, but they are very cool molecules.”

Like diamonds, the new molecules have great thermal stability, and the ChevronTexaco chemists took advantage of this property to isolate them. After first identifying a petroleum distillate fraction enriched in the molecules, the researchers heated the fraction to 450 °C to destroy less stable hydrocarbons. The diamondoids can be separated from one another by shape-selective chromatography and further purified by crystallization. Current procedures produce the materials "in quantities that would be useful for high-end applications," Carlson says.



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