Making chicken wire of molecular size.

Making chicken wire of molecular size

Nine years ago, chemist Orville L. Chapman had one question in mind when he traveled to Germany on a fellowship: "If God would give me one molecule to make, what would that molecule be?" His goal ever since: to build an 80-carbon molecular ball, a lesser-known member of a recently recognized class of spherical molecules called fullerenes.

In Honolulu this week, at the International Chemical Congress of Pacific Basin Societies, Chapman reported making molecular-scale "chicken wire" and other exotic materials, all of which have recently emerged from his efforts to develop a step-by-step laboratory method for building his chosen molecule.

Most famous of the fullerenes is buckminsterfullerene, a 60-carbon, soccer-ball-shaped molecule whose interlocking hexagonal and pentagonal facets resemble the geodesic domes designed by engineering visionary R. Buckminster Fuller. For about five years, researchers have been assembling evidence that the diminutive soccer balls and other carbon spheres form in the sooty wake of laserzapped graphite. The remarkably stable C-60 molecule forms in almost any soot-producing environment, notes fullerene chemist Richard E. Smalley of Rice University in Houston. It may even populate interstellar space (SN: 1/28/89, p.56).

Rather than relying on laser blasts, which produce fullerenes through unknown mechanisms, Chapman aims to design sequences of chemical reactions for methodically assembling both the C-60 and C-80 spheres. "We're trying to make halves of these molecules and then put those halves together" -- a goal that remains elusive, says the University of California, Los Angeles, researcher.

Those efforts have, however, led to the discovery of structurally exciting molecules that might serve as ingredients for new materials, such as selectively permeable membranes for filtering gases from liquids, or for very hard materials of remarkably low weight, Chapman told SCIENCE NEWS.

The molecular chicken wire -- which Chapman named azite because it incorporates nitrogen (once called azote) from the reaction environment during its formation -- begins as small, triangular molecules that virtually self-assemble when dissolved in an organic solvent. "It's a structure that wanted to be," Chapman says. The chicken-wire molecules aggregate into azite, a black, ceramic-like material about as hard as quartz yet only three-fifths as dense.

Chapman also reports making all-carbon polymers with molecular weights hovering around 18,000 daltons, comparable to small proteins. Growing in three dimensions, these soluble mega-molecules bind oxygen and, to a lesser degree, carbon dioxide.

"All we've ever had are infinite network polymers [of carbon], such as graphite and diamond," Chapman says. Though the all-carbon polymers contain well over 1,000 atoms, "these things are stable molecular forms of [pure] carbon," he says. And, while he has yet to coax any of his structures into a sphere, Chapman says he expects the pursuit to yield more surprises and, eventually, some practical new materials.